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. 
Q 2 
