METAMORPHIC STUDIES 603 



their composition, like shales, to the production of the necessary 

 schist-making minerals. But even in these cases the elimination 

 of water, carbon dioxide, and oxygen is recognized. We think it 

 can be likewise inferred, in cases where the composition of the 

 parent rock differs widely from the composition of the hornblende, 

 mica, or chlorite, that important chemical changes take place, 

 principally by the elimination of the substances present in excess 

 of these requirements, but possibly also in some cases by addition 

 of substances from without, and that there is therefore a chemical 

 as well as mineralogical convergence toward mineral types. 



Illustrative of such changes is the diminution of carbon dioxide, 

 and perhaps lime, when a marble becomes silicated. In extreme 

 cases an amphibolite has been described as the end-result of the 

 alteration of marble. This obviously means a considerable change 

 in chemical composition. For the purposes of our argument it is 

 immaterial whether this change consists dominantly of elimination 

 or of substitution of constituents from without along igneous con- 

 tacts. Rock ilowage and anamorphism are often accomplished 

 under conditions which make it impossible to determine the relative 

 importance of the purely mechanical and of the igneous influence 

 respectively in their production. The point we would emphasize 

 is that the characteristic product like amphibolite or silicated 

 marble resulting from rock flowage or anamorphism differs dis- 

 tinctly in composition from the primary limestones or marble. 



The development of talc schist from the anamorphism of a 

 dolomite indicates an important change of composition, like that 

 in the Menominee district of Michigan.^ 



A quartzite often becomes sheared and transformed into a seri- 

 cite schist. Where shearing follows impure phases of the quartz- 

 ite parallel to the bedding, it is difficult to prove any change in 

 composition, but where the sericite schist develops in shear zones, 

 crossing massive beds of quartzite, and samples can be collected 

 showing complete gradation of the massive quartzite into the 

 sericite schist, there is conclusive evidence of the elimination of 

 quartz. The authors have carefully sampled two good cases of 



' W. S. Bayley, "Menominee Iron-bearing District of Michigan," Mon. 46, U.S. 

 Geol. Survey, 1904. 



