340 THE PBNOKEE IRON BEARING SERIES. 



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

 feldspar has altered to mica the excess of silica has separated 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 this rock in such cases the mutual interlocking of these four minerals, 

 muscovite, biotite, quartz, and feldspai", could ilot possibly be more intri- 

 cate in any schist, gneiss, or granite. In other cases the decomposition 

 of a feldspar has resulted in the formation of many grains of quartz as 

 well as numerous folia of mica. In yet other cases the feldspar areas have 

 not altered to such an extent as described above. In almost every case the 

 rounded exteriors of the clastic grains are lost, but irregular areas of con- 

 siderable size often remain which include but few folia of biotite or little 

 replacing quartz, or both. The folia of mica in this rock are the largest 

 anywhere found in the mica-schists, many of them being 1 mm. in length. 

 The biotite has a remarkably strong dichroism, and both biotite and mus- 

 covite have well defined cleavages. 



(6) Crystalline muscovitic hiotite-schist (PI. xxxiv. Fig. 2). — Macroscopic- 

 ally, this rock is rather fine grained. It has a finely laminated structure, 

 along which the rock most readily breaks, although it is massive enough to 

 break quite easily across the <lirection of lamination. The individuals of 

 quartz and feldspar are too small to be seen with the naked eye, but are 

 distinguishable with a lens. The very abundant black, glittering flakes of 

 mica are large enough to be readily seen. A lens shows the rock to con- 

 tain a considerable quantity of pyrite. 



Under the microscope with a medium i)ower the sections show a 

 rather fine gi'ained, apparently completely crystalline, typical mica-schist. 

 The groundmass consists chiefly of quartz, mingled with which is con- 

 siderable feldspar, both orthoclase and plagioclase. The grains of quartz 

 vary considerably in size, but none are minute and none large, the largest 

 being not more than h nun. in diameter. Some few of the largest, which 

 are now very irregular in outline, contain rounded cores, proving them to be 

 fragmental, and these cores do not include folia of mica. ]\Iuch of the feld- 

 spar present is quite fresh, many of the plagioclases exhibiting sharply 



