266 EOCENE PERIOD. XIX. 
carried away by the force of running water, the cone whence 
the lava issued escaped destruction, because it stood upon a 
platform of gneiss several hundred feet above the level of the 
valley in which the force of running water was exerted. 
It is needless to multiply examples, or the Vivarais would 
supply many others equally striking. Among many we may 
instance the cone of Jaujac, and its lava current *» which is a 
counterpart of that near Pranal last mentioned. 
Lavas and Alluviums of different Ages. — We have seen that 
on the flanks of Etna, since the commencement of the present 
century, several currents of lava have flowed at the bottom of 
the Val del Bove, at the foot of precipices formed of more 
ancient lavas and tuffs. So we find in Auvergne that some 
streams of melted matter have flowed in valleys, the sides of 
which consist partly of older lavas. These are often seen 
capping the hills in broad sheets, resting sometimes on granite, 
sometimes on fresh-water strata. 
Many of the earlier lavas of Auvergne have flowed out upon 
the platform of granite before all the existing valleys had been 
excavated ; others again spread themselves in broad sheets over 
the horizontal lacustrine deposit, when these had been covered 
with gravel, probably soon after the drainage of the lakes. 
Great vicissitudes in the physical geography of the country 
must have taken place since the flowing of these ancient lavas ; 
and it is evident that the changes were gradual and suc- 
cessive, caused probably by the united agency of running- 
water and subterranean movements. We frequently observe 
one mass of lava capping a hill, and a second at a lower eleva- 
tion, forming a terrace on the side of a valley ; or sometimes 
occupying the bed of a river. 
It is a most interesting fact that we almost invariably find 
in these cases beds of gravel underlying the successive currents 
of lava, as in Catalonia before described (pp. 189, 190). Occa- 
sionally, when the highest platform of lava is 700 or 800 feet 
above the lowest, we cannot fail to be struck with the won- 
* See Scrope's Central France, plate 14. 
