lY. METAMORPHIC ROCKS 



Before proceeding to describe m detail the metamorpMe rocks, 

 it will be well to devote a brief space to a discussion of the 

 processes by which this metamorphism has been brought about. 



The word metamorphism as used in geology includes changes 

 in the structure of rocks induced through agencies in part 

 physical, and in part chemical, in their nature. It is, in fact, 

 a very general term, and indicates any transformation taking 

 place in the composition and structural features of rocks 

 of any kind, whether sedimentary or igneous, and from any 

 cause whatever. Rocks laid down in the form of sediments may 

 become so deeply buried as to be subject to intense heat from 

 the earth's interior, as well as to pressure from weight of 

 the overlying material. In this way, a partial or complete 

 fusion of the constituents takes place, which is followed by a 

 crystallization whereby the original fragmental nature may be 

 wholly or in part obscured. This form of change is included 

 under the general name of regional metamorphism. In this 

 manner, it was once assumed, were formed the gneisses, a part 

 of the granites, and the vast series of crystalline schists and 

 calcareous rocks (marbles, etc.). It has, however, been shown 

 that the banded and foliated structure shown by gneisses and 

 schists is not necessarily an indication of an original bedded 

 structure, but may be due to pressure acting throughout long 

 periods of time, accompanied by the heat thereby generated. A 

 common and readily understood illustration of this principle of 

 metamorphism by compression is offered by the roofing slates. 

 These, first laid down as fine silts, rarely show their eminent 

 cleavages whereby they are rendered so useful to man, parallel 

 to their original bedding, but inclined at any and all angles 

 thereto. In such eases the bedding is usually indicated by the 

 dark bands or '^ ribbons" which are so evident on a split surface. 



But it is not alone the fragmental rocks which thus become 

 schistose under pressure. Originally massive, igneous rocks, 

 in regions of profound disturbances have been found converted 

 into schistose aggregates, indistinguishable from rocks ordinarily 



135 



