Gh. 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 Barrat, for example, we find an assemblage of calca- 
reous 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 Charee, or other plants, 
or sometimes myriads of small jialudince and other fresh- 
water 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 
mattei*, 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 remarks. — So wonderfully minute are the sepa- 
rate parts of which some of the most massive geological monu- 
ments are made up ! When we desire to classify, it is neces- 
sary 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. We 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. 
