THE STORY OF THE EARTH. 



iture termed "contact metamorphism," 



which result from highly heated rocks acting 



upon the sediments over which or through which 

 they flow. The changes produced by the action 



of the atmosphere and infiltrating water, which 



lk up minerals originated by heat or pressure, 



and elaborate others in their place, give rise to 



metamorphism." 



In the central regions of mountain chains, such 



the Grampians and the central cutis of Devon 



and Cornwall, schists sometimes pass into the 

 condition termed granite ; ^<» that there has some- 

 times seemed to be a relation of cause and effect 

 between the position in which the granite occurs, 



the way in which its mineral matter is ar- 



je<L 



Elites VI much in the minerals they 



include that they form a family of rocks dis- 



by chemical and mineral composition 



and texture. The minerals depend upon the 

 che:: nStitUentS. 'I he silica 



cent. t( r cent. The alumina t';«>m 7 



1 that the quart/, is e-unmonly 

 1 a fifth t<> a third of the bulk of I nte, 



thouj two-thirds 1 



onally be only 1 ; • ., though it 



The 

 :n between 40 and 70 ; the 



hornblei 



1 



: s< hist! 



: immei 



hOW that the e, 



e them an 

 through which it have been 



upon schist Grai 



