﻿474 DR. WHITMAN CROSS 03 THE NATURAL [Aug. IQIO, 



that the observed variation in composition of the rocks of different 

 parts of the earth is of this origin. 



Differentiation and assimilation are in a measure 

 antithetical processes. The former must proceed by laws ; 

 the latter is not related to the former in any definite way, and its 

 effect may be regarded as determined by chance.' Assimilation 

 is not uncommonly assumed to have occurred in masses open to our 

 examination and at the place where the rock is now seen. My own 

 experience leads me to question the correctness of this assumption 

 except in rare instances, but it is difficult for me to conceive that 

 fusion does not take place at lower levels. If assimilation occurs 

 within a magma-basin w r here differentiation has been in progress, 

 it may affect only products of that basin, and thus merely disturb 

 the ordinary sequence of descendant magmas. Bat if other 

 materials, such as sediments, deeply infolded or faulted, should be 

 dissolved by a magma and assimilated, it is plain that the composi- 

 tion of the new solution may be radically changed. Quartzite, 

 limestone, shale, or old volcanic rocks of different composition would 

 modify the character of an assimilating magma in very different 

 ways, and quite independently of the earlier history of that magma. 



If the ideas of isostatic adjustment within the earth's crust, by 

 lateral flow of magma, now receiving much support from geo- 

 physicists, are correct, it appears quite possible that magmas of a 

 given character may be moved to places where they may attack and 

 assimilate materials of different character. When this new molten 

 solution resolves by differentiation into parts, the products may be 

 notably different from those which would have formed under the 

 earlier conditions. 



If the deep-seated magmas of large volume have 

 acquired their various chemical characters in different- 

 ways, it appears at once evident that this primary 

 genetic factor cannot be used in classification, unless 

 the characters of different origin can be distinguished 

 in the rocks. It seems to me clear that the antithetical and 

 wholly independent processes of differentiation and assimilation 

 cannot be recognized in their resultant rocks, except in the rela- 

 tively rare cases where they occurred at places subsequently re- 

 vealed to our examination. 



If certain rocks can be recognized as deriving their chemical 

 characters from magmatic differentiation (or from assimilation), it 

 is logically possible to introduce this factor into classification by a 

 primary division into : (a) rocks due to magmatic differentiation 

 (or to assimilation) ; and (b) rocks not demonstrably of such origin. 

 But such a proposition would hardly be seriously considered as 

 suitable for general petrographic system. 



Despite the inherent difficulties of genetic classification, many 

 attempts have been made to render systematic classification natural 

 by using factors of origin and occurrence, and it is now desired to 

 review the various propositions of this kind. 



