DENUDATION OF VOLCANIC REGIONS. 199 



Andes by Charles Darwin 1 and David Forbes 2 to recognise in the 

 vast inversion and incessant crumpling of the rocks, demonstration 

 that before the era of existing volcanic activity, enormous, almost 

 inconceivable, denudation must have taken place. This denudation, 

 which has bared the granitic rocks and exposed the rich metalliferous 

 veins at the surface, has reduced by no infinitesimal amount the 

 weight of superincumbent rock which had to be rent to form the 

 lines of volcanic fracture, and has lessened the height to which 

 volcanic forces heaved the products of their energy. 



The same phenomena may be observed in all volcanic regions, so 

 that the manifestations of volcanic action which we have been con- 

 sidering may be held to depend not only upon the internal heat of 

 the earth, but also in some degree upon the agencies which act upon 

 its surface. 



In the previous chapter we examined the ways in which heat acts 

 upon and changes the surface rocks ; we have now seen the nature 

 of the changes which are produced on the materials thus altered by 

 agencies at the earth's surface, and it only remains to inquire how 

 far an examination of the products of volcanic action justifies the 

 interpretation which has been here given. 



1 "Geological Observations on South America." 



2 Q. J. G. S., vol. xvii. p, 7, 



