Ch. XV.] LACUSTRINE STRATA — AUVERGNE. 199 



amassed near the northern shores, so in Auvergne the grits and con- 

 glomerates before mentioned were evidently formed near the borders. 



The entire thickness of these marls is unknown ; but it certainly ex- 

 ceeds, in some places, TOO 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 shells, or 

 carapace-valves, of that small animal called Cypris. This animal is pro- 

 vided with two small valves, not unlike those of a bivalve shell, and 

 moults its integuments periodically, which the conchiferous mollusks do 

 not. This circumstance may partly explain the countless myriads of the 

 shells of Cypris which were shed in the ancient lakes of Auvergne, so as 

 to give rise to divisions in the marl as thin as paper, and that, too, in 

 stratified masses several hundred feet thick. A more convincing proof of 

 the tranquillity and clearness of the waters, and of the slow and gradual 

 process by which the lake was filled up with fine mud, cannot be desired. 

 But we may easily suppose that, while this fine sediment was thrown 

 down in the deep and central parts of the basin, gravel, sand, and rocky 

 fragments were hurried into the lake, and deposited near the shore, form- 

 ing the group described in the preceding section. 



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 occurrence of these marls so near the 

 ancient margin may be explained by considering that, at the bottom of 

 the ancient lake, no coarse ingredients were deposited in spaces inter- 

 mediate between the points where rivers and torrents entered, but finer 



Fig. 177. 



Vertical strata of marl, at Champradelle, near Clermont. 



A. Granite. B. Space of 60 feet, in which no section is seen. 



C. Green marl, vertical and inclined. D. White marl. 



mud only was drifted there by currents. The verticality of some of the 

 beds in the above section bears testimony to considerable local disturb- 

 ance subsequent to the deposition of the marls ; but such inclined and 

 vertical strata are very rare. 



3. Limestone, travertin, oolite. — Both the preceding members of the 

 lacustrine deposit, the marls and grits, pass occasionally into limestone. 

 Sometimes only concretionary nodules abound in them ; but these, where 

 there is an increase in the quantity of calcareous matter, unite into reg- 

 ular beds. 



On each side of the basin of the Limagne, both on the west at Gan- 

 nat, and on the east at Vichy, a white oolitic limestone is quarried. At 



