VARIATION IN METAMORPHISM. 921 



It is jjlain from the foregoing that a series of rocks may be profoundly 

 metamorphosed locally and that its metamorphism in an adjacent area 

 may be unimportant. Not only is this so, but different parts of the same 

 fold are often metamorphosed differently, depending upon their positions 

 within the fold. Still further, one set of beds may be more readily 

 metamorphosed than another, so that one formation may be a perfect schist 

 while the adjacent formation may still be plainly fragmental; for instance, 

 a shale may be changed to a mica-schist and an interbedded quartzite be 

 but little affected by any process except cementation. As pegmatization 

 is so frequently dependent upon the presence of an intrusive rock, an 

 extreme phase of metamorphism engendered by volcanism, combined 

 with other processes, may have a very local character. 



Notwithstanding the variations of metamorphism, the amount and char- 

 acter of the metamorphism of a series of formations may be an important 

 guide in structural work. It may serve to separate one series from another 

 and assist in determining the structure within a series. The criterion has 

 limitations, which are readily deducible from the facts as to the variation 

 in metamorphism just considered. But if a set of formations be superim- 

 posed upon another set and the lower has become schistose throughout, while 

 the upper shows little or no change, it is probable that the lower sei'ies 

 underwent a period of metamorphism before the upper series was deposited. 

 In order to make this conclusion at all certain, however, the superior for- 

 mations should be of kinds which are equally likely to be metamorphosed, 

 further faulting must be excluded, and it must be certain that it is a case of 

 superposition, for laterally the metamorphosing forces may die out rapidly 

 and the altered rocks pass quickly into those but little changed. Still fur- 

 ther, if the metamorphism be due to pegmatization as a consequence of the 

 intrusion of great batholiths, the process may die out rapidly in a vertical 

 direction, so that the extremely metamorphosed lower formations graduallv 

 pass into the unmetamorphosed beds higher up, with no structural break. 

 Also, it has been seen that metamorphism is a direct function of depth of 

 burying. But probably the change in metamorphism as a result of varying 

 depth of burial is so slow that it ordinarily involves no practical difficulty 

 in separating series. 



