24 THEORY OF THE EARTH. 



long been considered possible to explain the more 

 ancient revolutions on its surface by means of these 

 still existing causes ; in the same manner as it is 

 found easy to explain past events in political his- 

 tory, by an acquaintance with the passions and 

 intrigues of the present day. But we shall pre- 

 sently see, that unfortunately the case is different 

 in physical history : the thread of operations is 

 here broken ; the march of Nature is changed ; 

 and none of the agents which she now employs, 

 would have been sufficient for the production of 

 her ancient works. 



There still exist, however, four causes in full 

 activity, which contribute to alter the surface of 

 our continents. These are, rains and thaws, which 

 waste down the steep mountains, and precipitate 

 the fragments to their bottoms ; running waters, 

 which carry off these fragments, and deposit them 

 in places where their current is abated ; the sea, 

 which undermines the foundations of elevated 

 coasts, forming steep cliffs, and which throws up 

 great banks of sand upon the low coasts; and, last- 

 ly, volcanoes, which pierce through the solid strata 

 from below, elevate these strata, or spread over the 

 surface vast quantities of ejected matter *. 



* Regarding the changes of the surface of the earth,, known 

 from history or tradition, and consequently dependent on 

 causes still in operation, see the German work of M. de Hof, 



