CHAPTER IV. 



METAMORPHIC ROCKS. 



WE have now finished both the stratified and the un- 

 stratified rocks, but there is yet an intermediate series 

 which must be described. These are stratified like the 

 stratified rocks, but crystalline in texture, and usually 

 destitute of fossils, like the igneous rocks. They are 

 supposed to have been formed from sediments like strati- 

 fied rocks, but have been subsequently changed by heat 

 and other agencies. They are therefore called meta- 

 morpfiic rocks. They may be traced by gradations, on 

 the one hand, into stratified, and, on the other, into 

 igneous rocks. 



Extent and Thickness. They cover large areas, es- 

 pecially among the oldest rocks and along axes of great 

 mountain-chains. The whole of Labrador, the larger por- 

 tion of Canada, the whole eastern slope of the Appala- 

 chian, and also the axes of the Colorado and Sierra, con- 

 sist of them. In Canada they are supposed to be 40,000 

 to 50,000 feet thick and very much crumpled. Meta- 

 morphism is nearly always associated with great thickness 

 and crumpling. 



Age. The oldest rocks are all metamorphic. Hence 

 many regard it as a sign of age. But it is probably more 

 correct to say that metamorphism is found in rocks of all 

 ages if only they be very thick and very much crumpled ; 

 but, since great thickness and complex crumplings are 

 most common in the oldest rocks, so also is metamorphism. 



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