OCR Text |
Show 230 EOCENE PERIOD. [Ch. XVII. -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 were derived. The red beds, although destitute of organic remains, pass upwards into strata containing Eocene fossils, and are certainly an integral part of the lacustrine for-mation. 3. Green and white foliated marls.-A great portion of what we term clay in ordinary language, consists of the same materials as sandstone, ·but the component parts are in a finer state of subdivision. The same primary rocks, therefore, of Auvergne, which, by the partial deg-radation of their harder parts, gave rise to the quartzose grits and conglomerates before mentioned, would, by the reduction of the same 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. 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 plates or scales of that small animal called cypris, a genus which comprises several species, of which some are recent, and may be seen swimming rapidly through the waters of our stagnant pools and ditches. 'J:'his animal resides within two small valves like those of a bivalve shell, and it moults its integuments annually, which the conchiferous • See vol. i. chap. xiii. Ch. XVII.] LACUSTRINE STRATA-AUVERGNE. 231 molluscs do not. This circumstance may partly explain the countless myriads of the shells of cypris which were shed in the Eocene lakes, 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 clearn~ss of the waters, and of the slow and gradual process by whiCh the lake was filJed up with fino mud, cannot be desired. We may easily suppose that, while in the deep and central parts of the basin, this fine sediment was thrown down gravel, sand, and rocky fragments were hurried into the lake' near the shore, and formed the group first described. 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 annexed section occurs at Champradelle, in a small ravine north of La petite Baraque, and above the bridge. No. 57. A Vertical strata of ma1·l nea'l' Clermont. A, Granite. C, Green marl, vertical and inclined. B, Space of 60 feet in which no section is seen. D. White marl. The occurrence of these marls so near the ancient margin may be explained by considering that, at the bottom of the a~cient lake in spaces intermediate between the points where nv~rs and torrents entered, no coarse ingredients were deposi~ ed,. but finer mud only was drifted by currents. The -~ertwaltty of some of the beds in the above section bears tesbmon!. to considerable local disturbance subsequent to the deposition of the marls, but such inclined and vertical strata are very rare. 4. Limestone, travertin, ~c.-Both the precedincr members of the I t • d · 0 acus l'lne epostt, the marls and grits, pass occasionally |