414 THE CRYSTAL FALLS IRON-BBAEING DISTRICT. 



which is certainly original and fragmental, although almost every slide 

 contains grains that may possibly be such. On the other hand, it is evident 

 that the large majority of the individual grains have formed in place. 



The micas are in most cases the most abundant constituent; sometimes 

 muscovite, though usually biotite, predominates. The two micas are often 

 intergrown. The biotite is usually very deeply colored, both brown and 

 green, and, except in the thinnest slides, is almost opaque even in cleavage 

 sections. The larger mica flakes do not exceed 0.5 mm. in length, and 

 average not more than 0.2'5 mm. 



Quartz generally occurs in irregular grains, full of fluid inclusions, and 

 inclosing the various accessories. It frequently appears in little triangles 

 in the interspaces between adjacent flakes of mica. Rarely part of the peri- 

 meter is rounded and embedded in a mica, thus suggesting a clastic origin. 



Feldspar is very abundant in some of the slides and entirely absent 

 from others. Both microcline and plagioclase occur, and in forms similar 

 to the quartz. Biotite sometimes penetrates in iiTegular shredded edges and 

 filaments into the interior of the feldspars, and in such cases may be a 

 metasomatic product, as described by Irving and Van Hise ^ in the mica- 

 schists of the Grogebic distinct. But much of the feldspar, as shown by its 

 form and freshness, has recrystallized. The alignment of these minerals is 

 with the schistosity of the rock, which they thus determine. When the 

 schistosity cuts the lines of stratification, as it frequently does, the latter are 

 but faintly marked in the thin section by very slight mineralogical difi'erences. 

 Thus a dark band, which may be very striking macroscopically, may be due 

 merely to the predominance of deeply colored biotite; a light band, to the 

 jDredominance of muscovite. Sometimes, however, in these bands a grain 

 of quartz, or a stout flake of muscovite, lies out of the general orientation 

 and with the direction of the band. Such grains are A^er)'- possibly original. 

 The schistose structure, as has already been stated, is determined by the 

 general parallelism of the long axes of the constituent grains. Since the 

 greater part, if not demonstrably all, of these grains have formed in this 

 position, and have not been forced mechanically into it, the cases in which 

 the schistosity cuts the bedding support the inference as to the time of 

 the general recrystallization of the series grounded on the facts observed 



' The Penokee-Gogebic iron-bearing district of Michigan and Wisconsin, by R. D. Irving and 

 C. E. Van Hise : Mon. U. S. Geol. Survey, No. XIX, 1892. 



