CHAPTER XIV.

OLDER PLIOCENE AND MIOCENE FORMATIONS.

Strata of Suffolk termed Red and Coralline crag — Fossils, and proportion of recent species — Depth of sea and climate — Reference of Suffolk crag to the older Pliocene period — Migration of many species of shells southwards during the glacial period — Fossil whales — Subapennine beds — Asti, Sienna, Rome — Miocene formations — Faluns of Touraine — Depth of sea and littoral character of fauna — Tropical climate implied by the testacea — Proportion of recent species of shells — Faluns more ancient than the Suffolk crag — Miocene strata of Bordeaux and Piedmont — Molasse of Switzerland — Tertiary strata of Lisbon — Older Pliocene and Miocene formations in the United States — Sewâlik Hills in India.

The older Pliocene strata, which next claim our attention, are chiefly confined, in Great Britain, to the eastern part of the county of Suffolk, where, like the Norwich beds already described, they are called "Crag," a provincial name given particularly to those masses of shelly sand which have been used from very ancient times in agriculture, to fertilize soils deficient in calcareous matter. The relative position of the "red crag" in Essex to the London clay, may be understood by reference to the accompanying diagram ([fig. 142.]).

Fig. 142.

These deposits, judging by the shells which they contain, appear, according to Professor Edward Forbes, to have been formed in a sea of moderate depth, generally from 15 to 25 fathoms deep, although in some few spots perhaps deeper. But they may, nevertheless, have been accumulated at the distance of 40 or 50 miles from land.

The Suffolk crag is divisible into two masses, the upper of which has been termed the Red, and the lower the Coralline Crag.[162-A] The upper deposit consists chiefly of quartzose sand, with an occasional intermixture of shells, for the most part rolled, and sometimes comminuted. The lower or Coralline crag is of very limited extent, ranging over an area about 20 miles in length, and 3 or 4 in breadth, between the rivers Alde and Stour. It is generally calcareous and marly—a mass of shells and small corals, passing occasionally into a soft building stone. At Sudbourn, near Orford, where it assumes this character, are large quarries, in which the bottom of it has not been reached at the depth of 50 feet. At some places in the neighbourhood, the softer mass is divided by thin flags of hard limestone, and corals placed in the upright position in which they grew.

The Red crag is distinguished by the deep ferruginous or ochreous colour of its sands and fossils, the Coralline by its white colour. Both formations are of moderate thickness; the red crag rarely exceeding 40, and the coralline seldom amounting to 20, feet. But their importance is not to be estimated by the density of the mass of strata or its geographical extent, but by the extraordinary richness of its organic remains, belonging to a very peculiar type, which seems to characterize the state of the living creation in the north of Europe during the older Pliocene era.

For a large collection of the fish, echinoderms, shells, and corals of the deposits in Suffolk, we are indebted to the labours of Mr. Searles Wood. Of testacea alone he has obtained from 230 species from the Red, and 345 from the Coralline crag, about 150 being common to each. The proportion of recent species in the new group is considered by Mr. Wood to be about 70[162-B] per cent., and that in the older or coralline about 60. When I examined these shells of Suffolk in 1835, with the assistance of Dr. Beck, Mr. George Sowerby, Mr. Searles Wood, and other eminent conchologists, I came to the opinion that the extinct species predominated very decidedly in number over the living. Recent investigations, however, have thrown much new light on the conchology of the Arctic, Scandinavian, British, and Mediterranean Seas. Many of the species formerly known only as fossils of the Crag, and supposed to have died out, have been dredged up in a living state from depths not previously explored. Other recent species, before regarded as distinct from the nearest allied Crag fossils, have been observed, when numerous individuals were procured, to be liable to much greater variation, both in size and form, than had been suspected, and thus have been identified. Consequently, the Crag fauna has been found to approach much more nearly to the recent fauna of the Northern, British, and Mediterranean Seas than had been imagined. The analogy of the whole group of testacea to the European type is very marked, whether we refer to the large development of certain genera in number of species or to their size, or to the suppression or feeble representation of others. The indication also afforded by the entire fauna of a climate not much warmer than that now prevailing in corresponding latitudes, prepares us to believe that they are not of higher antiquity than the Older Pliocene era.[163-A]

Fig. 143.

Section near Ipswich, in Suffolk.

The position of the red crag in Essex to the subjacent London clay and chalk has been already pointed out ([fig. 142.]). Whenever the two divisions are met with in the same district, the red crag lies uppermost; and, in some cases, as in the section represented in [fig. 143.], it is observed that the older or coralline mass b had suffered denudation before the newer formation a was thrown down upon it. At D there is not only a distinct cliff, 8 or 10 feet high, of coralline crag, running in a direction N.E. and S.W., against which the red crag abuts with its horizontal layers; but this cliff occasionally overhangs. The rock composing it is drilled everywhere by Pholades, the holes which they perforated having been afterwards filled with sand and covered over when the newer beds were thrown down. As the older formation is shown by its fossils to have accumulated in a deeper sea (15, and sometimes 25, fathoms deep or more), there must no doubt have been an upheaval of the sea-bottom before the cliff here alluded to was shaped out. We may also conclude that so great an amount of denudation could scarcely take place, in such incoherent materials, without many of the fossils of the inferior beds becoming mixed up with the overlying crag, so that considerable difficulty must be occasionally experienced by the palæontologist in deciding which species belong severally to each group. The red crag being formed in a shallower sea, often resembles in structure a shifting sand bank, its layers being inclined diagonally, and the planes of stratification being sometimes directed in the same quarry to the four cardinal points of the compass, as at Butley. That in this and many other localities, such a structure is not deceptive or due to any subsequent concretionary re-arrangement of particles, or to mere lines of colour, is proved by each bed being made up of flat pieces of shell which lie parallel to the planes of the smaller strata.

Some fossils, which are very abundant in the red crag, have never been found in the white or coralline division; as, for example, the Fusus contrarius ([fig. 144.]), and several species of Buccinum (or Nassa) and Murex (see [figs. 145], [146.]), which two genera seem wanting in the lower crag.

Fossils characteristic of the Red Crag.

Fig. 144. Fusus contrarius.

Fig. 145. Murex alveolatus.

Fig. 146. Nassa granulata.

Fig. 147. Cypræa coccinelloides.

Fig. 144. half nat. size; the others nat. size.

Among the bones and teeth of fishes are those of large sharks (Carcharias), and a gigantic skate of the extinct genus Myliobates, and many other forms, some common to our seas, and many foreign to them.

The distinctness of the fossils of the coralline crag arises in part from higher antiquity, and, in some degree, from a difference in the geographical conditions of the submarine bottom. The prolific growth of corals, echini, and a prodigious variety of testacea, implies a region of deeper and more tranquil water; whereas, the red crag may have formed afterwards on the same spot, when the water was shallower. In the mean time the climate may have become somewhat cooler, and some of the zoophytes which flourished in the first period may have disappeared, so that the fauna of the red crag acquired a character somewhat more nearly resembling that of our northern seas, as is implied by the large development of certain sections of the genera Fusus, Buccinum, Purpura, and Trochus, proper to higher latitudes, and which are wanting or feebly represented in the inferior crag.

Some of the corals of the lower crag of Suffolk belong to genera unknown in the living creation, and of a very peculiar structure; as, for example, that represented in the annexed [fig. (148.)], which is one of several species having a globular form. The great number and variety of these zoophytes probably indicate an equable climate, free from intense cold in winter. On the other hand, that the heat was never excessive is confirmed by the prevalence of northern forms among the testacea, such as the Glycimeris, Cyprina, and Astarte. Of the genus last mentioned (see [fig. 149.]) there are about fourteen species, many of them being rich in individuals; and there is an absence of genera peculiar to hot climates, such as Conus, Oliva, Mitra, Fasciolaria, Crassatella, and others. The cowries (Cypræa, [fig. 147.]), also, are small, and belong to a section (Trivia) now inhabiting the colder regions. A large volute, called Voluta Lamberti ([fig. 150.]), may seem an exception; but it differs in form from the volutes of the torrid zone, and may, like the living Voluta Magellanica, have been fitted for an extra-tropical climate.

Fig. 148.

Fascicularia aurantium, Milne Edwards. Family, Tubuliporidæ, of same author.

Coral of extinct genus, from the inferior or coralline crag, Suffolk.

Fig. 149.

Astarte (Crassina, Lam.); species common to upper and lower crag.

Astarte Omalii, Lajonkaire; Syn. A. bipartita, Sow. Min. Con. T. 521. f. 3.; a very variable species most characteristic of the coralline crag, Suffolk.

Fig. 150.

Voluta Lamberti, young individ.

The occurrence of a species of Lingula at Sutton is worthy of remark, as these Brachiopoda seem now confined to more equatorial latitudes, and the same may be said still more decidedly of a species of Pyrula, allied to P. reticulata. Whether, therefore, we may incline to the belief that the mean annual temperature was higher or lower than now, we may at least infer that the climate and geographical conditions were by no means the same at the period of the Suffolk crag as those now prevailing in the same region.

Of the echinoderms of the coralline crag about eleven species are known, but some of these are in too fragmentary a condition to admit of exact comparison. Of six which are the most perfect, Prof. E. Forbes has been able to identify three with recent species: one of which, of the genus Echinus, is British; a second, Echinocyamus, British and Mediterranean; and a third, Echinus monilis, a Mediterranean species, also found fossil in the faluns of Touraine.

One of the most interesting conclusions deduced from a careful comparison of the shells of these British Older Pliocene strata and those now inhabiting our seas, has been pointed out by Prof. E. Forbes. It appears that, during the glacial period, a period intermediate, as we have seen, between that of the crag and our own times, many shells, previously established in the temperate zone, retreated southwards to avoid an uncongenial climate. The Professor has given a list of fifty shells which inhabited the British seas while the coralline and red crag were forming, and which are wanting in the Pleistocene or glacial deposits. They must, therefore, after their migration to the south, have made their way northwards again. In corroboration of these views, it is stated that all these fifty species occur fossil in the Newer Pliocene strata of Sicily, Southern Italy, and the Grecian Archipelago, where they may have enjoyed, during the era of floating icebergs, a climate resembling that now prevailing in higher European latitudes.[166-A]

In the red crag at Felixstow, in Suffolk, Professor Henslow has found the ear-bones of no less than four species of cetacea, which, according to Mr. Owen, are the remains of true whales of the family Balænidæ. Mr. Wood is of opinion that these cetacea may be of the age of the red crag, or if not that they may be derived from the destruction of beds of coralline crag. I agree with him that the supposition of their having been washed out of the London clay, in which no Balænidæ have yet been met with, is improbable.

Strata containing fossil shells, like those of the Suffolk crag, above described, have been found at Antwerp, and on the banks of the Scheldt below that city. In 1840 I observed a small patch of them near Valognes, in Normandy; and there is also a deposit containing similar fossils at St. George Bohon, and several places a few leagues to the S. of Carentan, in Normandy; but they have never been traced farther southwards.

Subapennine strata.—The Apennines, it is well known, are composed chiefly of secondary rocks, forming a chain which branches off from the Ligurian Alps and passes down the middle of the Italian peninsula. At the foot of these mountains, on the side both of the Adriatic and the Mediterranean, are found a series of tertiary strata, which form, for the most part, a line of low hills occupying the space between the older chain and the sea. Brocchi, as we have seen ([p. 105.]), was the first Italian geologist who described this newer group in detail, giving it the name of the Subapennines; and he classed all the tertiary strata of Italy, from Piedmont to Calabria, as parts of the same system. Certain mineral characters, he observed, were common to the whole; for the strata consist generally of light brown or blue marl, covered by yellow calcareous sand and gravel. There are also, he added, some species of fossil shells which are found in these deposits throughout the whole of Italy.

We have now, however, satisfactory evidence that the Subapennine beds of Brocchi belong, at least, to three periods. To the Miocene we can refer a portion of the strata of Piedmont, those of the hill of the Superga, for example; to the Older Pliocene, part of the strata of northern Italy, of Tuscany, and of Rome; while the tufaceous formations of Naples, of Ischia, and the calcareous strata of Otranto, are referable to the Newer Pliocene, and in great part to the Post-Pliocene period.

That there is a considerable correspondence in the mineral composition of these different Italian groups is undeniable; but not that exact resemblance which should lead us to assume a precise identity of age, unless the fossil remains agreed very closely. It is now indispensable that a new scrutiny should be made in each particular district, of the fossils derived from the upper and lower beds—especially such localities as Asti and Parma, where the formation attains a great thickness; and at Sienna, where the shells of the incumbent yellow sand are generally believed to approach much more nearly, as a whole, to the recent fauna of the Mediterranean than those in the subjacent blue marl.

The greyish brown or blue marl of the Subapennine formation is very aluminous, and usually contains much calcareous matter and scales of mica. Near Parma it attains a thickness of 2000 feet, and is charged throughout with marine shells, some of which lived in deep, others in shallow water, while a few belong to freshwater genera, and must have been washed in by rivers. Among these last I have seen the common Limnea palustris in the blue marl, filled with small marine shells. The wood and leaves, which occasionally form beds of lignite in the same deposit, may have been carried into the sea by similar causes. The shells, in general, are soft when first taken from the marl, but they become hard when dried. The superficial enamel is often well preserved, and many shells retain their pearly lustre, part of their external colour, and even the ligament which unites the valves. No shells are more usually perfect than the microscopic foraminifera, which abound near Sienna, where more than a thousand full-grown individuals may be sometimes poured out of the interior of a single univalve of moderate dimensions.

The other member of the Subapennine group, the yellow sand and conglomerate, constitutes, in most places, a border formation near the junction of the tertiary and secondary rocks. In some cases, as near the town of Sienna, we see sand and calcareous gravel resting immediately on the Apennine limestone, without the intervention of any blue marl. Alternations are there seen of beds containing fluviatile shells, with others filled exclusively with marine species; and I observed oysters attached to many limestone pebbles. This appears to have been a point where a river, flowing from the Apennines, entered the sea when the tertiary strata were formed.

The sand passes in some districts into a calcareous sandstone, as at San Vignone. Its general superposition to the marl, even in parts of Italy and Sicily where the date of its origin is very distinct, may be explained if we consider that it may represent the deltas of rivers and torrents, which gained upon the bed of the sea where blue marl had previously been deposited. The latter, being composed of the finer and more transportable mud, would be conveyed to a distance, and first occupy the bottom, over which sand and pebbles would afterwards be spread, in proportion as rivers pushed their deltas farther outwards. In some large tracts of yellow sand it is impossible to detect a single fossil, while in other places they occur in profusion. Occasionally the shells are silicified, as at San Vitale, near Parma, from whence I saw two individuals of recent species, one freshwater and the other marine (Limnea palustris, and Cytherea concentrica, Lam.), both perfectly converted into flint.

Rome.—The seven hills of Rome are composed partly of marine tertiary strata, those of Monte Mario, for example, of the Older Pliocene period, and partly of superimposed volcanic tuff, on the top of which are usually cappings of a fluviatile and lacustrine deposit. Thus, on Mount Aventine, the Vatican, and the Capitol, we find beds of calcareous tufa with incrusted reeds, and recent terrestrial shells, at the height of about 200 feet above the alluvial plain of the Tiber. The tusk of the mammoth has been procured from this formation, but the shells appear to be all of living species, and must have been embedded when the summit of the Capitol was a marsh, and constituted one of the lowest hollows of the country as it then existed. It is not without interest that we thus discover the extremely recent date of a geological event which preceded an historical era so remote as the building of Rome.

MIOCENE FORMATIONS.

Faluns of Touraine.—The Miocene strata, corresponding with those named by many geologists "Middle Tertiary," will next claim our attention. Near the towns of Dinan and Rennes, in Brittany, and again in the provinces bordering the Loire, a tertiary formation, containing another assemblage of fossils, is met with, to which the name of Faluns has been long given by the French agriculturists, who spread the shelly sand and marl over the land, in the same manner as the crag was formerly much used in Suffolk. Isolated masses of these faluns occur from near the mouth of the Loire, near Nantes, as far as a district south of Tours. They are also found at Pontlevoy, on the Cher, about 70 miles above the junction of that river with the Loire, and 30 miles S.E. of Tours. I have visited all the localities above mentioned, and found the beds to consist principally of sand and marl, in which are shells and corals, some entire, some rolled, and others in minute fragments. In certain districts, as at Doué, in the department of Maine and Loire, 10 miles S.W. of Saumur, they form a soft building-stone, chiefly composed of an aggregate of broken shells, corals, and echinoderms, united by a calcareous cement; the whole mass being very like the coralline crag near Aldborough and Sudbourn in Suffolk. The scattered patches of faluns are of slight thickness, rarely exceeding 50 feet; and between the district called Sologne and the sea they repose on a great variety of older rocks; being seen to rest successively upon gneiss, clay-slate, and various secondary formations, including the chalk; and, lastly, upon the upper freshwater limestone of the Parisian tertiary series, which, as before mentioned ([p. 106.]), stretches continuously from the basin of the Seine to that of the Loire.

At some points, as at Louans, south of Tours, the shells are stained of a ferruginous colour, not unlike that of the red crag of Suffolk. The species are, for the most part, marine, but a few of them belong to land and fluviatile genera. Among the former, Helix turonensis ([fig. 45.] [p. 30.]) is the most abundant. Remains of terrestrial quadrupeds are here and there intermixed, belonging to the genera Deinotherium, Mastodon, Rhinoceros, Hippopotamus, Chæropotamus, Dichobune, Deer, and others, and these are accompanied by cetacea, such as the Lamantine, Morse, Sea-calf, and Dolphin, all of extinct species.

Professor E. Forbes, after studying the fossil testacea which I obtained from these beds; informs me that he has no doubt they were formed partly on the shore itself at the level of low water, and partly at very moderate depths, not exceeding 10 fathoms below that level. The molluscous fauna of the "faluns" is on the whole much more littoral than that of the red and coralline crag of Suffolk, and implies a shallower sea. It is, moreover, contrasted with the Suffolk crag by the indications it affords of an extra-European climate. Thus it contains seven species of Cypræa, some larger than any existing cowry of the Mediterranean, several species of Oliva, Ancillaria, Mitra, Terebra, Pyrula, Fasciolaria, and Conus. Of the cones there are no less than eight species, some very large, whereas the only European cone is of diminutive size. The genus Nerita, and many others, are also represented by individuals of a type now characteristic of equatorial seas, and wholly unlike any Mediterranean forms. These proofs of a more elevated temperature seem to imply the higher antiquity of the faluns as compared with the Suffolk crag, and are in perfect accordance with the fact of the smaller proportion of testacea of recent species found in the faluns.

Out of 290 species of shells, collected by myself, in 1840, at Pontlevoy, Louans, Bossée, and other villages 20 miles south of Tours; and at Savigné, about 15 miles north-west of that place; 72 only could be identified with recent species, which is in the proportion of 25 per cent. A large number of the 290 species are common to all the localities, those peculiar to each not being more numerous than we might expect to find in different bays of the same sea.

The total number of mollusca from the faluns, in my possession, is 302, of which 45 only were found by Mr. Wood to be common to the Suffolk crag. The number of corals obtained by me at Doué, and other localities before adverted to, amounts to 43, as determined by Mr. Lonsdale, of which 7 agree specifically with those of the Suffolk crag. Only one has, as yet, been identified with a living species. But it is difficult, if not impossible, to institute at present a satisfactory comparison between fossil and recent Polyparia, from the deficiency of our knowledge of the living species. Some of the genera occurring fossil in Touraine, as the Astrea, Lunulites, and Dendrophyllia, have not been found in European seas north of the Mediterranean; nevertheless the Polyparia of the faluns do not seem to indicate on the whole so warm a climate as would be inferred from the shells.

It was stated that, on comparing about 300 species of Touraine shells with about 450 from the Suffolk crag, 45 only were found to be common to both, which is in the proportion of only 15 per cent. The same small amount of agreement is found in the corals also. I formerly endeavoured to reconcile this marked difference in species with the supposed co-existence of the two faunas, by imagining them to have severally belonged to distinct zoological provinces or two seas, the one opening to the north, and the other to the south, with a barrier of land between them, like the Isthmus of Suez, separating the Red Sea and the Mediterranean. But I now abandon that idea for several reasons; among others, because I succeeded in 1841 in tracing the Crag fauna southwards in Normandy to within 70 miles of the Falunian type, near Dinan, yet found that both assemblages of fossils retained their distinctive characters, showing no signs of any blending of species or transition of climate.

On a comparison of 280 Mediterranean shells with 600 British species, made for me by an experienced conchologist in 1841, 160 were found to be common to both collections, which is in the proportion of 57 per cent., a fourfold greater specific resemblance than between the seas of the crag and the faluns, notwithstanding the greater geographical distance between England and the Mediterranean than between Suffolk and the Loire. The principal grounds, however, for referring the English crag to the older Pliocene and the French faluns to the Miocene epochs, consist in the predominance of fossil shells in the British strata identifiable with species, not only still living, but which are now inhabitants of neighbouring seas, while the accompanying extinct species are of genera such as characterize Europe. In the faluns, on the contrary, the recent species are in a decided minority, and many of them, like the associated extinct testacea, are much less European in character, and point to the prevalence of a warmer climate,—in other words, to a state of things receding farther from the present condition of Europe, geographically and climatologically, and doubtless, therefore, receding farther in time.

Bordeaux.—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 700 species of shells have been obtained. A large proportion of these shells belong to the same zoological type as those of Touraine; but many are peculiar, and the whole may possibly constitute a somewhat older division of the Miocene period than the faluns of the Loire. We must wait, however, for farther investigations, in order to decide this question with accuracy.

Piedmont.—Many of the shells peculiar to the hill of the Superga, near Turin, agree with those found at Bordeaux and Dax; but the proportion of recent species is much less. The strata of the Superga consist of a bright green sand and marl, and a conglomerate with pebbles, chiefly of green serpentine, and are inclined at an angle of more than 70°. This formation, which attains a great thickness in the valley of the Bormida, is probably one of the oldest Miocene groups hitherto discovered.

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, and probably divisible into several formations. How large a portion of these belong to the Miocene period cannot yet be determined, as fossil shells are often entirely wanting. In some places a decided agreement of the fossil fishes of the molasse and faluns has been observed. Among those common to both, M. Agassiz pointed out to me Lamna contortidens, Myliobates Studeri, Spherodus cinctus, Notidanus primigenius, and others.

Lisbon.—Marine tertiary strata near Lisbon contain shells which agree very closely with those of Bordeaux, and are therefore referred to the Miocene era. Thus, out of 112 species collected by Mr. Smith of Jordanhill, between 60 and 70 were found to be common to the strata of Bordeaux and Dax, the recent species being in the proportion of 21 per cent.

Older Pliocene and Miocene formations in the United States.—Between the Alleghany mountains, formed of older rocks, and the Atlantic, there intervenes, in the United States, a low region occupied principally by beds of marl, clay, and sand, consisting of the cretaceous and tertiary formations, and chiefly of the latter. The general elevation of this plain bordering the Atlantic does not exceed 100 feet, although it is sometimes several hundred feet high. Its width in the middle and southern states is very commonly from 100 to 150 miles. It consists, in the South, as in Georgia, Alabama, and South Carolina, almost exclusively of Eocene deposits; but in North Carolina, Maryland, Virginia, and Delaware, more modern strata predominate, which I have assimilated in age to the English crag and Faluns of Touraine.[172-A] If, chronologically speaking, they can be truly said to be the representatives of these two European formations, they may range in age from the Older Pliocene to the Miocene epoch, according to the classification of European strata adopted in this chapter.

The proportion of fossil shells agreeing with recent, out of 147 species collected by me, amounted to about 17 per cent., or one-sixth of the whole; but as the fossils so assimilated were almost always the same as species now living in the neighbouring Atlantic, the number may hereafter be augmented, when the recent fauna of that ocean is better known. In different localities, also, the proportion of recent species varied considerably.

Fig. 151.

Fulgur canaliculatus. Maryland.

Fig. 152.

Fusus quadricostatus, Say. Maryland.

On the banks of the James River, in Virginia, about 20 miles below Richmond, in a cliff about 30 feet high, I observed yellow and white sands overlying an Eocene marl, just as the yellow sands of the crag lie on the blue London clay in Suffolk and Essex in England. In the Virginian sands, we find a profusion of an Astarte (A. undulata, Conrad), which resembles closely, and may possibly be a variety of, one of the commonest fossils of the Suffolk crag (A. bipartita); the other shells also, of the genera Natica, Fissurella, Artemis, Lucina, Chama, Pectunculus, and Pecten, are analogous to shells both of the English crag and French faluns, although the species are almost all distinct. Out of 147 of these American fossils I could only find 13 species common to Europe, and these occur partly in the Suffolk crag, and partly in the faluns of Touraine; but it is an important characteristic of the American group, that it not only contains many peculiar extinct forms, such as Fusus quadricostatus, Say (see [fig. 152.]), and Venus tridacnoides, abundant in these same formations, but also some shells which, like Fulgur carica of Say, and F. canaliculatus (see [fig. 151.]), Calyptræa costata, Venus mercenaria, Lam., Modiola glandula, Totten, and Pecten magellanicus, Lam., are recent species, yet of forms now confined to the western side of the Atlantic, a fact implying that the beginning of the present geographical distribution of mollusca dates back to a period as remote as that of the Miocene strata.

Of ten species of zoophytes which I procured on the banks of the James River, two were identical with species of the Faluns of Touraine. With respect to climate, Mr. Lonsdale regards these corals as indicating a temperature exceeding that of the Mediterranean, and the shells would lead to similar conclusions. Those occurring on the James River are in the 37th degree of N. latitude, while the French faluns are in the 47th; yet the forms of the American fossils would scarcely imply so warm a climate as must have prevailed in France, when the Miocene strata of Touraine originated.

Among the remains of fish in these Post-Eocene strata of the United States are several large teeth of the shark family, not distinguishable specifically from fossils of the faluns of Touraine, and the Maltese tertiaries.

India.—The freshwater deposits of the Sub-Himalayan or Sewâlik Hills, described by Dr. Falconer and Captain Cautley, may perhaps be regarded as Miocene. Like the faluns of Touraine, they contain the Deinotherium and Mastodon. Whether any of the associated freshwater and land shells are of recent species is not yet determined. The occurrence in them of a fossil giraffe and hippopotamus, genera now only living in Africa, as well as of a camel, implies a geographical state of things very different from that now established in the same parts of India. The huge Sivatherium of the same era appears to have been a ruminating quadruped bigger than the rhinoceros, and provided with a large upper lip, or probably a short proboscis, and having two pair of horns, resembling those of antelopes. Several species of monkey belonged to the same fauna; and among the reptiles, several crocodiles, larger than any now living, and an enormous tortoise, Testudo Atlas, the curved shell of which measured 20 feet across.

CHAPTER XV.

UPPER EOCENE FORMATIONS.

Eocene areas in England and France — Tabular view of French Eocene strata — Upper Eocene group of the Paris basin — Same beds in Belgium and at Berlin — Mayence tertiary strata — Freshwater upper Eocene of Central France — Series of geographical changes since the land emerged in Auvergne — Mineral character an uncertain test of age — Marls containing Cypris — Oolite of Eocene period — Indusial limestone and its origin — Fossil mammalia of the upper Eocene strata in Auvergne — Freshwater strata of the Cantal, calcareous and siliceous — Its resemblance to chalk — Proofs of gradual deposition of strata.

Fig. 153. Map of the principal tertiary basins of the Eocene period.

N. B. The space left blank is occupied by secondary formations from the Devonian or old red sandstone to the chalk inclusive.

The tertiary strata described in the preceding chapters are all of them characterized by fossil shells, of which a considerable proportion are specifically identical with the living mollusca; and the greater the number, the more nearly does the entire fauna approach in species and genera to that now inhabiting the adjoining seas. But in the Eocene formations next to be considered, the proportion of recent species is very small, and sometimes scarcely appreciable, and those agreeing with the fossil testacea often belong to remote parts of the globe, and to various zoological provinces. This difference in conchological character implies a considerable interval of time between the Eocene and Miocene periods, during which the whole fauna and flora underwent other changes as great, and often greater, than those exhibited by the mollusca. In the accompanying map, the position of several Eocene areas is pointed out, such as the basin of the Thames, part of Hampshire, part of the Netherlands, and the country round Paris. The deposits, however, occupying these spaces comprise a great succession of marine and freshwater formations, which, although they may all be termed Eocene, as being newer than the chalk, and older than the faluns, are nevertheless divisible into separate groups, of high geological importance.

The newest of these, like the Faluns of the Loire, have no true representatives, or exact chronological equivalents, in the British Isles. Their place in the series will best be understood by referring to the order of superposition of the successive deposits found in the neighbourhood of Paris. The area which has been called the Paris basin is about 180 miles in its greatest length from north-east to south-west, and about 90 miles from east to west. This space may be described as a depression in the chalk, which has been filled up by alternating groups of marine and freshwater strata. MM. Cuvier and Brongniart attempted, in 1810, to distinguish five different formations, comprising three freshwater and two marine, which alternated with each other. It was imagined that the waters of the ocean had been by turns admitted and excluded from the same region; but the subsequent investigations of several geologists, especially of M. Constant Prevost,[175-A] have led to great modifications in these theoretical views; and now that the true order of succession is better understood, it appears that several of the deposits, which were supposed to have originated one after the other, were, in fact, in progress at the same time by the joint action of the sea and rivers.

The whole series of strata may be divided into three groups, as expressed in the following table:—

1. Upper Eocene { a. Upper freshwater limestone, marls, and siliceous millstone.
b. Upper marine sands, or Fontainebleau sandstone and sand.
2. Middle Eocene { a. Lower freshwater limestone and marl, or gypseous series.
b. Sandstone and sands with marine shells (Sables moyens, or grès de Beauchamp).
c. Calcaire grossier, limestone with marine shells.
d. Calcaire siliceux, hard siliceous freshwater limestone, for the most part contemporaneous with c.
3. Lower Eocene { a. Lower sands with marine shelly beds (Sables inférieurs et lits coquilliers).
b. Lower sands, with lignite and plastic clay (Sables inférieurs et argiles plastiques).

Postponing to the next chapter the consideration of the Middle and Lower Eocene groups, I shall now speak of the Upper Eocene of Paris, and its foreign equivalents.

The upper freshwater marls and limestone (1. a) seem to have been formed in a great number of marshes and shallow lakes, such as frequently overspread the newest parts of great deltas. It appears that many layers of marl, tufaceous limestone, and travertin, with beds of flint, continuous or in nodules, accumulated in these lakes. Charæ, aquatic plants, already alluded to (see [p. 32.]) left their stems and seed-vessels imbedded both in the marl and flint, together with freshwater and land shells. Some of the siliceous rocks of this formation are used extensively for millstones. The flat summits or platforms of the hills round Paris, large areas in the forest of Fontainebleau, and the Plateau de la Beauce, between the Seine and the Loire, are chiefly composed of these upper freshwater strata.

The upper marine sands (1. b), consist chiefly of micaceous and quartzose sands, 80 feet thick. As they succeed throughout an extensive area deposit of a purely freshwater origin (2 a.), they appear to mark a subsidence of the subjacent soil, whether it had formed the bottom of an estuary or a lake. The sea, which afterwards took possession of the same space, was inhabited by testacea, almost all of them differing from those found in the lower formations (2. b and 2. c) and equally or still more distinct from the Miocene Faluns of subsequent date. One of these upper Eocene strata in the neighbourhood of Paris has been called the oyster bed, "couche à Ostrea cyathula, Lamk.," which is spread over a remarkably wide area. From the manner in which the oysters 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 rashly 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.

Besides these oysters, M. Deshayes has described 29 species of shells, in his work (Coquilles fossiles de Paris), as belonging to this formation, all save one regarded by him as differing from fossils of the calcaire grossier. Since that time the railway cuttings near Etampes have enabled M. Hébert to raise the number to 90. I have myself collected fossils in that district, where the shells are very entire, and detachable from the yellow sandy matrix. M. Hébert first pointed out that most of them agree specifically with those of Kleyn Spauwen, Boom, and other localities of Limburg in Flanders, where they have been studied by MM. Nyst and De Koninck.[176-A]

The position in Belgium of this formation above the older Eocene group is well seen in the small hill of Pellenberg, rising abruptly from the great plain, half a mile south-east of the city of Louvain, where I examined it in company with M. Nyst in 1850. At the top of the hill, a thin bed of dark greyish green tile-clay is seen 11/2 foot thick, with casts of Nucula Deshaysiana. This clay rests on 12 feet of yellow sand, separated, by a band of flint and quartz pebbles, from a mass of subjacent white sand 15 feet thick, in which casts of the Kleyn Spauwen fossils have been met with. Under this is a bed of yellow sand 12 feet thick, and, at a lower level, the railway cuttings have passed through calcareous sands like those of Brussels, in which the Nautilus Burtini, and various shells common to the older Eocene strata of the neighbourhood of London, have been obtained. Every new fact which throws light on the true paleontological relations of the strata now under consideration, (the Upper Marine or Fontainebleau beds of the Paris basin, 1. b, [p. 175.]), deserves more particular attention, because geologists of high authority differ in opinion as to whether they should be classed as Eocene or Miocene.

Professor Beyrich has lately described a formation of the same age, occurring within 7 miles of the gates of Berlin, near the village of Hermsdorf, where, in the midst of the sands of which that country chiefly consists, a mass of tile-clay, more than 40 feet thick, and of a dark blueish grey colour, is found, full of shells, among which the genera Fusus and Pleurotoma predominate, and among the bivalves, Nucula Deshaysiana, Nyst, an extremely common shell in the Belgian beds above-mentioned. M. Beyrich has identified eighteen out of forty-five species of the Hermsdorf fossils with the Belgian species; and I believe that a much larger proportion agree with the Upper Eocene of Belgium, France, and the Rhine. On the other hand, eight of the forty-five species are supposed by him to agree with English Eocene shells. Messrs. Morris, Edwards, and S. Wood have compared a small collection, which I obtained of these Berlin shells, with the Eocene fossils of their museums, and confirmed the result of M. Beyrich, the species common to the English fossils belonging not simply to the uppermost of our marine beds, or those of Barton, but some of them to lower parts of the series, such as Bracklesham and Highgate. On the other hand, while these testacea, like those of Kleyn Spauwen and Etampes, present many analogies to the Middle and Lower Eocene group, they differ widely from the Falun shells,—a fact the more important in reference to Etampes, as that locality approaches within 70 miles of Pontlevoy, near Blois, and within 100 miles of Savigné, near Tours, where Falun shells are found. It is evident that the discordance of species cannot be attributed to distance or geographical causes, but must be referred to time, or the different epoch at which the upper marine beds of the Paris basin and the Faluns of the Loire originated.

Mayence.—The true chronological relation of many tertiary strata on the banks of the Rhine has always presented a problem of considerable difficulty. They occupy a tract from 5 to 12 miles in breadth, extending along the left bank of the Rhine from Mayence to the neighbourhood of Manheim, and are again found to the east, north, and south-west of Frankfort. In some places they have the appearance of a freshwater formation; but in others, as at Alzey, the shells are for the most part marine. Cerithia are in great profusion, which indicates that the sea where the deposit was formed was fed by rivers; and the great quantity of fossil land shells, chiefly of the genus Helix, confirm the same opinion. The variety in the species of shells is small, while the individuals are exceedingly numerous; a fact which accords perfectly with the idea that the formation may have originated in a gulf or sea which, like the Baltic, was brackish in some parts, and almost fresh in others. A species of Paludina ([fig. 154.]), very nearly resembling the recent Littorina ulva, is found throughout this basin. These shells are like grains of rice in size, and are often in such quantity as to form entire beds of marl and limestone. They are as thick as grains of sand, in stratified masses from 15 to 30 feet in thickness.

Fig. 154.

Paludina. Mayence.

That these Rhenish tertiary formations agree more nearly with the Upper Eocene deposits above enumerated, than with any others, I have no doubt, since I had the advantage of comparing (August, 1850), with the assistance of M. De Koninck of Liége, the fossils from Kleyn Spauwen, Boom, and other Limburg localities, with those from Mayence, Alzey, Weinheim, and other Rhenish strata. Among the common Belgian and Rhenish shells which are identical, I may mention Cassidaria depressa, Tritonium flandricum De Koninck, Cerithium tricinctum Nyst, Tornatella simulata, Rostellaria Sowerbyi, Nucula Deshaysiana, Corbula pisum, and Pectunculus terebratularis.

From these Upper Eocene deposits of the Rhine M. H. von Meyer has obtained a great number of characteristic fossil mammalia, such as Palæomæryx medius, Hyotherium Meissneri, Tapirus Helveticus, Anthracotherium Alsaticum, and others. The three first of these are species common to some of the lignite, or brown coal beds in Switzerland, commonly classed with the molasse, but of which the true age has not yet been distinctly made out.

The fossils of the sandy beds of Eppelsheim, comprising bones of the Deinotherium, Mastodon, and other quadrupeds, are regarded by H. von Meyer as belonging to a mammiferous fauna quite distinct from that of the Mayence basin, and they are probably referable to the Miocene period.

The upper freshwater strata (1. a, [p. 175.]), of the neighbourhood of Paris, stretch southwards from the valley of the Seine to that of the Loire, and in the last-mentioned region are seen to be older than the marine faluns, so that the perforating shells of the Miocene sea have sometimes bored the hard compact freshwater limestones; and fragments of the Upper Eocene rocks are found at Pontlevoy and elsewhere, which have been rolled in the bed of the Miocene sea.

Fig. 155.

Central France.—Lacustrine strata belonging, for the most part, to the same Upper Eocene series, are again met with in Auvergne, Cantal, and Velay, the sites of which may be seen in the annexed map. They appear to be the monuments of ancient lakes, which, like some of those now existing in Switzerland, once occupied the depressions in a mountainous region, 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 matter (see the map), the greater part of which is newer than the freshwater strata, and is sometimes seen to rest upon them, while a small part has evidently been of contemporaneous origin. Of these igneous rocks I shall treat more particularly in another part of this work.

Before entering upon any details, I may observe, that the study of these regions possesses a peculiar interest, very distinct in kind from that derivable from the investigation either of the Parisian or English tertiary strata. For we are presented in Auvergne with the evidence of a series of events of astonishing magnitude and grandeur, by which the original form and features of the country have been greatly changed, yet never so far obliterated but that they may still, in part at least, be restored in imagination. Great lakes have disappeared,—lofty mountains have been formed, by the reiterated emission of lava, preceded and followed by showers of sand and scoriæ,—deep valleys have been subsequently furrowed out through masses of lacustrine and volcanic origin,—at a still later date, new cones have been thrown up in these valleys,—new lakes have been formed by the damming up of rivers,—and more than one creation of quadrupeds, birds, and plants, Eocene, Miocene, and Pliocene, have followed in succession; yet the region has preserved from first to last its geographical identity; and we can still recall to our thoughts its external condition and physical structure before these wonderful vicissitudes began, or while a part only of the whole had been completed. There was first a period when the spacious lakes, of which we still may trace the boundaries, lay at the foot of mountains of moderate elevation, unbroken by the bold peaks and precipices of Mont Dor, and unadorned by the picturesque outline of the Puy de Dome, or of the volcanic cones and craters now covering the granitic platform. During this earlier scene of repose deltas were slowly formed; beds of marl and sand, several hundred feet thick, deposited; siliceous and calcareous rocks precipitated from the waters of mineral springs; shells and insects imbedded, together with the remains of the crocodile and tortoise, the eggs and bones of water birds, and the skeletons of quadrupeds, some of them belonging to the same genera as those entombed in the Eocene gypsum of Paris. To this tranquil condition of the surface succeeded the era of volcanic eruptions, when the lakes were drained, and when the fertility of the mountainous district was probably enhanced by the igneous matter ejected from below, and poured down upon the more sterile granite. During these eruptions, which appear to have taken place after the disappearance of the Eocene fauna, and in the Miocene epoch, the mastodon, rhinoceros, elephant, tapir, hippopotamus, together with the ox, various kinds of deer, the bear, hyæna, and many beasts of prey, ranged the forest, or pastured on the plain, and were occasionally overtaken by a fall of burning cinders, or buried in flows of mud, such as accompany volcanic eruptions. Lastly, these quadrupeds became extinct, and gave place to Pliocene mammalia, and these, in their turn, to species now existing. There are no signs, during the whole time required for this series of events, of the sea having intervened, nor of any denudation which may not have been accomplished by currents in the different lakes, or by rivers and floods accompanying repeated earthquakes, during which the levels of the district have in some places been materially modified, and perhaps the whole upraised relatively to the surrounding parts of France.

Auvergne.—The most northern of the freshwater 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 mountain ranges,—that of the Forèz, which divides the waters of the Loire and Allier, on the east; and that of the Monts Domes, which separates the Allier from the Sioule, on the west.[181-A] 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, and limestone, none of which observe a fixed and invariable order of superposition. The ancient borders of the lake, wherein the freshwater strata were accumulated, may generally be traced with precision, the granite and other ancient rocks rising up boldly from the level country. The actual junction, however, of the lacustrine and granitic beds is rarely seen, as a small valley usually intervenes between them. The freshwater 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 few instances vertical. The principal divisions into which the lacustrine series may be separated are the following:—1st, Sandstone, grit, and conglomerate, including red marl and red sandstone. 2dly, Green and white foliated marls. 3dly, Limestone or travertin, often oolitic. 4thly, Gypseous marls.

1. a. 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 these 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.

At Chamalieres, near Clermont, we have an example of one of these deltas, or littoral deposits, 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, and which could not have been there when the conglomerate was formed. Partial layers of lignite and pieces of wood are found in these beds.

At some localities on the margin of the basin quartzose grits are found; and, where these rest on granite, they are sometimes formed of separate crystals of quartz, mica, and felspar, derived from the disintegrated granite, the crystals having been subsequently bound together by a siliceous cement. In these cases the granite seems regenerated in a new and more solid form; and so gradual a passage takes place between the rock of crystalline and that of mechanical origin, that we can scarcely distinguish where one ends and the other begins.

In the hills called the Puy de Jussat and La Roche, we have the advantage of seeing a section continuously exposed for about 700 feet in thickness. At the bottom are foliated marls, white and green, about 400 feet thick; and above, resting on the marls, are the quartzose grits, cemented by calcareous matter, which is sometimes so abundant as to form imbedded nodules. These sometimes constitute spheroidal concretions 6 feet in diameter, and pass into beds of solid limestone, resembling the Italian travertins, or the deposits of mineral springs. This section is 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.

1. b. Red marl and sandstone.—But the most remarkable of the arenaceous groups is one of red sandstone and red marl, which are identical in all their mineral characters with the secondary New Red sandstone and marl of England. In these secondary rocks the red ground is sometimes variegated with light greenish spots, and the same may be seen in the tertiary formation of freshwater 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 are derived. The red beds, although destitute themselves of organic remains, pass upwards into strata containing Eocene fossils, and are certainly an integral part of the lacustrine formation. From this example the student will learn how small is the value of mineral character alone, as a test of the relative age of rocks.

2. Green and white foliated marls.—The same primary rocks 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 materials 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. 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.

Fig. 156.

Cypris unifasciata, a living species, greatly magnified.

Fig. 157.

Cypris vidua, a living species, greatly magnified.[183-A]

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 shells, or carapace-valves, of that small animal called Cypris; a genus which comprises several species, of which some are recent, and may be seen swimming swiftly through the waters of our stagnant pools and ditches. The antennæ, at the end of which are fine pencils of hair, are the principal organs of motion, and are seen to vibrate with great rapidity. This animal resides within two small valves, not unlike those of a bivalve shell, and moults its integuments periodically, which the conchiferous mollusks do not. This circumstance may partly explain the countless myriads of the shells of Cypris which were shed in the ancient lakes of Auvergne, 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. But we may easily suppose that, while this fine sediment was thrown down in the deep and central parts of the basin, gravel, sand, and rocky fragments were hurried into the lake, and deposited near the shore, forming the group described in the preceding section.

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 occurrence of these marls so near the ancient margin may be explained by considering that, at the bottom of the ancient lake, no coarse ingredients were deposited in spaces intermediate between the points where rivers and torrents entered, but finer mud only was drifted there 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.

Fig. 158.

Vertical strata of marl, at Champradelle, near Clermont.

3. Limestone, travertin, oolite.—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, where there is an increase in the quantity of calcareous matter, unite into regular beds.

On each side of the basin of the Limagne, both on the west at Gannat, and on the east 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. At Chadrat, in the hill of La Serre, the limestone is pisolitic, the small spheroids combining both the radiated and concentric structure.

Indusial limestone.—There is another remarkable form of freshwater limestone in Auvergne, called "indusial," from the cases, or indusiæ, of caddis-worms (the larvæ of Phryganea); great heaps of which have been incrusted, as they lay, by carbonate of lime, and formed into a hard travertin. The rock is sometimes purely calcareous, but there is occasionally an intermixture of siliceous matter. Several beds of it are frequently seen, either in continuous masses, or in concretionary nodules, one upon another, with layers of marl interposed. The annexed drawing ([fig. 159.]) will show the manner in which one of these indusial beds (a) is laid open at the surface, between the marls (b b), near the base of the hill of Gergovia; and affords, at the same time, an example of the extent to which the lacustrine strata, which must once have filled a hollow, have been denuded, and shaped out into hills and valleys, on the site of the ancient lakes.

Fig. 159.

Bed of indusial limestone, interstratified with freshwater marl, near Clermont (Kleinschrod.)

Fig. 160.

Larva of recent Phryganea.[185-A]

Fig. 161.

We may often observe in our ponds the Phryganea (or Caddis-fly), in its caterpillar state, covered with small freshwater shells, which they have the power of fixing to the outside of their tubular cases, in order, probably, to give them weight and strength. The individual figured in the annexed cut, which belongs to a species very abundant in England, has covered its case with shells of a small Planorbis. In the same manner a large species of caddis-worm, 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 often empty, the rest being filled up with thin concentric layers of travertin. The cases have been thrown together confusedly, and often lie, as in [fig. 161.], at right angles one to the other. 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 6 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 Phryganeæ lived on the spots where their cases are now found; they may have multiplied in the shallows near the margin of the lake, or in the streams by which it was fed, and their cases may have been drifted by a current far into the deep water.

In the summer of 1837, when examining, in company with Dr. Beck, a small lake near Copenhagen, I had an opportunity of witnessing a beautiful exemplification of the manner in which the tubular cases of Auvergne were probably accumulated. This lake, called the Fuure-Soe, occurring in the interior of Seeland, is about twenty English miles in circumference, and in some parts 200 feet in depth. Round the shallow borders an abundant crop of reeds and rushes may be observed, covered with the indusiæ of the Phryganea grandis and other species, to which shells are attached. The plants which support them are the bullrush, Scirpus lacustris, and common reed, Arundo phragmitis, but chiefly the former. In summer, especially in the month of June, a violent gust of wind sometimes causes a current by which these plants are torn up by the roots, washed away, and floated off in long bands, more than a mile in length, into deep water. The Cypris swarms in the same lake; and calcareous springs alone are wanting to form extensive beds of indusial limestone, like those of Auvergne.

4. 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 grit, 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, origin, and age of the freshwater formations of Auvergne.—The relations of the different groups above described cannot be learnt by the study of any one section; and the geologist 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 be seen in some places 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 both. The uppermost marls and sands are more calcareous than the lower; and we never meet with calcareous rocks covered by a considerable thickness of quartzose sand or green marl. From the resemblance of the limestones to the Italian travertins, we may conclude that they were derived from the waters of mineral springs,—such springs as even now exist in Auvergne, and which may be seen rising up through the granite, and precipitating travertin. They are sometimes thermal, but this character is by no means constant.

It seems that, when the ancient lake of the Limagne first began to be filled with sediment, no volcanic action had yet produced lava and scoriæ on any part of the surface of Auvergne. No pebbles, therefore, of lava were transported into the lake,—no fragments of volcanic rocks embedded 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 deposited, at some spots, alternately with the lacustrine strata. It is not improbable that cold and thermal springs, holding different mineral ingredients in solution, became more numerous during the successive convulsions attending this development of volcanic agency, and thus deposits of 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 freshwater strata to cease.

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 igneous action should break out in that country, and produce lava, scoriæ, 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 of rocks precipitated from the waters of mineral springs.

Although the freshwater 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 lacustrine 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 granite eminences. Nor need we be surprised at our inability to restore entirely the physical geography of the country after so great a series of volcanic eruptions; for it is by no means improbable that one part of it, the southern, for example, may have been moved upwards bodily, while others remained at rest, or even suffered a movement of depression.

Whether all the freshwater formations of the Limagne d'Auvergne belong to one period, I cannot pretend to decide, as large masses both of the arenaceous and marly groups are often devoid of fossils. Much light has been thrown on the mammiferous fauna by the labours of MM. Bravard and Croizet, and by those of M. Pomel. The last-mentioned naturalist has pointed out the specific distinction of all, or nearly all, the species of mammalia, from those of the gypseous series near Paris. Nevertheless, many of the forms are analogous to those of Eocene quadrupeds. The Cainotherium, for example, is not far removed from the Anoplotherium, and is, according to Waterhouse, the same as the genus Microtherium of the Germans. There are two species of marsupial animals allied to Didelphys, a genus also found in the Paris gypsum. The Amphitragulus elegans of Pomel, has been identified with a Rhenish species from Weissenau near Mayence, called by Kaup Dorcatherium nanum; and other Auvergne fossils, e.g., Microtherium Reuggeri, and a small rodent, Titanomys, are specifically the same with mammalia of the Mayence basin.

Cantal.—A freshwater formation, very analogous to that of Auvergne, is situated in the department of Haute Loire, near the town of Le Puy, in Velay, and another occurs near Aurillac, in Cantal. The leading feature of the formation last mentioned, as distinguished from those of Auvergne and Velay, is the immense abundance of silex associated with calcareous marls and limestone.

The whole series may be separated into two divisions; the lower, 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.

The resemblance of the freshwater limestone of the Cantal, and its accompanying flint, to the upper chalk of England, is very instructive, and well calculated to put the student upon his guard against relying too implicitly on mineral character alone as a safe criterion of relative age.

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, the surface is 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 in appearance 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 which rises before us, 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 dark vegetable mound. In these cavities we recognize an exact counterpart to those which are so numerous on the furrowed surface of our own white chalk. Advancing 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.

It will be remembered that the siliceous stone of Bilin, called tripoli, is a freshwater deposit, and has been shown, by Ehrenberg, to be of infusorial origin (see [p. 24.]). What is true of the Bohemian flint and opal, where the beds attain a thickness of 14 feet, may also, perhaps, be found to hold good respecting the silex of Aurillac, which may also have been immediately derived from the minute cases of microscopic animalcules. But even if this conclusion be established, the abundant supply both of siliceous, calcareous, and gypseous matter, which the ancient lakes of France received, may have been connected with the subterranean volcanic agency of which those regions were so long the theatre, and which may have impregnated the springs with mineral matter, even before the great outbreak of lava. It is well known that the hot springs of Iceland, and many other countries, contain silex in solution; and it has been lately affirmed, that steam at a high temperature is capable of dissolving quartzose rocks without the aid of any alkaline or other flux.[189-A]

Travellers not unfrequently mention, in their accounts of India, Australia, and other distant lands, that they have seen chalk with flints, which they have assumed to be of the same age as the Cretaceous system of Europe. A hasty observation of the white limestone and flint of Aurillac might convey the same idea; but when we turn from the mineral aspect and composition to the organic remains, we find in the flints of the Cantal the seed-vessels of the freshwater Chara, instead of the marine zoophytes so abundantly imbedded in chalk flints; and in the limestone we meet with shells of Limnea, Planorbis, and other lacustrine genera, instead of the oyster, terebratula, and echinus of the Cretaceous period.

Proofs of gradual deposition.—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 every one of them the flattened stems of Charæ, or other plants, or sometimes myriads of small Paludinæ and other freshwater shells. These minute foliations of the marl resemble precisely some of the recent laminated beds of the Scotch marl lakes, and 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 tints being white, green, and brown. Occasionally there is a parting layer of pure flint, or of black carbonaceous vegetable matter, about an 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.[190-A]

Thus 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 vast a succession of these strata unite to form a single group! We must remember, also, that piles of volcanic matter, like the Plomb du Cantal, which rises in the immediate neighbourhood of Aurillac, are themselves equally the result of successive accumulation, consisting of reiterated sheets of lava, showers of scoriæ, and ejected fragments of rock.—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 in succession during an indefinite lapse of ages, and superimposed upon each other.