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 Clermorit 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, 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 

 vertically 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, fyc. Both the preceding members of 

 the lacustrine deposit, the marls and grits, pass occasionally 



