A Manual of Elementary Geology Part 28

_Cantal._--A freshwater formation, very a.n.a.logous 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 a.s.sociated 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 pa.s.s 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 a.s.sumed 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 sh.e.l.ls 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 ama.s.sed. In the hill of Barrat, for example, we find an a.s.semblage 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 _Charae_, or other plants, or sometimes myriads of small _Paludinae_ and other freshwater sh.e.l.ls. 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 ma.s.sive geological monuments are made up! When we desire to cla.s.sify, 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 ma.s.s 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 acc.u.mulation, consisting of reiterated sheets of lava, showers of scoriae, 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 a.s.semblage of many such igneous and aqueous groups, formed in succession during an indefinite lapse of ages, and superimposed upon each other.

FOOTNOTES:

[175-A] Bulletin des Sci. de la Soc. Philom., May, 1825, p. 74.

[176-A] Hebert. Bulletin. 1849, vol. vi. 2d series, p. 459.

[181-A] Scrope, Geology of Central France, p. 15.

[183-A] See Desmarest's Crustacea, plate 55.

[185-A] I believe that the British specimen here figured is P. _rhombica_, Linn.

[189-A] See Proceedings of Roy. Soc., No. 44. p. 233.

[190-A] Lyell and Murchison, sur les Depots Lacust. Tertiaries du Cantal, &c. Ann. des Sci. Nat. Oct. 1829.

CHAPTER XVI.

EOCENE FORMATIONS--_continued_.

Subdivisions of the Eocene group in the Paris basin--Gypseous series--Extinct quadrupeds--Impulse given to geology by Cuvier's osteological discoveries--Sh.e.l.ly sands called sables moyens--Calcaire grossier--Miliolites--Calcaire siliceux--Lower Eocene in France--Lits coquilliers--Sands and plastic clay--English Eocene strata--Freshwater and fluvio-marine beds--Barton beds--Bagshot and Bracklesham division--Large ophidians and saurians--Lower Eocene and London Clay proper--Fossil plants and sh.e.l.ls--Strata of Kyson in Suffolk--Fossil monkey and opossum--Mottled clays and sands below London Clay--Nummulitic formation of Alps and Pyrenees--Its wide geographical extent--Eocene strata in the United States--Section at Claiborne, Alabama--Colossal cetacean--Orbitoid limestone--Burr stone.

From what was said in the two preceding chapters, it has already appeared that we have in England no true chronological representative of the Miocene faluns of the Loire, and none of the Upper Eocene group described in the last chapter. But, when we descend to the middle and inferior divisions of the Eocene system of France, we find that they have their equivalents in Great Britain.

MIDDLE EOCENE.--FRANCE.

_Gypseous series_ (2. _a_, Table, p. 175.).--Next below the upper marine sands of the neighbourhood of Paris, we find a series of white and green marls, with subordinate beds of gypsum. These are most largely developed in the central parts of the Paris basin, and, among other places, in the Hill of Montmartre, where its fossils were first studied by M. Cuvier.

The gypsum quarried there for the manufacture of plaster of Paris occurs as a granular crystalline rock, and, together with the a.s.sociated marls, contains land and fluviatile sh.e.l.ls, together with the bones and skeletons of birds and quadrupeds. Several land plants are also met with, among which are fine specimens of the fan palm or palmetto tribe (_Flabellaria_). The remains also of freshwater fish and of crocodiles and other reptiles, occur in the gypsum. The skeletons of mammalia are usually isolated, often entire, the most delicate extremities being preserved; as if the carca.s.ses, clothed with their flesh and skin, had been floated down soon after death, and while they were still swoln by the gases generated by their first decomposition. The few accompanying sh.e.l.ls are of those light kinds which frequently float on the surface of rivers, together with wood.

M. Prevost has therefore suggested that a river may have swept away the bodies of animals, and the plants which lived on its borders, or in the lakes which it traversed, and may have carried them down into the centre of the gulf into which flowed the waters impregnated with sulphate of lime. We know that the Fiume Salso in Sicily enters the sea so charged with various salts that the thirsty cattle refuse to drink of it. A stream of sulphureous water, as white as milk, descends into the sea from the volcanic mountain of Idienne on the east of Java; and a great body of hot water, charged with sulphuric acid, rushed down from the same volcano on one occasion, and inundated a large tract of country, destroying, by its noxious properties, all the vegetation.[191-A] In like manner the Pusanibio, or "Vinegar River," of Colombia, which rises at the foot of Purace, an extinct volcano, 7,500 feet above the level of the sea, is strongly impregnated with sulphuric and muriatic acids and with oxide of iron. We may easily suppose the waters of such streams to have properties noxious to marine animals, and in this manner the entire absence of marine remains in the ossiferous gypsum may be explained.[191-B] There are no pebbles or coa.r.s.e sand in the gypsum; a circ.u.mstance which agrees well with the hypothesis that these beds were precipitated from water holding sulphate of lime in solution, and floating the remains of different animals.

In this formation the relics of about fifty species of quadrupeds, including the genera _Paleotherium_, _Anoplotherium_, and others, have been found, all extinct, and nearly four-fifths of them belonging to a division of the order _Pachydermata_, which is now represented by only four living species; namely three tapirs and the daman of the Cape. With them a few carnivorous animals are a.s.sociated, among which are a species of fox and gennet. Of the _Rodentia_, a dormouse and a squirrel; of the _Insectivora_, a bat; and of the _Marsupialia_ (an order now confined to America, Australia, and some contiguous islands), an opossum, have been discovered.

Of birds, about ten species have been ascertained, the skeletons of some of which are entire. None of them are referable to existing species.[192-A]

The same remark applies to the fish, according to MM. Cuvier, and Aga.s.siz, as also to the reptiles. Among the last are crocodiles and tortoises of the genera _Emys_ and _Trionyx_.

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

[Ill.u.s.tration: Fig. 162. _Paleotherium magnum._]

The Paleothere, above alluded to, resembled the living tapir in the form of the head, and in having a short proboscis, but its molar teeth were more like those of the rhinoceros (see fig. 163.). _Paleotherium magnum_ was of the size of a horse, 3 or 4 feet high. The annexed woodcut, fig. 162., is one of the restorations which Cuvier attempted of the outline of the living animal, derived from the study of the entire skeleton. When the French osteologist declared in the early part of the present century, that all the fossil quadrupeds of the gypsum of Paris were extinct, the announcement of so startling a fact, on such high authority, created a powerful sensation, and from that time a new impulse was given throughout Europe to the progress of geological investigation. Eminent naturalists, it is true, had long before maintained that the sh.e.l.ls and zoophytes, met with in many ancient European rocks, had ceased to be inhabitants of the earth, but the majority even of the educated cla.s.ses continued to believe that the species of animals and plants now contemporary with man, were the same as those which had been called into being when the planet itself was created. It was easy to throw discredit upon the new doctrine by asking whether corals, sh.e.l.ls, and other creatures previously unknown, were not annually discovered? and whether living forms corresponding with the fossils might not yet be dredged up from seas. .h.i.therto unexamined? But from the era of the publication of Cuvier's Oss.e.m.e.nts Fossiles, and still more his popular Treatise called "A Theory of the Earth," sounder views began to prevail. It was clearly demonstrated that most of the mammalia found in the gypsum of Montmartre differed even generically from any now existing, and the extreme improbability that any of them, especially the larger ones, would ever be found surviving in continents yet unexplored, was made manifest. Moreover, the non-admixture of a single living species in the midst of so rich a fossil fauna was a striking proof that there had existed a state of the earth's surface zoologically unconnected with the present order of things.

[Ill.u.s.tration: Fig. 163. Upper molar tooth of _Paleotherium magnum_ from Isle of Wight. (Owen's Brit. Foss. p. 317.)

Reduced one-third.]

_Gres de Beauchamp_ (2. _b_, Table, p. 175.).--In some parts of the Paris basin, sands and marls, called the Gres de Beauchamp, or Sables Moyens, divide the gypseous beds from the underlying Calcaire grossier. These sands contain more than 300 species of marine sh.e.l.ls, many of them peculiar, but others common to the underlying marine deposit (No. 2. _c_.).

_Calcaire grossier_ (2. _c_, Table, p. 175.).--The formation called Calcaire grossier consists of a coa.r.s.e limestone, often pa.s.sing into sand.

It contains the greater number of the fossil sh.e.l.ls which characterize the Paris basin. No less than 400 distinct species have been procured from a single spot near Grignon, where they are embedded in a calcareous sand, chiefly formed of comminuted sh.e.l.ls, in which, nevertheless, individuals in a perfect state of preservation, both of marine, terrestrial, and freshwater species, are mingled together. Some of the marine sh.e.l.ls may have lived on the spot; but the _Cyclostoma_ and _Limnea_ must have been brought thither by rivers and currents, and the quant.i.ty of triturated sh.e.l.ls implies considerable movement in the waters.

Nothing is more striking in this a.s.semblage of fossil testacea than the great proportion of species referable to the genus _Cerithium_ (see fig.

164.). There occur no less than 137 species of this genus in the Paris basin, and almost all of them in the calcaire grossier. Now the living _Cerithia_ inhabit the sea near the mouths of rivers, where the waters are brackish; so that their abundance in the marine strata now under consideration is in harmony with the hypothesis, that the Paris basin formed a gulf into which several rivers flowed, the sediment of some of which gave rise to the beds of clay and lignite before mentioned; while a distinct freshwater limestone, called calcaire siliceux, which will presently be described, was precipitated from the waters of others situated farther to the south.

[Ill.u.s.tration: Fig. 164. Cerithium cinctum.[194-A]]

[4 Ill.u.s.trations: EOCENE FORAMINIFERA.

Fig. 165. _Calcarina rarispina_, Desh.

_b_. natural size.

_a_, _c_. same magnified.

Fig. 166. _Spirolina stenostoma_, Desh.

B. natural size.

A, C, D. same magnified.

Fig. 167. _Triloculina inflata_, Desh.

_b_. natural size.

_a_, _c_, _d_, same magnified.

Fig. 168. _Clavulina corrugata_, Desh.

_a_. natural size.

_b_, _c_. same magnified.]

In some parts of the calcaire grossier round Paris, certain beds occur of a stone used in building, and called by the French geologists "Miliolite limestone." It is almost entirely made up of millions of microscopic sh.e.l.ls, of the size of minute grains of sand, which all belong to the cla.s.s Foraminifera. Figures of some of these are given in the annexed woodcut. As this miliolitic stone never occurs in the Faluns, or Miocene strata of Brittany and Touraine, it often furnishes the geologist with a useful criterion for distinguishing the detached Eocene and Miocene formations, scattered over those and other adjoining provinces. The discovery of the remains of Paleotherium and other mammalia in some of the upper beds of the calcaire grossier shows that these land animals began to exist before the deposition of the overlying gypseous series had commenced.