RELATIONS OF METASOMATISM TO TEXTURE. 645 



emphasize them. This emphasizing of old textures and structures results 

 from the fact that solutions work along openings and surfaces of weakness. 

 At any place in which water is present in more than the average amount, 

 or is more than usually active, there may be greater than average solution 

 and deposition, and thus emphasis of the old texture or structure. Common 

 cases are the emphasis of perlitic cracks and bedding planes. 



However, where large individuals alter to many small particles, or 

 undergo a secondary enlargement with needle-like terminations, or are 

 altered in various other ways, the original textures may become much less 

 definite than they were originally, although the process of modification 

 rarely goes so far as to obliterate original textures. 



As a result of the preservation or emphasis of original textures and 

 structures during- metasomatism in the belt of cementation, it may happen 

 that somewhat extensive changes in a rock are\ overlooked or ignored. 

 Those who are most familiar with the recent little modified rocks are 

 inclined to explain the phenomena they see in them as original. Those 

 who have been working among the ancient and therefore more modified 

 rocks are inclined to explain similar phenomena as the result of alteration. 

 In each case the phenomena must be studied in the field and in the labora- 

 tory, taking into account all the evidence, in order to ascertain the actual 

 truth ; for it is certain that such phenomena as amphibole surrounding 

 pyroxene cores and pegmatitic textures may be due to primary crystalliza- 

 tion or to secondary alteration, and the appearance in the two cases be 

 much the same, if not identical. 



SEGREGATION OF INDIVIDUAL MINERALS. 



Under exceptional conditions, by alteration or substitution or the two 

 combined one mineral may very largely or wholly replace the minerals 

 previously occupying a certain space. The space thus taken may vaiy 

 from an insignificant amount to a considerable area for the entire thickness 

 of a formation. In the latter case the process of metasomatism is often 

 given a name dependent upon the mineral which replaces the other 

 minerals, or that into which the other minerals alter. If, for instance, the 

 segregating compound be silica, the process is called silicification; if it be 

 serpentine, it is called serpentinization; if it be chlorite, it is called 

 chloritization, etc. 



