6o4 C. K. LEITH AND W. J. MEAD 



this kind in the Marquette district of Michigan. Another case is 

 worked out in similar fashion by J. H. Warner on the Waterloo 

 quartzite of Wisconsin, where shear zones cross the bedding. 

 Analyses of these rocks/ show that the larger part of the free 

 quartz has been eliminated, presumably through solution aided 

 by granulation. 



Where shales are traced into slates or schists the change in 

 composition is less obvious, owing to the fact that the shale origin- 

 ally possesses pretty nearly the composition required by the end- 

 products, but even here, important changes in composition are 

 noted, like the elimination of CO2, water, and oxygen, and, in some 

 cases, the increase of potash. 



A greenstone in the Marquette district of Michigan has been 

 traced inch by inch into a sericite-chlorite schist and sampled on 

 a large scale, the resulting analyses showing elimination of silica, 

 an increase of alumina and ferric oxide, potash, and a considerable 

 decrease in lime. Similar changes are described and analyzed by 

 George H. Williams.^ 



It is not easy to find in the field satisfactory gradations from 

 massive to schistose rocks where one can be certain that the mass 

 was throughout of uniform composition. It is still more rare that 

 these gradations have been thoroughly sampled so that the analyses 

 furnish an adequate basis for comparison. It is significant, 

 however, that where this has been done — and we have searched the 

 literature carefully — important changes are to be noted. 



Fortunately we are not obliged in our conclusion to rely 

 entirely on the few well-sampled gradations from primary to 

 schistose rock. A study of the actual compositions of the principal 

 groups of schists brings out the fact that these vary in certain 

 essential respects from those of the primary rocks from which they 

 are supposed to have been derived. In another discussion'' of the 

 use of chemical criteria in the identification of schists and gneisses, 



' C. K. Leith and W. J. Mead, Textbook of Meiamorphism (in press, Henry Holt 

 & Co. 



^ G. H. Williams, "The Greenstone Schist Areas of the Menominee and Marquette 

 Regions of Michigan; A Contribution to the Subject of Dynamic Metamorphism in 

 Eruptive Rocks," Bull. 62, U.S. Geol. Survey, 1890. 



^ Textbook of Metamorphism, cited above. 



