Part 24 (1/2)

3. The _Aptien_ (or Greensand) consists generally of marls and clay. In France it is found in the department of Vaucluse, at Apt (whence the name Aptien), in the department of the Yonne, and in the Haute-Marne.

Fossils, _Ancyloceras Matheronia.n.u.s_, _Ostrea aquila_, and _Plicatula placunea_. These beds consist here of greyish clay, which is used for making tiles; there of bluish argillaceous limestone, in black or brownish flags. In the Isle of Wight it becomes a fine sandstone, greyish and slightly argillaceous, which at Havre, and in some parts of the country of Bray, become well-developed ferruginous sandstones.

[Ill.u.s.tration: Fig. 143.--Cypris spinigera.]

[Ill.u.s.tration: Fig. 144.--Cypris Valdensis.]

We have noted that the Lower Neocomian formation, although a marine deposit, is in some respects the equivalent of the _Weald Clay_, a fresh-water formation of considerable importance on account of its fossils. We have seen that it was either formed at the mouth of a great river, or the river was sufficiently powerful for the fresh-water current to be carried out to sea, carrying with it some animals, forming a fluviatile, or lacustrine fauna, on a small scale. These were small Crustaceans of the genus _Cypris_, with some molluscous Gasteropoda of the genera _Melania_, _Paludina_, and acephalous Mollusca of the five genera _Cyrena_, _Unio_, _Mytilus_, _Cyclas_, and _Ostrea_. Of these, _Cypris spinigera_ (Fig. 143) and _Cypris Valdensis_ (Fig. 144) may be considered as among the most characteristic fossils of this local fauna.

The Cretaceous series is not interesting for its fossils alone; it presents also an interesting subject for study in a mineralogical point of view. The white Chalk, examined under the microscope by Ehrenberg, shows a curious globiform structure. The green part of its sandstone and limestone const.i.tutes very singular compounds. According to the result of Berthier's a.n.a.lysis, we must consider them as silicates of iron. The iron shows itself here not in beds, as in the Jura.s.sic rocks, but in ma.s.ses, in a species of pocket in the Orgonian beds. They are usually hydrates in the state of hemat.i.tes, accompanied by quant.i.ties of ochre so abundant that they are frequently unworkable. In the south of France these veins were mined to a great depth by the ancient monks, who were the metallurgists of their age. But for the artist the important Orgonian beds possess a special interest; their admirable vertical fractures, their erect perpendicular peaks, each surpa.s.sing the other in boldness, form his finest studies. In the Var, the defiles of Vesubia, of the Esteron, and Tinea, are jammed up between walls of peaks, for many hundreds of yards, between which there is scarcely room for a narrow road by the side of the roaring torrent. ”In the Drome,” says Fournet, ”the entrance to the beautiful valley of the Vercors is closed during a part of the year, because, in order to enter, it is necessary to cross the two gullies, the _Great_ and _Little Goulet_, through which the waters escape from the valley. Even during the dry season, he who would enter the gorge must take a foot-bath.

”This state of things could not last; and in 1848 it was curious to see miners suspended on the sides of one of these lateral precipices, some 450 feet above the torrent, and about an equal distance below the summit of the Chalk. There they began to excavate cavities or niches in the face of the rock, all placed on the same level, and successively enlarged. These were united together in such a manner as to form a road practicable for carriages; now through a gallery, now covered by a corbelling, to look over which affords a succession of surprises to the traveller.

”This is not all,” adds M. Fournet: ”he who traverses the high plateaux of the country finds at every step deep diggings in the soil, designated pits or _scialets_, the oldest of which have their sides clothed with a curious vegetation, in which the _Aucolin_ predominates; shelter is found in these pits from the cutting winds which rage so furiously in these elevated regions. Others form a kind of cavern, in which a temperature obtains sufficient to freeze water even in the middle of summer. These cavities form natural _glaciers_, which we again find upon some of the table-lands of the Jura.

”The cracks and creva.s.ses of the limestone receive the waters produced by falling rain and melted snow; true to the laws of all fluid bodies, they filter through the rocks until they reach the lower and impervious marly beds, where they form sheets of water, which in course of time find some outlet through which they discharge themselves. In this manner subterranean galleries, sometimes of great extent, are formed, in which are a.s.sembled all the marvels which crumbling stalact.i.tes, stalagmites, placid lakes, and headlong torrents can produce; finally, these waters, forcing their way through the external orifices, give rise to those fine cascades which, with the first gus.h.i.+ng torrent, form an actual river.”

The _Albien_ of Alc. D'Orbigny, which Lyell considers to be the equivalent of the _Gault_, French authors treat as the ”_glauconie_”

formation, the name being drawn from a rock composed of chalk with greenish grains of _glauconite_, or silicate of iron, which is often mixed with the limestone of this formation. The fossils by which it is identified are very varied. Among its numerous types, we find Crustaceans belonging to the genera _Arcania_ and _Corystes_; many new Mollusca, _Buccinum_, _Solen_, _Pterodonta_, _Voluta_, _Chama_, &c.; great numbers of molluscous Brachiopods, forming highly-developed submarine strata; some Echinoderms, unknown up to this period, and especially a great number of Zoophytes; some Foraminifera, and many Polyzoa (Bryozoa). The glauconitic formation consists of two groups of strata: the _Gault_ Clay and the _glauconitic_ chalk, or Upper Greensand and Chloritic Marl.

UPPER CRETACEOUS PERIOD.

During this phase of the terrestrial evolutions, the continents, to judge from the fossilised wood which we meet with in the rocks which now represent it, would be covered with a very rich vegetation, nearly identical, indeed, with that which we have described in the preceding sub-period; according to Adolphe Brongniart, the ”age of angiosperms”

had fairly set in; the Cretaceous flora displays, he considers, a transitional character from the Secondary to the Tertiary vegetation; that the line between the gymnosperms, or naked-seeded plants, and the angiosperms, having their seeds enclosed in seed-vessels, runs between the Upper and Lower Cretaceous formations. ”We can now affirm,” says Lyell, ”that these Aix-la-Chapelle plants, called Credneria, flourished before the rich reptilian fauna of the secondary rocks had ceased to exist. The Ichthyosaurus, Pterodactyle, and Mosasaurus were of coeval date with the oak, the walnut, and the fig.”[80]

[80] Lyell's ”Elements of Geology,” p. 333.

The terrestrial fauna, consisting of some new Reptiles haunting the banks of rivers, and Birds of the genus Snipe, have certainly only reached us in small numbers. The remains of the marine fauna are, on the contrary, sufficiently numerous and well preserved to give us a great idea of its riches, and to enable us to a.s.sign to it a characteristic facies.

The sea of the Upper Cretaceous period bristled with numerous submarine reefs, occupying a vast extent of its bed--reefs formed of Rudistes (Lamarck), and of immense quant.i.ties of various kinds of corals which are everywhere a.s.sociated with them. The Polyps, in short, attain here one of the princ.i.p.al epochs of their existence, and present a remarkable development of forms; the same occurs with the Polyzoa (Bryozoa) and Amorphozoa; while, on the contrary, the reign of the Cephalopods seems to end. Beautiful types of these ancient reefs have been revealed to us, and we discover that they have been formed under the influence of submarine currents, which acc.u.mulated ma.s.ses of these animals at certain points. Nothing is more curious than this a.s.semblage of _Rudistes_--still standing erect, isolated or in groups--as may be seen, for instance, at the summit of the mountains of the _Cornes_ in the Corbieres, upon the banks of the pond of Berre in Provence, and in the environs of Martigues, at La Cadiere, at Figuieres, and particularly above Beausset, near Toulon.

”It seems,” says Alcide D'Orbigny, ”as if the sea had retired in order to show us, still intact, the submarine fauna of this period, such as it was when in life. There are here enormous groups of _Hippurites_ in their places, surrounded by Polyps, Echinoderms, and Molluscs, which lived in union in these animal colonies, a.n.a.logous to those which still exist in the coral-reefs of the Antilles and Oceania. In order that these groups should have been preserved intact, they must first have been covered suddenly by sediment, which, being removed by the action of the atmosphere, reveals to us, in their most secret details, this Nature of the past.”

In the Jura.s.sic period we have already met with these isles or reefs formed by the acc.u.mulation of Coral and other Zoophytes; they even const.i.tuted, at that period, an entire formation called the _Coral-rag_.

The same phenomenon, reproduced in the Cretaceous seas, gave rise to similar calcareous formations. We need not repeat what we have said already on this subject when describing the Jura.s.sic period. The coral or madrepore isles of the Jura.s.sic epoch and the reefs of Rudistes and Hippurites of the Cretaceous period have the same origin, and the _atolls_ of Oceania are reproductions in our own day of precisely similar phenomena.

The invertebrate animals which characterise the Cretaceous age are among

CEPHALOPODA.

_Nautilus sublaevigatus_ and _N. Danicus; Ammonites rostratus; Belemnitella mucronata._

GASTEROPODA.

_Voluta elongata; Phorus ca.n.a.liculatus; Nerinea bisulcata; Pleurotomaria Fleuriausa_, and _P. Santonensis; Natica supracretacea._

ACEPHALA.

_Trigonia scabra; Inoceramus problematicus_ and _I. Lamarckii; Clavigella cretacea; Pholadomya aequivalvis; Spondylus spinosus; Ostrea vesicularis; Ostrea larva; Janira quadricostata; Arca Gravesii; Hippurites Toucasia.n.u.s_ and _H. organisans; Caprina Aguilloni; Radiolites radiosus_, and _R. acuticostus._