A Manual of Elementary Geology Part 26

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 sh.e.l.ls, 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 pal.u.s.tris_ in the blue marl, filled with small marine sh.e.l.ls. 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 sh.e.l.ls, 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 sh.e.l.ls retain their pearly l.u.s.tre, part of their external colour, and even the ligament which unites the valves. No sh.e.l.ls 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, const.i.tutes, 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 sh.e.l.ls, 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 pa.s.ses 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 sh.e.l.ls are silicified, as at San Vitale, near Parma, from whence I saw two individuals of recent species, one freshwater and the other marine (_Limnea pal.u.s.tris_, 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 sh.e.l.ls, 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 sh.e.l.ls appear to be all of living species, and must have been embedded when the summit of the Capitol was a marsh, and const.i.tuted 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 a.s.semblage of fossils, is met with, to which the name of _Faluns_ has been long given by the French agriculturists, who spread the sh.e.l.ly sand and marl over the land, in the same manner as the crag was formerly much used in Suffolk.

Isolated ma.s.ses 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 princ.i.p.ally of sand and marl, in which are sh.e.l.ls and corals, some entire, some rolled, and others in minute fragments. In certain districts, as at Doue, 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 sh.e.l.ls, corals, and echinoderms, united by a calcareous cement; the whole ma.s.s 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 sh.e.l.ls 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, Chaeropotamus, 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 sh.o.r.e 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 _Cypraea_, 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 sh.e.l.ls, collected by myself, in 1840, at Pontlevoy, Louans, Bossee, and other villages 20 miles south of Tours; and at Savigne, 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 Doue, 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 inst.i.tute 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 sh.e.l.ls.

It was stated that, on comparing about 300 species of Touraine sh.e.l.ls 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 a.s.semblages of fossils retained their distinctive characters, showing no signs of any blending of species or transition of climate.

On a comparison of 280 Mediterranean sh.e.l.ls 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 princ.i.p.al 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 sh.e.l.ls 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 a.s.sociated 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 sh.e.l.ls have been obtained. A large proportion of these sh.e.l.ls belong to the same zoological type as those of Touraine; but many are peculiar, and the whole may possibly const.i.tute 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 sh.e.l.ls 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. .h.i.therto discovered.

_Mola.s.se of Switzerland._--If we cross the Alps, and pa.s.s 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 a.s.sociated in a similar manner with marls and conglomerate. This formation is called in Switzerland "mola.s.se," 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 sh.e.l.ls are often entirely wanting.

In some places a decided agreement of the fossil fishes of the mola.s.se and faluns has been observed. Among those common to both, M. Aga.s.siz pointed out to me _Lamna contortidens_, _Myliobates Studeri_, _Spherodus cinctus_, _Notida.n.u.s primigenius_, and others.

_Lisbon._--Marine tertiary strata near Lisbon contain sh.e.l.ls 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 princ.i.p.ally 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 a.s.similated 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 cla.s.sification of European strata adopted in this chapter.

The proportion of fossil sh.e.l.ls 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 a.s.similated 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.

[Ill.u.s.tration: Fig. 151. _Fulgur ca.n.a.liculatus._ Maryland.]

[Ill.u.s.tration: 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 Ess.e.x 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. bipart.i.ta_); the other sh.e.l.ls also, of the genera _Natica_, _Fissurella_, _Artemis_, _Lucina_, _Chama_, _Pectunculus_, and _Pecten_, are a.n.a.logous to sh.e.l.ls 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 sh.e.l.ls which, like _Fulgur carica_ of Say, and _F. ca.n.a.liculatus_ (see fig. 151.), _Calyptraea 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 sh.e.l.ls would lead to similar conclusions. Those occurring on the James River are in the 37th degree of N. lat.i.tude, 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 Sewalik 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 a.s.sociated freshwater and land sh.e.l.ls 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 sh.e.l.l of which measured 20 feet across.

FOOTNOTES:

[162-A] See paper by E. Charlesworth, Esq.; London and Ed. Phil. Mag. No.

x.x.xviii. p. 81., Aug. 1835.

[162-B] See Monograph on the Crag Mollusca. Searles Wood, Paleont.

Soc. 1848.

[163-A] In regarding the Suffolk crag, both red and coralline, as older Pliocene instead of Miocene, I am only returning to the cla.s.sification adopted by me in the Principles and Elements of Geology up to the year 1838.

[166-A] E. Forbes, Mem. Geol. Survey, Gt. Brit., vol. i. 386.

[172-A] Proceedings of the Geol. Soc. vol. iv. part 3. 1845, p. 547.

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.

[Ill.u.s.tration: Fig. 153. Map of the princ.i.p.al tertiary basins of the Eocene period.

N. B. The s.p.a.ce 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 sh.e.l.ls, 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 s.p.a.ces 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 s.p.a.ce 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:--

{ _a._ Upper freshwater limestone, marls, and siliceous 1. Upper Eocene { millstone.

{ _b._ Upper marine sands, or Fontainebleau sandstone { and sand.

{ _a._ Lower freshwater limestone and marl, or { gypseous series.

{ _b._ Sandstone and sands with marine sh.e.l.ls (_Sables_ 2. Middle Eocene { _moyens_, or _gres de Beauchamp_).

{ _c._ Calcaire grossier, limestone with marine sh.e.l.ls.

{ _d._ Calcaire siliceux, hard siliceous freshwater { limestone, for the most part contemporaneous { with _c_.

{ _a._ Lower sands with marine sh.e.l.ly beds (_Sables_ 3. Lower Eocene { _inferieurs et lits coquilliers_).

{ _b._ Lower sands, with lignite and plastic clay { (_Sables inferieurs et argiles plastiques_).