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 ma- 

 terials as sandstone, but the component parts are in a finer 

 state of subdivision. The same primary rocks, therefore, of 

 Auvergne, which, by the partial degradation 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 horn- 

 blende, 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 cer- 

 tainly 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 cyprisy 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. This 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. 



