234 EOCENE PERIOD. [Gh. XVII. 



We never meet with calcareous rocks covered by a consider- 

 able thickness of quartzose sand or green marl, and the up- 

 permost marls and sands are more calcareous than the lower. 

 From the resemblance of the Eocene limestones of Auvergne 

 to the Italian travertins, we may conclude that they were de- 

 rived from the waters of mineral springs, such springs as now 

 exist in Auvergne, and which rising up through the granite 

 precipitate travertin. They are sometimes thermal, but this 

 character is by no means constant. 



We suppose that, when the ancient lake of the Limagne first 

 began to be filled with sediment, no volcanic action had pn> 

 duced lava and scorias on any part of the surface of Auvergne. 

 No pebbles, therefore, of lava were transported into the lake, 

 no fragments of volcanic rocks imbedded in the conglomerate. 

 But at a later period, when a considerable thickness of sand- 

 stone and marl had accumulated, eruptions broke out, and lava 

 and tuff were alternately deposited, at some spots, with the lacus- 

 trine strata. Of this we shall give proofs in the 19th chapter. 

 It is not improbable that cold and thermal springs, holding 

 different mineral ingredients in solution, increased in number 

 during the successive convulsions attending this development 

 of volcanic agency, and thus carbonate and sulphate of lime, 

 silex, and other minerals, were produced. Hence these mine- 

 rals predominate in the uppermost strata. The subterranean 

 movements may then have continued until they altered the 

 relative levels of the country and caused the waters of the lakes 

 to be drained off, and the farther accumulation of regular 

 fresh-water strata to cease. The occurrence of these convul- 

 sions anterior to the Miocene epoch, and prolonged during 

 a succession of after-ages, may explain why no fresh-water for- 

 mations more recent than the Eocene are now found in this 

 country. 



We may easily conceive a similar series of events to give rise 

 to analogous results in any modern basin, such as that of Lake 

 Superior, for example, where numerous rivers and torrents are 

 carrying down the detritus of a chain of mountains into the 



