114 
NEWER PLIOCENE PERIOD. 
[Ch. IX. 
leys by rivers and land-floods would proceed in the same man- 
ner as in modern times in Calabria, according to our former 
description *. 
Before a tract could be upraised to the height of several 
thousand feet above the level of the sea, the joint operation of 
running water and subterranean movements must greatly mo- 
dify the physical geography ; but when the action of the vol- 
canic forces has been suspended, when a period of tranquillity 
succeeds, and the levels of the land remain fixed and stationary, 
the erosive power of water must soon be reduced to a state of 
comparative equilibrium. For this reason, a country that has 
been raised at a very remote period to a considerable height 
above the level of the sea, may present nearly the same external 
configuration as one that has been more recently uplifted to 
the same height. 
In other words, the time required for the raising of a mass 
of land to the height of several hundred yards must usually be 
so enormous (assuming as we do that the operation is effected by 
ordinary volcanic forces), that the aqueous and igneous agents 
will have time before the elevation is completed to modify the 
surface, and imprint thereon the ordinary forms of hill and 
valley, by which our continents are diversified. But after the 
cessation of earthquakes these causes of change will remain 
dormant, or nearly so. The greater part, therefore, of the 
earth's surface will at each period be at rest, simply retaining 
the features already imparted to it, while smaller tracts will 
assume, as they rise successively from the deep, a configuration 
perfectly analogous to that by which the more ancient lands 
were previously distinguished. 
Migration of animals and plants. — The changes which, 
according to the views already explained, have been brought 
about in the earth's crust by the agency of volcanic heat, can- 
not fail to strike the imagination, when we consider how recent 
in the calendar of nature is the epoch to which we refer them. 
But if we turn our thoughts to the organic world, we shall feel, 
* Chap, xxiv. 
