36 THEORY OF THE EARTH. 



Finally, if we examine with greater care these 

 remains of organized bodies, we shall discover, in 

 the midst even of the most ancient secondary stra- 

 ta, other strata that are crowded with animal or ve- 

 getable productions, which belong to the land and 

 to fresh water; and amongst the most recent stra- 

 ta, that is, the strata which are nearest the surface, 

 there are some of them in which land animals are 

 buried under heaps of marine productions. Thus 

 the various catastrophes of our planet have not 

 only caused the different parts of our continent to 

 rise by degrees from the basin of the sea, but it has 

 also frequently happened, that lands which had 

 been laid dry have been again covered by the water, 

 in consequence either of these lands sinking down 

 below the level of the sea, or of the sea being raised 

 above the level of the lands. The particular por- 

 tions of the earth also which the sea has abandoned 

 by its last retreat, had been laid dry once before, 

 and had at that time produced quadrupeds, birds, 

 plants, and all kinds of terrestrial productions ; it 

 had then been inundated by the sea, which has 

 since retired from it, and left it to be occupied by 

 its own proper inhabitants. 



The changes which have taken place in the pro- 

 ductions of the shelly strata have not, therefore, 

 been entirely owing to a gradual and general re- 

 treat of the waters, but to successive irruptions and 

 retreats, the final result of which, however, has 

 been an universal depression of the level of the sea. 



