Ch. XIV.] INDUSIAL LIMESTONE, AUVERGNE. 225 



Not far from Clermont, the green marls, containing the Cypris m 

 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. 165. 



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 

 Vichy, the oolite resembles our Bath stone in appearance and beauty ; 

 and, like it, is soft when first taken from the quarry, but soon hardens 

 on exposure to the air. At Gannat, the stone contains land-shells and 

 bones of quadrupeds. At Chadrat, in the hill of La Serre, the limestone 

 is pisolitic, the small spheroids combining both the radiated and concen- 

 tric structure. 



Indusial limestone. — There is another remarkable form of freshwater 

 limestone in Auvergne, called " indusial," from the cases, or inclusive, of 

 caddis-worms (the larvae of Phryganea) ; great heaps of which have 

 been incrusted, as they lay, by carbonate of lime, and formed into a hard 

 travertin. The rock is sometimes purely calcareous, but there is occa- 

 sionally an intermixture of siliceous matter. Several beds of it are fre- 

 quently seen, either in continuous masses, or in concretionary nodules, 

 one upon another, with layers of marl interposed. The annexed drawing 

 (fig. 166) will show the manner in which one of these indusial beds (a) 

 is laid open at the surface, between the marls (b b), near the base of the 

 hill of Gergovia ; and affords, at the same time, an example of the extent 

 to which the lacustrine strata, which must once have filled a hollow, have 

 been denuded, and shaped out into hills and valleys, on the site of the 

 indent lakes. 



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