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 Done, Croizet, 

 Jobert; Bouillet, and others. 



