458 C. R. Van Hise — Origin of the Mica-Schists and Black 



abundant muscovite and biotite. Many of the large quartz 

 grains include numerous folia of mica. The feldspar areas 

 include quartz and both muscovite and biotite. This section,, 

 by itself, if not examined closely would be taken to be of a 

 completely crystalline mica-schist in which the interlocking and 

 mutual inclusions of the different minerals are of the most 

 intricate kind. But, like the other mica-schists of the Penokee 

 series, it is an ordinary clastic rock in which the metasomatic 

 changes have gone very far. The large areas of quartz, includ- 

 ing folia of mica, are in the places of feldspar fragments. As 

 the feldspar has altered to mica the excess of silica has sepa- 

 rated as quartz. Frequently the alteration of a single feldspar 

 has resulted in the formation of a single ramifying individual 

 of quartz, with several or many included folia of mica, mingled 

 with which are detached remnants of the feldspar. In other 

 cases the decomposition of a feldspar has resulted in the for- 

 mation of many grains of quartz as well as numerous folia of 

 mica. In yet other cases the feldspar areas have not altered 

 to such extent as described above. In almost every case the 

 rounded exteriors of the clastic grains are lost, but irregular 

 areas of considerable size often remain considerable, which in- 

 clude but few folia of biotite or little replacing quartz, or both. 

 (5.) Muscovitic Biotite- Schist — Macroscopically this rock is 

 fine-grained to aphanitic, and dark grayish black, with rather 

 distinct lamination. Under the microscope the sections show 

 a rather fine-grained, apparently completely crystalline, typ- 

 ical mica schist. The ground-mass consists chiefly of quartz, 

 mingled with which is feldspar, both orthoclase and plagioclase. 

 Biotite in rather small, well-defined folia of quite uniform size is 

 very plentiful. Muscovite is much less abundant. That all the 

 mica is a secondary product cannot be proved from the sections 

 from this locality taken by themselves. A portion of it is, how- 

 ever, certainly of this nature. Many grains of feldspar are 

 partly surrounded and cut by folia of mica, while many of the 

 larger particles of feldspar contain throughout their areas num- 

 erous folia of mica which in magnitude and appearance are 

 exactly like the mass of the biotite in the rock. Quite numer- 

 ous black grains and crystals of a mineral which is taken to be 

 pyrite are seen, which are included alike in the quartz, feldspar 

 and mica. 



The rocks above described thus furnish us with a graded 

 series, from the slightly altered greywackes to the crystalline 

 mica-schists. There are many similar mica-schists in other 

 parts of the Lake Superior region which there is good reason 

 to think have had a similar origin ; but those wide-spread 

 mica-schists associated with the older gneisses are not now 

 referred to. 



