240 EOCENE PERIOD. [Ch. XVII, 



taining the remains of myriads of testacea and plants, fre- 

 quently enter into the composition of a single stratum, and 

 how great a succession of these strata unite to form a single 

 group ! We must remember, also, that volcanos like the 

 Plomb da Cantal, which rises in the immediate neighbour- 

 hood of Aurillac, are equally the result of successive accu- 

 mulation, consisting of reiterated flows of lava and showers of 

 scorine ; and we have shown, when we treated of the high an- 

 tiquity of Etna, how many distinct lava-currents and heaps of 

 ejected substances are required to make up one of the nume- 

 rous conical envelopes whereof a volcano is composed. Lastly, 

 we must not forget that continents and mountain- chains, 

 colossal as are their dimensions, are nothing more than an 

 assemblage of many such igneous and aqueous groups, formed 

 also in succession during an indefinite lapse of ages, and 

 superimposed upon each other. 



