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 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. 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. 
