200 



INDUSIAL LIMESTONE. 



[Ch. XV. 



Vicliy, 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 indusi^^ ot 

 caddis-worms (the larvse of Phrijganea) ; 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. 17 8) will show the manner in which one of these indusial beds (a) 

 is laid open at the surface, between the marls {h 6), 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 

 ancient lakes. 



Fiff. ITS. 



Bed of indusial limestone, interstratified with freshwater marl, near Clermont (Kleinschrod). 



We may often observe in our ponds the Phryganea (or Caddice-fly), 

 :n its caterpillar state, covered with small freshwater shells, which they 

 have the power of fixing to the outside of their tubular cases, in order, 

 probably, to give them weight and strength. The individual figured in 



