456 C. B. Van Hise — Origin of the Micc^Schists and Black 



a presumption which is rendered stronger by the fact that in 

 passing from east to west along the belt the biotite first appears 

 in minute quantities, becomes gradually more plentiful until it 

 equals in quantity the chlorite and farther west grows still more 

 abundant, until, at English Lake, little or no chlorite is found 

 and we have a typical mica-schist. 



In the detailed descriptions below given of these micaceous 

 rocks, I begin with one not greatly different from its original 

 condition, and proceed toward those which are more and more 

 completely crystalline. There is a considerable interval as to 

 degree of alteration between each description and the succeed- 

 ing one, but the gaps cannot be made shorter without unduly 

 extending this paper. In a study of all the sections of the 

 group, some sixty in number, these intervals are closed. 



(1.) Muscovitic and Bioiitic GreywacJce. — Microscopically this 

 rock is gray-colored, medium-grained, and massive. It breaks 

 with a conchoidal fracture. Under the microscope large frag- 

 ments of quartz and feldspar, with the alteration-products and 

 replacement products of the latter, compose the rock. The grains 

 of quartz are enlarged and consequently minutely angular, 

 although still retaining their general roundish forms. Much of 

 the feldspar is quite fresh, many individuals showing no alteration 

 further than a slight kaolinization. Other feldspar fragments, 

 however, include each many grains of quartz, or a single large, 

 reticulating quartz individual and numerous flakes of musco- 

 vite and biotite. Here the quartz, muscovite and biotite are 

 plainly replacements and alteration-products of the feldspar. 

 In some cases this alteration has proceeded so far as to leave 

 but irregular, partly replaced and altered cores of feldspar 

 which are entirely surrounded with the secondary quartz, mus- 

 covite and biotite. Again, in other places, the original rounded 

 outlines of the feldspars are distinct, the replacements and 

 alterations having occurred in spots through the grains. The 

 finer-grained parts of the rock are composed of quartz, a portion 

 of which is clastic; of feldspar — the proportion being smaller 

 than in the coarser parts ; and of biotite and muscovite. The 

 mica is here clearly also, to a very large extent, secondary to 

 feldspar, while there is little doubt that the small remaining 

 fraction of the mica is of the same origin. Scattered through 

 the finer portions of the section are numerous small particles of 

 a black substance which is taken to be partly altered pyrite or 

 marcasite, and perhaps also partly carbonaceous material. 



(2.) Bioiitic Qreywacke. — Macroscopically this rock differs 

 from (1) only in being of a darker gray color; and under the 

 microscope also it is much the same except that the micaceous 

 alteration of the feldspars has been carried farther. Well 

 rounded fragments of quartz and feldspar with the secondary 



