VARIATION IN METAMORPHISM. 917 



morphosed igneous rocks it is plain that all the criteria above mentioned 

 should be applied. But where there are intricate mixtures of metamorphosed 

 igneous and sedimentary rocks, as a result of injection, all these criteria 

 may fail to a greater or less extent, for the rocks are not wholly sedimentary 

 or igneous, but a combination of the two. 



It is therefore clear that there is frequently very great difficulty in 

 determining the origin of the schists and gneisses, and that the criteria 

 which are at present available are inadequate. However, it is believed 

 that a close study of the character of the changes in each of the well- 

 recognized igneous and sedimentary rocks will in the future furnish other 

 important guides which may be used to discriminate between the two great 

 classes of metamorphosed rocks. 



RELATIONS OF JIBTAMOEPHIC SEDIMENTARY ROCKS TO STRATIG- 

 RAPHY. 



VARIATION IN METAMORPHISM. 



In working out the stratigraphy of a metamorphosed region the fact 

 that the metamorphism varies in degree under different conditions is of the 

 utmost importance. 



UPON WHAT VARIATIONS ARE DEPENDENT. 



The variation in the amount and kind of metamorphism is dependent 

 upon many factors (see pp. 39-44), but from the point of view of structure 

 it is necessary to consider only variations depending upon the amount of 

 orogenic movement, upon the character of the rock, and upon the depth 

 of burying. 



That the degree of metamorphism is largely dependent upon orogenic 

 movement has been sufficiently shown on previous pages. 



It has been seen on pages 760-762 that where rocks consist of alternate 

 layers of different character, these are slow to be obliterated, and especially 

 is this so if the formations be thick. To illustrate, it may be said that it is 

 very difficult, and perhaps impossible, to completely destroy a great lime- 

 stone or quartzite formation by metamorphism. Also, coarse and strong 

 sedimentary formations are likely to retain their original features to a 

 recognizable degree unless the metamorphism be extreme. The existence 

 of such formations enables one to determine the sedimentary character of 



