70 IGNEOUS AGENCIES 



This conclusion we found strengthened by the oscillations of 

 level between land and sea, which, though extremely slow, are 

 seen to be still in progress. Historical geology will show us that 

 these changes of level have, in the course of ages, been effected 

 on the grandest scale. All the great continents are composed of 

 rocks, which, for the most part, were laid down in the sea and 

 still contain the fossils of marine animals, and this shows that 

 these continents have all been under the sea. Not that all the 

 continents, or even that all parts of any one continent, were sub- 

 merged at the same time, but now one part and now another was 

 overflowed and again emerged, until all have been covered in their 

 turn. 



In brief, the principal geological functions of the subterranean 

 agencies are two : (i) they bring up from below and form at the 

 surface, and at all depths beneath it, certain characteristic kinds of 

 rocks; and (2) they tend to increase the inequalities of the earth's 

 surface, and thus to counteract the agencies which are cutting 

 down the land and steadily tending to reduce it to the level of 

 the sea. 



