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 du 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 
scorias ; 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. 
