Principles of Geology, by Charles Lyell

Re: Principles of Geology, by Charles Lyell

Postby admin » Fri Jul 17, 2015 2:32 am

CHAPTER 13

Crag of Norfolk and Suffolk – Shown by its fossil contents to belong to the older Pliocene period – Heterogeneous in its composition – Superincumbent lacustrine deposits – Relative position of the crag – Forms of stratification – Strata composed of groups of oblique layers – Cause of this arrangement – Dislocations in the crag produced by subterranean movements – Protruded masses of chalk – Passage of marine crag into alluvium – Recent shells in a deposit at Sheppey, Ramsgate, and Brighton

CRAG OF NORFOLK AND SUFFOLK.

THE older Pliocene strata, described in the last chapter, are all situated in countries bordering the Mediterranean, but we shall now consider a group in our own island, which belongs to the same era. We have already alluded to this deposit under the provincial name of crag, [1] and pointed out its superposition to the London clay, a tertiary formation of much higher antiquity. [2] The crag is chiefly developed in the eastern parts of Norfolk and Suffolk, from whence it extends into Essex.

Its relative age. -- A collection of the shells of the 'crag' beds, which I formed in 1829, together with a much larger number sent me by my friend, Mr. Mantell of Lewes, were carefully examined by M. Deshayes, and compared to the tertiary species in his cabinet. This comparison gave the following result: out of 111 species, 66 were extinct or unknown, and 45 recent, the last, with one exception (Voluta Lamberti, Sow.), being now inhabitants of the German ocean. Such being the proportion of recent and extinct species, we may conclude, according to the rules before laid down, [3] that the crag belongs to the older Pliocene period.

Mineral composition. -- So heterogeneous is this deposit in mineral character, that we can scarcely convey any correct notions of its appearance, without describing the beds separately in the different localities where they occur. In general, they consist of sand, gravel, and blue or brown marl-the shells imbedded in the sand and marl being, for the most part, broken and sometimes finely comminuted. In a few spots we find the deposit in the form of a soft stratified rock, composed almost entirely of corals, sponges, and echini, [4] an assemblage of species which probably lived in a tranquil sea of some depth. In other parts of our coast it consists of alternations of sand and shingle, destitute of organic remains, and more than 200 feet in thickness, as in the Suffolk cliffs, between Dunwich and Yarmouth. In others, we meet with an enormous mass, more than 300 feet in thickness, of sand, loam, and clay, containing bones of terrestrial quadrupeds and drift wood, sometimes stratified regularly, at others consisting of a confused heap of rubbish, in which fragments of the chalk and its flints are imbedded in a chalky marl.

In this aggregate are also found many fragments of older rocks, the septaria of the London clay, together with ammonites, vertebrae of ichthyosauri, and other fossils from parts of the oolitic series. It has been questioned whether all the above-mentioned beds can be considered as belonging to the same era. The subject may admit of doubt, but after examining, in 1829, the whole line of coast of Essex, Suffolk, and Norfolk, I found it impossible to draw any line of separation between the different groups. Each seemed in its turn to pass into another, and those masses which approach in character to alluvium, and contain the remains of terrestrial quadrupeds, are occasionally intermixed with the strata of the crag.

There are, however, lacustrine deposits overlying the crag, which probably belong to a distinct zoological period. These are found in small cavities, which must have existed on the surface of the crag after its elevation, and which formed small lakes or ponds wherein recent fresh-water testacea were included in loamy strata. (See wood-cut, No. 30, c.)

Relative position. -- The crag is seen to rest on the chalk and on the London clay, but usually on the former. The strata are in great part horizontal, or slightly undulating; but at some points they are much disturbed, especially where several masses of chalk appear to have been protruded from below.

The annexed section may give a general idea of the manner in which the crag may be supposed to rest on the chalk as we pass from the Norfolk cliffs, at Trimmingham, into the interior, where the country rises gradually.

Image
No. 30.
a, Chalk.
b, Crag.
c, Lacustrine deposit.
D, Trimmingham beacon.
E, Interior and higher parts of Norfolk. [5]


The outline of the surface of the subjacent chalk, in this section, is imaginary, but is such as might explain the relations of those protruded masses, three of which appear in the cliffs near Trimmingham, and which some geologists have too hastily assumed to be unconnected with the great mass of chalk below. We shall treat of these presently, when we describe the disturbances which the crag appears to have suffered since its original deposition.

In the interior, at E, there is a thick covering of sand and gravel upon the chalk, having the characters of an alluvium, partly, perhaps, marine, and partly terrestrial, and which seems to pass gradually in this district into the regular marine strata of the crag.

Forms of stratification. -- In almost every formation the individual strata are rarely persistent for a great distance, the superior and inferior planes being seldom precisely parallel to each other; and if the materials are very coarse, the beds often thin out if we trace them for a few hundred yards. There are also many cases where all the layers are oblique to the general direction of the strata, and the crag affords most interesting illustrations of this phenomenon.

In the sea-cliff near Walton, in Suffolk, opposite the Martello Tower, called R, the section represented in the annexed diagram is seen. The vertical height is about 20 feet, and the beds consist alternately of sets of inclined and horizontal layers of sand and comminuted shells. The sand is siliceous and of a ferruginous colour, but the layers are sometimes made up of small plates of bivalve shells, arranged with their flat sides parallel to the plane of each layer, like mica in micaceous sandstones.

Image
No. 31: Section of shelly crag near Walton, Suffolk.

The number of laminae in the thickness of an inch, both in the siliceous and shelly sand, varies from seven to ten in number, so that it is impossible to express them all ill the diagram. The height of the uppermost stratum is, in this instance, remarkable, as it extends to twelve feet. The inclination of the laminae is about 30°; but in the cliffs of Bawdesey, to the eastward, they are sometimes inclined at an angle of 45°, and even more.

Image
No. 32: Section at the lighthouse near Happisborough. Height sixteen feet.
a, Pebbles of chalk flint, and of rolled pieces of white chalk.
b, Loam overlying a.
c, c, Blue and brown clay.


This diagonal arrangement of the layers, sometimes called 'false stratification,' is not confined to deposits of fine sand and comminuted shells, for we find beds of shingle disposed in the same manner as is seen in the annexed section (No. 32).

The direction of the dip of the inclined layers, throughout the Suffolk coast, is so uniformly to the south, that I only saw two or three instances of a contrary nature, where the inclination was northerly. One of the best examples of this variation is exhibited in a cliff between Mismer and Dunwich, wood-cut No. 33. In this case, there are about six layers in the thickness of an inch, and the part of the cliff represented is about six feet high.

Image
No. 33: Section of part of Little Cat cliff, composed of quartzose sand, showing the inclination of the layers in opposite directions.

Another example may be seen near Walton, where the layers, which are of extreme tenuity, consist of ferruginous sand, brown loam, and comminuted shells. It is not uncommon to find in this manner sets of perfectly horizontal strata resting upon and covered by groups of wavy and transverse layers.

Image
No. 34: Lamination Of shelly sand and loam. near the Signal-house, Walton. Vertical height four feet.

The appearances exhibited in the diagrams are not peculiar to the crag, and I have seen sand and pebble-beds of all ages, including the old red sandstone, greywacke, and clay-slate, exhibit the same arrangement.

When we inquire into the causes of such a disposition of the materials of each bed or group of layers, we may, in the first place, remark, that however numerous may be the successive layers a, b, c, the layer a must have been deposited before b, b before c, and so of the rest.

Image
No. 35.

We must suppose that each thin seam was thrown down on a slope, and that it conformed itself to the side of the steep bank, just as we see the materials of a talus arrange themselves at the foot of a cliff when they have been cast down successively from above. If the transverse layers are cut off by a nearly horizontal line, as in many of the above sections, it may arise from the denuding action of a wave which has carried away the upper portion of a submarine bank and truncated the layers of which it was composed. But I do not conceive this hypothesis to be necessary; for if a bank have a steep side, it may grow by the successive apposition of thin strata thrown down upon its slanting side, and the removal of matter from the top may proceed simultaneously with its lateral extension. The same current may borrow from the top what it gives to the sides, a mode of formation which I had lately an opportunity of observing on the rippled surface of the hills of blown sand near Calais. The undulating ridges and intervening furrows on the dunes of blown sand resembled exactly in form those caused by the waves on a sea-beach, and were always at right angles to the direction of the wind which had produced them. Each ridge had one side slightly inclined and the other steep, the lee side being always steep, as b c, d e, the windward side a gentle slope, as a b, c, d. When a gust of wind blew with sufficient force to drive along a cloud of sand, all the ridges were seen to be in motion at once, each encroaching on the furrow before it, and, in the course of a few minutes, filling the place which the furrows had occupied. Many grains of sand were drifted along the slopes a b, and c d, which, when they fell over the scarps b c, and d e, were under shelter from the wind, so that they remained stationary, resting, according to their shape and momentum, on different parts of the descent. In this manner each ridge was distinctly seen to move slowly on as often as the force of the wind augmented. We think that we shall not strain analogy too far if we suppose the same laws to govern the subaqueous and subaerial phenomena; and if so, we may imagine a submarine bank to be nothing more than one of the ridges of ripple on a larger scale, which may increase in the manner before suggested, by successive additions to the steep scarps.

Image
No. 36.

The set of tides and currents, in opposite directions, may account for sudden variations in the direction of the dip of the layers, as represented in the wood-cut, No. 33, while the general prevalence of a southerly inclination in the Crag of Suffolk may indicate that the matter was brought by a current from the north.

We may refer to a drawing given in the first volume, [6] to show the analogy of the arrangement of the submarine strata, just considered, to that exhibited by deposits formed in the channels of rivers where a considerable transportation of sediment is in progress.

Derangement in the Crag strata. -- In the above examples we have explained the want of parallelism or horizontality in the subordinate layers of different strata, by reference to the mode of their original deposition; but there are signs of disturbance which can only be accounted for by subsequent movements. The same blue and brown clay, or loam, which is often perfectly horizontal, and as regularly bedded as any of our older formations, is, in other places, curved and even folded back upon itself, in the manner represented in the annexed diagrams.

Image
No. 37: Bent strata of loam in the cliffs between Cromer and Runton.

Image
No. 38: Folding of the strata between East and West Runton.

In the last of these cuts a central nucleus of sand is surrounded by argillaceous and sandy layers. This phenomenon is very frequent, and there are instances where the materials thus enveloped consist of broken flints mingled with pieces of chalk, forming a white mass encircled by dark laminated clay. The diameter of these included masses, as seen in sections laid open in the sea-cliffs, varies from five to fifteen feet.

East of Sherringham, a heap of partially-rounded flints, about five feet in diameter, is nearly enveloped by finely-laminated strata of sand and loam, and some of the loam is entangled in the midst of the flints.

Image
No. 39: Section in the Cliffs east of Sherringham.
a, Sand and loam in thin layers.


In this and similar instances, we may imagine the yielding strata, a, to have subsided into a cavity, and the flints belonging to a superincumbent bed. to have pressed down with their weight, so as to cause the strata to fold round them.

That some masses of stratified sand and loam have actually sunk down into cavities, or have fallen like landslips into ravines, seems indicated by other appearances. Thus, near Sherringham, the argillaceous beds, a, represented in the annexed diagram (No. 40), are cut off abruptly, and succeeded by the vertical and contorted series, b, c. The face of the cliff here represented, is 24 feet in height. Some of the layers in b, b, are composed of pebbles, and these alternate with thin beds of loose sand. The whole set must once have been horizontal, and must have moved in a mass, or the relative position of the several parts would not have been preserved. Similar appearances may, perhaps, be produced when chasms open during earthquakes and portions of yielding strata fall in from above and are engulphed.

Image
No. 40: Section east of Sherringham, Norfolk.
a, Sand, loam, and blue clay.
b, b, Sand and gravel.
c, Twisted beds of loam.


Protruded masses of chalk. -- But whatever opinion we may entertain on this point, we cannot doubt that subterranean movements have given rise to some of the local derangements in this formation, particularly where masses of solid chalk pierce, as it were, the crag. Thus, between Mundesley and Trimmingham we see the appearances exhibited in the accompanying view (No. 41). The chalk, of which the strata are highly inclined, or vertical, projects in a promontory, because it offers more resistance to the action of the waves than the tertiary beds which, on both sides, constitute the whole of the cliff. The height of the soft crag strata immediately above the chalk is, in this place, about 130 feet. Those which are in contact (see the wood-cut) are inclined at an angle of 45:), and appear more disturbed than in other parts of the cliffs, as if they had been displaced by the movement by which the chalk was protruded.

Image
No. 41: Side view of a promontory of chalk and crag, Trimmingham, Norfolk.,
a, Gravel and ferruginous sand, rounded and angular pieces of chalk flint, with some quartz pebbles, 3 feet.
b, Laminated blue clay, 8 feet.
c, Yellow sand, 1 foot 6 inches.
d, Dark blue clay with fragments of marine shells, 6 feet.
e, Yellow loam and flint gravel, 3 feet.
f, Light blue clay, 1 foot.
g, Sand and loam, 12 feet.
h, Yellow and white sand, loam, and gravel, about 100 feet.


Very similar appearances are exhibited by the northernmost of the three protuberances of chalk, of which a front view is given in the annexed diagram. It occupies a space of about 100 yards along the shore, and projects about 60 yards in advance of the general line of cliff. One of its edges, at c, rests upon the blue clay beds of the crag, in such a manner as to imply that the mass had been undermined when the crag was deposited, unless we suppose, as some have done, that this chalk is a great detached mass enveloped by crag. For, as one of the 'Needles', or insulated rocks of chalk, which projected 120 feet above high water-mark, at the western extremity of the Isle of Wight, fell into the sea in 1772, [7] so a pinnacle of chalk may have been precipitated into the tertiary sea, at a point where some strata of the crag had previously accumulated. The beds of flint and chalk in the above diagram appear nearly horizontal, but they are in fact highly inclined inwards towards the cliff. The rapid waste of the Norfolk coast might soon enable us to understand the true position of this mass, if observations and drawings are made from time to time of the appearances which present themselves.

Image
No. 42: Northern protuberance of chalk, Trimmingham.
a. Chalk with flints.
b. Gravel, of broken and half-rounded flints.
c. Laminated blue clay.
d. Sand and yellow loam.


Perhaps it may be necessary to suppose, that subterranean movements were in progress during the deposition of the crag, and the extraordinary dislocations of the beds, in some places, which in others are perfectly regular and horizontal, may be most easily accounted for by introducing an alternate rise and depression of the bed of the sea, such as we know to be usually attendant on a series of subterranean convulsions. Several of the contortions may also have been produced by lateral movements.

Passage of marine crag into alluvium. -- By supposing the adjoining lands to have participated in this movement, we may explain the origin of those masses of an alluvial character which contain the detritus of many rocks, the bones of land animals and of drift timber, which were evidently swept down into the sea. The land-floods which accompany earthquakes are, as we have seen, capable of transporting such materials to great distances, [8] and, as part of these alluviums must be left somewhere upon the land, we may expect to find, on exploring the interior, a gradual passage from the terrestrial alluvium to that which was carried down into the sea, and which alternates with marine beds.

The fossil quadrupeds imbedded in the crag appear to be the same as those of a great part of the alluviums of the interior of England, which may, therefore, have been formed when the testacea of the older Pliocene period were in existence.

Upon the whole, we may imagine the crag strata to bear a great resemblance to the formations which may now be in progress in the sea between the British and Dutch coasts, -- a sea for the most part shallow, yet having here and there a depth of 50 or 60 fathoms, and where strong tides and currents prevail; where shells, also, and zoophytes abound, and where matter drifted from wasting cliffs must be thrown down in certain receptacles in the form of sand, shingle, and mud.

In conclusion we may observe that the history of the crag requires further elucidation, and the author is by no means satisfied with the sketch above given; but as the country is so accessible and the formation so interesting both in its structure and zoological characters, it is hoped that these remarks may excite curiosity and lead to fuller investigation.

Sheppey. -- Ramsgate. -- Brighton. -- Deposits have lately been observed by Mr. Crow [9] resting on the London clay, in the Isle of Sheppey, at the height of 140 feet above the sea, and by Captain Kater at Pegwell Bay, near Ramsgate, at the height of a few yards, and by Mr. Mantell, in the cliffs near Brighton, all containing recent marine shells. But as there are only five or six species yet discovered in these localities, we cannot decide, till we obtain further information, whether these strata belong to the crag or to a more recent formation.

_______________

Notes:

1. Chap. ii. p. 19.

2. See above, Diagram No.4, p. 21.

3. Page 54.

4. R. Taylor, Geol. of East Norfolk.

5. This section is compiled principally from one by Mr. Murchison, the others in this chapter are from drawings by the Author.

6. Chap. xiv., Diag. No. 6.

7. Dodsley's Annual Register, vol. xv. p. 140.

8. Vol. i. chap. 25.

9. Of Christ Church College, Cambridge.
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Re: Principles of Geology, by Charles Lyell

Postby admin » Fri Jul 17, 2015 2:36 am

CHAPTER 14

Volcanic rocks of the older Pliocene period – Italy – Volcanic region of Olot in Catalonia – Its extent and geological structure – Map – Number of cones – Scoriae – Lava currents – Ravines in the latter cut by water – Ancient alluvium underlying lava – Jets of air called 'Bufadors' – Age of the Catalonian volcanos uncertain – Earthquake which destroyed Olot in 1421 – Sardinian volcanos – District of the Eifel and Lower Rhine – Map – Geological structure of the country – Peculiar characteristics of the Eifel volcanos – Lake craters – Trass – Crater of the Roderberg – Age of the Eifel volcanic rocks uncertain – Brown coal formation

VOLCANIC ROCKS OF THE OLDER PLIOCENE PERIOD.

Italy. -- IT is part of our proposed plan to consider the igneous as well as the aqueous formations of each period, but we are far from being able as yet to assign to each of the numerous groups of volcanic origin scattered over Europe a precise place in the chronological series. We have already stated that the volcanic rocks of Tuscany belong, in great part at least, to the older Pliocene period, -- those for example of Radicofani, Viterbo, and Aquapendente, which have been chiefly erupted beneath the sea. The same observation would probably hold true in regard to the igneous rocks of the Campagna di Roma.

But several other districts, of which the dates are still uncertain, may be mentioned in this chapter as being possibly referrible to the period now under consideration. It will at least be useful to explain to the student the points which require elucidation before the exact age of the groups about to be described can be accurately determined.

Volcanos of Olat, in Catalonia. -- I shall first direct the reader's attention to a district of extinct volcanos in the north of Spain, which is little known, and which I visited in the summer of 1830.

The whole extent of country occupied by volcanic products in Catalonia is not more than fifteen geographical miles from north to south, and about six from east to west. The vents of eruption range entirely within a narrow band running north and south, and the branches which we have represented as extending eastward in the map are formed simply of two lava-streams, those of Castell Follit and Cellent.

Image
No. 43: Volcanic district of Catalonia.

Dr. Maclure, the American geologist, was the first who made known the existence of these volcanos; [1] and, according to his description, the volcanic region extended over twenty square leagues, from Amer to Massanet. I searched in vain in the environs of Massanet, in the Pyrenees, for traces of a lava-current; and I can say with confidence that the adjoined map gives a correct view of the true area of the volcanic action.

Geological structure of the district. -- The eruptions have burst entirely through secondary rocks, composed in great part of grey and greenish sandstone and conglomerate, with some thick beds of nummulitic limestone. The conglomerate contains pebbles of quartz, limestone, and Lydian stone. The limestone is not only replete with nummulites, but occasionally includes oysters, pectens, and other shells. This system of rocks is very extensively spread throughout Catalonia, one of its members being a red sandstone, to which the celebrated salt-rock of Cardona is subordinate. It is conjectured that the whole belongs to the age of our green-sand and chalk.

Near Amer, in the Valley of the Ter, on the southern borders of the region delineated in the map, primary rocks are seen consisting of gneiss, mica-schist, and clay-slate. They run in a line nearly parallel to the Pyrenees, and throw off the secondary strata from their flanks, causing them to dip to the north and north-west. This dip, which is towards the Pyrenees, is connected with a distinct axis of elevation, and prevails through the whole area described in the map, the inclination of the beds being sometimes at an angle of between 40 and 50 degrees.

It is evident that the physical geography of the country has undergone no material change since the commencement of the era of the volcanic eruptions, except such as has resulted from. the introduction of new hills of scoriae and currents of lava upon the surface. If the lavas could be remelted and poured out again from their respective craters, they would descend the same valleys in which they are now seen, and reoccupy the spaces which they at present fill. The only difference in the external configuration of the fresh lavas would consist in this, that they would nowhere be intersected by ravines, or exhibit marks of erosion by running water.

Volcanic cones and lavas. -- There are about fourteen distinct cones with craters in this part of Spain, besides several points whence lavas may have issued; all of them arranged along a narrow line running north and south, as will be seen in the map. The greatest number of perfect cones are in the immediate neighbourhood of Olot, some of which are represented in the frontispiece, and the level plain on which that town stands has clearly been produced by the flowing down of many lava-streams from those hills into the bottom of a valley, probably once of considerable depth like those of the surrounding country.

In the frontispiece an attempt is made to represent by colours the different geological formations of which the country is composed. The blue line of mountains in the distance are the Pyrenees, which are to the north of the spectator, and consist of primary and ancient secondary rocks. In front of these are the secondary formations described in this chapter, coloured grey. Different shades of this colour are introduced, to express various distances. The flank of the hill, in the foreground, called Costa de Pujou, is composed partly of secondary rocks and partly of volcanic, the red colour expressing lava and scoriae.

The Fluvia, which passes near the town of Glot, has only cut to the depth of forty feet through the lavas of the plain before mentioned. The bed of the river is hard basalt, and at the bridge of Santa Madalena, are seen two distinct lava-currents, one above the other, separated by a horizontal bed of scoriae eight feet thick.

In one place, to the south of Olot, the even surface of the plain is broken by a mound of lava, called the 'Bosque de Tosca,' the upper part of which is scoriaceous, and covered with enormous heaps of fragments of basalt more or less porous. Between the numerous hummocks thus formed, are deep cavities, having the appearance of small craters. The whole precisely resembles some of the modern currents of Etna, or that of Come, near Clermont, the last of which, like the Bosque de Tosca, supports only a scanty vegetation.

Most of the Catalonian volcanos are as entire as those in the neighbourhood of Naples, or on the flanks of Etna. One of these, figured in the frontispiece, called Montsacopa, is of a very regular form, and has a circular depression or crater at the summit. It is chiefly made up of red scoriae, undistinguishable from that of the minor cones of Etna. The neighbouring hills of Olivet and Garrinada, also figured in the frontispiece, are of similar composition and shape. The largest crater of the whole district occurs farther to the east of Olot, and is called Santa Margarita. It is 455 feet deep, and about a mile in circumference. Like Astroni, near Naples, it is richly covered with wood, wherein game of various kinds abound.

Although the volcanos of Catalonia have broken out through sandstone, shale, and limestone, as have those of the Eifel, in Germany, to be described in the sequel, there is a remarkable difference in the nature of the ejections composing the cones in these two regions. In the Eifel, the quantity of pieces of sandstone and shale thrown out from the vents, is often so immense as far to exceed in volume the scoriae, pumice, and lava; but I sought in vain in the cones near Olot for a single fragment of any extraneous rock, and Don Francisco Bolos informs me that he has never been able to detect any. Volcanic sand and ashes are not confined to the cones, but have been sometimes scattered by the wind over the country, and drifted into narrow valleys, as is seen between Olot and Cellent, where the annexed section is exposed. The light cindery volcanic matter rests in thin regular layers, just as it alighted on the slope formed by the solid conglomerate. No flood could have passed through the valley since the scoriae fell, or these would have been for the most part removed.

Image
No. 44
a, Secondary conglomerate.
b, Thin seams of volcanic sand and scoriae.


The currents of lava in Catalonia, like those of Auvergne, the Vivarais, Iceland, and all mountainous countries, are of considerable depth in narrow defiles, but spread out into comparatively thin sheets in places where the valleys widen. If a river has flowed on nearly level ground, as in the great plain near Olot, the water has only excavated a channel of slight depth; but where the declivity is great, the stream has cut a deep section, sometimes by penetrating directly through the central part of a lava-current, but more frequently by passing between the lava and the secondary rock which bounds the valley. Thus, in the accompanying section, at the bridge of Cellent, six miles east of Olot, we see the lava on one side of the small stream, while the inclined stratified rocks constitute the channel and opposite bank. The upper part of the Java at that place is scoriaceous; farther down it becomes less porous, and assumes a spheroidal structure; still lower it divides in horizontal plates, each about two inches in thickness, and is more compact. Lastly, at the bottom is it mass of prismatic basalt about five feet thick. The vertical columns often rest immediately on the subjacent secondary rocks; but there is sometimes an intervention of such sand and scoriae as cover a country during volcanic eruptions, and which when unprotected, as here, by superincumbent lava, is washed away from the surface of the land. Sometimes the bed d contains a few pebbles and angular fragments of rock; in other places fine earth, which may have constituted an ancient vegetable soil.

Image
No. 45: Section above the bridge of Cellent.
a, Scoriaceous lava.
b, Schistose basalt.
c, Columnar basalt.
d, Scoriae, vegetable soil, and alluvium.
e, Nummulitic limestone.
f, Micaceous grey sandstone.


In several localities, beds of sand and ashes are interposed between the lava and subjacent stratified rock, as may be seen if we follow the course of the lava-current which descends from Las Planas towards Amer, and stops two miles short of that town. The river there has often cut through the lava, and through eighteen feet of underlying limestone. Occasionally an alluvium, several feet thick, is interposed between the igneous and marine formation; and it is interesting to remark, that in this, as in other beds of pebbles occupying a similar position, there are no rounded fragments of lava, whereas, in the modern gravel beds of rivers in this country, volcanic pebbles are abundant.

The deepest excavation made by a river through lava, which I observed in this part of Spain, is that seen in the bottom of a valley near San Feliu de Pallerols, opposite the Castell de Stolles. The lava there has filled up the bottom of a valley, and a narrow ravine has been cut through it to the depth of 100 feet. In the lower part the lava has a columnar structure. A great number of ages were probably required for the erosion of so deep a ravine; but we have no reason to infer that this current is of higher antiquity than those of the plain near Olot. The fall of the ground, and consequent velocity of the stream, being in this case greater, a more considerable volume of rock may have been removed in an equal quantity of time.

We shall describe one more section to elucidate the phenomena of this district. A lava-stream, flowing from a ridge of hills to the east of Olot, descends a considerable slope until it reaches the valley of the river Fluvia. Here, for the first time, it comes in contact with running water, which has removed a portion, and laid open its internal structure in a precipice about 130 feet in height, at the edge of which stands the town of Castell Follit.

By the junction of the rivers Fluvia and Teronel the mass of lava has been cut away on two sides; and the insular mass B (No. 46) has been left, which was probably never so high as the cliff A, as it may have constituted the lower part of the sloping side of the original current.

Image
No. 46: Section at Castell Follit.
A, Church and town of Castell Follit, overlooking precipices of basalt.
B, Small island, on each side of which branches of the river Teronel flow to meet the Fluvia.
c, Precipice of basaltic lava, chiefly columnar.
d, Ancient alluvium underlying the lava-current.
e, Inclined strata of secondary sandstone.


From an examination of the vertical cliffs, it appears that the upper part of the lava on which the town is built is scoriaceous, passing downwards into a spheroidal basalt; some of the huge spheroids being no less than six feet in diameter. Below this is a more compact basalt with crystals of olivine. There are in all about four distinct ranges of prismatic basalt, separated by thinner beds not columnar, and some of which are schistose. The whole mass rests on alluvium, ten or twelve feet in thickness, composed of pebbles of limestone and quartz, but without any intermixture of igneous rocks; in which circumstance alone it appears to differ from the modern gravel of the Fluvia.

Bufadors. -- The volcanic rocks near Olot have often a cavernous structure like some of the lavas of Etna; and in many parts of the hill of Batet, in the environs of the town, the sound returned by the earth, when struck, is like that of an archway. At the base of the same hill are the mouths of several subterranean caverns, about twelve in number, which are called in the country 'bufadors,' from which a current of cold air issues during summer; but which in winter is said to be scarcely perceptible. I visited one of these bufadors in the beginning of August, 1830, when the heat of the season was. unusually intense, and found a cold wind blowing from it, which may easily be explained, for as the external air when rarefied by heat ascends, the cold air from the interior of the mountain rushes in to supply its place.

Age of the Catalonian volcanos uncertain. -- It now only remains to offer some remarks on the probable age of these Spanish volcanos. Attempts have been made to prove, that in this country, as well as in Auvergne and the Eifel, the earliest inhabitants were eye-witnesses to the volcanic action. In the year 1421 it is said, when Olot was destroyed by an earthquake, an eruption broke out near Amer, and consumed the town. The researches of Don Francisco Bolos have, I think, shown, in the most satisfactory manner, that there is no good historical foundation for the latter part of this story; and any geologist who has visited Amer must be convinced that there never was any eruption on that spot. It is true that, in the year above-mentioned, the whole of Olot, with the exception of a single house, was cast down by an earthquake; one of those shocks which at distant intervals, during the last five centuries, have shaken the Pyrenees, and particularly the country between Perpignan and Olot, where the movements, at the period alluded to, were most violent.

Some houses are said to have sunk into the earth; and this account has been corroborated by the fact that, within the memory of persons now living, the buried arches of a Benedictine monastery were found at a depth of six feet beneath the surface; and still later, some houses were dug out in the street called Aigua. Don Bolos informed me, that he was present when the latter excavation was made, and when the roof of a buried house, nearly entire, was found six feet beneath the surface, the interior being in a great part empty, so that it was necessary to fill it up with earth and stones, in order to form a sure foundation for the new edifice.

The annihilation of the ancient Olot may, perhaps, be ascribed, not to the extraordinary violence of the movement on that spot, but to the cavernous nature of the subjacent rocks; for Catalonia is beyond the line of those modern European earthquakes which destroy towns throughout extensive areas.

As we have no historical records, then, to guide us in regard to the extinct volcanos, we must appeal to geological monuments. We have little doubt that some fossil land-shells, and bones of quadrupeds, will hereafter reward the industry of collectors. If such remains are found imbedded in volcanic ejections, the period of the eruptions may be inferred; but at present we have no evidence beyond that afforded by superposition, in regard to which the annexed diagram will present to the reader, in a synoptical form, the results obtained from numerous sections.

Image
No. 47: Superposition of rocks in the volcanic district of Catalonia.
a, Sandstone and nummulitic limestone.
b, Older alluvium with volcanic pebbles.
c, Cones of scoriae and lava.
d, Newer alluvium.


The more modern alluvium d is partial, and has been formed by the action of rivers and floods upon the lava; whereas the older gravel, b, was strewed over the country before the volcanic eruptions. In neither have any organic remains been discovered, so that we can merely affirm, as yet, that the volcanos broke out after the elevation of some of the newest rocks of the secondary series, and before the formation of an alluvium, d, of unknown date. The integrity of the cones merely shows that the country has not been agitated by violent earth quakes, nor subjected to the action of any great transient flood since their origin.

East of Olot, on the Catalonian coast, marine tertiary strata occur, which, near Barcelona, attain the height of about 500 feet. It appears probable, from a small number of shells which I collected, that these strata may correspond with the Subapennine beds, so that if the volcanic district had extended thus far, we might be able to determine the age of the igneous products, by observing their relation to these older Pliocene formations. [2]

Sardinian volcanos. -- The line of extinct volcanos in Sardinia, described by Captain Smyth, [3] is also of uncertain date, as, notwithstanding the freshness of some of the cones and lavas, they may be of high antiquity. They rest, however, on a tertiary formation, supposed by some to correspond to the Subapennine strata, but of which the fossil remains have not been fully described.

VOLCANIC ROCKS OF THE EIFEL.

The volcanos of the Lower Rhine and the Eifel are of no less uncertain date than those of Catalonia; but we are desirous of pointing out some of their peculiar characters, and shall, therefore, treat of them in this chapter, trusting that future investigations will determine their chronological relations more accurately.

For the geographical details of this volcanic region, we refer the reader to the annexed map, for which I am indebted to Mr. Leonard Horner, whose residence in the country has enabled him to verify the maps of MM. Noeggerath and Von Oyenhausen, from which that now given has been principally compiled.

Image
No. 48.

There has been a long succession of eruptions in this country, and some of them must have occurred when its physical geography was in a very different state, while others have happened when the whole district had nearly assumed its present configuration.

The fundamental rock of the Eifel is an ancient secondary sandstone and shale, to which the obscure and vague appellation of 'graywacke' has been given. The formation has precisely the characters of a great part of those grey and red sandstones and shales which are called 'old red sandstone' in England and Scotland, where they constitute the inferior member of the carboniferous series. In the Eifel they occupy the same geological position, and in some parts alternate with a limestone, containing trilobites and other fossils of our moun tain and transition limestones. The strata are inclined at all angles from the horizontal to the vertical, and must have undergone reiterated convulsions before the country was moulded into its present form.

Lake-Craters. -- The volcanos have broken out sometimes at the bottom of deep valleys, sometimes on the summit of hills, and frequently on intervening platforms. The traveller often falls upon them unexpectedly in a district otherwise extremely barren of geological interest. Thus, for example, he might arrive at the village of Gemunden, immediately south of Daun, without suspecting that he was in the immediate vicinity of some of the most remarkable vents of eruption. Leaving a stream which flows at the bottom of a deep valley in a sandstone country, he climbs the steep acclivity of a hill where he observes the edges of strata of sandstone and shale dipping inwards towards the mountain. When he has ascended to a considerable height he sees fragments of scoriae sparingly scattered over the surface, till at length on reaching the summit he finds himself suddenly on the edge of a tarn, or deep circular lake-basin.

Image
No. 49: The Gemunden Maar.

This, which is called the Gemunden Maar, is the first of three lakes which are in immediate contact, the same ridge forming the barrier of two neighbouring cavities (see diag. No. 50). On viewing the first of these we recognize the ordinary form of a crater, for which we have been prepared by the occurrence of scoriae scattered over the surface of the soil. But on examining the walls of the crater, we find precipices of sandstone and shale which exhibit no signs of the action of heat, and we look in vain for those beds of lava and scoriae, dipping in opposite directions on every side, which we have been accustomed to consider as characteristic of volcanic craters. As we proceed, however, to the opposite side of the lake, and afterwards visit the craters c and d, we find a considerable quantity of scoriae and some lava, and see the whole surface of the soil sparkling with volcanic sand and ejected fragments of half-fused shale, which preserves its laminated texture in the interior, while it has a vitrified or scoriform coating.

Image
No. 50.
a, Village of Gemunden.
b, Gemunden Maar.
c, Weinfelder Maar.
d, Schalkenmehren Maar.


We cannot, therefore, doubt that these great hollows have been formed by gaseous explosions; in other words, that parts of the summits of hills composed of sandstone and shale were blown up during a copious discharge of gas or steam, accompanied by the escape of a small quantity of lava. It is a peculiar feature of the Eifel volcanos that aeriform discharges have been violent, and the quantity of melted matter poured out from the vents proportionably insignificant. In this respect they differ, as a group, from any assemblage of extinct volcanos which I have seen in France, Italy, or Spain.

In some of the Eifel lavas, as in Auvergne and the Vivarais, fragments of granite, gneiss, and clay-slate are found inclosed; pieces of these rocks having probably been torn off by the melted matter and gases as they rose from below. A few miles to the south of the lakes above-mentioned occurs the Pulvermaar of Gillenfeld, an oval lake of very regular form, and surrounded by an unbroken ridge of fragmentary materials. consisting of ejected shale and sandstone, and preserving an uniform height of about one hundred and fifty feet above the water. The side slope in the interior is at an angle of about 45°; on the exterior, of 35°. Volcanic substances are intermixed very sparingly with the ejections which in this place entirely conceal from view the stratified rocks of the country. [4]

The Meerfelder Maar is a cavity of far greater size and depth, hollowed out of similar strata; the sides presenting some abrupt sections of inclined secondary rocks, which in other places are buried under vast heaps of pulverised shale. I could discover no scoriae amongst the ejected materials, but balls of olivine, and other volcanic substances are mentioned as having been found. [5] This cavity, which we must suppose to have discharged an immense volume of gas, is nearly a mile in diameter, and is said to be more than one hundred fathoms deep. In the neighbourhood is a mountain called the Moseberg, which consists of red sandstone and shale in its lower parts, but supports on its summit a triple volcanic cone, while a distinct current of lava is seen descending the flanks of the mountain. The edge of the crater of the largest cone reminded me much of the form and characters of that of Vesuvius.

If we pass from the Upper to the Lower Eifel we find the celebrated lake-crater of Laach, which has a greater resemblance than any of those before-mentioned to the Lago di Bolsena, and others in Italy-being surrounded by a ridge of gently sloping hills, composed of loose tuffs, scoriae, and blocks of a variety of lavas.

Trass and its origin. -- It appears that in the Lower Eifel eruptions of trachytic lava preceded the emission of currents of basalt, and that immense quantities of pumice were thrown out wherever trachyte issued. In this district, also, we find the tufaceous alluvium of the Rhine volcanos called trass, which has covered large areas, and choked up some valleys now partially re-excavated. This trass is, like the loess, unstratified. The base is composed almost entirely of pumice, in which are included fragments of basalt and other lavas, pieces of burnt shale, slate, and sandstone, and numerous trunks and branches of trees.

If an eruption, attended by a copious evolution of gases, should now happen in one of the lake basins, we might suppose the water to remain for weeks in a state of violent ebullition, until it became of the consistency of mud, just as the sea became charged with red mud round the new island of Sciacca, in the Mediterranean, in the year 1831. If a breach should then be made in the side of the cone, the flood would sweep away great heaps of ejected fragments of shale and sandstone, which would be borne down into the adjoining valleys. Forests would be torn up by such a flood, which would explain the occurrence of the numerous trunks of trees dispersed irregularly through the trass.

Crater of the Roderberg. -- One of the most interesting volcanos on the left bank of the Rhine is called the Roderberg. It forms a circular crater nearly a quarter of a mile in diameter, and one hundred feet deep, now covered with fields of corn. The highly inclined graywacke strata rise even to the rim of one side of the crater, but they are overspread by quartzose gravel, and this again is covered by volcanic scoriae and tufaceous sand. The opposite wall of the crater is a scoriaceous rock, like that at the summit of Vesuvius. It is quite evident that the eruption in this case burst through the graywacke and alluvium which immediately overlies it; and I observed some of the quartz pebbles mixed with scoriae on the flanks of the mountain, so placed as if they had been cast up into the air, and had fallen again with the volcanic ashes.

On the opposite, or right bank of the Rhine, are the Siebengebirge, a group of mountains wherein analogous phenomena are exhibited. There also trachytic lavas have flowed out and covered the graywacke; and basaltic currents of a somewhat later date have followed.

There is, however, such a connexion between these rocks that a suite might be procured from the Siebengebirge, showing an insensible gradation from highly crystalline trachyte into compact basalt, with the accompanying passage of the hornblende in the former, into augite in the latter.

Age of the volcanic rocks of the Eifel uncertain. -- Besides the ancient inclined graywacke, we have in the immediate vicinity of the valley of the Rhine, a nearly horizontal tertiary formation, called brown coal, from the association with it of beds of lignite worked for coal. The great mass of the igneous rocks are seen to be newer than this formation; and thus we obtain a relative date of much local importance for the volcanos of the whole region. This brown coal consists of beds of sand and sandstone, with nodules of clay- ironstone, and siliceous conglomerate. Beds of lignite of various thickness are interstratified with the clays and sands, and often irregularly diffused through them. This deposit was classed with the plastic clay at a time when every group of tertiary strata was referred to the age of some one of the subdivisions of the Paris basin, but as no shells, either marine, fresh-water, or land have yet been found imbedded, it is not easy to decide the age of the formation. Near Marienforst, in the vicinity of Bonn, large blocks are found on the surface of a white opaque quartz rock, containing numerous casts of fresh-water shells which appear to belong to Planorbis rotundatus and Limnea longiscatus, two well-known Eocene species; [6] but this rock is not in situ, and may possibly have been a local deposit in some small lake, fed by a spring holding silica in solution. Yet, as there are beds of the brown coal at Marienforst, and this formation contains in other places subordinate beds of silex, it seems to me most probable that the quartzose blocks alluded to were derived from some member of that tertiary group.

The other organic remains of the brown coal are principally fishes; they are found in a bituminous shale, called paper-coal, from being divisible into extremely thin leaves. The individuals are extremely numerous, but they appear to belong to about five species, which M. Agassiz informs me are all extinct, and hitherto peculiar to the brown coal. They belong to the fresh-water genera Leuciscus, Aspius, and Perca. The remains of frogs also, of an extinct species, have been discovered in the paper coal, and a perfect series may be seen in the museum at Bonn, from the most imperfect state of the tadpole to that of the full-grown animal. With these a salamander, scarcely distinguishable from the recent species, has been found.

All the distinguishable remains of plants in the lignite and associated beds are said to belong to dicotyledonous trees and shrubs, bearing a close resemblance to those now existing in the country. The same is declared to be the case with the remains found in the trachytic tuffs and in the trass; but the absolute identification of species on which some geologists have insisted must be received with great caution.

As trachytic tuff has been observed at several places interstratified with the clay beds of the brown coal formation, and containing the same impressions of plants, there can be no doubt that the oldest eruptions began when the fresh-water deposits were still in progress, and when the geographical features of the country must have been extremely different from those which it has now assumed.

We have stated that the volcanic ejections of the Roderberg repose upon a bed of gravel. This gravel forms part of an ancient alluvium which is quite distinct in character from that now found in the plains of the valley of the Rhine. It consists chiefly of quartz pebbles, and is found at considerable elevations both on the graywacke and brown coal beds. It forms indeed a general capping to the latter, varying from ten to thirty-five feet in thickness, and was probably an alluvium formed at that period when the ancient lake, in which the brown coal strata were deposited, was drained; for the disappearance of that great body of fresh water may naturally be supposed to have taken place when the country was undergoing great changes in its physical geography.

Beds and large veins of quartz are found in the Hundsruck, Taunus, and Eifel, the nearest mountain-chains which border this part of the Rhine, and their degradation may have supplied the quartz found in this gravel called Kiesel gerolle by the Germans.

It has been supposed by some writers that the latest volcanic eruptions of the Eifel and Rhine coincided in epoch with the deposition of the Loess before described (chap. xi.). Such an association, if established, would give a comparatively recent date to the most modern igneous eruptions; but I looked in vain for any clear indications of such a connexion, and all the sections which I saw appeared to indicate the posteriority of the Loess. The integrity of the volcanic cones is, for reasons before explained, a character to which we attach no value.

We have, therefore, in this region, graywacke covered by brown coal, and some volcanic formations so blended with the latter as to prove the igneous eruptions to have been contemporaneous. Yet when we endeavour to assign a chronological position to anyone part of the series by reference to organic remains, we discover that the evidence is vague and inconclusive. I have as yet been unable to obtain satisfactory proof that anyone species of fossil animal or plant has been found in the brown coal, or superimposed formations which was common to a tertiary group of known date in any other part of Europe; whereas the reader will bear in mind that the relative age of different tertiary formations, of which we have before spoken, was usually determined by reference to a comparison of several hundred, often more than a thousand, species of testacea. [7]

_______________

Notes:

1. Maclure, Journ. de Phys., vol. lxvi. p. 219, 1808; cited by Daubeny, Descriptionor Volcanos, p. 24.

2. For some account of the Olot volcanos see 'Noticia de Los Estinguidos Volcanes de Ia Villa de Olot,' by Francisco Bolos. Barcelona. No date, -- but the observations, I am told, preceded those of Dr. Maclure.

3. Present state of Sardinia, &c., pp. 69, 70.

4. Scrape, Edin. Journ. of Sci., June 1826, p. 145.

5. Hibbert, Extinct Volcanos of the Rhine, p. 24.

6. M. Deshayes, to whom I showed the specimens, said he felt as confident of the above identifications as mere casts would warrant.

7. A memoir has lately been communicated to the Geological Society of London, by Mr. Homer, on the geology of this district. For fuller details consult Noeggerath's Rheinland Westphalen, and the works of Von Dechen, Oyenhausen, Von Buch, Steinger, Van der Wyck, Scrope, Duubeny, Leonhard, and Hibbert.
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Re: Principles of Geology, by Charles Lyell

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CHAPTER 15

Miocene period – Marine formations – Faluns of Touraine – Comparison of the Faluns of the Loire and the English Crag – Basin of the Gironde and Landes – Fresh-water limestone of Saucats – Position of the limestone of Blaye – Eocene strata in the Bordeaux basin – Inland cliff near Dax – Strata of Piedmont – Superga – Valley of the Bormida – Molasse of Switzerland – Basin of Vienna – Styria – Hungary – Volhynia and Podolia – Montpellier

MIOCENE FORMATIONS-MARINE.

HAVING treated in the preceding chapters of the older and newer Pliocene formations, we shall next consider those members of the tertiary series which we have termed Miocene. The distinguishing characters of this group, as derived from its imbedded fossil testacea, have been explained in the fifth chapter (p. 54). In regard to the relative position of the strata, they underlie the older Pliocene, and overlie the Eocene formations, when any of these happen to be present.

The area covered by the marine, fresh-water, and volcanic rocks of the Miocene period, in different parts of Europe, can already be proved to be very considerable, for they occur in Touraine, in the basin of the Loire, and still more extensively in the south of France, between the Pyrenees and the Gironde. They have also been observed in Piedmont, near Turin, and in the neighbouring valley of the Bormida, where the Apennines branch off from the Alps. They are largely developed in the neighbourhood of Vienna and in Styria; they abound in parts of Hungary; and they overspread extensive tracts in Volhynia and Podolia.

Shells characteristic of the Miocene strata are found in all these countries, figures of some of which are given in Plate 2 in this volume. They characterize the period, because they are either wanting or extremely rare in the Eocene or Pliocene formations.

We shall now proceed to notice briefly some of the countries before enumerated as containing monuments of the era under consideration.

Touraine. -- We have already alluded to the proofs of superposition adduced by M. Desnoyers, to show that the shelly strata provincially called 'the Faluns of the Loire' were posterior to the most recent fresh-water formation of the basin of the Seine. Their position, therefore, shows that they are of newer origin than the Eocene strata,-more recent, at least, than the uppermost beds of the Paris basin. But an examination of their fossil contents proves also that they are referrible to that type which distinguishes the Miocene period. When three hundred of the Touraine shells were compared with more than eleven hundred of the Parisian species, there were scarcely more than twenty which could be identified; and, on the other hand, the fossil shells of the Touraine beds agree far less with the testacea now inhabiting our seas than does the group occurring in the older Pliocene strata of northern Italy.

The Miocene strata of the Loire have been observed to repose on a great variety of older rocks between Sologne and the sea, in which line they are seen to rest successively upon gneiss, clay-slate, coal-measures, Jura limestone, greenstone, chalk, and lastly upon the upper fresh-water deposits of the basin of the Seine. They consist principally of quartzose gravel, sand, and broken shells. The beds are generally incoherent, but sometimes agglutinated together by a calcareous or earthy cement, so as to serve as a building-stone. Like the shelly portion of the crag of Norfolk and Suffolk, the faluns and associated strata are of slight thickness, not exceeding seventy feet. They often bear a close resemblance to the crag in appearance, the shells being stained of the same ferruginous colour, and being in the same state of decay; serving in Touraine, just as in Norfolk and Suffolk, to fertilize the arable land. Like the crag, also, they contain mammiferous remains, which are not only intermixed with marine shells, but sometimes encrusted with serpulae, flustra, and balani. These terrestrial quadrupeds belong to the genera Mastodon, Rhinoceros, Hippopotamus, &c., the assemblage, considered as a whole, being very distinct from those of the Paris gypsum.

I examined several detached patches of the Touraine beds, where they rest on primary strata in the environs of Nantes, particularly one locality at Les Cleans, about eight miles southeast of that town, and was struck with the evidence afforded by them of the emergence of large intervening tracts of granitic schist since the Miocene era, which we might otherwise have supposed to have been raised at a very remote epoch. It is probable that these patches of tertiary deposits were originally local, having been thrown down wherever the set of the tides and currents permitted an accumulation to take place.

The faluns and contemporary strata of the basin of the Loire may be considered generally as having been formed in a shallow sea, into which a river, flowing perhaps from some of the lands now drained by the Loire, introduced from time to time fluviatile shells, wood, and the bones of quadrupeds, which may have been washed down during floods. Some of these bones have precisely the same black colour as those found in the peaty shell-marl of Scotland; and we might imagine them to have been dyed black in Miocene peat which was swept down into the sea during the waste of cliffs, did we not find the remains of cetacea in the same strata, bones, for example, of the lamantine, morse, sea-calf, and dolphin, having precisely the same colour.

Comparison of the Faluns of the Loire and the English Crag. -- The resemblance which M. Desnoyers has pointed out as existing between the English crag and the French faluns is one which ought by no means to induce us to ascribe a contemporaneous origin to these two groups, but merely a similarity of geographical circumstances at the respective periods when each was deposited. In every age, where there is land and sea, there must be shores, shallow estuaries, and rivers; and near the sea-coasts banks of marine shells and corals may accumulate. It must also be expected that rivers will drift in fresh-water shells, together with sand and pebbles, and occasionally, perhaps, sweep down the carcasses of land quadrupeds into the sea. If the sand and shells, both of the 'crag' and the 'faluns' have each acquired the same ferruginous colour, such a coincidence would merely lead us to infer that, at each period, there happened to be springs charged with iron, which flowed into some part of the sea or basin of the river, by which the sediment was carried down into the sea.

Even had the French and English strata which we are comparing shared a greater number of mineral characters in common, that identity could not have justified us in inferring the synchronous date of the two groups, where the discordance of fossil remains is so marked. The argument which infers a contemporaneous origin from correspondence of mineral contents, proceeds on the supposition that the materials were either washed down from a common source, or from different sources into a common receptacle. If, according to the latter hypothesis, the crag and the faluns were thrown down in one continuous sea, the testacea could not have been so distinct in two very contiguous regions, unless we assume that the laws which regulated the geographical distribution of species were then distinct from those now prevailing. But if it be said that the two basins may have been separated from each other, as are those of the Mediterranean and Red Sea, by an isthmus, and that distinct assemblages of species may have flourished in each, as ill the example above-mentioned is actually the case, [1] we may reply that such narrow lines of demarcation are extremely rare now, and must have been infinitely more so in remoter tertiary epochs, because there can be no doubt that the proportion of land... to sea has been greatly on the increase in European latitudes during the more modern geological eras.

In the faluns, and in certain groups of the same age, which occur not far to the west of Orleans, M. Desnoyers has discovered the following mammiferous quadrupeds. Palaeotherium magnum, Mastodon angustidens, Hippopotamus major, and H. minutus, Rhinoceros leptorhinus, and R. minutus, Tapir gigas, Anthracotherium (small species), Sus, Equus (small species), Cervus, and an undetermined species of the Rodentia.

The first species on this list is common to the Paris gypsum, and is therefore an example of a land quadruped common to the Miocene and Eocene formations, an exception perfectly in harmony with the results obtained from the study of fossil shells. [1]

Basin of the Gironde and district of the Landes. -- A great extent of country between the Pyrenees and the Gironde is overspread by tertiary deposits which have been more particularly studied in the environs of Bordeaux and Dax, from whence about 600 species of shells have been obtained. These shells belong to the same type as those of Touraine. -- See Appendix I. [2]

Most of the beds near Dax, whence these shells are procured, consist of incoherent quartzose sand, mixed for the most part with calcareous matter, which has often bound together the sand into concretionary nodules. A great abundance of fluviatile shells occur in many places intermixed with the marine; and in some localities microscopic shells are in great profusion.

The tertiary deposits in this part of France are often very inconstant in their mineralogical character, yet admit generally of being arranged in four groups, which are enumerated in the explanation of diagram No. 51.

In some places the united thickness of these groups is considerable, but in the country between the Pyrenees and the valley of the Adour around Dax, the disturbed secondary rocks are often covered by a thin pellicle only of tertiary strata, which rests horizontally on the chalk and does not always conceal it.

Image
No. 51: Tertiary strata overlying chalk in the environs of Dax.
a, Siliceous sand without shells.
b, Gravel.
c, Sand and marl with shells.
d, Blue marl with shells.
E, Chalk and volcanic tuff.


In the valleys of the Adour and Luy, sections of all the members of the tertiary series are laid open, but the lowest blue marl, which is sometimes 200 feet thick, is not often penetrated. On the banks of the Luy, however, to the south of Dax, the subjacent white chalk is exposed in inclined and vertical strata. In the hill called Puy Arzet the chalk, characterized by its peculiar fossils, is accompanied by beds of volcanic tuff, which are conformable to it, and which may be considered as the product of submarine eruptions which took place in the sea wherein the chalk was formed.

About a mile west of Orthes, in the Bas Pyrenees, the blue marl is seen to extend to the borders of the tertiary formation, and rises to the height probably of six or seven hundred feet. In that locality many of the marine Miocene shells preserve their original colours. This marl is covered by a considerable thickness of ferruginous gravel, which seems to increase in volume near the borders of the tertiary basin on the side of the Pyrenees.

In an opposite direction, to the north of Dax, the shelly sands often pass into calcareous sandstone, in which there are merely the casts of shells as at Carcares, and into a shelly breccia resembling some rocks of recent origin which I have received from the coral reefs of the Bermudas.

Fresh-water limestone at Saucats. -- Associated with the Miocene strata near Bordeaux, at a place called Saucats, is a compact fresh- ater limestone, of slight thickness, which is perforated on the upper surface by marine shells, for the most part of extinct species. It is evident that the space must have been alternately occupied by salt and fresh water. First, a lagoon may have been formed, in which the water may have become fresh; then a barrier of sand, by which the sea was excluded for a time, may have been breached, whereby the salt water again obtained access.

Eocene strata in the Bordeaux basin. -- The relations of some of the members of the tertiary series, in the basin of the Gironde, have of late afforded matter of controversy. A limestone, resembling the calcaire grossier of Paris, and from 100 to 200 feet in thickness, occurs at Pauliac and Blaye, and extends on the right bank of the Gironde, between Blaye and La Roche. It contains many species of fossils identical with those of the Paris basin. This fact was pointed out to me by M. Deshayes before I visited Blaye in 1830; hut although I recognized the mineral characters of the rock to be very different from those of the Miocene formations in the immediate neighbourhood of Bordeaux, I had not time to verify its relative position. I inferred, however, the inferiority of the Blaye limestone to the Miocene strata, from the order in which each series presented itself as I receded from the chalk and passed to the central parts of the Bordeaux basin.

Upon leaving the white chalk with flints, in travelling from Charente by Blaye to Bordeaux, I first found myself upon overlying red clay and -sand (as at Mirambeau); I then came upon the tertiary limestone above alluded to, at Blaye; and lastly, on departing still farther from the chalk, reached the strata which at Bordeaux and Dax contain exclusively the Miocene shells.

The occurrence both of Eocene and Miocene fossils in the same basin of the Gironde, had been cited by M. Boue as a fact which detracted from the value of zoological characters as a means of determining the chronological relations of tertiary groups. But on farther inquiry, the fact, on the contrary, has furnished additional grounds of confidence in these characters.

M. Ch. Desmoulins replied, in answer to M. Boue's objections, that the assemblage of Eocene shells are never intermixed with those found in the 'moellon,' as he calls the sandy calcareous rock of the environs of Bordeaux and Dax; and M. Dufrenoy farther stated, that the hills of limestone which border the right bank of the Gironde, from Marmande as far as Blaye, present several sections wherein the Parisian (or Eocene) limestone is seen to be separated from the shelly strata called 'faluns,' or 'moellon,' by a fresh-water formation of considerable thickness. It appears, therefore, that as the marine faluns of Touraine rest on a fresh ..water formation, which overlies the marine calcaire grossier of Paris, so the marine Miocene strata of Bordeaux are separated from those of Blaye by a freshwater deposit. [4]

The following diagram, therefore, will express the order of position of the groups above alluded to.

Image
No. 52.
a, Red clay and sand.
b, Limestone like calcaire grossier, sometimes alternating with green marl and containing Eocene shells.
c, Fresh-water formation, same as that of the department of Lot and Garonne.
d, Tertiary strata of the Landes, with Miocene fossils.


Inland cliff near Dax. -- A few miles west from Dax, and at the distance of about twelve miles from the sea, a steep bank is seen running in a direction nearly north-east and south-west, or parallel to the contiguous coast. This steep declivity, or brae, which is about 50 feet in height, conducts us from the higher platform of the Landes to a lower plain which extends to the sea. The outline of the ground might suggest to every geologist the opinion, that the bank in question was once a sea-cliff, when the whole country stood at a lower level relatively to the sea. But this can no longer be regarded as matter of conjecture. In making excavations recently for the foundation of a building at Abesse, a quantity of loose sand, which formed the slope d, e, was removed, and a perpendicular cliff exposed about 50 feet in height. The bottom of this cliff consists of limestone, b, which contains shells and corals of Miocene species, and is probably a calcareous form of the division c (diagram No. 51, p. 207). Immediately below this limestone is the clay c (probably d, diagram No. 51, p. 207), and above it the usual tertiary sand a of the department of the Landes. At the base of the precipice are seen large, partially-rounded, masses of rock, evidently detached from the stratum b. The face of the limestone is hollowed out and weathered into such forms as are seen in the calcareous cliffs of the adjoining coast, especially at Biaritz, near Bayonne. [5] It is evident that, when the country was at a somewhat lower level, the sea advanced along the surface of the argillaceous stratum c, which, by its yielding nature, favoured the waste and undermining of the more solid superincumbent limestone b. Afterwards, when the country had been elevated, part of the sand a fell down, or was drifted by the winds, so as to form the talus d, e, which masked the inland cliff until it was artificially laid open to view.

Image
No. 53: Section of Inland cliff at Abesse, near Dax.
a, Sand of the Landes.
b, Limestone.
c, Clay.


The situation of this cliff is interesting, as marking one of the pauses which intervened between the successive movements of elevation whereby the marine tertiary strata of this country were upheaved to their present height, a pause which allowed time for the sea to advance and strip off the upper beds a, b, from the denuded clay c.

Hills of Mont Ferrat and the Superga. -- The late Signor Bonelli of Turin was the first who remarked that the tertiary shells found in the green sand and marl of the Superga near Turin differed, as a group, from those generally characteristic of the Subapennine beds. The same naturalist had also observed, that many of the species peculiar to the Superga were identical with those occurring near Bordeaux and Dax. The strata of which the hill of the Superga is composed, are inclined at an angle of more than 70 degrees. They consist partly of fine sand and marl, and partly of a conglomerate composed of primary boulders, which forms a lower part of the series, and not, as represented by M. Brongniart by mistake, [6] an unconformable and overlying mass. [7] This same series of beds is more largely developed in the chain of Mont Ferrat, especially in the basin of the Bormida. The high road which leads from Savona to Alessandria intersects them in its northern descent, and the formation may be well studied along this line at Carcare, Cairo, and Spinto, at all which localities fossil shells occur in a bright green sand. At Piana, a conglomerate, interstratified with this green sand, contains rounded blocks of serpentine and chlorite schist, larger than those near the summit of the Superga, some of the blocks being not less than nine feet in diameter.

When we descend to Aqui, we find the green sand giving place to bluish marls, which also skirt the plains of the Tanaro at lower levels. These newer marls are associated with sand, and are near1y horizontal, and appear to belong to the older Pliocene Subapennine strata. [8] The shells which characterize the latter, abound in various parts of the country near Turin; but that region has not yet been examined with sufficient care to enable us to give exact sections to illustrate the superposition of the Miocene and older Pliocene beds. It is, however, ascertained, that the highly-inclined green sand, which comes immediately in contact with the primary rocks, is the oldest part of the series. [9]

Molasse of Switzerland. -- If we cross the Alps, and pass from Piedmont to Savoy, we find there, at the northern base of the great chain and throughout the lower country of Switzerland, a soft green sandstone, much resembling some of the beds of the basin of the Bormida, above described, and associated in a similar manner with marls and conglomerate. This formation is called, in Switzerland, 'molasse,' said to be derived from 'mol,' 'soft,' because the stone is easily cut in the quarry. It is of vast thickness, but shells have so rarely been found in it that they do not supply sufficient data for correctly determining its age. M. Studer, in his treatise on the 'molasse,' enumerates some fossil shells found near Lucerne, agreeing, apparently, with the testacea of the Subapennine hills. The correspondence in mineral character between the green sand of Piedmont and that of Switzerland can in nowise authorise us to infer identity of age, but merely to conclude that both have been derived from the degradation of similar ancient rocks.

Until the place of the 'molasse' in the chronological series of tertiary formations has been more rigorously determined, the application of this provincial name to the tertiary groups of other countries must prove a source of ambiguity, and we regret that the term has been so vaguely employed by M. Boue.

Styria, Vienna, Hungary, &c. -- Of the various groups which have hitherto been referred to the Miocene era, none are so important in thickness and geographical extent as those which are found at the eastern extremity of the Alps, in what have been termed the basins of Vienna and Styria, and which spread thence into the plains of Hungary. The collection of shells formed by M. Constant Prevost, in the neighbourhood of Vienna, and described by him in 1820, [10] were alone sufficient to identify a great part of the formations of that country with the Miocene beds of the Loire, Gironde, and Piedmont. The fossil remains subsequently procured by that indefatigable observer M. Boue have served to show the still greater range of the same beds through Hungary and Transylvania.

It appears from the recently published memoirs of Professor Sedgwick and Mr. Murchison t, that the formations in Styria may be divided into groups corresponding to those adopted by M. Partsch for the Vienna beds; the basin of Vienna exhibiting nearly the same phenomena as that of Styria. These regions have evidently formed, during the Miocene period, two deep bays of the same sea, separated from each other by a great promontory connected with the central ridge of the eastern Alps.

The English geologists, above mentioned, describe a long succession of marine strata intervening between the Alps and the plains of Hungary, which are divisible into three natural groups, each of vast thickness, and affording a great variety of rocks. All these groups are of marine origin, and lie in nearly horizontal strata, but have a slight prevailing easterly dip, so that, in traversing them from west to east, we commence with the oldest and end with the youngest beds. At their western extremity they fill an irregular trough-shaped depression, through which the waters of the Mur, the Raab, and the Drave, make their way to the lower Danube. [11] They here consist of conglomerate, sandstone, and marls, some of the marls containing marine shells. Beds also of lignite occur, showing that wood was drifted down in large quantities into the sea. In parts of the series there are masses of rounded siliceous pebbles resembling the shingle banks which are forming on some of our coasts.

The second principal group is characterized by coralline and concretionary limestone of a yellowish white colour: it is finely exposed in the escarpments of Wildon, and in the hills of Ehrenhausen, on the right bank of the Mur. [12] This coralline limestone is not less than 400 feet thick at Wildon, and exceeds, therefore, some of the most considerable of our secondary groups in England, as, for example, the 'Coral Rag.' [13]

Beds of sandstone, sand, and shale, and calcareous marls, are associated with the above-mentioned limestone.

The third group, which occurs at a still greater distance from the mountains, is composed of sandstone and marl, and of beds of limestone, exhibiting here and there a perfectly oolitic structure. In this system fossil shells are numerous.

It is by no means clear that the coralline limestones of the second group, are posterior in origin to all the beds of the first division; they may possibly have been formed at some distance from land, while the head of the gulf was becoming filled up with enormous deposits of gravel, sand, and mud, which may, in that quarter, have rendered the waters too turbid for the fullest development of testaceous and coralline animals.

In regard to the age of the formations above described, we may observe that the middle group, both in the basins of Styria and Vienna, belongs indisputably to the Miocene period, for the species of shells are the same as those of the Loire, Gironde, and other contemporary basins before noticed. Whether the lowest and uppermost systems are referrible to the same, or to distinct tertiary epochs, is the only question. We cannot doubt that the accumulation of so vast a succession of beds required an immense lapse of ages, and we are prepared to find some difference in the species characterizing the different members of the series; nevertheless, all may belong to different subdivisions of the Miocene period. Professor Sedgwick and Mr. Murchison have suggested that the inferior, or first group, which comprises the strata between the Alps and the coralline limestone of Wildon, may correspond in age to the Paris basin; but the list of fossils which they have given, seems rather to favour the supposition, that the deposit is of the Miocene era. They enumerate four characteristic Miocene fossils, -- Mytilus Brardii, Cerithium pictum, C. pupaeforme, and C. plicatum, -- and if there are some few of the associated shells common to the Paris basin, such a coincidence is no more than holds true in regard to all the European Miocene formations.

On the other hand, the third or newest system, which overlies the coralline limestone, contains fossils which do not appear to depart so widely from the Miocene type as to authorize us to separate them. They appear to agree with the tertiary strata of a great part of Hungary and Transylvania, which will be seen, by the tables of shells in Appendix I., to be referrible to the Miocene period.

Volhynia and Podolia. -- We may expect to find many other districts in Europe composed of Miocene strata, and there appears already to be sufficient evidence that the marine deposits of the platform of Volhynia and Podolia were of this era. The fossils of that region, which is bounded by Galicia on the west, and the Ukraine on the east, and comprises parts of the basins of the Bog and the Dniester, has been investigated by Von Buch, Eichwald, and Du Bois, and the latter has given excellent plates of more than one hundred fossil shells of the country, which M. Deshayes finds to agree decidedly with the fossils of the Miocene period. [14]

The formation consists of different rocks, sand and sandstone, clay, coarse limestone, and a white oolite, the last of which is of great extent.

Montpellier. -- The tertiary strata of Montpellier contain many of the Dax and Bordeaux species of shells, so that they are probably referrible to the Miocene epoch; but in the catalogue given by M. Marcel de Serres, many Pliocene species, similar to those of the Subapennine beds, are enumerated. This subject requires fuller investigation, and it would be highly interesting if the Montpellier beds should be found to indicate a passage from the fossils of the Miocene type to those of the older Pliocene. We are fully prepared for the discovery of such intermediate links, and we have endeavoured to provide a place for them in the classification proposed in the fifth chapter. [15]

_______________

Notes:

1. Sec above, chap. x.

2. For further details respecting the basin of the Loire, see M. Desnoyers, Ann. des Sci. Nat., tome xvi. pp. 171 and 402, where full references to other authors are given.

3. M. de Basterot has given a description of more than 300 shells of Bordeaux and Dax, and figures of the greater number of them. Mem. de la Soc. d'Hist. Nat. de Paris, tome ii.

4. Bulletin de la Soc, Geol. de France, tome ii. p. 440.

5. This spot was pointed out to me by the proprietor of the lands of Abesse in 1830.

6. Terrains du Vicentin, p. 26.

7. I examined the Superga in company with Mr. Murchison in 1828.

8. See section, wood-cut No. 4, p. 21.

9. We trust that MM. Pareto, Passini, Sismonda, and La Marmora, will devote their attention to the relative position of the several groups of tertiary strata in Piedmont, by instituting a comparison between their respective organic remains.

10. Journal de Physique, Novembre, 1820.

11. Geol. Trans., Second Series, vol. iii. p. 301.

12. Ibid., p. 382.

13. Geol. Trans., Second Series, vol. iii. p. 385.

14. Ibid., p. 390.

15. Conch. Foss. du Plateau Wolhyni-Podo, par F. du Bois. Berlin, 1831.
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Re: Principles of Geology, by Charles Lyell

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CHAPTER 16

Miocene alluviums – Auvergne – Mont Perrier – Extinct quadrupeds – Velay – Orleanais – Alluviums contemporaneous with Faluns of Touraine – – Miocene fresh – water formations – Upper Val d'Arno – Extinct mammalia – Coal of Cadibona – Miocene volcanic rocks – Hungary – Transylvania – Styria – Auvergne – Velay

IN the present chapter we shall offer some observations on the alluviums and fresh-water formations of the Miocene era, and shall afterwards point out the countries in Europe where the volcanic rocks of the same period may be studied.

MIOCENE ALLUVIUMS.

Auvergne. -- The annexed drawing will explain to the reader the position of two ancient beds of alluvium, c and e, in Auvergne, in which the remains of several quadrupeds characteristic of the Miocene period have been obtained. In order to account for the situation of these beds of rounded pebbles and sand, we must suppose that after the tertiary strata g, covered by the basaltic lava f, had been disturbed and exposed to aqueous denudation, a valley was excavated, wherein the alluvium e accumulated, and in which the remains of quadrupeds then inhabiting the country were buried. The trachytic breccia d was then superimposed; this breccia is an aggregate of shapeless and angular fragments of trachyte, cemented by volcanic tuff and pumice, resembling some of the breccias which enter into the composition of the neighbouring extinct volcano of Mont Dor in Auvergne, or those which are found in Etna. Upon this rests another alluvium c, which also contains the bones of Miocene species, and this is covered by another enormous mass of tufaceous breccia. We suppose the breccias to have resulted from the sudden rush of large bodies of water down the sides of an elevated volcano at its moments of eruption, when snow perhaps was melted by lava. Such floods occur in Iceland, sweeping away loose blocks of lava and ejections surrounding the crater, and then strewing the plains with fragments of igneous rocks, enveloped in mud or 'moya.' The abrupt escarpment presented by the above-described beds, b, c, d, e, towards the valley of the Couze, must have been caused by subsequent erosion, whereby a large portion of those masses has been carried away. [1]

Image
No. 54: Position of the Miocene alluviums of Mont Perrier (or Boulade).
a, Newer alluvium.
b, Second trachytic breccia.
c, Second Miocene alluvium with bones.
d, First trachytic breccia.
e, First Miocene alluvium with bones.
f, Compact basalt.
g, Eocene lacustrine strata.


In the alluviums c and e, MM. Croizet, Jobert, Chabriol, and Bouillet have discovered the remains of about forty species of extinct mammalia, the greater part of which are peculiar as yet to this locality; but some of them characteristic of the Miocene period, being common to the faluns of Touraine, and associated in other localities with marine Miocene strata. Among these species may be enumerated Mastodon minor and M. arvernensis, Hippopotamus major, Rhinoceros leptorhinus and Tapir arvernensis. The Elephas primigenius, a species common to so many tertiary periods, is also stated to accompany the rest. In some cases the remains are not sufficiently characteristic to indicate the exact species, but the following genera can be determined: the boar, horse, ox, hyaena (two species), felis (three or four species), bear (three species), deer, a great variety, canis, otter, beaver, hare, and water-rat. [2]

Velay. -- In Velay a somewhat similar group of mammiferous remains were found by Dr. Hibbert, [3] in a bed of volcanic scoriae and tuff, inclosed between two beds of basaltic lava, at Saint-Privat d'Allier. Some of the bones were found adhering to the slaggy lava. Among the animals were Rhinoceros leptorhinus, Hyaena spelaea, and another species allied to the spotted hyaena of the Cape, together with four undetermined species of deer. [4]

At Cussac and Solilhac, one league from Puy en Velay, M. Robert discovered, in an ancient alluvium covered with lava, the remains of Elephas primigenius, Rhinoceros leptorhinus, Tapir arvernensis, horse (two species), deer (seven species), ox (two species), and an antelope.

Orleanais. -- In the Orleanais, at Avaray, Chevilly, les Aides, and les Barres, fossil land quadrupeds have been found associated with fluviatile shells and reptiles, identical with those found in the marine faluns of Touraine. [5] These are supposed, with great probability, by M. Desnoyers, to mark the passage of streams which flowed towards the sea in which the faluns were deposited. They bear the same relation to the Miocene strata of Touraine, as part of the ancient gravel and silt of England, containing the bones of elephants and other extinct animals, probably bear to the crag.

MIOCENE FRESH-WATER FORMATIONS.

Upper Val d'Arno. -- There are a great number of isolated tertiary formations, of fresh-water origin, resting on primary and secondary rocks in different parts of Europe, in the same manner as we now find small lakes scattered over our continents and islands wherein deposits are forming, quite detached from all contemporary marine strata. To determine the age of such groups with reference to the great chronological series established for the marine strata, must often be a matter of difficulty, since we cannot always enjoy an opportunity of studying a locality where the fresh-water species are intermixed with marine shells, or where they occur in beds alternating with marine strata.

The deposit of the Upper Val d'Arno before alluded to, (p. 161) was evidently formed in an ancient lake; but although the fossil testaceous and mammiferous remains preserved therein are very numerous, it is scarcely possible, at present, to decide with certainty the precise era to which they belong. I collected six species of lacustrine shells, in an excellent state of preservation, from this basin belonging to the genera Anodon, Paludina and Neritina; but M. Deshayes was unable to identify them with any recent or fossil species known to him. If the beds belonged to the older Pliocene formations we might expect that several of the fossils would agree specifically with living testacea; and we are therefore disposed to believe that they belong to an older epoch. If we consider the terrestrial mammalia of the same beds, we immediately perceive that they cannot be assimilated to the Eocene type, as exhibited in the Paris basin, or in Auvergne and Velay: but some of them agree with Miocene species. Mr. Pentland has obligingly sent me the following list of the fossil mammifers of the Upper Val d'Arno which are in the museums of Paris. -- Ferae. -- Ursus cultridens, Viverra Valdarnensis, Canis lupus, and another of the size of the common fox. Hyaena radiata, H. fossilis. Felis (a new species of the size of the panther). Rodentia. -- Histrix, nearly allied to dorsalis, Castor. Pachydermata. -- Elephas Italicus, Mastodon angustidens, M. Taperoides, Tapir- -, Equus --, Sus scrofa, Rhinoceros leptorhinus, Hippopotamus major, fossilis. Ruminantia. -- Cervus megaceros, (?) C. Valdarnensis, C. --, new species, Bos, bubalo affinis, B. urus and B. taurus.

Cuvier also mentions the remains of a species of lophiodon as occurring among the hones in the Upper Val d'Arno. [6] The elephant of this locality has been called by Nesti [7] meridionalis, and is considered by him as distinct from the Siberian fossil species E. primigenius, with which, however, some eminent comparative anatomists regard it as identical. The skeletons of the hippopotamus are exceedingly abundant; no less than forty had been procured when I visited Florence in 1828. Remains of the elephant, stag, ox, and horse, are also extremely numerous. In winter the superficial degradation of the soil is so rapid, that bones which the year before were buried are seen to project from the surface of the soil, and are described by the peasants as growing. In this manner the tips of the horns of stags, or of the tusks of hippopotamuses often appear on the surface, and thus lead to the discovery of an entire head or skeleton.

Cadibona. -- Another example of an isolated lacustrine deposit, belonging possibly to the Miocene period, is that which occurs at Cadibona, between Savona and Carcare. Its position is described in the annexed section, which does not however pretend to accuracy in regard to the relative heights of the different rocks, or the distances of the places from each other. The lacustrine strata are composed of gravel, grit, and micaceous sandstone, of such materials as were derivable from the surrounding primary rocks; and so great is the thickness of this mass, that some valleys intersect it to the depth of seven or eight hundred feet without penetrating to the subjacent formations. In one part of the series, carbonaceous shales occur, and several seams of coal from two to six feet in thickness, but no impressions of plants of which the species could be deter.. mined, and no shells have been discovered. Many entire jaws and other bones of an extinct mammifer, called by Cuvier Anthracotherium, have been found in the coal-beds, the bone being itself changed into a kind of coal; but as this species does not occur elsewhere in association with organic remains of known date, it affords us no aid in our attempt to assign a place to the lignites of Cadibona. [8]

Image
No. 55: Section of the fresh-water formation of Cadibona.
a. Blue marl and yellow sand (older Pliocene).
b. Sand, shale and coal of Cadibona (Miocene?).
c. Green sand, &c. of the Bormida (Miocene).
d. Chloritic and micaceous schist, serpentine, &c.


MIOCENE VOLCANIC ROCKS.

Hungary. -- M. Beudant, in his elaborate work on Hungary, describes five distinct groups of volcanic rocks, which, although rarely of great extent, form striking features in the physical geography of that country, rising as they do abruptly from extensive plains composed of tertiary strata. They may have constituted islands in the ancient sea, as Santorin and Milo now do in the Grecian archipelago; and M. Beudant has remarked that the mineral products of the last-mentioned islands resemble remarkably those of the Hungarian extinct volcanos, where many of the same minerals, as opal, calcedony, resinous silex (silex resinite), pearlite, obsidian, and pitchstone abound.

The Hungarian lavas are chiefly felspathic, consisting of different varieties of trachyte; many are cellular and used as millstones; some so porous and even scoriform as to resemble those which have issued in the open air. Pumice occurs in great quantity, and there are conglomerates, or rather breccias, wherein fragments of trachyte are bound together by pumiceous tuff or sometimes by silex.

It is probable that these rocks were permeated by the waters of hot springs, impregnated, like the Geysers, with silica; or, in some instances perhaps, by aqueous vapours, which, like those of Lancerote, may have precipitated hydrate of silica. [9]

By the influence of such springs or vapours the trunks and branches of trees washed down during floods, and buried in tuffs on the flanks of the mountains, may have become silicified. It is scarcely possible, says M. Beudant, to dig into any of the pumiceous deposits of these mountains without meeting with opalized wood, and sometimes entire silicified trunks of trees of great size and weight.

It appears from the species of shells collected principally by M. Boue, and examined by M. Deshayes, that the fossil remains imbedded in the volcanic tuffs, and in strata alternating with them in Hungary, are of the Miocene type, and no identical, as was formerly supposed, with the fossils of the Paris basin.

Transylvania. -- The igneous rocks of the eastern part of Transylvania described by M. Boue, are probably of the same age. They cover a considerable area, and bear a close resemblance to the Hungarian lavas, being chiefly trachytic. Several large craters, containing shallow lakes like the Maars of the Eifel, are met with in some regions; and a rent in the trachytic mountains of Budoshagy exhales hot sulphureous vapours, which convert the trachyte into alum-stone, a change which that rock has undergone at remote periods in several parts of Hungary.

Styria. -- Many of the volcanic groups of this country bear a similar relation to the Styrian tertiary deposits, as do the Hungarian rocks to the marine strata of that country. The shells are found imbedded in the volcanic tuffs in such a manner as to show that they lived in the sea when the volcanic eruptions were in progress, as many of the Val di Noto lavas in Sicily, before described, were shown to be contemporaneous with newer Pliocene strata. [10]

Auvergne -- Velay. -- We believe that part of the volcanic eruptions of Auvergne took place during the Miocene period; those, for example, which cover, or are interstratified with the alluviums mentioned in this chapter, and some of the ancient basaltic cappings of hills in Auvergne, which repose on gravel characterized by similar organic remains. A part also of the igneous rocks of Velay belong to this epoch, but to these we shall again refer when we treat more fully of the volcanic rocks of Central France; the older part of which are referrible to the Eocene period.

_______________

Notes:

1. For an account of the position and age of the volcanic breccias of Mont Perrier and Boulade, see Lyell and Murchison on the beds of Mont Perrier, Ed. New Phil Journ., July, 1829, p. 15.

2. Recherches sur les Oss. Foss. du Dept. du Puy de Dome, 4to., 1828. -- Essai Geol. et Mineral. sur les Environs d'Issoire, Dept. du Puy de Dome, folio, 1827.

3. Edin. Journ. of Sci., No. 4, New Series, p. 276.

4. Figures of some of these remains are given by M. Bertrand de Doue, Ann. de la Soc. d'Agricult. de Puy, 1828.

5. MM. Desnoyers and Lockart, Bulletin de la Soc. Geol., tom. ii. p. 336.

6. Oss. Foss., vol. v. p. 504.

7. Lettere sopra alcune Ossa Fossili del Val d'Arno, &c. Pisa, 1825.

8. The author visited Cadibona in August, 1828, in company with Mr. Murchison.

9. See above, vol. i. chap. xxii.

10. Sedgwick and Murchison, Geol. Trans. Second Series, vol. iii. p. 400. -- Daubeny, Extinct Volcanos, p. 92.
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Re: Principles of Geology, by Charles Lyell

Postby admin » Fri Jul 17, 2015 2:42 am

CHAPTER 17

Eocene period – Fresh-water formations – Central France – Map – Limagne d'Auvergne – Sandstone and conglomerate – Tertiary Red marl and sandstone like the secondary 'new red sandstone' – Green and white foliated marls – Indusial limestone – Gypseous marls – General arrangement and origin of the Travertin – Fresh-water formation of the Limagne – Puy en Velay – Analogy of the strata to those of Auvergne – Cantal – Resemblance of Aurillac limestone and its flints to our upper chalk – Proofs of the gradual deposition of marl – Concluding remarks

EOCENE FRESH-WATER FORMATIONS.

WE have now traced back the history of the European formations to that period when the seas and lakes were inhabited by a few only of the existing species of testacea, a period which we have designated Eocene, as indicating the dawn of the present state of the animate creation. But although a small number only of the living species of animals were then in being, there are ample grounds for inferring that all the great classes of the animal kingdom, such as they now exist, were then fully represented. In regard to the testacea, indeed, it is no longer a matter of inference, for 1400 species of this class have been obtained from that small number of detached Eocene deposits which have hitherto been examined in Europe.

The celebrated Paris basin, the position of which was pointed out in the former part of this volume, (see wood-cut, p. 16) first presents itself, and seems to claim our chief attention when we treat of the phenomena of this era. But in order more easily to explain to the student the peculiar nature and origin of that group, it will be desirable, first, to give a brief sketch of certain deposits of Central France, which afford many interesting points of analogy, both in organic remains and mineral composition, and where the original circumstances under which the strata were accumulated may more easily be discerned.

Auvergne. -- We allude to the lacustrine basins of Auvergne, Cantal, and Velay, the site of which may be seen in the annexed Map. [1] They appear to be the monuments of ancient lakes which may have resembled in geographical distribution some of those now existing in Switzerland, and may like them have occupied the depressions in a mountainous country, and have been each fed by one or more rivers and torrents. The country where they occur is almost entirely composed of granite, and different varieties of granitic schist, with here and there a few patches of secondary strata much dislocated, and which have probably suffered great denudation. There are also some vast piles of volcanic rock, (see the Map,) the greater part of which are newer than the fresh-water strata, often resting upon them, whilst a small part were evidently of contemporaneous origin. Of these igneous rocks we shall treat more particularly in the nineteenth chapter, and shall first turn our attention exclusively to the lacustrine beds.

Image
No. 56

The most northern of the fresh-water groups is situated in the valley-plain of the Allier, which lies within the department of the Puy de Dome, being the tract which went formerly by the name of the Limagne d'Auvergne. It is inclosed by two parallel primitive ranges, -- that of the Forez, which divides the waters of the Loire and Allier, on the east, and that of the Monts Domes, which separates the latter river from the Sioule, on the west. [2] The average breadth of this tract is about 20 miles, and it is for the most part composed of nearly horizontal strata of sand, sandstone, calcareous marl, clay, limestone, and some subordinate groups, none of which observe a fixed and invariable order of superposition. The ancient borders of the lake, wherein the fresh-water strata were accumulated, may generally be traced with precision, the granite and other ancient rocks rising up boldly from the level country. The precise junction, however, of the lacustrine and granitic beds is rarely seen, as a small valley usually intervenes between them. The fresh-water strata may sometimes be seen to retain their horizontality within a very slight distance of the. border-rocks, while in some places they are inclined, and in a few instances vertical. The principal divisions into which the lacustrine series may be separated are the following: 1st, Sandstone, grit, and conglomerate. 2ndly, green and white foliated marls. 3dly, limestone or travertin, oolite, &c. 4thly, gypseous marls.

1. Sandstone and conglomerate. -- Strata of sand and gravel, sometimes bound together into a solid rock, are found in great abundance around the confines of the lacustrine basin, containing, in different places, pebbles of all the ancient rocks of the adjoining elevated country, namely, granite, gneiss, mica-schist, clay-slate, porphyry, and others. But the arenaceous strata do not form one continuous band around the margin of the basin, being rather disposed like the independent deltas which grow at the mouths of torrents along the borders of existing lakes. [3]

At Chamalieres, near Clermont, we have an example of one of these littoral groups of local extent where the pebbly beds slope away from the granite as if they had formed a talus beneath the waters of the lake near the steep shore. A section, of about 50 feet in vertical height, has been laid open by a torrent, and the pebbles are seen to consist throughout of rounded and angular fragments of granite, quartz, primary slate, and red sandstone, but without any intermixture of those volcanic rocks which now abound in the neighbourhood. Partial layers of lignite and pieces of wood are found in these beds, but no shells, a fact which probably indicates that testacea could not live where the turbid waters of a stream were frequently hurrying down uprooted trees, together with sand and pebbles, or, that if they existed, they were triturated by the transported rocks.

There are other localities on the margin of the basin where quartzose grits are found, composed of white sand bound together by a siliceous cement.

Occasionally, when the grits rest on granite, as at Chamalieres before mentioned, and many other places, the separate crystals of quartz, mica, and felspar, of the disintegrated granite, are bound together again by the silex, so that the granite seems regenerated in a new and even more solid form, and thus so gradual a passage may sometimes be traced between a crystalline rock and one of mechanical origin, that we can scarcely distinguish where one ends and the other begins.

In the Puy de Jussat, and the neighbouring hill of La Roche, are white quartzose grits, cemented into a sandstone by calcareous matter, which is sometimes so abundant as to form imbedded nodules. These sometimes constitute spheroidal concretions six feet in diameter, and pass into beds of solid limestone resembling the Italian travertins, or the deposits of mineral springs.

In the hills above mentioned, we have the advantage of seeing a section continuously exposed for about 700 feet in thickness. t the bottom are foliated marls, white and green, about 400 feet thick, and above, resting on the marls, are the quartzose grits before mentioned with the associated travertins. This section is observed close to the confines of the basin, so that the lake must here have been filled up near the shore with fine mud, before the coarse superincumbent sand was introduced. There are other cases where sand is seen below the marl.

2. Red marl and sandstone. -- But the most remarkable of the arenaceous groups is a red sandstone and red marl, identical in all their characters with the secondary new red sandstone and marl of England. In the latter, the red ground is sometimes variegated with light greenish spots, and the same may be seen in its tertiary counterpart of fresh-water origin at Coudes, on the Allier. The marls are sometimes of a purplish-red colour, as at Champheix, and are accompanied by a reddish limestone, like the well-known 'cornstone,' which is associated with the old red sandstone of English geologists. The red sandstone and marl of Auvergne have evidently been derived from the degradation of gneiss and mica-schist, which are seen in situ on the adjoining hills, decomposing into a soil very similar to the tertiary red sand and marl. We also find pebbles of gneiss, mica-schist, and quartz, in the coarser sandstones of this group, clearly pointing to the parent rocks from which the sand and marl were derived. The red beds, although destitute of organic remains, pass upwards into strata containing Eocene fossils, and are certainly an integral part of the lacustrine formation.

3. Green and white foliated marls. -- A great portion of what we term clay in ordinary language, consists of the same materials as sandstone, but the component parts are in a finer state of subdivision. The same primary rocks, therefore, of Auvergne, which, by the partial degradation of their harder parts, gave rise to the quartzose grits and conglomerates before mentioned, would, by the reduction of the same Into powder, and by the decomposition of their felspar, mica, and hornblende, produce aluminous clay, and, if a sufficient quantity of carbonate of lime was present, calcareous marl. This fine sediment would naturally be carried out to a greater distance from the shore, as are the various finer marls now deposited in Lake Superior. [4] And, as in the American lake, shingle and sand are annually amassed near the northern shores, so in Auvergne the grits and conglomerates before mentioned were evidently formed near the borders.

The entire thickness of these marls is unknown, but it certainly exceeds, in some places, 700 feet. They are for the most part either light- green or white, and usually calcareous. They are thinly foliated, a character which frequently arises from the innumerable thin plates or scales of that small animal called cypris, a genus which comprises several species, of which some are recent, and may be seen swimming rapidly through the waters of our stagnant pools and ditches. This animal resides within two small valves like those of a bivalve shell, and it moults its integuments annually, which the conchiferous molluscs do not. This circumstance may partly explain the countless myriads of the shells of cypris which were shed in the Eocene lakes, so as to give rise to divisions in the marl as thin as paper, and that too in stratified masses several hundred feet thick A more convincing proof of the tranquillity and clearness of the waters, and of the slow and gradual process by which the lake was filled up with fine mud, cannot be desired. We may easily suppose that, while in the deep and central parts of the basin, this fine sediment was thrown down, gravel, sand, and rocky fragments were hurried into the lake near the shore, and formed the group first described.

Not far from Clermont the green marls, containing the cypris in abundance, approach to within a few yards of the granite which forms the borders of the basin. The annexed section occurs at Champradelle, in a small ravine north of La petite Baraque, and above the bridge.

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No. 57: Vertical strata of marl near Clermont.
A, Granite.
B, Space of 60 feet in which no section is seen.
C, Green marl, vertical and inclined.
D. White marl.


The occurrence of these marls so near the ancient margin may be explained by considering that, at the bottom of the ancient lake in spaces intermediate between the points where rivers and torrents entered, no coarse ingredients were deposited, but finer mud only was drifted by currents. The verticality of some of the beds in the above section bears testimony to considerable local disturbance subsequent to the deposition of the marls, but such inclined and vertical strata are very rare.

4. Limestone, travertin, &c. -- Both the preceding members of the lacustrine deposit, the marls and grits, pass occasionally into limestone. Sometimes only concretionary nodules abound in them; but these, by an additional quantity of calcareous matter, unite, as already noticed (p. 229), into regular beds.

On each side of the basin of the Limagne, both on the east at Gannat, and on the west at Vichy, a white oolitic limestone is quarried. At Vichy, the oolite resembles our Bath stone in appearance and beauty, and, like it, is soft when first taken from the quarry, but soon hardens on exposure to the air. At Gannat, the stone contains land-shells and bones of quadrupeds, resembling those of the Paris gypsum. In several places in the neighbourhood of Gannat, at Marculot among others, this stone is divided by layers of clay.

At Chadrat, in the hill of La Serre, the limestone is pisolitic, and in this and other respects resembles the travertin of Tivoli. It presents the same combination, of a radiated and concentric structure, and the coats of the different segments of spheroids have the same undulating surface. (See wood-cut No. 5, chap. xii. vol. i.)

Indusial limestone. -- There is another remarkable form of fresh-water limestone in Auvergne, called 'indusial,' from the cases, or indusiae, of the larvae of Phryganea, great heaps of which have been encrusted, as they lay, by hard travertin, and formed into a rock. We may often see, in our ponds, some of the living species of these insects, covered with small fresh-water shells, which they have the power of fixing to the outside of their tubular cases, in order, probably, to give them weight and strength. It appears that, in the same manner, a large species which swarmed in the Eocene lakes of Auvergne, was accustomed to attach to its dwelling the shells of a small spiral univalve of the genus Paludina. A hundred of these minute shells are sometimes seen arranged around one tube, part of the central cavity of which is still occasionally empty, the rest being filled up with thin concentric layers of travertine. When we consider that ten or twelve tubes are packed within the compass of a cubic inch, and that some single strata of this limestone are six feet thick, and may be traced over a considerable area, we may form some idea of the countless number of insects and mollusca which contributed their integuments and shells to compose this singularly constructed rock. It is unnecessary to suppose that the Phryganeae lived on the spots where their cases are now found; they may have multiplied in the shallows near the margin of the lake, and their buoyant cases may have been drifted by a current far into the deep water.

The calcareous strata of the Limagne, like the other members of the lacustrine formation, are for the most part horizontal, or inclined at a very slight angle, but instances of local dislocation are sometimes seen. At the town of Vichy, for example, the strata dip at an angle of between 30 and 40 degrees; in an ancient quarry behind the convent of Celestines, and near the hot spring at the same place, the beds of limestone are seen first inclined at an angle of 80°, and then vertical.

5. Gypseous marls. -- More than 50 feet of thinly-laminated gypseous marls, exactly resembling those in the hill of Montmartre, at Paris, are worked for gypsum at St. Romain, on the right bank of the Allier. They rest on a series of green cypriferous marls which alternate with grits, the united thickness of this inferior group being seen, in a vertical section on the banks of the river, to exceed 250 feet.

General arrangement and origin of the fresh-water formations of Auvergne. -- The relations of the different groups above described cannot be learnt by the study of any one section, and he who sets out with the expectation of finding a fixed order of succession may perhaps complain that the different parts of the basin give contradictory results. The arenaceous division, the marls and the limestone, may all he seen in some localities to alternate with each other, yet it can by no means be affirmed that there is no order of arrangement. The sands, sandstone, and conglomerate, constitute in general a littoral group; the foliated white and green marls a contemporaneous central deposit, and the limestone is for the most part subordinate to the newer portions of the above groups. We never meet with calcareous rocks covered by a considerable thickness of quartzose sand or green marl, and the uppermost marls and sands are more calcareous than the lower. From the resemblance of the Eocene limestones of Auvergne to the Italian travertins, we may conclude that they were derived from the waters of mineral springs, -- such springs as now exist in Auvergne, and which rising up through the granite precipitate travertin. They are sometimes thermal, but this character is by no means constant.

We suppose that, when the ancient lake of the Limagne first began to be filled with sediment, no volcanic action had produced lava and scoriae on any part of the surface of Auvergne. No pebbles, therefore, of lava were transported into the lake, -- no fragments of volcanic rocks imbedded in the conglomerate. But at a later period, when a considerable thickness of sandstone and marl had accumulated, eruptions broke out, and lava and tuff were alternately deposited, at some spots, with the lacustrine strata. Of this we shall give proofs in the 19th chapter. It is not improbable that cold and thermal springs, holding different mineral ingredients in solution, increased in Dumber during the successive convulsions attending this development of volcanic agency, and thus carbonate and sulphate of lime, silex, and other minerals, were produced. Hence these minerals predominate in the uppermost strata. The subterranean movements may then have continued until they altered the relative levels of the country and caused the waters of the lakes to be drained off, and the farther accumulation of regular fresh-water strata to cease. The occurrence of these convulsions anterior to the Miocene epoch, and prolonged during a succession of after-ages, may explain why no fresh-water formations more recent than the Eocene are now found in this country.

We may easily conceive a similar series of events to give rise to analogous results in any modern basin, such as that of Lake Superior, for example, where numerous rivers and torrents are carrying down the detritus of a chain of mountains into the lake. The transported materials must be arranged according to their size and weight, the coarser near the shore, the finer at a greater distance from land; but in the gravelly and sandy beds of Lake Superior no pebbles of modern volcanic rocks can be included, since there are none of these at present in the district. If the igneous action should break out in that country and produce lava, scoriae, and thermal springs, the deposition of gravel, sand, and marl, might still continue as before; but in addition, there would then be an intermixture of volcanic gravel and tuff, and rocks precipitated from the waters of mineral springs.

Although the fresh-water strata of the Limagne approach generally to a horizontal position, the proofs of local disturbance are sufficiently numerous and violent to allow us to suppose great changes of level since the Eocene period. We are unable to assign a northern barrier to the ancient lake, although we can still trace its limits to the east, west, and south, where they were formed of bold granitic eminences. But we need not be surprised at our inability to restore the physical geography of the country after so great a series of volcanic eruptions. It is by no means improbable that one part of the district may have been moved upwards bodily, while the others remained at rest, or even suffered a movement of depression.

Puy en Velay. -- In the department of the Haute Loire, a fresh-water formation, very analogous to that of Auvergne, is situated in the basin of the Loire, and is exposed in the valley in which stands the town of Le Puy. Since the deposition of the lacustrine strata, there have been so many volcanic eruptions in this country, and such immense quantities of lava and scoriae poured out upon the surface, that the aqueous rocks are almost buried and concealed. We are indebted, however, to the researches of M. Bertrand de Doue for having distinctly ascertained the succession of strata, and we have had opportunities of verifying his observations during a visit to Le Puy.

In this basin we find, as in Auvergne, two great divisions, consisting of grits and marls; the former composed of quartzose grit, sometimes granitiform, reddish and mottled sands and conglomerates, all evidently derived from the degradation of granitic rocks, and resembling exceedingly the arenaceous group of the Limagne before described. This formation is almost confined to the borders of the basin, and was evidently a littoral deposit. The other member of the formation, the marls, are more or less calcareous, and are associated with limestone and gypsum, which last is worked for agricultural uses, and exactly resembles that of Paris.

The analogy in the mineral character of the Velay and Paris basins is rendered more complete by the presence in both of silex in regular beds. In the limestone I found gyrogonites, or seeds of the ehara, of the same species as those most common in the Paris basin; and M. Bertrand de Doue has discovered the bones of several mammiferous animals of the same genera as those which characterize the basins of Auvergne and Paris. [5] The shells also of this formation correspond specifically with those of Eocene formations in other parts of France.

The sand and conglomerate of the fresh-water basin of Velay is entirely free from volcanic pebbles, agreeing in this respect with the analogous group of the Limagne; but the fact is the more striking in Velay, because the masses of trachyte, clinkstone, and other igneous rock now abounding in that country, have an aspect of extremely high antiquity, and constitute a most prominent feature in the geological structure of the district. Yet the non-intermixture of volcanic products with the lacustrine sediment, is just what we should expect when we have ascertained that the imbedded organic remains of those strata are Eocene; whereas the lavas belong in part, if not entirely, to the Miocene period. [6]

Cantal. -- Near Aurillac, in CantaI, another series of freshwater strata occurs, which resembles, in mineral character and organic remains, those of Auvergne and Velay already described. The leading feature of this group, as distinguished from the two former, is the immense abundance of silex associated with the calcareous marls and limestone, which last, like the limestone of Auvergne, constitutes an upper member of the fresh-water series.

The formation of the Cantal may be divided into two groups, the lowest composed of gravel, sand, and clay, such as might have been derived from the wearing down and decomposition of the granitic schists of the surrounding country; the upper system consisting of siliceous and calcareous marls, contains subordinately gypsum, silex, and limestone-deposits such as the waters of springs charged with carbonate and sulphate of lime, and with silica, may have produced.

Fresh-water limestone and flints resembling chalk. -- To the English geologist, the most interesting feature in the eantal is the resemblance of the fresh-water limestone, and its accompanying flint, to our upper chalk, a resemblance which, like that of the red sandstone of Auvergne to our secondary 'new red,' is the more important, as being calculated to put the student upon his guard against too implicit a reliance on lithological characters as tests of the relative ages of rocks. When we approach Aurillac from the west, we pass over great heathy plains, where the sterile mica-schist is barely covered with vegetation. Near Ytrac, and between La Capelle and Viscamp, we begin to see the surface strewed over with loose broken flints, some of them black in the interior, but with a white external coating, others stained with tints of yellow and red, and looking precisely like the flint gravel of our chalk districts. When heaps of this gravel have thus announced our approach to a new formation, we arrive at length at the escarpment of the lacustrine beds. At the bottom of the hill we see strata of clay and sand resting on mica-schist; and above, in the quarries of Belbet, Leybros, and Bruel, a white limestone, in horizontal strata, the surface of which has been hollowed out into irregular furrows, since filled up with broken flint, marl, and vegetable mould. We recognize in these cavities, filled with dark mould and flint gravel, an exact counterpart to the appearances so frequently presented on the furrowed surface of our white chalk. Proceeding onwards from these quarries, along a road made of the white limestone, which reflects as glaring a light in the sun, as do our roads composed of chalk, we reach, at length, in the neighbourhood of Aurillac, hills of limestone and calcareous marl, in horizontal strata, separated in some places by regular layers of flint in nodules, the coating of each nodule being of an opaque white colour, like the exterior of the flinty nodules of our chalk. In these last the hard white substance has been ascertained to consist, in some instances, wholly of siliceous matter, and sometimes to contain a small admixture of carbonate of lime, [7] and the analysis of those of the Cantal would probably give the same results. The Aurillac flints have precisely the appearance of having separated from their matrix after the siliceous and calcareous matter had been blended together. The calcareous marl sometimes occupies small sinuous cavities in the flint, and the siliceous nodule, when detached, is often as irregular in form as those found in our chalk.

By what means, then, can the geologist at once decide that the limestone and silex of Aurillac are referrible to an epoch entirely distinct from that of the English chalk? It is not by reference to position, for we can merely say of the lacustrine beds, as we should have been able to declare of the true chalk had it been present, that they overlie the granitic rocks of this part of France. It is by reference to the organic remains that we are able to pronounce the formation to belong to the Eocene tertiary period. Instead of the marine Alcyonia of our cretaceous system, the silicified seed-vessels of the Chara, a plant which grows at the bottom of lakes, abound in the flints of Aurillac, both in those which are in situ and those forming the gravel. Instead of the Echinus and marine testacea of the chalk, we find in the marls and limestones the shells of the Planorbis, and other lacustrine testacea, all of them, like the gyrogonites, agreeing specifically with species of the Eocene type.

Proofs of the gradual deposition of marl. -- Some sections of the foliated marls in the valley of the Cer, near Aurillac, attest, in the most unequivocal manner, the extreme slowness with which the materials of the lacustrine series were amassed. In the hill of Barrat, for example, we find an assemblage of calcareous and siliceous marls, in which, for a depth of at least 60 feet, the layers are so thin that thirty are sometimes contained in the thickness of an inch; and when they are separated we see preserved in each the flattened stems of Charm, or other plants, or sometimes myriads of small paludinae and other freshwater shells. These minute foliations of the marl resemble precisely some of the recent laminated beds of the Scotch marl lakes, and when divided may be compared to the pages of a book, each containing a history of a certain period of the past. The different layers may be grouped together in beds from a foot to a foot and a half in thickness, which are distinguished by differences of composition and colour, the latter being white, green, and brown. Occasionally there is a parting layer of pure flint, or of black carbonaceous vegetable matter, one inch thick, or of white pulverulent marl. We find several hills in the neighbourhood of Aurillac composed of such materials for the height of more than 200 feet from their base, the whole sometimes covered by rocky currents of trachytic or basaltic lava. [8]

Concluding remarks. -- So wonderfully minute are the separate parts of which some of the most massive geological monuments are made up! When we desire to classify, it is necessary to contemplate entire groups of strata in the aggregate; but if we wish to understand the mode of their formation, and to explain their origin, we must think only of the minute subdivisions of which each mass is composed. We must bear in mind how many thin, leaf-like seams of matter, each containing the remains of myriads of testacea and plants, frequently enter into the composition of a single stratum, and how great a succession of these strata unite to form a single group! We must remember, also, that volcanos like the Plomb du Cantal, which rises in the immediate neighbourhood of Aurillac, are equally the result of successive accumulation, consisting of reiterated flows of lava and showers of scoriae; and we have shown, when we treated of the high antiquity of Etna, how many distinct lava-currents and heaps of ejected substances are required to make up one of the numerous conical envelopes whereof a volcano is composed. -- Lastly, we must not forget that continents and mountain-chains, colossal as are their dimensions, are nothing more than an assemblage of many such igneous and aqueous groups, formed also in succession during an indefinite lapse of ages, and superimposed upon each other.

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Notes:

1. The following account of the fresh-water formations of Central France is the result of observations made in the summer of 1828, in company with Mr. Murchison.

2. Scrope, Geology of Central France, p. 15.

3. See vol. i. chap. xiv. p. 249; and 2nd. Ed. p. 286.

4. See vol. i. chap. xiii.

5. Descrip. Geognos. des Euv. du Puy en Velay, 1823.

6. See above, p. 219, and below, Chap. xix.

7. Phillips, Geol. Trans. First Series, vol. v. p. 22. -- Outlines of Geology, p. 95.

8. Lyell and Murchison, sur les Depots Lacust. Tertiaires du Cantal, &c. Ann. des Sci. Nat., Oct. 1829.
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Re: Principles of Geology, by Charles Lyell

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CHAPTER 18

Marine formations of the Eocene period – Strata of the Paris basin how far analogous to the lacustrine deposits of Central France – Geographical connexion of the Limagne d'Auvergne and the Paris basin – Chain of lakes in the Eocene period – Classification of groups in the Paris basin – Observations of M. C. Prevost – Sketch of the different subdivisions of the Paris basin – Contemporaneous marine and fresh-water strata – Abundance of Cerithia in the Calcaire grossier – Upper marine formation indicates a subsidence – Part of the Calcaire grossier destroyed when the upper marine strata originated – All the Parisian groups belong to one great epoch – Microscopic shells – Bones of quadrupeds in gypsum – In what manner entombed – Number of species – All extinct – Strata with and without organic remains alternating – Our knowledge of the physical geography, fauna, and flora of the Eocene period considerable – Concluding remarks

EOCENE FORMATIONS-PARIS BASIN.

THE geologist who has studied the lacustrine formations described in the last chapter cannot enter the tract usually termed 'the Paris Basin' without immediately recognizing a great variety of rocks with which his eye has already become familiar. The green and white marls of Auvergne, Cantal, and Velay, again present themselves, together with limestones and quartzose grits, siliceous and gypseous marls, nodules and layers of flint, and saccharoid gypsum; lastly, in addition to all this identity of mineral character, we find an assemblage of the same species of fossil animals and plants.

When we consider the geographical proximity of the two districts, we are the more prepared to ascribe this correspondence in the mineral composition of these groups to a combination of similar circumstances in the same era. From the map (No. 56, p. 226) in the last chapter, it will be seen that the united waters of the Allier and Loire, after descending from the valleys occupied by the fresh-water formations of Central France, flow on till they reach the southern extremity of what is called the Paris basin. M. Omalius d'Halloy long ago suggested the very natural idea that there existed formerly a chain of lakes, reaching from the highest part of the central mountain- roup of France, and terminating in the basin of Paris, which he supposes was at that time an arm of the sea.

Notwithstanding the great changes which the physical geography of that part of France must since have undergone, we may easily conceive that many of the principal features in the configuration of the country may have remained unchanged, or but slightly modified. Hills of volcanic matter have indeed been formed since the Eocene formations were accumulated, and the levels of large tracts have been altered in relation to the sea; lakes have been drained, and a gulf of the sea turned into dry land, but many of the reciprocal relations of the different parts of the surface may still remain the same. The waters which flowed from the granitic heights into the Eocene lakes may now descend in the same manner into valleys once the basins of those lakes. Let us, for example, suppose the great Canadian lakes, and the gulf into which their waters are discharged, to be elevated and laid dry by subterranean movements. The whole hydrographical basin of the St. Lawrence might be upraised during these convulsions, yet that river might continue, after so extraordinary a revolution, to drain the same elevated regions, and might continue to convey its waters in the same direction from the interior of the continent to the Atlantic. Instead of traversing the lakes, it would hold its course through deposits of lacustrine sand and shelly marl, such as we know to be now forming in Lakes Superior and Erie; and these fresh-water strata would occupy the site and bear testimony to the pristine existence of the lakes. Marine strata might also be brought into view in the space where an inlet of the sea, like the estuary of the St. Lawrence, had once received the continental waters; and in such formations we might discover shells of lacustrine and fluviatile species intermingled with marine testacea and zoophytes.

Subdivisions of strata in the Paris basin. -- The area which has been called the Paris basin is about one hundred and eighty miles in its greatest length from north-east to southwest, and about ninety miles from east to west. This space may be described as a depression in the chalk (see diagram No. 2, p. 16), which has been filled up by alternating groups of marine and fresh-water strata. MM. Cuvier and Brongniart attempted in 1811 to distinguish five different formations, and to arrange them in the following order, beginning with the lowest: --

Image
1. First fresh-water formation ... {Plastic clay; Lignite; First sandstone.
2. First marine formation ... {Calcaire grossier.
3. Second fresh-water formation ... {Siliceous limestone; Gypsum, with bones of animals; Fresh-water marls.
4. Second marine formation ... {Gypseous marine marls; Upper marine sands and sandstones; Upper marine marls and limestones.
5. Third fresh-wafer formation ... {Siliceous millstone, without shells; Siliceous millstone, with shells; Upper fresh-water marls.


These formations were supposed to have been deposited in succession upon the chalk; and it was imagined that the waters of the ocean had been by turns admitted into and excluded from the same region. But the subsequent investigations of several geologists, especially of M. Constant Prevost, [1] have led to great modifications in the theoretical views entertained respecting the order in which the several groups were formed; and it now appears that the formations Nos. 1, 2, and 3, of the table of MM. Cuvier and Brongniart, instead of having originated one after the other, are divisible into four nearly contemporaneous groups.

Image
No. 58. No. 59.

Superposition of different formations in the Paris basin. -- A comparison of the two accompanying diagrams will enable the reader to comprehend at a glance the different relations which the several sets of strata bear to each other, according to the original, as well as the more modern classification. We shall now proceed to lay before the reader a brief sketch of the several sets of strata referred to in the above systems.

Immediately upon the chalk a layer of broken chalk flints, often cemented into a breccia by siliceous sand, is very commonly found. These flints probably indicate the action of the sea upon reefs of chalk when a portion of that rook had emerged and before the regular tertiary beds were superimposed. To this partial layer no reference is made in the annexed sections.

Plastic clay and sand. -- Upon this flinty stratum, or, if it be wanting, upon the chalk itself, rests frequently a deposit of clay and lignite (No.1 of the above tables). It is composed of fresh-water shells and drift-wood, and was, at first, regarded as a proof that the Paris basin had originally been filled with fresh water. But it has since been shown that this group is not only of very partial extent, but is by no means restricted to a fixed place in the series; for it alternates with the marine calcaire grossier (No.2 of the tables), and is repeated in the very middle of that limestone at Veaugirard, Bagneux, and other places, where the same Planorbes, Paludinae, and Limnei occur. [2] M. Desnoyers pointed out to me a section in the suburbs of Paris, laid open in 1829, where a similar intercalation was seen in a still higher part of the calcaire grossier. These observations relieve us from the difficulty of seeking a cause why vegetable matter, and certain species of fresh-water shells and a particular kind of clay, was first introduced into the basin, and why the same space was subsequently usurped by the sea. A minute examination of the phenomena leads us simply to infer, that a river charged with argillaceous sediment entered a bay of the sea and drifted down, from time to time, fresh-water shells and wood.

Calcaire grossier. -- The calcaire grossier above alluded to, is composed of a coarse limestone, often passing into sand, such as may perhaps have been derived from the aqueous degradation of a chalk country. It contains by far the greater number of the fossil shells which characterize the Paris basin. No less than 400 distinct species have been derived from a single locality near Grignon. They are imbedded in a calcareous sand, chiefly formed of comminuted shells, in which, nevertheless, individuals in a perfect state of preservation, both of marine, terrestrial, and fresh-water species, are mingled together' and were evidently transported from a distance. Some of the marine shells may have lived on the spot, but the Cyclostoma and Limnea must have been brought there by rivers and currents, and the quantity of triturated shells implies considerable movement in the waters.

Nothing is more remarkable in this assemblage of fossil testacea than the astonishing proportion of species referrible to the genus Cerithium. [3] There occur no less than 137 species of this genus in the Paris basin, and almost all of them in the calcaire grossier. Now the living testacea of this genus inhabit the sea near the mouths of rivers, where the waters are brackish, so that their abundance in the marine strata of the Paris basin is in perfect harmony with the hypothesis before advanced, that a river flowed into the gulf, and gave rise to the beds of clay and lignite before mentioned. But there are ample data for inferring that the gulf was supplied with fresh water by more than one river, for while the calcaire grossier occupies the northern part of the Paris basin, another contemporaneous deposit, of fresh-water origin, appears at the southern extremity.

Calcaire siliceux. -- This group (No. 3 of the foregoing tables) is a compact siliceous limestone, which resembles a precipitate from the waters of mineral springs. It is, for the most part, devoid of organic remains, but in some places it contains fresh-water and land species, and never any marine fossils. The siliceous limestone and the calcaire grossier occupy distinct parts of the basin, the one attaining its fullest development in those places where the other is of slight thickness. They also alternate with each other towards the centre of the basin, as at Sergy and Osny, and there are even points where the two rocks are so blended together, that portions of each may be seen in hand specimens. Thus in the same bed, at Triel, we have the compact fresh-water limestone, characterized by its Limnei, mingled with the coarse marine limestone through which the small multilocular shell, called milliolite, is dispersed in countless numbers. These microscopic testacea are also accompanied by Cerithia and other shells of the calcaire grossier. It is very extraordinary that, although in this instance both kinds of sediment must have been thrown down together on the same spot, each still contains its own peculiar organic remains. [4]

These facts lead irresistibly to the conclusion, that while to the north, where the bay was probably open to the sea, a marine limestone was formed, another deposit of fresh-water origin was introduced to the southward, or at the head of the bay. For it appears that during the Eocene period, as now, the ocean was to the north, and the continent, where the great lakes existed, to the south. From the latter region we may suppose a body of fresh water to have descended charged with carbonate of lime and silica, the water being perhaps in sufficient volume to convert the upper end of the bay into fresh water, like some of the gulfs of the Baltic.

Gypsum and marls. -- The next group to be considered is the gypsum, and the white and green marls, subdivisions of No. 3 of the table of Cuvier and Brongniart. These were once supposed to be entirely subsequent in origin to the two groups already considered; but M. Prevost has pointed out that in some localities they alternate repeatedly with the calcaire siliceux, and in others with some of the upper members of the calcaire grossier. The gypsum, with its associated marls and limestone, is in greatest force towards the centre of the basin, where the two groups before mentioned are less fully developed; and M. Prevost infers, that while those two principal deposits were gradually in progress, the one towards the north, and the other towards the south, a river descending from the east may have brought down the gypseous and marly sediment.

It must be admitted, as highly probable, that a bay or narrow sea, 180 miles in length, would receive, at more points than one, the waters of the adjoining continent; at the same time we must observe, that if the gypsum and associated green and white marls of Montmartre were derived from an hydrographical basin distinct from that of the southern chain of lakes before adverted to, this basin must nevertheless have been placed under circumstances extremely similar; for the identity of the rocks of Velay and Auvergne with the fresh- water group of Montmartre, is such as can scarcely be appreciated by geologists who have not carefully examined the structure of both these countries.

Some of our readers may think that the view above given of the arrangement of four different sets of strata in the Paris basin is far more obscure and complicated than that first presented to them in the system of MM. Cuvier and Brongniart. We admit that the relations of the several sets of strata are less simple than the first observers supposed, being much more analogous to those exhibited by the lacustrine groups of Central France before described.

The simultaneous deposition of two or more groups of strata in one basin, some of them fresh-water and others marine, must always produce very complex results; but in proportion as it is more difficult in these cases to discover any fixed order of superposition in the associated mineral masses, so also is it more easy to explain the manner of their origin and to reconcile their relations to the agency of known causes. Instead of the successive irruptions and retreats of the sea, and changes in the chemical nature of the fluid and other speculations of the earlier geologists, we are now simply called upon to imagine a gulf, into one extremity of which the sea entered, and at the other a large river, while other streams may have flowed in at different points, whereby an indefinite number of alternations of marine and fresh-water beds were occasioned.

Second or Upper marine group. -- The next group, called the second or Upper marine formation (No. 4 of the tables), consists in its lower division of green marls which alternate with the fresh-water beds of gypsum and marl last described. Above this division the products of the sea exclusively predominate, the beds being chiefly formed of micaceous sand, 80 feet or more in thickness, surmounted by beds of sandstone with scarcely any limestone. The summits of a great many platforms and hills in the Paris basin consist of this upper marine series, but the group is much more limited in extent than the calcaire grossier. Although we fully agree with M. C. Prevost that the alternation of the various marine and fresh-water formations before described admit of a satisfactory explanation without supposing different retreats and subsequent returns of the sea, yet we think a subsidence of the soil may best account for the position of the upper marine sands. Oscillations of level may have occurred whereby for a time the sea and a river prevailed each in their turn, until at length a more considerable sinking down of part of the basin took place, whereby a tract previously occupied by fresh water was converted into a sea of moderate depth.

In one part of the Paris basin there are decisive proofs that during the Eocene period, and before the upper marine sand was formed, parts of the calcaire grossier were exposed to the action of denuding causes. At Valmondois, for example, a deposit of the upper marine sandstone is found, [5] in which rolled blocks of the calcaire grossier with its peculiar fossils, and fragments of a limestone resembling the calcaire siliceux, occur. These calcareous blocks are rolled and pierced by perforating shells belonging to no less than fifteen distinct species, and they are imbedded, as well as worn shells washed out from the calcaire grossier, with the ordinary fossils of the upper marine sand.

We have seen that the same earthquake in Cutch could raise one part of the delta of the Indus and depress another, and cause the river to cut a passage through the upraised strata and carry down the materials removed from the new channel into the sea. All these changes, therefore, might happen within a short interval of time between the deposition of two sets of strata in the same delta. [6]

It is not improbable, then, that the same convulsions which caused one part of the Paris basin to sink down so as to let in the sea upon the area previously covered by gypsum and freshwater marl, may have lifted up the calcaire grossier and the siliceous limestone, so that they might be acted upon by the waves, and fragments of them swept down into the contiguous sea, there to be drilled by boring testacea.

It is observed that the older marine formation at Laon is now raised 300 metres above the sea, whereas the upper marine sands never attain half that elevation. Such may possibly have been the relative altitude of the two groups when the newest of them was deposited.

Third fresh-water formation. -- We have still to consider another formation, the third fresh-water group (No. 5 of the preceding tables). It consists of marls interstratified with beds of flint and layers of flinty nodules. One set of siliceous layers is destitute of organic remains, the other replete with them.

Gyrogonites, or fossil seed-vessels of charae, are found abundantly in these strata, and all the animal and vegetable remains agree well with the hypothesis, that after the gulf or estuary had been silted up with the sand of the upper marine forma tion, a great number of marshes and shallow lakes existed, like those which frequently overspread the newest parts of a delta. These lakes were fed by rivers or springs which contained, in chemical solution or mechanical suspension, such kinds of sediment as we have already seen to have been deposited in the lakes of Central France during the Eocene period.

The Parisian groups all Eocene. -- Having now given a rapid sketch of the different groups of the Paris basin, we may observe generally that they all belong to the Eocene epoch, although the entire series must doubtless have required an immense lapse of ages for its accumulation. The shells of the different fresh-water groups, constituting at once some of the lowest and uppermost members of the series, are nearly all referrible to the same species, and the discordance between the marine testacea of the calcaire grossier and the upper marine sands is very inconsiderable.

A curious observation has been made by M. Deshayes, in reference to the changes which one species, the Cardium porulosum, has undergone during the long period of its existence in the Paris basin. Different varieties of this cardium are characteristic of different strata. In the oldest sand of the Soissonnais (a marine formation underlying the regular beds of the calcaire grossier), this shell acquires but a small volume, and has many peculiarities which disappear in the lowest beds of the calcaire grossier. In these the shell attains its full size, and many peculiarities of form, which are again modified in the uppermost beds of the calcaire grassier, and these last characters are preserved throughout the whole of the 'upper marine' series. [7]

Microscopic shells. -- In some parts of the calcaire grossier microscopic shells are very abundant, and of distinct species from those before mentioned of the older Pliocene beds of Italy. We may remind those readers who are not familiar with these minute fossil bodies, that they belong to the order Cephalopoda, the animals of which are most free in their movements, and most advanced in their organization, of all the mollusca. The multilocular cephalopods have been separated, by d'Orbigny, into two subdivisions: first, those having a syphon or internal tube connecting the different chambers, such as the nautilus and ammonite; and, secondly, those without a syphon, to which the microscopic species now under consideration belong. They are often in an excellent state of preservation, and their forms are singularly different from those of the larger testacea. We have given a plate of some of these, from unpublished drawings by M. Deshayes, who has carefully selected the most remarkable types of form.

The natural size of each species figured in plate 4, is indicated by minute points, to which we call the reader's attention, as they might be easily overlooked.

Bones of quadrupeds in gypsum. -- We have already considered the position of the gypsum which occurs in the form of a saccharoid rock in the hill of Montmartre at Paris, and other central parts of the basin. At the base of that hill it is seen distinctly to alternate with soft marly beds of the calcaire grossier, in which cerithia and other marine shells occur. But the great mass of gypsum may be considered as a purely fresh-water deposit, containing land and fluviatile shells, together with fragments of palm-wood, and great numbers of skeletons of quadrupeds and birds, an assemblage of organic remains which has given great celebrity to the Paris basin. The bones of fresh-water fish, also, and of crocodiles, and many land and fluviatile reptiles occur in this rock. The skeletons of mammalia are usually isolated, often entire, the most delicate extremities being preserved as if the carcasses clothed with their flesh and skin had been floated down soon after death, and while they were still swoln by the gases generated by their first decomposition. The few accompanying shells are of those light kinds which frequently float on the surface of rivers together with wood.

M. Prevost has, therefore, suggested that a river may have swept away the bodies of animals, and the plants which lived on its borders, or in the lakes which it traversed, and may have carried them down into the centre of the gulf into which flowed the waters impregnated with sulphate of lime. We know that the Fiume Salso in Sicily enters the sea so charged with various salts that the thirsty cattle refuse to drink of it. A stream of sulphureous water, as white as milk, descends into the sea from the volcanic mountain of Idienne, on the east of Java; and a great body of hot water, charged with sulphuric acid, rushed down from the same on one occasion, and inundated a large tract of country, destroying, by its noxious properties, all the vegetation". [8] In like manner the Pusanibio, or 'Vinegar river' of Colombia, which rises at the foot of Purace, an extinct volcano 7500 feet above the level of the sea, is strongly impregnated with sulphuric and muriatic acid, and with oxide of iron. We may easily suppose the waters of such streams to have properties noxious to marine animals, and in this manner we may explain the entire absence of marine remains in the ossifferous gypsum. [9]

There are no pebbles or coarse sand in the gypsum, a circumstance which agrees well with the hypothesis that these beds were precipitated from water holding sulphate of lime in solution, and floating the remains of different animals. The bones of land quadrupeds however are not confined entirely to the fresh-water formation to which the gypsum belongs, for the remains of a Palreotherium, together with some fresh-water shells have been found in a marine stratum belonging to the calcaire grossier at Beauchamp.

In the gypsum the remains of about fifty species of quadrupeds have been found all extinct and nearly four-fifths of them belonging to a division of the order Pachydermata, which is now only represented by four living species, namely by three tapirs and the daman of the Cape. A few carnivorous animals are associated, among which are a species of fox and gennet. Of the Rodentia, a dormouse and a squirrel; of the Insectivora, a bat; and of the Marsupialia, (an order now confined to America, Australia, and some contiguous islands,) an opossum, have been discovered.

Of birds about ten species have been ascertained, the skeletons of some of which are entire. None of them are referrible to existing species. [10] The same remark applies to the fish, according to MM. Cuvier and Agassiz, as also to the reptiles. Among the last are crocodiles and tortoises of the genera Emys and Trionix.

The tribe of land quadrupeds most abundant in this formation is such as now inhabits alluvial plains and marshes and the banks of rivers and lakes, a class most exposed to suffer by river inundations. Whether the disproportion of carnivorous animals can be ascribed to this cause, or whether they were comparatively small in number and dimensions, as in the indigenous fauna of Australia, when first known to Europeans, is a point on which it would be rash perhaps to offer an opinion in the present state of our knowledge.

We have no reason to be surprised that all the species of vertebrated animals hitherto observed are extinct, when we recollect that out of 1122 species of fossil testacea obtained from the Paris basin, 38 only can be identified with species now living. We have more than once adverted to the fact that extinct mammalia are often found associated with assemblages of recent shells, a fact from which we have inferred the inferior duration of species in mammalia as compared to the testacea; and it is not improbable that the higher order of animals in general may more readily become extinct than the marine molluscs. Some of the thirty-eight species of testacea above alluded to, as having survived from the Eocene period to our own times, have now a wide geographical range, as, for example, Lucina divaricata, and are therefore fitted to exist under a great variety of circumstances. On the other hand, the great proportion of the Eocene marine testacea which have become extinct sufficiently demonstrates that the loss of species has been due to general laws, and that a sudden catastrophe, such as the invasion of a whole continent by the sea-a cause which could only annihilate the terrestrial and fresh-water tribes, is an hypothesis wholly inadequate to account for the phenomenon.

Strata with and without organic remains alternating. -- Between the gypsum of the Paris basin and the upper marine sands a thin bed of oysters is found, which is spread over a remarkably wide area. From the manner in which they lie, it is inferred that they did not grow on the spot, but that some current swept them away from a bed of oysters formed in some other part of the bay. The strata of sand which immediately repose on the oyster-bed are quite destitute of organic remains; and nothing is more common in the Paris basin and in other formations, than alternations of shelly beds with others entirely devoid of them. The temporary extinction and renewal of animal life at successive periods have been inferred from such phenomena, which may nevertheless be explained, as M. Prevost justly remarks, without appealing to any such extraordinary revolutions in the state of the animate creation. A current one day scoops out a channel in a bed of shelly sand and mud, and the next day, by a slight alteration of its course, ceases to prey upon the same bank. It may then become charged with sand unmixed with shells, derived from some dune, or brought down by a river. In the course of ages an indefinite number of transitions from shelly strata to those without shells may thus be caused.

Concluding remarks. -- It will be seen by our observations on Auvergne and other parts of Central France, and on the district round Paris, that geologists have already gained a considerable insight into the state of the physical geography of part of Europe during the Eocene period. We can point to some districts where lakes and rivers then existed, and to the site of some of the lands encircling those lakes, and to the position of a great bay of the sea, into which their surplus waters were discharged. We can also show, as we shall endeavour to explain in the next chapter, the points where some volcanic eruptions took place. We have acquired much information respecting the quadrupeds which inhabited the land at that period, and concerning the reptiles, fishes, and testacea which swarmed in the waters of lakes and rivers; and we have a collection of the marine Eocene shells more complete than has yet been obtained from any existing sea of equal extent in Europe. Nor are the contemporary fossil plants altogether unknown to us, which, like the animals, are of extinct species, and indicate a warmer climate than that now prevailing in the same latitudes.

When we reflect on the tranquil state of the earth implied by some of the lacustrine and marine deposits of this age, and consider the fullness of all the different classes of the animal kingdom, as deduced from the study of the fossil remains, we are naturally led to conclude, that the earth was at that period in a perfectly settled state, and already fitted for the habitation of man.

The heat of European latitudes during the Eocene period does not seem to have been superior if equal to that now experienced between the tropics; some living species of molluscous animals both of the land, the lake, and the sea, existed when the strata of the Paris basin were formed, and the contrast in the organization of the various tribes of Eocene animals when compared to those now co-existing with man, although striking, is not, perhaps, so great as between the living Australian and European types. At the same time we are fully aware that we cannot reason with any confidence on the capability of our own or any other contemporary species to exist under circumstances so different as those which might be caused by an entirely new distribution of land and sea; and we know that in the earlier tertiary periods the physical geography of the northern hemisphere was very distinct. Our inability to account for the atmospheric and other latent causes, which often give rise to the most destructive epidemics, proves the extent of our ignorance of the entire assemblage of conditions requisite for the existence of anyone species on the globe.

_______________

Notes:

1. Bulletin des Sci. de la Soc. Philom., May, 1825, p. 74.

2. Prevost, Sur les Submersions Iteratives, &c. Mem. de la Soc. d'Hist. Nat. de Paris, tome iv. p. 74.

3. See the tables of M. Deshayes, Appendix I., p. 26.

4. M. Prevost has pointed out this limestone to me, both in situ at Triel, and in hand specimens in his cabinet.

5. M. Deshayes, Memoires de la Soc. d'Hist. Nat. de Paris, tom. i. p. 243. -- The sandstone is called, by mistake, gres marin inferieur, instead of superieur, to which last the author has since ascertained it to belong.

6. Vol. i. 2d Edit. chap. xxiii.; vol. ii 1st Edit. chap. xvi.

7. Coquilles characterist. des Terrains, 1831.

8. Leyde Magaz. voor Wetensch Konst en Lett., partie v. cahier i. p. 71. Cited by Rozet, Journ. de Geologie, tom. i. p.43.

9. M. C. Prevost, Submersions Iteratives, &c. Note 23.

10. Cuvier, Oss. Foss. tom. iii. p. 255.
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Re: Principles of Geology, by Charles Lyell

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CHAPTER 19

Volcanic rocks of the Eocene period – Auvergne – Igneous formations associated with lacustrine strata – Hill of Gergovia – Eruptions in Central France at successive periods – Mont Dor an extinct volcano – Velay – Plomb du Cantal – Train of minor volcanos stretching from Auvergne to the Vivarais – Monts Domes – Puy de Côme – Puy Rouge – Ravines excavated through lava – Currents of lava at different heights – Subjacent alluviums of distinct ages – The more modern lavas of Central France may belong to the Miocene period – The integrity of the cones not inconsistent with this opinion – No eruptions during the historical era – Division of volcanos into ante-diluvian and post-diluvian inadmissible – Theories respecting the effects of the Flood considered – Hypothesis of a partial flood – Of a universal deluge – Theory of Dr. Buckland as controverted by Dr. Fleming – Recapitulation

EOCENE VOLCANIC ROCKS.

WHEN we treated in the seventeenth chapter of the lacustrine deposits of Central France, we purposely omitted to give a detailed account of the associated volcanic rocks, to which we now recall the attention of the reader. (See the Map, p. 226.) We stated that, in the arenaceous and pebbly group of the lacustrine basins of Auvergne, Cantal, and Velay, no volcanic pebbles had ever been detected, although massive piles of igneous rocks are now found in the immediate vicinity. As this observation has been confirmed by minute research, we are warranted in inferring, as we before explained, that the volcanic eruptions had not commenced when the older subdivisions of the fresh-water groups originated.

In Cantal and Velay we believe no decisive proofs have yet been brought to light that any of the igneous out-bursts happened during the deposition of the fresh-water strata; but there can be no doubt that in Auvergne some volcanic explosions took place before the drainage of the lakes, and at a time when the Eocene species of animals and plants still flourished. We shall first advert to these proofs as relating to the history of the period under consideration, and shall then proceed to show that there are in the same country volcanic rocks of much newer date, some of which appear to be referrible to the Miocene era.

Volcanic rocks associated with Lacustrine in Auvergne. -- The first locality to which we shall call the reader's attention is Pont du Chateau near Clermont, where a section is seen in a precipice on the right bank of the river Allier. [1] Beds of volcanic tuff alternate with a fresh-water limestone, which is in some places pure, but in others spotted with fragments of volcanic matter, as if it were deposited while showers of sand and scoriae were projected from a neighbouring vent. [2] This limestone contains the Helix Ramondi and other shells of Eocene species. It is immaterial to our present argument whether the volcanic sand was showered down from above, or drifted to the spot by a river, for the latter opinion must presuppose the country to have been covered with volcanic ejections during the Eocene period.

Another example occurs in the Puy de Marmont, near Veyres, where a fresh-water marl alternates with volcanic tuff containing Eocene shells. The tuff or breccia in this locality is precisely such as is known to result from volcanic ashes falling into water, and subsiding together with ejected fragments of marl and other stratified rocks. These tuffs and marls are highly inclined, and traversed by a thick vein of basalt which, as it rises in the hill, divides into two branches.

Gergovia. -- The hill of Gergovia near Clermont affords a third example. We agree with MM. Dufrenoy and Jobert that there is no alternation here of lava and fresh-water strata, in the manner supposed by some other observers; [3] but the position and contents of some of the tuffs prove them to have been derived from volcanic eruptions which occurred during the deposition of the Eocene formations.

The bottom of the hill consists of slightly inclined beds of white and greenish marls, more than three hundred feet in thickness, which are intersected by a dike of basalt, which may be studied in the ravine above the village of Merdogne. The dike here cuts through the marly strata at a considerable angle, producing, in general, great alteration and confusion in them for some distance from the point of contact. Above the white and green marls, a series of beds of limestone and marl, containing fresh-water shells, are seen to alternate with volcanic tuff. In the lowest part of this division, beds of pure marl alternate with compact fissile tuff resembling some of the subaqueous tuffs of Italy and Sicily called peperinos. Occasionally fragments of scoriae are visible in this rock. Still higher is seen another group of some thickness, consisting exclusively of tuff, upon which lie other marly strata intermixed with volcanic matter.

Image
No. 60: Hill of Gergovia.

There are many points in Auvergne where igneous rocks have been forced by subsequent injection through clays and marly limestones, in such a manner that the whole has become blended in one confused and brecciated mass, between which and the basalt there is sometimes no very distinct line of demarcation. In the cavities of such mixed rocks we often find calcedony and crystals of mesotype, stilbite and arragonite. To formations of this class may belong some of the breccias immediately adjoining the dike in the hill of Gergovia; but it cannot be contended that the volcanic sand and scoriae interstratified with the marls and limestones in the upper part of that hill were introduced, like the dike, subsequently by intrusion from below. They must have been thrown down like sediment from water, and can only have resulted from igneous action which was going on contemporaneously with the deposition of the lacustrine strata.

The reader will bear in mind that this conclusion agrees well with the proofs, adverted to in the seventeenth chapter, of the abundance of silex, travertin and gypsum precipitated when the upper lacustrine strata were formed: for these rocks, as we have pointed out, are such as the waters of mineral and thermal springs might generate.

The igneous products above mentioned, as associated with the lacustrine strata, form the lowest members of the great series of volcanic rocks of Auvergne, Cantal, and Velay, which repose for the most part on the granitic mountains (see Map, above, p. 226). There was evidently a long succession of eruptions, beginning with those of the Eocene period, and ending, so far as we can yet infer from the evidence derived from fossil remains, with those of the Miocene epoch. The oldest part of the two principal volcanic masses of Mont Dor and the Plomb du Cantal may perhaps belong to the Eocene period, -- the newer portion of the same mountains to the Miocene; just as Etna commenced its operations during the newer Pliocene era, and has continued them down to the Recent epoch, and still retains its energy undiminished. There are some parts of the Mont Mezen, in Velay, which are perhaps of the same antiquity as the oldest parts of Mont Dor. Besides these ancient rocks, of which the lavas are in a great measure trachytic, there are many minor cones in Central France, for the most part of posterior origin, which extend from Auvergne, in a direction north-west and south-east, through Velay, into the Vivarais, where they are seen in the basin of the Ardeche. This volcanic line does not pass by the Plomb du Cantal; it was formed, as nearly as we can conjecture in the present imperfect state of our knowledge, during the Miocene period; but there may probably be found, among these cones and their accompanying lavas, rocks of every intermediate age between the oldest and newest volcanic formations of Central France.

We shall first give a brief description of the Mont Dor and the Plomb du Cantal, and then pass on to the train of newer cones, examining the evidence at present obtained respecting their relative ages, and the light which they throw on the successive formation of alluviums and on the excavation of valleys.

Mont Dor. -- Mont Dor, the most conspicuous of the volcanic masses of Auvergne, rests immediately on the granitic rocks standing apart from the fresh-water strata. [4] This volcano rises suddenly to the height of several thousand feet above the surrounding platform, and retains the shape of a flattened and somewhat irregular cone, all the sides sloping more or less rapidly, until their inclination is gradually lost in the high plain around. It is composed of layers of scoriae, pumice-stones, and their fine detritus, interstratified with beds of trachyte and basalt, which descend often in uninterrupted currents, till they reach and spread themselves around the base of the mountain. [5] Conglomerates also, composed of angular and rounded fragments of igneous rocks, are observed to alternate with the above; and the various masses are seen to dip off from the central axis, and to lie parallel to the sloping flanks of the great cone, in the same manner as we have described when treating of Etna.

The summit of the mountain terminates in seven or eight rocky peaks, where no regular crater can be traced, but where we may easily imagine one to have existed which may have been shattered by earthquakes, and have suffered degradation by aqueous causes. Originally, perhaps, like the highest crater of Etna, it may have formed an insignificant feature in the great pile, and may frequently have been destroyed and renovated.

We cannot at present determine the age of the great mass of Mont Dor, because no organic remains have yet been found in the tuffs, except impressions of the leaves of trees of species not determined. Some of the lowest parts of the great mass are formed of white pumiceous tuffs, in which animal remains may perhaps be one day found. In the mean time, we conclude that Mont Dor had no existence when the grits and conglomerates of the Limagne, which contain no volcanic materials, were formed; but some of the earliest eruptions may perhaps have been contemporary with those described in the commencement of this chapter. To the latest of these eruptions, on the other hand, we refer those trachytic breccias of Mont Perrier which were shown in the sixteenth chapter, p. 217, to alternate with Miocene alluviums.

Velay. -- The observations of M. Bertrand de Done have not yet established that any of the most ancient volcanos of Velay were in action during the Eocene period, although it is very probable that some of them may have been contemporaneous with the oldest of the Auvergne lavas. There are beds of gravel in Velay, as in Auvergne, covered by lava at different heights above the channels of the existing rivers. In the highest and most ancient of these alluviums the pebbles are exclusively of granitic rocks; but in the newer, which are found at lower levels, they contain an intermixture of volcanic substances. We have already shown, in the sixteenth chapter, that, in the volcanic ejections and alluviums covered by the lavas of Velay, the bones of animals of Miocene species have been found, in which respect the phenomena accord perfectly with those of Auvergne.

Plomb du Cantal. -- In regard to the age of the igneous rocks of the Cantal we are still Jess informed, and at present can merely affirm that they overlie the Eocene lacustrine strata of that country. The Plomb du Cantal (see Map, wood-cut No. 56) is a conical mass, which has evidently been formed, like the cone of Etna, by a long series of eruptions. It is composed of trachytic and basaltic lavas, tuff's, and conglomerates, or breccias, forming a mountain several thousand feet in height. This volcano evidently broke out precisely on the site of the lacustrine deposit before described (Chapter xvii.), which had accumulated in a depression of a tract composed of micaceous schist. In the breccias, even to the very summit of the mountain, we find ejected masses of the fresh-water beds, and sometimes fragments of flint, containing Eocene shells. Deep valleys radiate in all directions from the central heights of the mountain, especially those of the Cer and Jourdanne, which are more than twenty miles in length, and lay open the geological structure of the mountain. No alternation of lavas with undisturbed Eocene strata have been observed, nor any tuffs containing fresh-water shells; on the northern side of the Plomb du Cantal, at La Vissiere, near Murat, we have pointed out on the Map (wood-cut, p. 226) a spot where fresh-water limestone and marl are seen covered by a thickness of about 800 feet of volcanic rock. Shifts are here seen in the strata of limestone and marl. [6]

Although it appears that the lavas of the Cantal are more recent than the fresh-water formation of that country, it does not follow that they may not belong to the Eocene period. The lake may possibly have been drained by the earthquakes which preceded or accompanied the first eruptions, but the Eocene animals and plants may have continued to exist for a long series of ages, while the cone went on increasing in dimensions.

Train of minor Volcanos. -- We shall next consider those minor volcanos before alluded to, which stretch in a long range from Auvergne to the Vivarais, and which appear for the most part to be of newer origin than the mountains above described. They have been thrown up in a great number of isolated points, and much resemble those scattered over the Phlegraean fields and the flanks of Etna. They have given rise chiefly to currents of basaltic lava, whereas those of Mont Dor and the Cantal are in great part trachytic. There are perhaps about three hundred of these minor cones in Central France; but a part of them only occur in Auvergne, where some few are found at the bottom of valleys excavated through the more ancient lavas of Mont Dor, as the Puy de Tartaret, for example, whence issues a current of lava which, flowing into the bed of the river Couze, gave rise to the lake of Chambon. Here the more ancient columnar basalts of Auvergne are seen forming the upper portion of the precipices which bound the valley.

But the greater part of the minor cones of Auvergne are placed upon the granitic platform, where they form an irregular ridge about eighteen miles in length and two in breadth. They are usually truncated at the summit, where the crater is often preserved entire, the lava having issued from the base of the hill. But frequently the crater is broken down on one side, where the lava has flowed out. The hills are composed of loose scoriae, blocks of lava, lapilli, and puzzuolana, with fragments of trachyte and granite.

The lavas may be often traced from the crater to the nearest valley, where they usurp the channel of the river, which has often excavated a deep ravine through the basalt. We have thus an opportunity of contrasting the enormous degradation which the solid and massive rock has suffered by aqueous erosion and the integrity of the cone of sand and ashes which has, in the mean time, remained uninjured on the neighbouring platform, where it was placed beyond the reach of the power of running water.

Puy de Come. -- We may mention the Puy de Come and its lava current, near Clermont, as one of the numerous illus trations of the phenomenon here alluded to. This conical hill rises from the granitic platform at an angle of about 40 degrees to the height of more than 900 feet. Its summit presents two distinct craters, one of them with a vertical depth of 250 feet. A stream of lava takes its rise at the western base of the hill, instead of issuing from either crater, and descends the granitic slope towards the present site of the town of Pont Gibaud. Thence it pours in a broad sheet down a steep declivity into the valley of the Sioule, filling the ancient river-channel for the distance of more than a mile. The Sioule, thus dispossessed of its bed, has worked out a fresh one between the lava and the granite of its western bank; and the excavation has disclosed, in one spot, a wall of columnar basalt about fifty feet high. [7]

The excavation of the ravine is still in progress, every winter some columns of basalt being undermined and carried down the channel of the river, and in the course of a few miles rolled to sand and pebbles. Meanwhile the cone of Come remains stationary, its loose materials being protected by a dense vegetation, and the hill standing on a ridge not commanded by any higher ground whence floods of rain-water may descend.

Puy Rouge. -- At another point, farther down the course of the Sioule, we find a second illustration of the same phenomenon in the Puy Rouge, a conical hill to the north of the village of Pranal. The cone is composed entirely of red and black scoriae, tuff, and volcanic bombs. On its western side there is a worn-down crater whence a powerful stream of lava has issued and flowed into the valley of the Sioule. The river has since excavated a ravine through the lava and subjacent gneiss, to the depth of 400 feet.

On the upper part of the precipice forming the left side of this ravine, we see a great mass of black and red scoriaceous lava; below this a thin bed of gravel, evidently an ancient river-bed, now at an elevation of 50 feet above the channel of the Sioule. The gravel again rests upon gneiss, which has been eroded to the depth of 50 feet. [8] It is quite evident in this case that, while the basalt was gradually undermined and carried away by the force of running water, the cone whence the lava issued escaped destruction, because it stood upon a platform of gneiss several hundred feet above the level of the valley in which the force of running water was exerted.

It is needless to multiply examples, or the Vivarais would supply many others equally striking. Among many we may instance the cone of Jaujac, and its lava current, [9] which is a counterpart of that near Pranal last mentioned.

Lavas and Alluviums of different Ages. -- We have seen that on the flanks of Etna, since the commencement of the present century, several currents of lava have flowed at the bottom of the Va1 del Bove, at the foot of precipices formed of more ancient lavas and tuffs. So we find in Auvergne that some streams of melted matter have flowed in valleys, the sides of which consist partly of older lavas. These are often seen capping the hills in broad sheets, resting sometimes on granite, sometimes on fresh-water strata.

Many of the earlier lavas of Auvergne have flowed out upon the platform of granite before all the existing valleys had been excavated; others again spread themselves in broad sheets over the horizontal lacustrine deposit, when these had been covered with gravel, probably soon after the drainage of the lakes. Great vicissitudes in the physical geography of the country must have taken place since the flowing of these ancient lavas; and it is evident that the changes were gradual and successive, caused probably by the united agency of running water and subterranean movements. We frequently observe one mass of lava capping a hill, and a second at a lower elevation, forming a terrace on the side of a valley; or sometimes occupying the bed of a river.

It is a most interesting fact that we almost invariably find in these cases beds of gravel underlying the successive currents of lava, as in Catalonia before described (pp. 189, 190). Occasionally, when the highest platform of lava is 700 or 800 feet above the lowest, we cannot fail to be struck with the wonderful alterations effected in the drainage of the country since the first current flowed; for the most elevated alluviums must originally have been accumulated in the lowest levels of the then existing surface. As some geologists have referred almost all the superficial gravels to one era, and have supposed them to be the result of one sudden catastrophe, the phenomena of Auvergne above alluded to are very important. The flows of volcanic matter have preserved portions of the surface in the state in which they existed at successive periods, so that it is impossible to confound together the alluviums of different ages. The reader will see at once by reference to the wood-cut (No. 61) that a considerable interval of time occurred between the formation of the uppermost bed of gravel and that next below it; during which interval the uppermost lava was poured out and a valley excavated, at the bottom of which the second bed of gravel accumulated. In like manner the pouring out of a second current of lava, and a farther deepening of the valley, took place between the date of the second gravel and that of the modern alluvium which now fills the channel of the river. [10]

Image
No. 61: Lavas of Auvergne resting on alluviums of different ages.

When rivers are dispossessed of their channels by lava, they usually flow between the mass of lava and one side of the original valley. They then eat out a passage, partly through the volcanic and partly through the older formation; but as the soft tertiary marls in Auvergne give way more readily than the basalt, it is usually at the expense of the former that the enlarging and deepening of the new valley is effected, and all the remaining lava is then left on one side, in the manner represented in the above wood-cut.

Age of the more modern lavas. -- The only organic remains found as yet in the ancient alluviums appear to belong to the Miocene period; but we have heard of none discovered in the gravel underlying the newest lavas, - those which either occupy the channels of the existing rivers or are very slightly elevated above them. We think it not improbable that even these may be of Miocene date, although the conjecture will appear extremely rash to some who are aware that the cones and craters whence the lavas issue, are often as fresh in their aspect as the majority of the cones of the forest zone of Etna.

The brim of the crater of the Puy de Pariou, near Clermont, is so sharp, and has been so little blunted by time, that it scarcely affords room to stand upon. This and other cones in an equally remarkable state of integrity have stood, we conceive, uninjured, not in spite of their loose porous nature, as some geologists might think, but in consequence of it. No rills can collect where all the rain is instantly absorbed by the sand and scoriae, as we have shown to be the case on Etna (see above, p. 102), and nothing but a waterspout breaking directly upon the Puy de Pariou could carry away a portion of the hill, so long as it is not rent by earthquakes or engulphed.

Attempt to divide Volcanos into ante-diluvian and post-diluvian. -- The opinions above expressed are entirely at variance with the doctrines of those writers who have endeavoured to arrange all the volcanic cones of Europe under two divisions, those of ante-diluvian and those of post-diluvian origin. To the former they attribute such hills of sand and scoriae as exhibit on their surface evident signs of aqueous denudation; to the latter, such as betray no marks of having been exposed to such aqueous action. According to this classification almost all the minor cones of Central France must be called postdiluvian; although, if we receive this term in its ordinary acceptation as denoting posteriority of date to the Noachian deluge~ we are forced to suppose that all the volcanic eruptions occurred within a period of little more than twenty centuries, or between the era of the flood, which happened about 4000 years ago, and the earliest historical records handed down to us respecting the former state of Central France. Dr. Daubeny has justly observed, that had any of these French volcanos been in a state of activity in the age of Julius Caesar, that general, who encamped upon the plains of Auvergne, and laid siege to its principal city, (Gergovia, near Clermont,) could hardly have failed to notice them. Had there been even any record of their existence in the time of Pliny or Sidonius Apollinaris, the one would scarcely have omitted to make mention of it in his Natural History, nor the other to introduce some allusion to it among the descriptions of this his native province. This poet's residence was on the borders of the Lake Aidat, which owed its very existence to the damming up of a river by one of the most modern lava currents. [11]

The ruins of several Roman bridges and of the Roman baths at Royat confirm the conclusion that no sensible alteration has taken place in the physical geography of the district, not even in the chasms excavated through the newest lavas since ages historically remote. We have no data at present for presuming that anyone of the Auvergne cones has been produced within the last 4000 or 5000 years; and the same may be said of those of Velay. Until the bones of men or articles of human workmanship are found buried under some of their lavas, instead of the remains of extinct animals, which alone have hitherto been met with, we shall consider it probable, as we before hinted, that the latest of the volcanic eruptions may have occurred during the Miocene period.

Supposed effects of the Flood.

They who have used the terms ante-diluvian and postdiluvian in the manner above adverted to, proceed on the assumption that there are clear and unequivocal marks of the passage of a general flood over all parts of the surface of the globe. It had long been a question among the learned, even before the commencement of geological researches, whether the deluge of the Scriptures was universal in reference to the whole surface of the globe, or only so with respect to that portion of it which was then inhabited by man. If the latter interpretation be admissible, the reader will have seen, in former parts of this work, that there are two classes of phenomena in the configuration of the earth's surface, which might enable us to account for such an event. First, extensive lakes elevated above the level of the ocean; secondly, large tracts of dry land depressed below that level. When there is an immense lake, having its surface, like Lake Superior, raised 600 feet above the level of the sea, the waters may be suddenly let loose by the rending or sinking down of the barrier during earthquakes, and hereby a region as extensive as the valley of the Mississippi, inhabited by a population of several millions, might be deluged. [12] On the other hand, there may be a country placed beneath the mean level of the ocean, as we have shown to be the case with part of Asia, [13] and such a region must be entirely laid under water should the tract which separates it from the ocean be fissured or depressed to a certain depth. The great cavity of western Asia is 18,000 square leagues in area, and is occupied by a considerable population. The lowest parts, surrounding the Caspian Sea, are 300 feet below the level of the Euxine, -- here, therefore, the diluvial waters might overflow the summits of hills rising 300 feet above the level of the plain; and if depressions still more profound existed at any former time in Asia, the tops of still loftier mountains may have been covered by a flood.

But it is undeniable, that the great majority of the older commentators have held the deluge, according to the brief account of the event given by Moses, to have consisted of a rise of waters over the whole earth, by which the summits of the loftiest mountains on the globe were submerged. Many have indulged in speculations concerning the instruments employed to bring about the grand cataclysm; and there has been a great division of opinion as to the effects which it might be expected to have produced on the surface of the earth. According to one school, of which De Luc in former times, and more recently Dr. Buckland, have been zealous and eloquent supporters, the passage of the flood worked a considerable alteration in the external configuration of our continents. By the last-mentioned writer the deluge is represented as a violent and transient rush of waters which tore up the soil to a great depth, excavated valleys, gave rise to immense beds of shingle, carried fragments of rock and gravel from one point to another, and, during its advance and retreat, strewed the valleys, and even the tops of many hills, with alluvium. [14]

But we agree with Dr. Fleming, [15] that in the narrative of Moses there are no terms employed that indicate the impetuous rushing of the waters, either as they rose or when they re- treated, upon the restraining of the rain and the passing of a wind over the earth. On the contrary, the olive-branch, brought back by the dove, seems as clear an indication to us that the vegetation was not destroyed, as it was then to Noah that the dry land was about to appear.

We have been led with great reluctance into this digression, in the hope of relieving the minds of some of our readers from groundless apprehension respecting the bearing of many of the views advocated in this work. They have been in the habit of regarding the diluvial theory above controverted as alone capable of affording an explanation of geological phenomena in accordance with Scripture, and they may have felt disapprobation at our attempt to prove, in a former chapter, [16] that the minor volcanos on the flanks of Etna may, some of them, be more than 10,000 years old. How, they would immediately ask, could they have escaped the denuding force of a diluvial rush of waters? The same objection may have presented itself when we quoted, with so much respect, the opinion of a distinguished botanist, that some living specimens of the Baobab tree of Africa, or the Taxodium of Mexico, may be five thousand years old. [17] Our readers may also have been astonished at the high antiquity assigned by us to the greater part of the European alluviums, and the many different ages to which we refer them, [18] as they may have been taught to consider the whole as the result of one recent and simultaneous inundation. Lastly, they may have felt some disappointment at observing, that we attach no value whatever to the hypothesis of M. Elie de Beaumont, adopted by Professor Sedgwick, that the sudden elevation of mountain-chains 'has been followed again and again by mighty waves desolating whole regions of the earth,' [19] a phenomenon which, according to the last-mentioned of these writers, has 'taken away all anterior incredibility from the fact of a recent deluge.' [20]

For our own part, we have always considered the flood, if we are required to admit its universality in the strictest sense of the term, as a preternatural event far beyond the reach of philosophical inquiry, whether as to the secondary causes employed to produce it, or the effects most likely to result from it. At the same time, it is evident that they who are desirous of pointing out the coincidence of geological phenomena with the occurrence of such a general catastrophe, must neglect no one of the circumstances enumerated in the Mosaic history, least of all so remarkable a fact as that the olive remained standing while the waters were abating.

Recapitulation. -- We shall now briefly recapitulate some of the principal conclusions to which we have been led by an examination of the volcanic districts of Central France.

1st. Some of the volcanic eruptions of Auvergne took place during the Eocene period, others at an era long subsequent, probably during the Miocene period.

2ndly. There are no proofs as yet discovered that the most recent of the volcanos of Auvergne and Velay are subsequent to the Miocene period, the integrity of many cones and craters not opposing any sound objection to the opinion that they may be of indefinite antiquity.

3rdly. There are alluviums in Auvergne of very different ages, some of them belonging to the Miocene period. Many of these have been covered by Java-currents which have been poured out in succession while the excavation of valleys was in progress.

4thly. There are a multitude of cones in Auvergne, Velay, and the Vivarais, which have never been subjected to the action of a violent rush of waters capable of modifying considerably the surface of the earth.

5thly. If, therefore, the Mosaic deluge be represented as universal, and as having exercised a violent denuding force, all these cones, several hundred in number, must be post-diluvian.

6thly. But since the beginning of the historical era, or the invasion of Gaul by Julius Caesar, the volcanic action in Auvergne has been dormant, and there is nothing to countenance the idea that, between the date usually assigned to the Mosaic deluge and the earliest traditional and historical records of Central France (a period of little more than twenty centuries), all or anyone of the more entire cones of loose scoriae were thrown up.

Lastly, it is the opinion of some writers, that the earth's surface underwent no great modification at the era of the Mosaic deluge, and that the strictest interpretation of the scriptural narrative does not warrant us in expecting to find any geological monuments of the catastrophe, an opinion which is consistent with the preservation of the volcanic cones, however high their antiquity.

_______________

Notes:

1. This place, and all the others in Auvergne, mentioned in this chapter, were examined by the author, in company with Mr. Murchison, in 1828.

2. See Scrope's Central France, p. 21.

3. Scrope, ibid. p. 7.

4. See the Map, p. 226.

5. Scrope's Central France, p. 98.

6. See Lyell and Murchison, Ann. des Sci. Nat., Oct. 1829.

7. Scrope's Central France, p. 60, and plate.

8. See Lyell and Murchison on the Excavation of Valleys, Edin. New Phil Journ., July 1829.

9. See Scrope's Central France, plate 14.

10. For localities in Central France where lavas or sheets of basalt repose on alluviums at different elevations above the present valleys, consult the works of MM. Le Grand d'Aussi, Montlosier, Ramond, Scrope, Bertrand de Doue, Croizet, Jobert, Bouillet, and others.

11. Daubeny on Volcanos, p. 14.

12. See vol. i. p. 89, and Second Edition, p. 101.

13. Vol. ii. p. 163, and Second Edition, p. 169.

14. Buckland, Reliquiae Diluvianae.

15. See a Memoir by the Rev. John Fleming, D. D, on the Geological Deluge, Edin. Phil. Journ., vol. xiv. p. 205, whose opinions were reviewed by the author in Oct. 1827, in an article in the Quarterly Review, No. lxxii. p. 481.

16. Chap. viii. p. 100.

17. See above, p. 99.

18. P. 147.

19. P. 101.

20. Anniv. Address to the Geol. Soc., Feb. 18th, 1831.
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Re: Principles of Geology, by Charles Lyell

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CHAPTER 20

Eocene formations, continued – Basin of the Cotentin, or Valognes – Rennes – Basin of Belgium, or the Netherlands – Aix in Provence – Fossil insects – Tertiary strata of England – Basins of London and Hampshire – Different groups – Plastic clay and sand – London clay – Bagshot sand – Fresh-water strata of the Isle of Wight – Palaeotherium and other fossil mammalia of Binstead – English Eocene strata conformable to chalk – Outliers on the elevated parts of the chalk – Inferences drawn from their occurrence – Sketch of a theory of the origin of the English tertiary strata

HAVING in the last three chapters treated of the Eocene formations of different parts of France, we now propose to examine those which are found in the south-eastern division of England; but before we pass from the continent to our own island, we may briefly advert to several other spots where Eocene deposits have been observed. Their geographical position will be found delineated on the annexed map. [1]

MAP OF THE PRINCIPAL TERTIARY BASINS OF THE EOCENE PERIOD.

Image
No. 62.

Basin of the Cotentin, or Valognes. -- The strata in the environs of Valognes, in the department of La Manche, consist chiefly of a coarse limestone resembling the calcaire grossier of Paris, of which M. Desnoyers has given an elaborate description. It is occasionally covered with a compact fresh-water limestone alternating with fresh-water marls. In these Eocene strata more than 300 species of fossil shells have been discovered, almost all identical with species of the Paris basin. (See Tables, Appendix I.) Superimposed upon the Eocene strata of this basin is a newer marine deposit, extending over a limited area, the fossils of which agree with those of the Faluns of the Loire. [2] Here, therefore, the geologist has an opportunity of observing the superposition of the Miocene deposits upon those of the age of the Paris basin.

Rennes. -- Several small patches, also, of marine strata, have been found by M. Desnoyers, in the neighbourhood of Rennes, which are characterised by Eocene fossils and repose on ancient rocks, as will be seen in the map.

Basin of Belgium, or the Netherlands. -- The greater part of the tertiary formations of the Low Countries consist of clay and sand, much resembling those of the basin of London, afterwards to be described. The fossil shells, also, are of the same species, 49 of which will be found referred to by M. Deshayes, in the tables, Appendix I.

Aix, in Provence. -- The tertiary strata of Aix and Fuveau in Provence are of great thickness and extent, the lower members being remarkable for containing coal grit and beds of compact limestone, such as we only find in England in ancient secondary groups. Yet these strata are for the most part of fresh-water origin, and contain several species of Eocene shells, together with many which are peculiar to this basin. It would require a fuller comparison than has yet been made of the fossil remains of Aix and Fuveau, before we can determine with accuracy the relative age of that formation. Some of the plants seem to agree with those of the Paris basin, while many of the insects have been supposed identical with species now living. [3] These insects have been almost exclusively procured from a thin bed of grey calcareous marl, which passes into an argillaceous limestone found in the quarries of gypsum near Aix. The rock in which they are imbedded is so thinly laminated that there are sometimes more than 70 layers in the thickness of an inch. The insects are for the most part in an extraordinary state of preservation, and an impression of their form is seen both on the upper and under laminae, as in the case of the Monte Bolca fishes. M. Marcel de Serres enumerates 62 genera belonging chiefly to the orders Diptera, Hemiptera and Coleoptera. On reviewing a collection brought from Aix, Mr. Curtis observes that they are all of European forms and most of them referrible to existing genera. [4] With the single exception of an Hydrobius, none of the species are aquatic. The antennae, tarsi, and trophi are generally very obscure, or distorted, yet in a few the claws are visible, and the sculpture, and even some degree of local colouring, are preserved. The nerves of the wings, in almost all the Diptera, are perfectly distinct, and even the pubescence on the head of one of them. Several of the beetles have the wings extended beyond the elytra, as if they had made an effort to escape by flying, or had fallen into the water while on the wing. [5]

The reader will see in the small map above given (No. 62, p. 275,) the position of the two districts usually called the basins of London and Hampshire, to which the Eocene formations of England are confined. These tracts are bounded by rising grounds composed of chalk, except where the sea intervenes. That the chalk passes beneath the tertiary strata, we can not only infer from geological data, but can prove by numerous artificial sections at points where wells have been sunk, or borings made through the overlying beds. The Eocene deposits are chiefly marine, and have generally been divided into three groups: 1st, the Plastic clay and sand, which is the lowest group; 2dly, the London clay; and, 3rdly, the Bagshot sand. Of all these the mineral composition is very simple, for they consist almost entirely of clay, sand, and shingle, the great mass of clay being in the middle, and the upper and lower members of the series being more arenaceous.

Plastic clay and sand. -- The lowest formation, which sometimes attains a thickness of from 400 to 500 feet, consists principally of an indefinite number of beds of sand, shingle, clay, and loam, irregularly alternating, some of the clay being used in potteries, in reference to which the name of Plastic clay has been given to the whole formation. The bens of shingle are composed of perfectly rolled chalk flints, with here and there small pebbles of quartz. Heaps of these materials appear sometimes to have remained for a long time covered by a tranquil sea. Dr. Buckland mentions that he observed a large pebble in part of this formation at Bromley, to which five full-grown oyster- hells were affixed, in such a manner as to show that they had commenced their first growth upon it, and remained attached through life. [6]

In some of the associated clays and sand, perfect marine shells are met with, which are of the same species as those of the London clay. The line of separation, indeed, between the superincumbent blue clay last alluded to, and the Plastic clay and sand, is quite arbitrary, as any geologist may be convinced who examines the celebrated section in Alum Bay, in the Isle of Wight, [7] where a distinct alternation of the two groups is observable, each marked with their most characteristic peculiarities. In the midst of the sands of the lower series a mass of clay occurs 200 feet thick, containing septaria, and replete with the usual fossils of the neighbourhood of London. [8]

The arenaceous beds are chiefly laid open on the confines of the basins of London and Hampshire, in following which we discover at many places great beds of perfectly rounded flints. Of this description, on the southern borders of the basin of London, are the hills of Comb Hurst and the Addington hills, which form a ridge stretching from Blackheath to Croydon. Here they have much the appearance of banks of sand and shingle formed near the shores of the tertiary sea; but whether they were really of littoral origin cannot be determined for want of a sufficient number of sections which might enable us to compare the tertiary strata at the edges with those in the central parts of each basin.

We have ample opportunities in the basin of Paris of examining steep cliffs of hard rock which bound many of the valleys, and innumerable excavations have been made for building-stone, limestone, and gypsum; but when we attempt to obtain a connected view of any considerable part of the tertiary series in the basin of London, we are almost entirely limited to a single line of coast-section; for in the interior the regular beds are much concealed by an alluvial covering of flint gravel spread alike over the summits and gentle slopes of the hills, and over the bottoms of the valleys.

Organic remains are extremely scarce in the Plastic clay; but when any shells occur they are of Eocene species. Vegetable impressions and fossil wood sometimes occur, and even beds of lignite, but none of the species of plants have, we believe, as yet been ascertained.

London clay. -- This formation consists of a bluish or blackish clay, sometimes passing into a calcareous marl, rarely into a solid rock. Its thickness is very great, sometimes exceeding 500 feet. [9] It contains many layers of ovate or flattish masses of argillaceous limestone, which, in their interior, are generally traversed in various directions by cracks, partially or wholly filled by calcareous spar. These masses, called septaria, are sometimes continued through a thickness of 200 feet. [10]

A great number of the marine shells of this clay have been identified with those of the Paris basin, and are mentioned by name in Appendix I. It is quite evident, therefore, that these two formations belong to the same epoch.

No remains of terrestrial mammalia have as yet been found in this clay, but the occurrence of bones and skeletons of crocodiles and turtles prove, as Mr. Conybeare justly remarks, the existence of neighbouring dry land. The shores, at least, of some islands were accessible, whither these creatures may have resorted to lay their eggs. In like manner, we may infer the contiguity of land from the immense number of ligneous seed-vessels of plants, some of them resembling the cocoa-nut, and other spices of tropical regions, which have been found fossil in great profusion in the Isle of Sheppey. Such is the abundance of these fruits, that they have been supposed to belong to several hundred distinct species of plants.

Bagshot sand. -- The third and uppermost group, usually termed the Bagshot sand, rests conformably upon the London clay, and consists of siliceous sand and sandstone devoid of organic remains, with some thin deposits of marl associated. From these marls a few marine shells have been obtained which are in an imperfect state, but appear to belong to Eocene species common to the Paris basin. [11]

Fresh-water strata of the Hampshire basin. -- In the northern part of the Isle of Wight, and part of the opposite coast of Hampshire, fresh-water strata occur resting on the London clay. They are composed chiefly of calcareous and argillaceous marls, interstratified with some thick beds of siliceous sand, and a few layers of limestone sometimes slightly siliceous. The marls are often green, and bear a considerable resemblance to the green marls of Auvergne and the Paris basin. The shells and gyrogonites also agree specifically with some of those most common in the French deposits. Mr. Webster, who first described the fresh-water formation of Hampshire, divided it into an upper and lower series separated by intervening beds of marine origin. There are undoubtedly certain intercalated strata, both in the Isle of Wight and coast of Hampshire, marked by a slight intermixture of marine and fresh-water shells, sufficient to imply a temporary return of the sea, before and after which the waters of a lake, or rather, perhaps, some large river, prevailed. [12] The united thickness of the fresh-water and intercalated upper marine beds, exposed in a vertical precipice in Headen Hill, in the Isle of Wight, is about 400 feet, the marine series appearing about half way up in the cliff.

Eocene mammiferous remains. -- Very perfect remains of tortoises and the teeth of crocodiles have been procured from the fresh-water strata, but a still more interesting discovery has recently been made. The bones of mammalia corresponding to those of the celebrated gypsum of Paris, have been disinterred at Binstead, near Ryde, in the Isle of Wight. In the ancient quarries near this town a limestone, belonging to the lower fresh-water formation, is worked for building. Solid beds alternate with marls, wherein a tooth of an Anoplotherium, and two teeth of the genus Palaeotherium, were found. These remains were accompanied not only by several other fragments of the bones of Pachydermata (chiefly in a rolled and injured state), but also by the jaw of a new species of Ruminantia, apparently closely allied to the genus Moschus. [13] Mr. T. Allan of Edinburgh had several years before found the tooth of an Anoplotherium at the same spot, and when we alluded to this in our first volume, [14] we threw out some doubts as to the authenticity of his specimen, stating at the same time, that in the Binstead beds, if anywhere in our island, we should expect such remains to be found. Although we carried our scepticism too far, it has been attended with good results, for it induced Mr. Pratt to visit Binstead, where he verified and extended the discovery of Mr. Allan.

These newer strata of the Isle of Wight bear a certain degree of resemblance to some of the green marls and limestones in the Paris basin, yet, as a whole, no formations can be more dissimilar in mineral character than the Eocene deposits of England and Paris. In our own island the tertiary strata are more exclusively marine, and it might be said that the Parisian series differs chiefly from that of London in the very points in which it agrees with the formations of Auvergne, Cantal, and Velay. The tertiary formations of England arc, in fact, almost exclusively of mechanical origin, and their composition be speaks the absence of those mineral and thermal waters to which we have attributed the origin of the compact and siliceous limestones, the gypsum, and beds of pure flint, common to the Paris basin and Central France.

English tertiary strata conformable to the chalk. -- The British Eocene strata are nearly conformable to the chalk on which they rest, being horizontal where the strata of the chalk are horizontal, and vertical where they are vertical. On the other hand, there are evident signs that the surface of the chalk had, in many places, been furrowed by the action of the waves and currents, before the Plastic clay and its sands were superimposed. In the quarries near Rochester and Gravesend, for instance, fine examples are seen of deep indentations on the surface of the chalk, into which sand, together with rolled and angular pieces of chalk-flint, have been swept. [15] But these appearances may be referred to the action of water when the chalk began to emerge during the Eocene period, and they by no means warrant the conclusion, that the chalk had undergone any considerable change of position before the tertiary strata were superimposed.

In this respect there is a marked difference between the reciprocal relations of our secondary and tertiary rocks and those which exist between the same groups throughout the greater part of the continent, especially in the neighbourhood of mountain-chains. Near the base, for example, of the Alps, Apennines, and Pyrenees, we find the newer formations reposing unconformably upon the truncated edges of the older beds, and it is c1ear that, in many cases, the latter had been subjected to a complicated series of movements before the more modern strata were formed. The latter rise only to a certain height on the flanks of the mountains which usually tower above them, and are recognized at once by the geologist as having been upraised into land when the tertiary formations were still forming in the sea. The ancient borders also of that sea can often be defined with certainty, and the outline of some of its bays and sea-cliffs traced.

In England, although undoubtedly the greater portion of the tertiary strata is confined to certain spaces, we find outlying patches here and there at great distances beyond the general limits, and at great heights upon the chalk which separates the basins of London and Hampshire. [16] I have seen masses of clay extending in this manner to near the edge of the western escarpment of the chalk in Wiltshire, and Mr. Mantell has pointed out the same to me in the South Downs. Near the escarpment at Lewes, for example, there is a fissure in the chalk filled with sand, and with a ferruginous breccia, such as usually marks the lower members of the Plastic clay formation. From the fact of these tertiary outliers Dr. Buckland inferred, 'that the basins of London and Hants were originally united together in one continuous deposit across the now intervening chalk of Salisbury Plain in Wilts, and the plains of Andover and Basingstoke in Hants, and that the greater integrity in which the tertiary strata are preserved within the basins has resulted from the protection which their comparatively low position has afforded them from the ravages of diluvial denudation.' [17]

We agree so far with this conclusion as to believe that the basins of London and Hampshire were not separated until part of the tertiary strata were deposited, but we do not think it probable that the tertiary beds ever extended continuously over those spaces where the outliers above mentioned occur, nor that the comparative thinness of those deposits in the higher chalk countries should be attributed chiefly to the greater degree of denudation which they have there suffered.

Origin of the English tertiary strata. -- In explanation of the phenomena above described, we shall endeavour, in the two next chapters, to lay before the reader a view of the series of events which may have produced the leading geological and geographical features of the south-east of England.

A preliminary outline of these views may be useful in this place. We conceive that the chalk, together with many subjacent rocks, may have remained undisturbed and in horizontal stratification until after the commencement of the Eocene period. When at length the chalk was upheaved and exposed to the action of the waves and currents, it was rent and shattered, so that the subjacent secondary strata were exposed at the same time to denudation. The waste of these rocks, composed chiefly of sandstone and clay, supplied materials for the tertiary sands and clays, while the chalk was the source of flinty shingle, and of the calcareous matter which we find intermixed with the Eocene clays. The tracts now separating the basins of London and Hampshire were those first elevated, and which contributed by their gradual decay to the production of the newer strata. These last were accumulated in deep submarine hollows, formed probably by the subsidence of certain parts of the chalk, which sank while the adjoining tracts were rising.

_______________

Notes:

1. This map is copied from one given by M. Desnoyers, Mem. de la Soc. d'Hist. Nat. de Paris, 1825, pl. 9; compiled partly from that author's observations, and partly from Mr. Webster's map, Geol. Trans., 1st series, vol. ii. plate 10.

2. Desnoyers, Mem. de 1a Soc. d'Hist. Nat. de Paris, 1825.

3. M. Marcel de Serres, Geog. des Ter. Tertiaires du Midi de la France.

4. Murchison and Lyell. Ed. New Phil. Journ., Oct. 1829.

5. Curtis, ibid., where figures of some of the insects are given.

6. Geol. Trans., First Series, vol. iv. p. 300.

7. See Mr. Webster's Memoir, Geol. Trans., vol. ii., First Series, and his Letters in Sir H. Englefield's Isle of Wight.

8. See Mr. Webster's sections, plate 11. Geol. Trans., vol. ii., First Series.

9. Con. and Phil. Outlines of Geol., p. 33.

10. Outlines of Geol., p. 27.

11. Warburton, Geol. Trans., vol. i. Second Series.

12. See Memoirs of Mr. Webster, Geol. Trans., vol. ii., First Series, vol. i. part i., Second Series, and Englefield's Isle of Wight. -- Professor Sedgwick, Ann. of Phil., 1822, and Lyell, Geol. Trans., vol. ii. Second Series.

13. Pratt, Proceedings of Geol. Soc., No. 18, p.239.

14. First Edition, p. 153.

15. Con. and Phil., Outlines of Geol., p. 62.

16. Dr. Buckland, Geol. Trans., Second Series, vol. ii.p.125.

17. lbid., p. 126.
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Re: Principles of Geology, by Charles Lyell

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CHAPTER 21

Denudation of secondary strata during the deposition of the English Eocene formations – Valley of the Weald between the North and South Downs – Map – Secondary rocks of the Weald divisible into five groups – North and South Downs – Section across the valley of the Weald – Anticlinal axis – True scale of heights – Rise and denudation of the strata gradual – Chalk escarpments once sea-cliffs – Lower terrace of 'firestone,' how caused – Parallel ridges and valleys formed by harder and softer beds – No ruins of the chalk on the central district of the Weald – Explanation of this phenomenon – Double system of valleys, the longitudinal and the transverse – Transverse how formed – Gorges intersecting the chalk – Lewes Coomb – Transverse valley of the Adur

Denudation of the Valley of the Weald. -- IN order to understand the theory of which we sketched an outline at the close of the last chapter, it will be necessary that the reader should be acquainted with the phenomena of denudation exhibited by the chalk and some of the older secondary rocks in parts of England most nearly contiguous to the basins of London and Hampshire. It will be sufficient to consider one of the denuded districts, as the appearances observable in others are strictly analogous; we shall, therefore, direct our attention to what we may call the Valley of the Weald, or the region intervening between the North and South Downs.

Map. -- In the coloured map given in Plate V., [1] the district alluded to is delineated, and it will be there seen that the southern portion of the basin of London, and the north-eastern limits of that of Hampshire, are separated by a tract of secondary rocks, between 40 and 50 miles in breadth, comprising within it the whole of Sussex and parts of the counties of Kent, Surrey, and Hampshire.

There can be no doubt that the tertiary deposits of the Hampshire basin formerly extended much farther along our southern coast towards Beachy Head, for patches are still found near Newhaven, and at other points, as will be seen by the map. These are now wasting away, and will in time disappear, as the sea is constantly encroaching and undermining the subjacent chalk.

The secondary rocks, depicted on the map, may be divided into five groups: --

1. Chalk and Upper green-sand. -- This group is the uppermost of the series; it includes the white chalk with and without flints, and an inferior deposit called, provincially, 'Firestone,' and by English geologists the 'Upper green-sand.' It sometimes consists of loose siliceous sand, containing grains of silicate of iron, but often of firm beds of sandstone and chert.

2. Blue clay or calcareous marl, called provincially Gault.

3. Lower green-sand, a very complex group consisting of grey, yellowish, and greenish sands, ferruginous sand and sandstone, clay, chert, and siliceous limestone.

4. Weald clay, composed for the most part of clay without intermixture of calcareous matter, but sometimes including thin beds of sand and shelly limestone.

5. Hastings sands, composed chiefly of sand, sandstone, clay, and calcareous grit, passing into limestone. [2]

The first three formations above enumerated are of marine origin, the last two, Nos. 4 and 5, contain almost exclusively the remains of fresh-water and amphibious animals. But it is not our intention at present to enlarge upon the organic remains of these formations, as we have merely adverted to the rocks in order that we may describe the changes of position which they have undergone, and the denudation to which they have been exposed since the commencement of the Eocene period, -- mutations which, if our theory be well founded, belong strictly to the history of tertiary phenomena.

By a glance at the map, the reader may trace at once the superficial area occupied by each of the five formations above mentioned. On the west will be seen a large expanse of chalk, from which two branches are sent off; one through the hills of Surrey and Kent to Dover, forming the ridge called the North Downs, the other through Sussex to the sea at Beachy Head, constituting the South Downs. The space comprised between the North and South Downs, or 'the Valley of the Weald,' consists of the formations Nos. 2, 3, 4, 5, of the above table. It will be observed that the chalk terminates abruptly, and with a well-defined line towards the country occupied by those older strata. Within that line is a narrow band coloured blue, formed by the gault, and within this again is the Lower green sand, next the Weald clay, and then, in the centre of the district, a ridge formed by the Hastings sands.

Section of the Valley of the Weald. -- It has been ascertained by careful investigation, that if a line be drawn from any part of the North to the South Downs, which shall pass through the central group, No. 5, the beds will be found arranged in the order described in the annexed section (No. 63, p. 288).

We refer the reader at present to the dark lines of the section, as the fainter lines represent portions of rock supposed to have been carried away by denudation.

At each end of the diagram the tertiary strata a are exhibited reposing on the chalk. In the centre are seen the Hastings sands (No. 5), forming an anticlinal axis, on each side of which the other formations are arranged with an opposite dip. It has been necessary however, in order to give a clear view of the different formations, to exaggerate the proportional height of each in comparison to its horizontal extent, and we have subjoined a true scale in another diagram (No. 61) in order to correct the erroneous impression which might otherwise be made on the reader's mind. In this section the distance between the North and South Downs is represented to exceed 40 miles; for we suppose the valley of the Weald to be here intersected in its longest diameter, in the direction of a line between Lewes and Maidstone.

Image
No. 63. Section from the London to the Hampshire basin across the Valley of the Weald.
a, Tertiary strata.
1, Chalk and firestone.
2, Gault.
3, Lower green-sand.
4, Weald clay.
5, Hastings sands.
Highest point of South Downs, 858 feet.
Anticlinal axis of the Weald. Crowborough Hill, 804 feet.
Highest point of North Downs, 880 feet. [3]
Section of the country from the confines of the basin of London to that of Hants, with the principal heights above the level of the sea on a true scale. [4]


In attempting to account for the manner in which the five secondary groups above mentioned may have been brought into their present position, the following hypothesis has been very generally adopted. Suppose the five formations to lie in horizontal stratification at the bottom of the sea; then let a movement from below press them upwards into the form of a flattened dome, and let the crown of this dome be afterwards cut off, so that the incision should penetrate to the lowest of the five groups. The different beds would then be exposed on the surface in the manner exhibited in the map, plate 5. [5]

It will appear from former parts of this work, that the amount of elevation here supposed to have taken place is not greater than we can prove to have occurred in other regions within geological periods of no great duration. On the other hand, the quantity of denudation or removal by water of vast masses which are assumed to have once reached continuously from the North to the South Downs is so enormous, that the reader may at first be startled by the boldness of the hypothesis. nut he will find the difficulty to vanish when once sufficient time is allowed for the gradual and successive rise of the strata, during which the waves and currents of the ocean might slowly accomplish an operation, which no sudden diluvial rush of waters could possibly have effected.

Escarpments of the chalk once sea-cliffs. -- In order to make the reader acquainted with the physical structure of the Valley of the Weald, we shall suppose him first to travel southwards from the London basin. On leaving the tertiary strata he will first ascend a gently- inclined plane, composed of the upper flinty portion of the chalk, and then find himself on the summit of a declivity consisting, for the most part, of different members of the chalk formation, below which the upper green-sand, and sometimes also the gault crop out. [6] This steep declivity is called by geologists 'the escarpment of the chalk,' which overhangs a valley excavated chiefly out of the argillaceous or marly bed, termed Gault (No.2). The escarpment is continuous along the southern termination of the North Downs, and the reader may trace it from the sea at Folkstone, westward to Guildford and the neighbourhood of Petersfield, and from thence to the termination of the South Downs at Beachy Head. In this precipice or steep slope the strata are cut off abruptly, and it is evident that they must originally have extended farther. In the accompanying wood-cut (No. 65), part of the escarpment of the South Downs is faithfully represented, where the denudation at the base of the declivity has been somewhat more extensive than usual, in consequence of the upper and lower green-sand being formed of very incoherent materials, the former, indeed, being extremely thin and almost wanting.

Image
No. 65: View of the chalk escarpment of the South Downs. Taken from the Devil's Dike, looking towards the west and south-west,
a, The town of Steyning is hidden by this point.
b, Edburton church.
c, Road.
d, River Adur.


The geologist cannot fail to recognize in this view the exact likeness of a sea-cliff, and if he turns and looks in an opposite direction, or eastward, towards Beachy Head, he will see the same line of height prolonged. Even those who are not accustomed to speculate on the former changes which the surface has undergone, may fancy the broad and level plain to resemble the flat sands which were laid dry by the receding tide, and the different projecting masses of chalk to be the headlands of a coast which separated the different bays from each other.

Image
No. 66: Chalk escarpment as seen from the hill above Steyning, Sussex. The castle and village of Bramber in the foreground.

Lower terrace of firestone. -- We have said that the upper green-sand ('firestone,' or 'malm rock,' as it is sometimes called) is almost absent in the tract here alluded to. It is, in fact, seen at Beachy Head to thin out to an inconsiderable stratum of loose green-sand; but farther to the west ward it is of great thickness, and contains hard beds of blue chert and limestone. Here, accordingly, we find that it produces a corresponding influence on the scenery of the country, for it runs out like a step beyond the foot of the chalk-hills, and constitutes a lower terrace varying in breadth from a quarter of a mile to three miles, and following the sinuosities of the chalk escarpment. [7]

Image
No. 67.
a, Chalk with flints.
b, Chalk without flints.
c, Upper green sand, or firestone.
d, Gault.


It is impossible to desire a more satisfactory proof that the escarpment is due to the excavating power of water during the gradual rise of the strata. For we have shown, in our account of the coast of Sicily, [8] in what manner the encroachments of the sea tend to efface that succession of terraces which must otherwise result from the successive rises of a coast preyed upon by the waves. During the interval between two elevatory movements, the lower terrace will usually be destroyed, wherever it is composed of incoherent materials; whereas the sea will not have time entirely to sweep away another part of the same terrace, or lower platform, which happens to be composed of rocks of a harder texture and capable of offering a firmer resistance to the erosive action of water.

Valleys where softer strata, ridges where harder crop out. -- It is evident that the Gault No. 2 (see the map) could not have opposed any effectual resistance to the denuding force of the waves; its outcrop, therefore, is marked by a valley, the breadth of which is often increased by the loose incoherent nature of the uppermost beds of the lower green-sand, which lie next to it, and which have often been removed with equal facility.

The formation last mentioned has been sometimes entirely smoothed off like the gault; but in those districts where chert, limestone, and other solid materials enter largely into its composition, it forms a range of hills parallel to the chalk, which sometimes rival the escarpment of the chalk itself in height, or even surpass it, as in Leith Hill. This ridge often presents a steep escarpment towards the Weald clay which crops out from under it. (See the strong lines in diagram No. 63, p. 288.)

The clay last mentioned forms, for the most part, a broad valley, separating the lower green-sand from the Hastings sands, or Forest ridge; but where subordinate beds of sandstone of a firmer texture occur, the uniformity of the plain is broken by waving irregularities and hillocks. [9]

In the central region, or Forest ridge, the strata have been considerably disturbed and are greatly fractured and shifted. One fault is known where the vertical shift of a bed of calcareous grit is no less than 60 fathoms. [10] It must not be supposed that the anticlinal axis, which we have described as running through the centre of the weald, is by any means so simple as is usually represented in geological sections. There are, on the contrary, a series of anticlinal and synclinal [11] lines, which form ridges and troughs running nearly parallel to each other.

Image
No. 68.
a, Anticlinal lines.
b, Synclinal lines.


Much of the picturesque character of the scenery of this district arises from the depth of the narrow valleys and ridges to which the sharp bends and fractures of the strata have given rise; but it is also in part to be attributed to the excavating power exerted by water, especially on the interstratified argillaceous beds.

From the above description it will appear that, in the tract intervening between the North and South Downs, there are a series of parallel valleys and ridges; the valleys appearing evidently to have been formed principally by the removal of softer materials, while the ridges are due to the resistance offered by firmer beds to the destroying action of water.

Rise and denudation of the strata gradual. -- Let us then consider how far these phenomena agree with the changes which we should naturally expect to occur during the gradual rise of the secondary strata. Suppose the line of the most violent movements to have coincided with what is now the central ridge of the Weald Valley; in that case, the first land which emerged must have been situated where the Forest ridge is now placed. Here a number of reefs may have existed, and islands of chalk, which may have been gradually devoured by the ocean in the same manner as Heligoland and other European isles have disappeared in modern times, as related in our first volume. [12]

Suppose the ridge or dome first elevated to have been so rent and shattered on its summit as to give more easy access to the waves, until at length the masses represented by the fainter lines in the annexed diagram were removed. Two strips of land might then remain on each side of a channel, in the same manner as the opposite coasts of France and England, composed of chalk, present ranges of white cliffs facing each other. A powerful current might then rush, like that which now ebbs and flows through the straits of Dover, and might scoop out a channel in the gault. We must bear in mind that the intermittent action of earthquakes would accompany this denuding process, fissuring rocks, throwing down cliffs, and bringing up, from time to time, new stratified masses, and thus greatly accelerating the rate of waste. If the lower bed of chalk on one side of the channel should be harder than on the other, it would cause an under terrace, as represented in the above diagram, resembling that presented by the upper green-sand in parts of Sussex and Hampshire. When at length the gault was entirely swept away from the central parts of the channel, the lower green-sand (3, diagram No. 70,) would be laid bare, and portions of it would become land during the continuance of the upheaving earth quakes. Meanwhile the chalk cliffs would recede farther from one another, whereby four parallel strips of land, or perhaps rows of islands, would be caused.

Image
No. 69

Image
No. 70.
The dotted line represents the sea-level.


The edges of the argillaceous strata, No. 2, are still exposed to erosion by the waves, and a portion of the clay, No. 4, is already removed. This clay, as it gradually rises, will be swept off from part of the subjacent group, No. 5, which will then be laid bare, and may afterwards become land by subsequent elevation.

Why no ruins of chalk on central district. -- By this theory of the successive emergence and denudation of the groups, 1, 2, 3, 4, 5, we may account for an alluvial phenomenon which seems inexplicable on any other hypothesis. The summits of the chalk downs are covered everywhere with flint gravel, which is often entirely wanting on the surface of the clay at the foot of the chalk escarpment, and no traces of chalk flint have ever been found in the alluvium of the central district, or Forest ridge. It is rare, indeed, to see any wreck of the chalk, even at the distance of two or three miles from the escarpments of the North and South Downs. To this general rule, however, an exception occurs near Barcombe, about three miles to the north of Lewes, where we obtain the accompanying section. [13] It will be seen that the valley at the foot of the escarpment extends, in this case, not only over the gault, but over the 'lower green-sand' to the "Weald clay. On this clay a thick bed of flints, evidently derived from the waste of chalk, remains in the position above described.

Image
No. 71: Section from the North escarpment of the South Downs to Barcombe.
1, Gravel composed of partially-rounded chalk flints.
2, Chalk with and without flints.
3, Lowest chalk or chalk marl (upper green-sand wanting).
4, Gault.
5, Lower green-sand.
6, Weald clay.


We say that there is no detritus of the chalk and its flints on the central ridge of the Weald. I have sought in vain for a vestige of such fragments, and Mr. Mantell, who has had greater opportunities of minute investigation, assures me that he has never been able to detect any. Now whether we embrace or reject the theory of the former continuity of the chalk and other groups over the whole space intervening between the North and South Downs, we cannot certainly imagine that any transient and tumultuous rush of waters could have swept over this country, which should not have left some fragments of the chalk and its flints in the deep valleys of the Forest ridge. Indeed, if we adopted the diluvial hypothesis of Dr. Buckland, we should expect to find vast heaps of broken flints drifted more frequently into the valleys of the Gault and Weald clay, instead of being so frequently confined to the summit of the chalk downs. On the other hand, it is quite conceivable that the slow agency of oceanic currents may have cleared away, in the course of ages, the matter which fell into the sea from wasting cliffs. The reader will recollect our account of the manner in which the sea has advanced, within the last century, upon the Norfolk coast at Sherringham. [14]

Image
No. 72: Section of cliffs west of Sherringham.
a, Crag.
b, Ferruginous flint breccia on the surface of the chalk.
c, Chalk with flints.


The beach, at the foot of the cliff, is composed of bare chalk with flints, as is the bed of the sea near the shore. No one would suspect, from the appearance of the beach at low water, that a few years ago beds of solid chalk, together with sand and loam of the superincumbent crag, formed land on the very spot where the waves are now rolling; still less that these same formations extended, within the last 50 years, to a considerable distance from the present shore, over a space where the sea has now excavated a channel 20 feet deep.

As in this recent instance the ocean has cleared away part of the chalk, and its capping of crag, so the tertiary sea may have swept away not only the chalk, but the layer of broken flints on its surface, which was probably a marine alluvium of the Eocene period. Hence these flints might naturally occur on the downs, and be wanting in the valleys below.

If the reader will refer to the preceding diagrams (Nos. 69 and 70), and reflect not only on the successive states of the country there delineated, but on all the intermediate conditions which the district must have passed through during the process of elevation and denudation before supposed, he will understand why no wreck of the chalk (No. 1) should occur at great distances from the chalk escarpments. For it is evident that when the ruins of the uppermost bed (No. 1, diagram 69) had been thrown down upon the surface of the bed immediately below, those ruins would subsequently be carried away when this inferior stratum itself was destroyed. And in proportion to the number and thickness of the groups, thus removed in succession, is the probability lessened of our finding any remnants of the highest group strewed over the bared surface of the lowest.

Transverse valleys. -- There is another peculiarity in the geographical features of the south-east of England which must not be overlooked when we are considering the action of the denuding causes. By reference to the map (Plate 5), the reader will perceive that the drainage of the country is not effected by water-courses following the great valleys excavated out of the argillaceous strata (Nos. 2 and 4), but by valleys which run in a transverse direction, passing through the chalk to the basin of the Thames on the one side, and to the English channel on the other.

In this manner the chain of the North Downs is broken by the rivers Wey, Mole, Darent, Medway, and Stour; the South Downs by the Arun, Adur, Ouse, and Cuckmere. [15]

If these transverse hollows could be filled up, all the rivers, observes Mr. Conybeare, would be forced to take an easterly course, and to empty themselves into the sea by Romney Marsh and Pevensey levels. [16]

Mr. Martin has suggested that the great cross fractures of the chalk which have become river channels have a remarkable correspondence on each side of the valley of the Weald; in several instances the gorges in the North and South Downs appearing to be directly opposed to each other. Thus, for example, the defiles of the Wey, in the North Downs, and of the Arun in the South, seem to coincide .s in direction; and, in like manner, the Ouse is opposed to the Darent, and the Cuckmeloe to the Medway. [17] But we think it very possible that these coincidences may be accidental. It is, however, by no means improbable, as hinted by the author above mentioned, that the great amount of elevation towards the centre of the Weald district gave rise to transverse fissures. And as the longitudinal valleys were connected with that linear movement which caused the anticlinal lines running east and west, so the cross fissures might have been occasioned by the intensity of the upheaving force towards the centre of the line, whereby the effect of a double axis of elevation was in some measure produced.

Image
No. 73: Transverse valley of the Adur in the South Downs.
a, Town of Steyning.
b, River Adur.
c, Old Shoreham.


In order to give a clearer idea of the manner in which the chalk-hills are intersected by these transverse valleys, we subjoin a sketch (No. 73) of the gorge of the river Adur, taken from the summit of the chalk-downs, at a point in the bridle-way leading from the towns of Bramber and Steyning to Shoreham. If the reader will refer again to the view given in a former wood-cut (No. 65, p. 290), he will there see the exact point where the gorge, of which we are now speaking, interrupts the chalk escarpment. A projecting hill, at the point a, hides the town of Steyning, near which the valley commences where the Adur passes directly to the sea at Old Shoreham. The river flows through a. nearly level plain, as do most of the others which intersect the hills of Surrey, Kent, and Sussex; and it is evident that these openings, so far at least as they are due to aqueous erosion, have not been produced by the rivers, many of which, like the Ouse near Lewes, have filled up arms of the sea, instead of deepening the hollows which they traverse.

In regard to the origin of the transverse ravines, there can be no doubt that they are connected with lines of fracture, and perhaps, in some places, there may be an anticlinal dip on both sides of the valley, as suggested by a local observer. [18] But this notion requires confirmation.

Image
No. 74: Supposed section of Transverse Valley.

The ravine, called the Coomb, near Lewes, affords a beautiful example of the manner in which narrow openings in the chalk may have been connected with shifts and dislocations in the strata. This coomb is seen on the eastern side of the valley of the Ouse, in the suburbs of the town of Lewes. The steep declivities on each side are covered with green turf, as is the bottom, which is perfectly dry. No outward signs of disturbance are visible, and the connexion of the hollow with subterranean movements would not have been suspected by the geologist, had 110t the evidence of great convulsions been clearly exposed in the escarpment of the valley of the Ouse, and in the numerous chalk pits worked at the termination of the Coomb. By aid of these we discover that the ravine coincides precisely with a line of fault, on one side of which the chalk with flints a, appears at the summit of a hill, while it is thrown down to the bottom on the other.

Image
No. 75: The Coomb, near Lewes.

Image

No. 76: Fault in the cliff-hills near Lewes.
a, Chalk with flints.
b, Lower chalk. [19]


The fracture here alluded to is one of those which run east and west, and of which there are many in the Weald district, parallel to the central axis of the Forest ridge.

In whatever manner the transverse gorges originated, they must evidently have formed ready channels of communication between the submarine longitudinal valleys and those deep parts of the sea wherein we imagine the tertiary strata to have been accumulated. If the strips of land which first rose had been unbroken, and there had been no free passage through the cross fractures, the currents would not so easily have drifted away the materials detached from the wasting cliffs, and it would have been more difficult to understand how the wreck of the denuded strata could have been so entirely swept away from the base of the escarpments.

In the next chapter we shall resume the consideration of these subjects, especially the proofs of the former continuity of the chalk of the North and South Downs, and the probable connexion of the denudation of the Weald valley with the origin of the Eocene strata.

_______________

Notes:

1. This map has been chiefly taken from Mr. Greenough's Map of England.

2. For an account of these strata in the southeast of England, see Mantell's Geology of Sussex, and Dr. Fitton's Geology of Hastings, where the memoirs of all the writers on this part of England are referred to.

3. Lieutenant H. Murphy, R. E., informs me that Botley Hill, near Godstone, in Surrey, was found by trigonometrical measurement to be 880 feet above the level of the sea; and Wrotham Hill, near Maidstone, which appears to be next in height of the North Downs, 795 feet.

4. My friend Mr. Mantell, of Lewes, has kindly drawn up this scale at my request.

5. See illustrations of this theory by Dr. Fitton, Geol. Sketch of Hastings.

6. We use this term, borrowed from our miners, to express the coming up to the surface of one stratum from beneath another.

7. Mr. Murchison, Geol. Sketch of Sussex, &c., Geol. Trans., 2nd Series, vol. ii. p. 98.

8. See p. 111, and wood-cut No. 24.

9. Martin, Geol. of Western Sussex. Fitton, Geol. of Hastings, p.31.

10. Fitton, ibid., p. 55.

11. We adopt this term, first used, we believe, by Professor Sedgwick; its signification will best be understood by reference to the accompanying diagram.

12. Page 289, and Second Edition, page 330.

13. The author visited this locality with Mr. Mantell, to whom he is indebted for this section.

14. Vol. i. p. 268, and Second Edition, p. 307.

15. Conybeare, Outlines of Geol., p. 81.

16. Ibid., p. 145.

17. Geol. of Western Sussex, p. 61.

18. Martin, Geol. of Western Sussex, p. 64, plate III. fig. 3.

19. I examined this spot in company with Mr. Mantell, to whom I am indebted for the above section.
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Re: Principles of Geology, by Charles Lyell

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CHAPTER 22

Denudation of the Valley of the Weald, continued – The alternative of the proposition that the chalk of the North and South Downs were once continuous, considered – Dr. Buckland on the Valley of Kingsclere – Rise and denudation of secondary rocks gradual – Concomitant deposition of tertiary strata gradual – Composition of the latter such as would result from the wreck of the secondary rocks – Valleys and furrows on the chalk how caused – Auvergne, the Paris basin, and south-east of England one region of earthquakes during the Eocene period – Why the central parts of the London and Hampshire basins rise nearly as high as the denudation of the Weald -- Effects of protruding force counteracted by the levelling operations of water – Thickness of masses removed from the central ridge of the Weald – Great escarpment of the chalk having a direction north-east and south-west – Curved and vertical strata in the Isle of Wight – These were convulsed after the deposition of the fresh-water beds of Headen Hill – Elevations of land posterior to the crag – Why no Eocene alluviums recognizable – Concluding remarks on the intermittent operations of earthquakes in the south-east of England, and the gradual formation of valleys – Recapitulation

Extent of denudation in the Valley of the Weald. -- 'IT would be highly rash,' observes Mr. Conybeare, speaking of the denudation of the Weald, 'to assume that the chalk at any period actually covered the whole space in which the inferior strata are now exposed, although the truncated form of its escarpment evidently shows it to have once extended much farther than at present.' [1]

We believe that few geologists who have considered the extent of country supposed to have been denuded, and who have explored the hills and valleys of the central, or Forest ridge, without being able to discover the slightest vestige of chalk in the alluvium, [2] will fail to participate, at first, in the doubts here expressed as to the original continuity of the upper secondary formations over the anticlinal axis of the Weald. For our own part, we never traversed the wide space which separates the North and South Downs, without desiring to escape from the conclusions advocated in the last chapter; and yet we have been invariably brought back again to the opinion, that the chalk was originally continuous, on a more deliberate review of the whole phenomena.

It may be useful to consider the only other alternative of the hypothesis before explained. If the marine groups, Nos. 1, 2, 3, were not originally continuous, it is necessary to imagine that they each terminated at some point between their present outgoings and the secondary strata of the Forest ridge. Thus we might suppose them to have thinned out one after the other, as in the above diagram, and never to have covered the entire area occupied by the fresh-water strata, Nos. 4 and 5.

Image
No. 77.
1, Chalk and Upper green-Sand. } Marine.
2, Gault. } Marine.
3, Lower green-sand. } Marine.
4, Weald clay. } Fresh-water.
5, Hastings sands. } Fresh-water.


We grant that had such been the original disposition of the different groups, they might, as they gradually emerged from the sea, have become denuded in the manner explained in the last chapter, so that the country might equally have assumed its present configuration. But, although we know of no invincible objection to such an hypothesis, there are certainly no appearances which favour it. If the strata Nos. 4 and 5 had been unconformable to the Lower green-sand No. 3, then, indeed, we might have imagined that the older groups had been disturbed by a series of movements antecedently to the deposition of No. 3, and, in that case, some parts of them might be supposed to have emerged or formed shoals in the ancient sea, interrupting the continuity of the newer marine deposits. But the group No. 4 is conformable to No. 3, and the only change which has been observed to take place at the junction, is an occasional intermixture of the Weald clay with the superior marine sand, such as might have been caused by a slight superficial movement in the waters when the sea first overflowed the fresh-water strata.

On the other hand, the green-sand and chalk, as they approach the central axis of the Weald, are not found to contain littoral shells, or any wreck of the fresh-water strata, such as might indicate the existence of an island with its shores or wasting cliffs. Had any such signs been discovered, we might have been inclined to suppose the geography of the region to have once borne some resemblance to that exhibited in the diagram No. 77.

Dr. Buckland on Valleys of Elevation. -- We are indebted to Dr. Buckland for an able memoir in illustration of several districts of similar form and structure to the Weald, which occur at no great distance in the south of England. His paper is intitled, 'On the formation of the Valley of Kingsclere and other valleys by the elevation of the strata which enclose them.' [3]

The valley of Kingsclere, situate a few miles south of Newbury, in Berkshire, is about five miles long and two in breadth. The upper and lower chalk, as will be seen in the accompanying section, [4] and the upper green-sand dip in opposite directions from an anticlinal axis which passes through the middle of the valley along the line a, b, of the ground-plan (No. 78).

Image
No. 78: Valley of Kingsclere
a,b, Anticlinal line marking the opposite dip of the strata on each side of it.


Image
No. 79: Section across the Valley of Kingsclere from north to south.
1, Chalk with flints.
2, Lower chalk without flints.
3, Upper green-sand, or firestone, containing beds of chert.


We subjoin an additional wood-cut, as conveying a scale of heights more nearly approaching to that of nature, although the altitudes, in proportion to the horizontal extent, are even in this, perhaps, somewhat in excess. On each side of the valley we find escarpments of chalk, the strata of which dip in opposite directions, in the northern escarpment to the north, and in the southern to the south. At the eastern and western extremities of the valley, the two escarpments become confluent, precisely in the same manner as do those of the North and South Downs, at the eastern end of the Weald district, near Petersfield. And as, a few miles east of the town last mentioned (see Map, plate V.), the firestone, or upper green-sand, is laid open in the sharp angle between the escarpment of the Alton Hills and the western termination of the South Downs; [5] so in the valley of Kingsclere the same formation is seen to crop out from beneath the chalk.

Image
No. 80: Valley of Kingsclere.

The reader might imagine, on regarding Dr. Buckland's section (No. 79), where, for the sake of elucidating the geological phenomena, the heights are greatly exaggerated in proportion to the horizontal extent, that the solution of continuity of the strata bounding the valley of Kingsclere had been simply due to elevation and fracture, unassisted by aqueous causes; but by reference to the true scale (No. 80), it will immediately appear, that a considerable mass of chalk must have been removed by denudation.

If the anticlinal dip had been confined to the valley of Kingsclere, we might have supposed that the upheaving force had acted on a mere point, forcing upwards the superincumbent strata into a small dome-shaped eminence, the crown of which had been subsequently cut off. But Dr. Buckland traced the line of opposite dip far beyond the confluence of the chalk escarpments, and found that it was prolonged in a more northwest direction far beyond the point a, diagram No. 78. In following the line thus extended, the strata are seen in numerous chalk-pits to have an opposite dip on either side of a central axis, from which we may clearly infer the linear direction of the movement. Perhaps the intensity of the disturbing force was greatest where the denudation of the valley of Kingsclere took 'place; but this cannot be confidently inferred, for the quantity of matter removed by aqueous agency must depend on the set of the tides and currents at the period of emergence, and not solely on the amount of elevation and derangement of the strata.

Many of the valleys enumerated by Dr. Buckland as having a similar conformation to that of Kingsclere, run east and west, like the anticlinal ridge of the Weald valley. Several of these occur in Wiltshire and Dorsetshire, and they are all circumscribed by an escarpment whose component strata dip outwards from an anticlinal line running along the central axis of the valley. One of these, distant about seven miles to the north-east of Weymouth, is nearly elliptical in shape, and in size does not much exceed that of the Coliseum at Rome. Their drainage is generally effected by an aperture in one of their lateral escarpments, and not at either extremity of their longer axis, as would have happened had they been simply excavated by the sweeping force of rapid water. [6]

'It will be seen,' continues Dr. Buckland, 'if we follow on Mr. Greenough's map the south-western escarpment of the chalk in the counties of Wilts and Dorset, that, at no great distance from these small elliptical valleys of elevation, there occur several longer and larger valleys, forming deep notches, as it were, in the lofty edge of the chalk. These are of similar structure to the smaller valleys we have been considering, and consist of green-sand, inclosed by chalk at one extremity, and flanked by two escarpments of the same, facing each other with an opposite dip; but they differ in the circumstance of their other and broader extremity being without any such inclosure, and gradually widening till it is lost in the expanse of the adjacent country.

The cases I now allude to are the Vale of Pewsey, to the east of Devizes, that of the Wily, to the east of Warminster, and the valley of the Nadder, extending from Shaftsbury to Barford, near Salisbury; in which last not only the strata of green-sand are brought to the surface, but also the still lower formations of Purbeck and Portland beds, and of Kimmeridge clay.

It might at first sight appear that these valleys are nothing more than simple valleys of denudation; but the fact of the strata composing their escarpments having an opposite and outward dip from the axis of the valley, and this often at a high angle, as near Fonthill and Barford, in the Vale of the Nadder, and at Oare, near the base of Martinsell Hill, in the Vale of Pewsey, obliges us to refer their inclination to some antecedent violence, analogous to that to which I have attributed the position of the strata in the inclosed valleys near Kingsclere, Ham, and Burbage. Nor is it probable that, without some preexisting fracture or opening in the lofty line of the great chalk escarpment, which is here presented to the north-west, the power of water alone would have forced open three such deep valleys as those in question, without causing them to maintain a more equable breadth, instead of narrowing till they end in a point in the body of the chalk. [7]

Rise and denudation of the secondary rocks gradual. -- To return to the valley of the Weald, the strata of the North Downs are inclined to the north, at an angle of from 10° to 15°, and in the narrow ridge of the Hog's back, west of Guildford, in Surrey, about 45°; those in the South Downs dip to the south at a slight angle. It is superfluous to dwell on the analogy which in this respect the two escarpments bear to those which flank the valleys above alluded to; and in regard to the greater distance which separates the hills of Surrey from those of Sussex, the difficulty may be reduced simply to a question of time. If the rise of the land and its degradation by aqueous causes was accomplished by an indefinite number of minor convulsions, during an immense lapse of ages, we behold in the ocean a power fully adequate to perform the work of demolition. If, on the other hand, we embrace the hypothesis of paroxysmal elevation, or, in other words, suppose a submarine tract to have been converted instantaneously into high land, we may seek in vain for any known cause capable of sweeping away even those portions of chalk and other rocks which, all are agreed, must once have formed the prolongation of the existing escarpments. It is common in such cases to call in one arbitrary hypothesis to support another, and as the upheaving force operated with sudden violence, so a vast diluvial wave is introduced to carry away, with almost equal celerity, the mountain mass of strata assumed to have been stripped off.

Materials of the tertiary strata whence derived. -- If, then, we conclude that the wreck of the denuded district was removed gradually, it follows that it was deposited by degrees elsewhere. If any part of the sea immediately adjacent to the district which was then emerging, was of considerable depth, the drift matter would be consigned to that submarine region, since every current charged with sediment must purge itself in the first deep cavity which it traverses, as does a turbid river in a lake. Suppose that while the wave sand currents were excavating the longitudinal valleys, D and C (No. 81, p. 310), the deposits a were thrown down to the bottom of the contiguous deep water E, the sediment being drifted through transverse fissures, as before explained. In this case, the rise of the formations Nos. 1, 2, 3, 4, 5, may have been going on contemporaneously with the excavation of the valleys C and D, and with the accumulation of the strata a. There must be innumerable points on our own coast where the sea is of great depth near to islands and cliffs now exposed to rapid waste, and in all these the denuding and reproductive processes must be going on in the immediate proximity of each other. Such may have been the case during the rise of the Valley of the Weald, and the deposition of the beds of the London and Hampshire basins.

Image
No. 81.

The theory above proposed requires that the deposits a should be composed, for the most part, of a mixture of such mineral ingredients as would result from the degradation of the secondary groups, Nos. 1, 2, 3, 4, 5. Now the tertiary strata answer extremely well to these conditions. They consist, as we have before seen, of alternations of variously-coloured sands and clays, as do the secondary strata from the group No. 5 to No. 2 inclusive, the principal difference being, that the latter are more consolidated.

If it be asked, where do we find the ruins of the white chalk among our Eocene strata? We reply, that the flint pebbles which are associated in such immense abundance with the sands of the plastic clay, are derived evidently from the destruction of chalk; and as to the soft white calcareous matrix, we may suppose it to have been reduced easily to fine sediment, and to have contributed, when in a state of perfect solution, to form the shells of Eocene testacea; or when mixed with the waste of the argillaceous groups, Nos. 2 and 4, which have been peculiarly exposed to denudation, it may have entered into the composition of the London clay, which contains no slight proportion of calcareous matter. In the crag of Norfolk, undoubtedly, we find great heaps of broken pieces of white chalk, with slightly-worn and angular flints; but in this case we may infer that the attrition was not continued for a long time; whereas the large accumulations of perfectly-rolled shingle, which are interstratified with our Eocene formations, proves that they were acted upon for a protracted period by the waves. We have many opportunities of witnessing the entire demolition of the chalk on our southern coast, as at Sea· ford, for example, in Sussex, where large masses are, year after year, detached from the cliffs, and soon disappear, leaving nothing behind but a great bank of flint shingle. [8]

Valleys and furrows in the chalk how caused. -- The furrows which occur on the surface of the chalk, filled with sand and pebbles of the plastic clay, may be easily explained if we suppose the English Eocene strata to have been formed during a period of local convulsion. For if portions of the secondary rocks emerged from the sea in the south-east of our island during that period, it is probable that the chalk underwent many oscillations of level, and that certain tracts became land and then sea, and then land again, so that parts of the surface, first excavated by currents or rivers, were occasionally submerged, and, after being covered by tertiary deposits, upraised again. We must also remember, that almost every part of the chalk must have been exposed for some time to the action of the waves, if we assume the elevation to have been slow and by successive movements. The valleys seen everywhere on the surface, and the layers of partially-rolled and broken flints which very generally overspread it, may be referred to the sea breaking upon the reefs and shoals when the rocks were about to emerge. We apprehend, indeed, that no formidable difficulty will be encountered in explaining the position of the tertiary sand which sometimes fills rents and furrows in the chalk, or the occurrence of banks of shingle at the junction of the tertiary strata and the chalk, if we once admit that the Eocene deposits originated while the chalk and other secondary rocks were rising from the deep and wasting away.

Earthquakes during the Eocene period. -- We have pointed out, in a former chapter, our reasons for concluding that the Paris basin was a theatre of subterranean convulsions during the Eocene period, the older beds of the calcaire grossier having been raised and exposed to the action of the waves before, or at least during, the deposition of the upper or second marine group. [9] These convulsions were doubtless connected with that depression which let in the sea upon the second freshwater formation, and gave rise to the superposition of the upper marine beds. We have also demonstrated, in a preceding chapter, that some of the earlier volcanic eruptions in Auvergne happened before the Eocene species of animals were extinct, and we suggested that the great lakes of Central France may have been drained by alterations of level which accompanied the outbreak of those earlier Eocene volcanos of Auvergne. We ought not, therefore, to be surprised if we discover proofs that the south-east of England participated in the earthquakes which seem to have extended at that time over a considerable part of the neighbouring continent; and we may refer the alternation of marine and fresh-water beds in the Isle of Wight and coast of Hampshire, to changes of level analogous to those which gave rise to the intercalation of the upper marine formation in the Paris basin.

Why the English Eocene strata rise nearly as high as the denuded secondary districts. -- Those geologists who have hitherto regarded the rise and denudation of the lands in the south-east of England as events posterior in date to the deposition of the London clay, will object to the foregoing reasoning, that not only certain outlying patches of tertiary strata, but even the central parts of the London and Hampshire basins, attain very considerable altitudes above the level of the sea. Thus the London clay at Highbeach, in Essex, reaches the height of 750 feet, an elevation exceeding that of large districts of the chalk and other denuded secondary rocks. But these facts do not, we think, militate against the theory above proposed, for we have assumed a long-continued series of elevatory movements in a region where the degradation and reproduction of strata were in progress.

If this be granted, it is evident that the great antagonist powers, the igneous and the aqueous, would, throughout the whole period, be brought into play in their fullest energy, the igneous labouring continually to produce the greatest inequality of surface, by uplifting certain lines of country and depressing others; the aqueous no less incessantly engaged in reducing the whole to a level, by cutting off the summits of the upraised tracts, and throwing the materials thence removed into the adjoining hollows. If the volcanic forces eventually prevail, so as to convert the whole region into land, we must expect that some of the materials drifted into the hollows, and forming the newer strata, will be brought up to view, while the denuded districts are raised at the same time. If these last continue, in general, to occupy a higher position above the level of the sea, it is all that can be expected after the levelling operations before alluded to.

Now the tracts occupied by our Eocene formations are low, not so much with reference to the secondary rocks which remain, as to these masses which must be supposed by our theory to have disappeared, having been carried away by denudation. Let the portions removed from the space intervening between the North and South Downs, and which are expressed by faint lines in our section, wood- cut No. 63, be restored, and we may readily conceive that those masses may have formed shoals and dry land for ages before any part of our tertiary basins emerged.

The estimate of Mr. Martin is not, perhaps, exaggerated, when he computes the probable thickness of strata removed from the highest part of the Forest ridge to be about 1900 feet. So that if we restore to Crowborough Hill, in Sussex, the beds of Weald clay, Lower green-sand, Gault, and chalk, which have been removed by denudation, that hill, instead of rising to the height of 800 feet, would be more than trebled in altitude, [10] and be about 2700 feet high. It would then tower far above the highest outlyers of tertiary strata which are scattered over our chalk, for Inkpen Hill, the greatest elevation of chalk in England, rises only 1011 feet above the level of the sea.

Some geologists who have thought it necessary to suppose all the strata of the London and Hampshire basins to have been once continuous, have estimated the united thickness of the three marine Eocene groups before described, as amounting to 1300 feet, and have been bold enough to imagine a mass of this height to have been once superimposed upon the chalk which formerly covered the axis of the Weald. [11] Hence they were led to infer that Crowborough Hill was once 4000 feet high, and was then cut down from 4000 to 800 feet by diluvial action.

We, on the contrary, deem it wholly unnecessary to suppose any removal of rocks newer than the secondary from the central parts of the valley of the Weald; and we suppose the waste of the older rocks to have been caused gradually during the emergence of the country. The small strips of land which were first protruded in an open sea above the level of the waves, may have been entirely carried away, again and again, in the intervals between successive movements, until at last a great number of reefs and islands rising at once, afforded protection to each other against the attacks of the waves, and the lands began to increase. We do not conceive, therefore, that a mountain ridge first rose to the height of more than 2000 feet, and was then lowered to less than half that elevation; but that a stratified mass, more than 2000 feet thick, was, by the continual stripping off of the uppermost beds as they rose, diminished to a thickness of about 800 feet.

It is not our intention, at present, to point out the application of the above theory to the region immediately westward of the great line of chalk escarpment which runs through the central parts of England from Dorsetshire to Cambridgeshire. The denudation in that country has doubtless been on a great scale, and was, perhaps, effected during the Eocene period; for we know of no reason why one line of movements should not have been in progress in a direction north-east and southwest, while others were heaving up the strata in lines running east and west. We may remark, at the same time, that if the chalk in the interior of England, in those tracts from which it has been extensively swept away, began to rise during the tertiary epoch, and before the emergence of the chalk which once extended over the central axis of the Weald, some tertiary deposits may, in that case, have been thrown down upon that central ridge. We have at present, however, no data to lend countenance to such conjectures.

Vertical strata of the Isle of Wight. -- A line of vertical and inclined strata running east and west, or parallel to the central axis of the Weald, extends through the isles of Wight and Purbeck, and through Dorsetshire, and has been observed by Dr. Fitton to reappear in France, north of Dieppe. The same strata which are elevated in the Weald Valley are upheaved also on this line in the centre of the Isle of Wight, where all the tertiary strata appear to have partaken in the same movement. [12]

From the horizontality of the fresh-water series in Alum Bay, as contrasted with the vertical position of the marine tertiary beds, Mr. Webster was at first led very naturally to conclude, that the latter had undergone great derangement before the deposition of the former. It appears, however, from the subsequent observations of Professor Sedgwick, [13] that these appearances are deceptive, and that at the eastern extremity of the Isle of Wight, part of the fresh-water series is vertical, like the marine. Hence it is now ascertained that as the chalk is horizontal at the southern extremity of the Isle of Wight, while it is vertical in the centre of that island, so the Eocene strata are horizontal in the north of the island, and vertical in the centre. We have only to imagine that the great flexure of the secondary and tertiary beds, so ingeniously suggested by Mr. Webster in his theoretical section, [14] extended to the fresh-water formations, in order to comprehend how a very simple series of movements may have brought the whole of the Isle of Wight groups into their present position.

We are unable to assign a precise date to the convulsions which produced this great curve in the stratified rocks of the Isle of Wight; but we may observe that, although subsequent to the deposition of the fresh-water beds, it does not follow that it was not produced in the Eocene period. It may have been contemporaneous with those movements which raised the central parts of the London and Hampshire basins, which, as we before explained, were subsequent to the principal elevation and denudation of the central axis of the Weald.

Land has certainly been elevated on our eastern coast since the commencement of the older Pliocene period, as is attested by the moderate height attained by the crag strata. [15] But these changes of level may have been partial, and if the crag does not extend farther over the Eocene formations, and into the Weald Valley, it is probably because those regions were dry land when the strata of crag were forming in the sea.

The first land that rose in the south-eastern extremity of England may have been placed, as we before hinted, where we now find the central axis of elevation in the Weald. Perhaps the chalk islands there formed may have supported that tropical vegetation whereof we find memorials in the fossil plants of Sheppey; and the shores of those islands may have been frequented, during the ovipositing season, by the turtles and crocodiles, of which the teeth and skeletons are imbedded in the London clay. [16]

Eocene alluviums. -- The river which produced that body of water in which the fresh-water strata of Hampshire originated, must have drained some contiguous lands which may have emerged during the Eocene period. On these lands we may suppose the Paleothere, Anoplothere, and Moschus of Binstead to have lived. The discovery of the two former genera, associated as they are with well-known Eocene species of testacea, is most interesting. It shows that in England, or rather on the space now occupied by part of our island, as well as in the Paris basin, Auvergne, Cantal, and Velay, there were mammalia of a peculiar type during the Eocene period. Yet we have never found a single fragment of the bones of any of these quadrupeds in our alluviums or cave breccias. In these formations we find the bones of the mastodon and mammoth, of the rhinoceros, hippopotamus, lion, hyaena, bear, and other quadrupeds, all of extinct species. They are accompanied by recent fresh-water shells, or by the marine fossils of the crag, and evidently belong to an epoch posterior to the Eocene. Where, then, are the terrestrial alluviums of that surface which was inhabited by the Paleothere and its congeners? Have the remains which were buried at so remote a period decomposed, so that they no longer afford any zoological characters which might enable us to distinguish the Eocene from more modern alluviums?

It seems clear that a peculiar and rare combination of favourable circumstances is required to preserve mammiferous or other remains in terrestrial alluviums in sufficient quantity to afford the geologist the means of assigning the date of such deposits. For this reason we are scarcely able, at present, to form any conjecture as to the relative ages of the numerous alluviums which cover the surface of Scotland, a country which probably became land long before the commencement of the tertiary epochs.

Elevation of land gradual. -- As we have assumed, throughout this and the preceding chapter, that the elevatory force was developed in a succession of minor convulsions in the southeast of England, we may seem called upon to answer an objection which has been drawn from the verticality of the strata in the Isles of Wight and Purbeck. Mr. Conybeare has remarked, that the vertical strata are traced through a district nearly 60 miles in length, so that 'if their present position were the effect of a single convulsion, no disturbance in the least comparable with it has occurred in modern times.' [17] As we can by no means dissent from this proposition, we only ask where is the evidence that a single effort of the subterranean force, rather than reiterated movements, produced that sharp flexure of which we suppose the vertical strata of the Isle of Wight to form a part, the remainder of the arc having been carried away by denudation.

It appears extremely probable that the Cutch earthquake of 1819, so often alluded to by us, [18] may have produced an incipient curve, running in a linear direction through a tract at least 60 miles in length. The strata were upraised in the Ullah Bund, and depressed below the level of the sea in the adjoining tract, where the fort of Sindree was submerged. It would be impossible, if the next earthquake should raise the Bund still higher, and sink to a lower depth the adjoining tract, to discriminate, by any geological investigations, the different effects of the two earthquakes, unless a minute survey of the effects of the first shock had been made and put on record. In this manner we may suppose the strata to be bent, again and again, in the course of future ages, until parts of them become perpendicular.

To some it may appear, that there is a unity of effect in the line of deranged strata in the isles of Wight and Purbeck, as also in the central axis of the Weald, which is inconsistent with the supposition of a great number of separate movements recurring after long intervals of time. But we know that earthquakes are repeated throughout a long series of ages, in the same spots, like volcanic eruptions. The oldest lavas of Etna were poured out many thousand, perhaps myriads, of years, before the newest, and yet they have produced a symmetrical mountain; and if rivers of melted matter thus continue to flow in the same direction, and towards the same points, for an indefinite lapse of ages, what difficulty is there in conceiving that the subterranean volcanic force, occasioning the rise or fall of certain parts of the earth's crust, may, by reiterated movements, produce the most perfect unity of result?

Excavation of Valleys. -- In our attempt to explain the origin of the existing valleys in the south-east of England, it will be seen that we refer their excavation chiefly to the ocean. We are aware that we cannot generalize these views and apply them to the valleys of all parts of the world. In Central France, for example, rivers and land-floods, co-operating with earthquakes, have deeply intersected the lacustrine and volcanic deposits, and have hollowed out valleys as deep, perhaps, as any in our Weald district. In what manner these effects may be brought about in the course of time, by the action of running water, even without the intervention of the sea, may be understood by what we have said of the removal of rock by aqueous agency during the Calabrian earthquakes. [19]

Those geologists who contend that the valleys in England are not due to what they term 'modern causes,' are in the habit of appealing to the fact, that the rivers in the interior of England are working no sensible alterations, and could never, in their present state, not even in millions of years, excavate the valleys through which they now flow. We suspect that a false theory is involved even in the term 'modern causes,' as if it could be assumed that there were ancient causes differing from those which are now in operation. But if we substitute the phrase, existing causes, we shall find that the argument now controverted amounts to little more than this, ' that in a country free from subterranean movements, the action of running water is so trifling that it could never hollow out, in any lapse of ages, a deep system of valleys, and, therefore, no known combination of existing causes could ever have given rise to our present valleys!?

The advocates of these doctrines, in their anxiety to point out the supposed absurdity of attributing to ordinary causes those inequalities of hill and dale, which now diversify the earth's surface, have too often kept entirely out of view the many recorded examples of elevations and subsidences of land during earthquakes, the frequent fissuring of mountains, and opening of chasms, the damming up of rivers by landslips, the deflection of streams from their original courses, and more important, perhaps, than all these, the denuding power of the ocean, during the rise of our continents from the deep. Few of the ordinary causes of change, whether igneous or aqueous, can be observed to act with their full intensity in anyone place at the same time; hence it is easy to persuade those who have not reflected long and profoundly on the working of the numerous igneous and aqueous agents, that they are entirely inadequate to bring about any important fluctuations in the configuration of the earth's surface.

Recapitulation. -- We shall now briefly recapitulate the conclusions to which we have arrived respecting the geology of the south-east of England, in reference to the nature and origin of the Eocene formations considered in this and the two preceding chapters.

1. In the first place, it appears that the tertiary strata rest exclusively upon the chalk, and consist, with some trifling exceptions, of alternations of clay and sand.

2. The organic remains agree with those of the Paris basin, but the mineral character of the deposit is extremely different, those rocks in particular, which are common to the Paris basin and Central France, being wanting, or extremely rare, in the English tertiary formations.

3. The Eocene deposits of England are generally conformable to the chalk, being horizontal where the beds of chalk are horizontal, and vertical where they are vertical; so that both series of rocks appear to have participated in nearly the same movements.

4. It is not possible to define the limits of the ancient borders of the tertiary sea in the south-east of England, in the same manner as can be frequently done in those countries where the secondary rocks are unconformable to the tertiary.

5. Although the tertiary deposits are chiefly confined to the tracts called the basins of London and Hampshire, insulated patches of them are, nevertheless, found on some of the highest summits of the chalk intervening between these basins.

6. These outliers, however, do not necessarily prove that the great mass of tertiary strata was once continuous between the basins of London and Hampshire, and over other parts of the south-east of England now occupied by secondary rocks.

7. On the contrary, it is probable that these secondary districts were gradually elevated and denuded when the basins of London and Hampshire were still submarine, and while they were gradually becoming filled up with tertiary sand and clay.

8. If, in illustration of this theory, we examine one of the districts thus supposed to have been denuded, we find in the Valley of the Weald decided proofs, that since the emergence of the secondary rocks, an immense mass of chalk and subjacent formations has been removed by the force of water.

9. We infer from the existence of large valleys along the outcrop of the softer beds, and of parallel chains of hills where harder rocks come up to the surface, that water was the removing cause; and from the shape of the escarpments presented by the harder rocks, and the distribution of alluvium over different parts of the surface of the Weald district, we conclude that the denudation was successive and gradual during the rise of the strata.

10. We may suppose that the materials carried away from the denuded district were conveyed into the depths of the contiguous sea, through channels produced by cross fractures which have since become river-channels, and which now intersect the chalk in a direction at right angles to the general axis of elevation of the country.

11. The analogous structure of the Valley of Kingsclere, and other valleys which run east and west, like the Valley of the Weald, but are much narrower, accord also with the hypothesis, that they were all produced by the denuding power of water co-operating with elevatory movements.

12. The mineral composition of the materials thus supposed to have been removed in immense abundance from the Valley of the Weald, are precisely such as would, by degradation, form the English Eocene strata.

13. It is probable that there were many oscillations of level during the Eocene period, so that some tracts were alternately land and then sea, and then land again. These fluctuations may account for the furrowed surface of the chalk on which the tertiary strata sometimes repose, for the valleys on its surface, for the banks of shingle associated with the Plastic clay, for the partial deposits of sand and clay on elevated tracts of chalk, and for the alternations of marine and fresh-water strata in the Hampshire basin.

14. The volcanic eruptions of the Eocene period in Auvergne, the changes of level which took place at the same time in the Paris basin, and those above alluded to in the south-east of England, may all have belonged to one theatre of subterranean convulsion.

15. The basins of London and Hampshire may have been partly formed by subsidences in the bed of the sea, contemporaneously with the elevation and emergence of the Weald district.

16. The movements which threw the chalk and the tertiary strata of the isles of Wight and Purbeck into a vertical position, were subsequent to the formation of the Eocene freshwater strata of the Isle of Wight, but may possibly have occurred during the Eocene period.

17. The masses of secondary rock which have been removed by denudation from the central axis of the Weald would, if restored, rise to more than double the height now attained by any patches of tertiary strata in England.

18. If, therefore, the Eocene strata do not appear to occupy a much lower level than the secondary rocks, from the destruction of which they have been formed, it is because the highest summits of the latter have been cut off during the rise of the land, and thrown into those troughs where we now find the tertiary deposits.

19. The upheaving of the strata of the London and Hampshire basins may have been in great part effected towards the close of the Eocene period; but it must also have been in some part due to the movements which raised the crag.

_______________

Notes:

1. Outlines, p. 144.

2. See above, p. 295.

3. Geol. Trans., 2nd Series, vol. ii. p. 119.

4. Copied by permission from Dr. Buckland's plate XVII., Geol. Trans., 2nd Series, vol. ii.

5. See Mr. Murchison's map, plate XIV., ibid.

6. Dr. Buckland, Geol. Trans., 2nd Series, vol. ii. p. 122.

7. Dr. Buckland, Geol. Trans., 2nd Series, vol. ii. p. 123.

8. Vol. i. p. 279, and Second Edition, p. 319.

9. See above, p. 248.

10. Phil. Mag. and Annals, No. 26, New Series, p. 117.

11. Martin, ibid.

12. See Mr. Webster's section, Geol. Trans., vol. ii. First Series, plate XI.

13. Anniv. Address to the Geol. Soc., Feb. 1831, p. 9. Professor Sedgwick informs me that his observations, made six years ago, have recently been confirmed by Professor Henslow.

14. Englefield's Isle of Wight, plate XLVII. fig. I.

15. We alluded, at p. 182, to the supposed discovery of recent marine shells at the height of 140 feet above the sea in Sheppey; but we have since learnt from Professor Sedgwick, that the information communicated to the Geological Society on this subject was erroneous.

16. We have introduced these islands into the map of Europe, in the 2nd volume, which may be supposed to relate to the commencement of the Eocene period.

17. Phil. Mag. and Annals, No. 49, new Series, p. 21.

18. Vol. i., Second Edition, p. 465, and vol. ii., First Edition, p. 265.

19. Vol. i. chap. xxiv.
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