268 EOCENE PERIOD. [Ch. XIX. 
When rivers are dispossessed of their channels by lava, they 
usually flow between the mass of Java 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. 
Age of the more modem 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 
either occupy the channels of the existing rivers or are 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 are 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 thePuy 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 spite 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 scorias, as we have shown to be the case on Etna (see 
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. 
Attempt to divide Volcanos into ante-diluvian and post-di- 
luvian. — The opinions above expressed are entirely at variance 
with the doctrines of those writers who have endeavoured to 
arrange all the volcanic cones of Europe under two divisions, 
