134 THEORY OP THE EARTH. 



since it has been laid dry for the last time, and 

 its continents have assumed their present form, 

 at least in such parts as are somewhat elevated 

 above the level of the ocean, it may be clearly seen 

 that this last revolution, and consequently the es- 

 tablishment of our existing societies, could not 

 have been very ancient. Thik result is one of the 

 best established and least attended to, in rational 

 zoology ; and it is so much the more valuable, as 

 it connects natural and civil history together ia 

 one uninterrupted series. 



When we endeavour to estimate the quantity 

 of effects produced in a given time by any causes 

 still acting, by comparing them with the effects 

 which these causes have produced since they be- 

 gan to operate, we may determine nearly the pe- 

 riod at which their action commenced 5 which 

 must necessarily be the same period with that in 

 which our continents assumed their presently exis- 

 ting forms, or with that of the last retreat of the wa- 

 ters. It must have been since that last retreat of 

 the waters, that the acclivities of our mountains 

 have begun to disintegrate, and to form slopes or 

 taluses of the debris at their bottoms and upon 

 their sides ; that our rivers have begun to flow in 

 their present courses, and to form alluvial depo- 

 sitions ; that our existing vegetation has begun to 

 extend itself, and to form vegetable soil ; that 

 our present cliflfe, or steep sloping coasts, have 

 feegun to be worn away by the waters of the sea ; 



