Mica-Slates of the Penokee-Gogebic Iron Series. 457 



products of the latter compose most of the section. A few of 

 the grains of quartz are finely complex, and nearly all of them 

 are enlarged by renewed growths. The alteration of feldspar 

 to biotite is nicely shown. The freshest of the feldspar grains 

 are surrounded by and more or less deeply penetrated by 

 secondary biotite. These grains yet retain their well-rounded 

 forms. However, in many cases, the original outlines of the 

 grains of feldspar are lost. Often the entire surfaces of the 

 feldspars include very numerous particles of the biotite, there 

 remaining through such areas, here and there, little spots of 

 feldspar which, act as a unit in each area. With a low power 

 such areas appear to be roundish aggregates of biotite. It is 

 only with a higher power that the remaining feldspar and its 

 true relations to the biotite are discovered. The rather sparse 

 matrix of the rock does not differ materially from the matrix 

 of(l). 



(3.) Biotite- schist. — Macroscopically this rock is mottled dark- 

 gray and black, fine-grained, and quite massive, showing round- 

 ish cleavage- areas. Under the microscope the cleavage-areas 

 seen macroscopically are found to be well-rounded, partly-altered 

 feldspars. These feldspars are set in a ground-mass which con- 

 sists of intimately mingled grains of quartz and small brown 

 folia of biotite with a considerable quantity of ferrite. The 

 partial decomposition of the feldspar has resulted in the forma- 

 tion of very numerous small folia of biotite and a few larger 

 ones of muscovite. In places also the feldspar is replaced by 

 quartz. In each of the feldspar areas the secondary mica is found 

 most plentifully at or near the exteriors of the grains, although 

 in almost every case the alteration has proceeded to a greater or 

 less degree quite to the center. In the matrix of the rock it is 

 quite impossible to determine what part, if any, of the quartz is 

 fragmental. The biotite of the matrix is in part plainly sec- 

 ondary to feldspar and is precisely like that found in the larger 

 feldspars. The folia are deep brown and very strongly 

 dichroic, and therefore probably bear a large percentage of iron. 

 The feldspar plainly shows this rock to have been fragmental, 

 and the alteration of feldspar to both biotite and muscovite 

 upon a large scale is most beautifully shown. The large quan- 

 tity of dark brown and black ferrite has doubtless furnished 

 the iron required for the formation of the biotite. The peculiar 

 spotted appearance of the section, viewed without the micro- 

 scope, gives a clear idea, when taken in connection with its ap- 

 pearance under the microscope, of the processes by which the 

 rock reached its present condition. 



(4.) Muscovilic Biotite- Schist. — Macroscopically this rock is 



very fine-grained, grayish and quartzose, with very fine mica 



flakes visible. Under the microscope the thin section shows a 



fine grained ground-mass of quartz and feldspar in which are 



Am. Jour. Sci.— Third Series, Vol. XXXT, No. 186, June, 1886. 

 29 



