918 A TREATISE ON METAMORPHISM. 



the rocks and often the true strike and dip. In a metamorphosed series the 

 contacts between formations of different character are the most reliable 

 criteria for correct structural observations. 



It has been explained (Chapter IV, p. 187) that a formation in the zone 

 of katamorphism is merely fractured, instead of being mashed throughout, 

 and therefore if rocks are deformed under conditions of moderate burying 

 it is usually easy to determine the origin of the rocks with which one is 

 dealing. However, where rocks are deformed at a small depth, faulting is 

 likely to occur, and thus while the problem of stratigraphy is made easier, 

 in that the formations are not so greatly altered, it is made more difficult in 

 that faulting is always a possibility which has to be taken into account. 



Rocks which were deeply buried when deformed are mashed, and 

 therefore under . these conditions the profoundest kind of metamorphism 

 occurs and all the difficulties which result from extreme metamorphism are 

 introduced. On the other hand, where mashing is the typical kind of 

 deformation faulting is not likely to occur. But it is to be remembered 

 that a series which is mashed before it reaches the surface must ]jass through 

 the upper zone, and during this passage may again be subjected to dynamic 

 movements and thus be broken and faulted. Consequently, combined with 

 extreme metamorphism we may have also displacement, and in such cases 

 the difficulties of stratigraphy are at a maximum. 



RESULTING VARIATIONS. 



In handling the various difficulties in stratigraphy which result from 

 metamorphism the variations in amount and kind are of great assistance. 

 Such changes in the nature of metamorphism are to be considered both (1) 

 along the strike, and (2) across the strike. 



(1) A series or formation which is profoundly metamorphosed in one part 

 of a district or region may be but slightly metamorphosed in another part. 

 The change in metamorphism is likely to be gradual along the strike. Con- 

 sequently, by following a formation along the strike, one may solve the 

 problem of the origin of completely metamorphosed formations, for some- 

 where he may find a locality where it is so little metamorphosed that its 

 original character may be determined. In the less altered area the true 

 succession may be made out, and thus it may be possible to interpret an 



