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 hun- 
dred feet thick. A more convincing proof of the tranquillity 
and clearness of the waters, and of the slow and gradual pro- 
cess by which the lake was filled up with fine 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. 
Vertical strata of marl near Clermont, 
A t 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 
ancient lake in spaces intermediate between the points where 
rivers and torrents entered, no coarse ingredients were de- 
posited, but finer mud only was drifted by currents. The 
verticality of some of the beds in the above section bears tes- 
timony to considerable local disturbance subsequent to the 
deposition of the marls, but such inclined and vertical strata 
are very rare. 
4. Limestone, travertin, Spc. — Both the preceding members of 
the lacustrine deposit, the marls and grits, pass occasionally 
