2(38 EOCENE PERIOD. [Ch. XIX. 



"When rivers arc dispossessed of their channels hy lava, they 

 usually flow between the mass of lava and one side of the 

 original valley. They then eat out a passage, partly through 

 the volcanic and partly through the older formation ; but as 

 the soft tertiary marls in Auvergne give way more readily 

 than the basalt, it is usually at the expense of the former that 

 the enlarging and deepening of the new valley is effected, and 

 all the remaining lava is then left on one side, in the manner 

 represented in the above wood-cut. 



A(jc of the more modern lavas. The only organic remains 

 found as yet in the ancient alluviums appear to belong to 

 the Miocene period ; but we have heard of none discovered 

 in the gravel underlying the newest lavas, those which 

 cither occupy the channels of the existing rivers or arc very 

 slightly elevated above them. We think it not improbable 

 that even these may be of Miocene date, although the con- 

 jecture will appear extremely rash to some who arc aware 

 that the cones and craters whence the lavas issue, are often 

 as fresh in their aspect as the majority of the cones of the forest 

 zone of Etna. 



The brim of the crater of the Puy de Pariou, near Clermont, 

 is so sharp, and has been so little blunted by time, that it scarcely 

 affords room to stand upon. This and other cones in an 

 equally remarkable state of integrity have stood, we conceive, 

 uninjured, not in spifc of their loose porous nature, as some 

 geologists might think, but in consequence of it. No rills can 

 collect where all the rain is instantly absorbed by the sand 

 and scoria: 1 , as we have shown to be the case on Etna (sec 

 above, p. 102), and nothing but a waterspout breaking directly 

 upon the Puy de Pariou could carry away a portion of the 

 hill, so long as it is not rent by earthquakes or engulphed. 



Atlrmjrf to divide Vulwnios into ante-diluvian and post-di- 

 luvian. The opinions above expressed arc entirely at variance 

 with the doctrine's of those writers who have endeavoured to 

 arrange all the volcanic cones of Europe under two divisions. 



