CHAPTER XVI 



METAMORPHISM AND METAMORPHIC ROCKS 



By the term metamorphism is meant the profound transforma- 

 tion of a rock from its original condition by means other than those 

 of disintegration. The incipient changes of the latter class may- 

 very greatly modify a rock and its constituent minerals, but such 

 changes are distinguished from metamorphism under the term 

 alteration. Metamorphism usually implies an increase in hardness 

 and in the degree of crystallization, and very frequently also the 

 generation of an entirely new set of minerals, which take on a 

 characteristic arrangement. The degree of metamorphism varies 

 according to circumstances, and from the mere consolidation of 

 loose sediments to the most radical reconstruction of the rock, 

 there is every possible transition. Fossils may be found in those 

 metamorphic rocks of sedimentary origin, which have not been 

 completely changed. The more thorough the reconstruction of 

 the rock, the more obscure do the fossils become, and in advanced 

 stages nearly all trace of them is obliterated. 



For many years it was supposed that the metamorphic rocks 

 were one and all transformed sediments, but later investigations 

 have shown that many of them were originally igneous. Indeed, 

 it is often quite impossible to decide whether a given metamorphic 

 rock has been derived from a sedimentary or an igneous original. 

 This is not surprising, for the ultimate chemical (not the minera- 

 logical) composition of a basalt, a volcanic tuff, or a clay shale, 

 may be the same, and the metamorphic processes may produce 

 an identical rock from any one of these three as a starting-point. 

 Much yet remains to be learned regarding the modes, causes, and 

 results of metamorphism and some of the most far-reaching prob- 

 lems of geology are bound up with these questions. 



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