OUTLINES OF GEOLOGY. 



91 



sited in basins, forming the beds of fresh water lakes. The 

 formations of the superior order are also of the basin form, 

 and manifest the alternate prevalence of fresh and salt water. 

 Neither of these, however, lie in the position in which they 

 must have been at first deposited. 



The occurrence of similar basins, in which all the sedi- 

 mentary rocks have been deposited, is the only mode in 

 which it appears possible to account for the limited extent of 

 these several formations, and the fact that they often exclude 

 the basins of more modern origin. It is therefore evident, 

 that the relative levels of the surface of the land and ocean, 

 must have been many times changed, between the time to 

 which we can trace back the earliest geological formations, 

 and the last great catastrophe. 



That such changes of level have been of frequent occur- 

 rence, and are sufficient to account for these alternating in- 

 fluxes of the ocean is now demonstrated by the brilliant dis- 

 covery made by de Beaumont, of a method of determining 

 the geological age of mountains. 



This method rests upon the simplest principles : 



(I.) The ancient rocks of aqueous origin must have been 

 originally deposited in layers, nearly horizontal. No other 

 variation from this could have occurred, except in the great- 

 er thickness of the strata in the deeper parts of the basin in 

 which the formation took place. If, then, we find such rocks, 

 elevated to the tops of mountains, or lying on their sides in in- 

 clined positions, the inference is direct, that the mountains 

 must have been raised since the rocks were deposited. 



(2.) If, on examining the pebbles of gravel, or of conglo- 

 merate rocks, we find all the longer axes lying in a horizon- 

 tal position, it is obvious that the bed has not been disturbed ; 

 but if any of the axes occur in inclined positions, a disturb- 

 ance is to be inferred. 



