Ch.XXIV.] OF MOUNTAIN-CHAINS. 351 



mountain -chain to the base of another, the beds being vertical 

 or inclined in one chain, and horizontal in the other. We 

 might then decide with confidence, according to the method 

 proposed by M. de Beaumont, on the relative eras when these 

 chains had undergone disturbance ; and from one point thus 

 securely established, we might proceed to another, until we 

 had determined the dates of many neighbouring lines of 

 convulsion. 



We fear that the cases are rare where such evidence can be 

 obtained; and, for the most part, we can identify the age 

 of strata, not by their continuity and homogeneous mineral 

 character, but by organic remains. When by their aid we 

 prove strata to be contemporaneous, we must generally speak 

 with great latitude, merely intending that they were deposited 

 in the same geological epoch during which certain animals and 

 plants flourished. 



