Ch. XVII.] 



LACUSTRINE STRATA AUVERGNE. 229 



Occasionally, when the grits rest on granite, as at Chama- 

 lieres before mentioned, and many other places, the separate 

 crystals of quartz, mica, and felspar, of the disintegrated 

 granite, are bound together again by the silex, so that the 

 granite seems regenerated in a new and even more solid form, 

 and thus so gradual a passage may sometimes be traced be- 

 tween a crystalline rock and one of mechanical origin, that we 

 can scarcely distinguish where one ends and the other begins. 



In the Puy de Jussat, and the neighbouring hill of La 

 Roche, are white quartzose grits, cemented into a sandstone 

 by calcareous matter, which is sometimes so abundant as to 

 form imbedded nodules. These sometimes constitute sphe- 

 roidal concretions six feet in diameter, and pass into beds of 

 solid limestone resembling the Italian travertins, or the de- 

 posits of mineral springs. 



In the hills above mentioned, we have the advantage of see- 

 ing a section continuously exposed for about 700 feet in thick- 

 ness. At the bottom are foliated marls, white and green, 

 about 400 feet thick, and above, resting on the marls, are the 

 quartzose grits before mentioned with the associated travertins. 

 This section is observed close to the confines of the basin, so 

 that the lake must here have been filled up near the shore with 

 fine mud, before the coarse superincumbent sand was intro- 

 duced. There are other cases where sand is seen below the 

 marl. 



2. Red marl and sandstone. But the most remarkable of the 

 arenaceous groups is a red sandstone and red marl, identical in 

 all their characters with the secondary new red sandstone and 

 marl of England. In the latter, the red ground is sometimes 

 variegated with light greenish spots, and the same may be seen 

 in its tertiary counterpart of fresh- water origin at Coudes, on 

 the Allier. The marls are sometimes of a purplish-red colour, 

 as at Champheix, and are accompanied by a reddish limestone, 

 like the well-known ( cornstone,' which is associated with the 

 old red sandstone of English geologists. The red sandstone 

 and marl of Auvergne have evidently been derived from the 



