220 LOWER MIOCENE STRATA OF FRANCE. [Ch. XIV. 



From the first I had advocated the doctrine that there has been a con- 

 tinual coming in of new species, and dying out of old ones, and a 

 gradual change in the physical geography and climate of the earth, 

 and not such a recurrence of sudden revolutions in the animate and 

 inanimate worlds, as was, in 1833, insisted upon by many English ge- 

 ologists of note, and is still maintained by some eminent continental 

 writers. I therefore foretold that from time to time new sets of strata 

 would come to light, and require to be intercalated between those al- 

 ready described, in which case the fossils of some of the newly-found 

 beds would " deviate from the normal types first selected, and approxi- 

 mate more and more to the types of the antecedent or subsequent 

 epochs." According to this view, .it was obvious from the first that 

 the oldest Miocene records, whenever they were detected, would not be 

 easily distinguishable from the youngest members of the Eocene series, 

 especially in the proportion of the living to the extinct species of 

 fossil shells. The importance, indeed, of the latter test must diminish 

 rapidly the more we recede from the Pliocene and approach the 

 Miocene, and still more the Eocene formations, although it is never 

 without its value, and often furnishes the only common standard of 

 comparison between strata of very distant countries. To this subject 

 of classification, or the line of demarcation between the Eocene and 

 Miocene strata, T shall again refer in this and the sixteenth chapter. 



Lower Miocene strata of Central France. — Lacustrine strata, belong- 

 ing, for the most part, to the same Miocene system as the Calcaire de 

 la Beauce, are again met with in Auvergne, Cantal, and Yelay, the sites 

 of which may be seen in the annexed map. They appear to be the 

 monuments of ancient lakes, which, like some of those now existing in 

 Switzerland, once occupied the depressions in a mountainous region, 

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

 country were 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 suf- 

 fered great denudation. There are also some vast piles of volcanic 

 matter (see the map), the greater part of which is newer than the 

 freshwater strata, and is sometimes seen to rest upon them, while a 

 small part has evidently been of contemporaneous origin. Of these 

 igneous rocks I shall treat more particularly in another part of this 

 work. 



Before entering into any details, I may observe that the study of 

 these regions possesses a peculiar interest, very distinct in kind from that 

 derivable from the investigation either of the Parisian or English ter- 

 tiary areas. For we are presented in Auvergne with the evidence of a 

 series of events of astonishing magnitude and grandeur, by which the 

 original form and features of the country have been greatly changed, 

 yet never so far obliterated but that they may still, in part at least, be 

 restored in imagination. Great lakes have disappeared — lofty moun- 

 tains have been formed, by the reiterated emission of lava, preceded and 



