Ch. XIX.] 
ALLUVIUMS OF DIFFERENT AGES. 
267 
derful alterations effected in the drainage of the country since 
the first current flowed ; for the most elevated alluviums must 
No. 61. 
Lavas of Auvergne resting on alluviums of different ages, 
originally have been accumulated in the lowest levels of the 
then existing surface. As some geologists have referred 
almost all the superficial gravels to one era, and have supposed 
them to be the result of one sudden catastrophe, the phe- 
nomena of Auvergne above alluded to are very important. 
The flows of volcanic matter have preserved portions of the 
surface in the state in which they existed at successive periods., 
so that it is impossible to confound together the alluviums of 
different ages. The reader will see at once by reference to 
the wood-cut (No. 61) that a considerable interval of time 
occurred between the formation of the uppermost bed of gravel 
and that next below it ; during which interval the uppermost 
lava was poured out and a valley excavated, at the bottom of 
which the second bed of gravel accumulated. In like manner 
the pouring out of a second current of lava, and a farther 
deepening of the valley, took place between the date of the 
second gravel and that of the modern alluvium which now fills 
the channel of the river *. 
* For localities in Central France where lavas or sheets of basalt repose on 
alluviums at different elevations above the present valleys, consult the works of 
MM. Le Grand d'Aussi, Montlosier, Ramond, Scrope, Bertrand de Doue, Croizet, 
Jobertj Bouillet, and others. 
