876 



THE WONDERS OF GEOLOGY. Lect. VIII. 



organic, he can scarcely entertain any doubt upon the 

 subject. 



38. Chronology of Mountain-chains. — We have 

 seen that the intrusions of molten rocks have not only 

 altered the chemical nature of the strata through which 

 they were erupted, but have also changed their positions 

 and relations, and produced corresponding modifications in 

 the physical geography of the dry land ; having in some 

 instances transformed plains into mountain-peaks, and 

 in others occasioned the subsidence of elevated regions to 

 the bottom of the ocean. As these changes took place at 

 various epochs, separated from each other by periods of 

 repose, sometimes considerable, sometimes brief, it is mani- 

 fest that the existing mountain-chains are of very different 

 ages. By a careful examination of the phenomena which 

 bear upon this question, the relative antiquity of many of 

 the principal ranges has been determined ; or, in other 

 terms, it has been ascertained during what geological epochs 

 the Alps, Pyrenees, Andes, &c. were elevated above the 

 waters. 



My observations on this subject must, however, be 

 restricted to an explanation of the mode of induction 

 employed, and a brief notice of some of the results. The 

 positions of the older secondary strata in relation to the 

 protruded plutonic rocks and the newer sedimentary de- 

 posits, are the principal data by which this problem may 

 be solved ; for, as the secondary and tertiary formations 

 have been deposited in directions either nearly or entirely 

 horizontal, it is obvious, that when they are found highly 

 inclined, and in contact with mountain masses of primary 

 or volcanic rocks, the latter must have been protruded 

 since the sedimentary were formed, and of course during 

 the secondary or tertiary epochs, as the case may be. On 

 the contrary, if we find other strata in contact with the 

 same masses, but only touching them with their edges, or 



