CHAPTER IX. 



ROCKS. 



In the preceding chapters I have considered the forces and agents of 

 metamorphism, the divisions of the lithosphere into zones of metamorphism 

 upon a physical and chemical basis, the alterations of each of the important 

 rock-making minerals in reference to the zones in which they occur, and 

 the general nature of the alterations of rocks in the belt of weathering, 

 the belt of cementation, and the zone of anamorphism. It now remains 

 to consider the metamorphism of individual rocks, but for a number of 

 reasons it is impossible to do this part of the work satisfactorily at the 

 present time. 



In papers upon metamorphism there has in general been no attempt 

 whatever to consider the subject from the points of view set forth in the 

 previous chapters. The usual practice has been to describe the metamor- 

 phism of a given region or district and give various conjectures as to the 

 causes of the alterations, often with little or no reference to the different 

 belts and zones of metamorphism. To some extent the belt of weathering 

 has been recognized as having distinctive- reactions. The general neglect 

 of the principles of physics and chemistry in the consideration of meta- 

 morphism renders it impossible in most cases to interpret the descriptions 

 of the phenomena from the points of view discussed in this volume. If 

 the principles of metamorphism set forth in the foregoing chapters are well 

 founded they should be considered in the study of individual instances. 

 If these principles be recognized it will be comparatively easy to describe 

 a given case of metamorphism in reference to them. When this work is 

 done for a sufficient number of districts it will then be possible to write a 

 satisfactory chapter upon the metamorphisrn of individual rocks. 



A second insuperable difficulty preventing- a s}^stematic treatment of the 

 metamorphic igneous rocks is the status of the classification of these rocks. 

 At the present time there is not only no consensus of opinion concerning 



