Ch. XVII.] LACUSTRINE STRATA AUVERGNE. 227 



annexed Map *. They appear to be the monuments of ancient 

 lakes which may have resembled in geographical distribution 

 some of those now existing in Switzerland, and may like them 

 have occupied the depressions in a mountainous country,, and 

 have been each fed by one or more rivers and torrents. The 

 country where they occur is almost entirely composed of 

 granite, and different varieties of granitic schist, with here 

 and there a few patches of secondary strata much dislocated, 

 and which have probably suffered great denudation. There 

 are also some vast piles of volcanic rock, (see the Map,) the 

 greater part of which are newer than the fresh-water strata, 

 often resting upon them, whilst a small part were evidently of 

 contemporaneous origin. Of these igneous rocks we shall treat 

 more particularly in the nineteenth chapter, and shall first turn 

 our attention exclusively to the lacustrine beds. 



The most northern of the fresh -water groups is situated in 

 the valley-plain of the Allier, which lies within the department 

 of the Puy de Dome, being the tract which went formerly by 

 the name of the Limagne d'Auvergne. It is inclosed by two 

 parallel primitive ranges, that of the Forez, which divides the 

 waters of the Loire and Allier, on the east, and that of the Monts 

 Domes, which separates the latter river from the Sioule, on the 

 westf . The average breadth of this tract is about ^20 miles, 

 and it is for the most part composed of nearly horizontal strata 

 of sand, sandstone, calcareous marl, clay, limestone, and some 

 subordinate groups, none of which observe a fixed and inva- 

 riable order of superposition. The ancient borders of the lake, 

 wherein the fresh-water strata were accumulated, may generally 

 be traced with precision, the granite and other ancient rocks 

 rising up boldly from the level country. The precise junc- 

 tion, however, of the lacustrine and granitic beds is rarely seen, 

 as a small valley usually intervenes between them. The fresh- 



* The following account of the fresh-water formations of Central France is the 

 result of observations made in the summer of 1828, in company with Mr. Mur- 

 chison. 



f Scrope, Geology of Central France, p. 15. 



Q2 



