234 
EOCENE PERIOD. 
[Ch. 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. 
W e suppose that, when the ancient lake of the Limagne first 
began to be filled with sediment, no volcanic action had pro- 
duced lava and scorise 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 
