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, xxiy, 



