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Show 238 EOCF:NE PERIOD• (Ch. XVII. the appearances so frequently presented nn the furrowed sur. face of our white chalk. Proceeding onwards from these quai.n :e s, a1 o no('/' a road made of the white limestone, which re. fleets as glaring a light in the sun, as do our roads composed of chalk, we reach, at length, in the neighbourhood of Am·illac, hills of limestone and calcareous marl, in horizontal strata, separated in some places by regular layers of flint in nodules, the coating of each nodule being of an opaque white colour, like the exterior of the flinty nodules of our chalk. In these last the hard white substance has been ascertained to consist, in some instances, wholly of siliceous matter, and sometimes to contain a small admixture of carbonate of lime*, and the ana· lysis of those of the Cantal would probably give the same results. The Aurillac flints have precisely the appearance of having separated from their matrix after the siliceous and cal· careous matter had been blended together. The calcareous marl sometimes occupies small sinuous cavities in the flint, and the siliceous nodule, when detached, is often as irregular in form as those found in our chalk. By what means, then, can the geologist at once decide that the limestone and silex of Am·illac are referrible to an epoch entirely distinct from that of the English chalk? It is not by reference to position, for we can merely say of the lacustrine beds, as we should have been able to declare of the true chalk had it been present, that they overlie the granitic rocks of this part of France. It is by reference to the organic remains that we are able to pronounce the formation to belong to the Eocene tertiary period. Instead of the marine Alcyonia of our cretaceous system, the silicified seed·vessels of the Chara, a plant which gt·ows at the bottom of lakes, abound in the flints of Aurillac, both in those which are in situ and those forming the gravel. Instead of the Echinus and marine testacea of the chalk, we find in the marls and limestones the shells of the Planorbis, and other lacustrine testacea, all of * Phillips, GeoL . Trans. First Series, vol. v. p. 22.-0utlines of Geology, p. 95. ~h. XVII.] LACUSTRINE STRATA-CANTAL. 239 them, like the gyrogonites, agreeing specifically with species of the Eocene type . Proofs of the gradual deposition of marl.-Some sections of the foliated marls in the valley of the Cer, near Aurillac, attest, in the most unequivocal manner, the extreme slowness with which the materials of the lacustrine series were amassed. In the hill of Harrat, for example} we find an assemblage of calcareous and siliceous marls, in which, for a depth of at least 60 feet, the layers are so thin that thirty are sometimes contained in the thickness of an inch; and when they are separated we see preserved in each the flattened stems of Charoo, or other plants, or sometimes myriads of small paludinm and other freshwater shells. These minute foliations of the marl resemble precisely some of the recent laminated beds of the Scotch marl lakes, and when divided may be compared to the pages of a book, each containing a history of a certain period of the past. The different layers may be grouped together in beds from a foot to a foot and a half in thickness, which are distinguished by differences of composition and colour, the latter being white, green, and brown. Occasionally there is a parting layer of pure flint, or of black carbonaceous vegetable matter, one inch thick} or of white pulverulent marl. We find several hills in the neighbourhood of Aurillac composed of such materials for the height .of more than 200 feet from their base, the whole sometimes covered by rocky currents of trachytic or basaltic lava *. Concluding remarlcs.-So wonderfully minute are the separate parts of which some of the most massive geological monuments are made up ! When we desire to classify, it is necessary to contemplate entire groups of strata in the aggregate; but if we wish to understand the mode of their formation, and to explain their origin, we must think only of the minute subdivisions of which each mass is composed. 'Ve must bear in mind how many thin, leaf.like seams of matter, each con- * Lyell and Murchison, sur les Depots Lacust. Tertiaires du Cantal, &c. Ann, des Sci. Nat., Oct. 1829. - |
