400 THE PENOKEE IllON-EEAKING SERIES. 



The sections ure composed almost wliolly of exceedingly finely crystiilliue inter- 

 locking quartz, with iierha])s some amorphous silica. Ferro-dolomite, sericite, and 

 ferrite are accessories. In phices, the sections are cut by veins of coarser crystalline 

 quartz. 



19. Ferruginous day -slate. Specimen 7.373 (slide 1.S41), MO N., 30 W., Sec. l.">, 

 T. 47 N., K. 43 W., Michigau. 



The roclc is dark greenish gray and lui.s an a))hanitic texture. 



The thin section consists of an exceedingly tine mixture of quartz, cherty silica, 

 kaolin, chlorite, and dark brown and black iron oxide. The section res(}mbles 9 but 

 is somewhat coarser. 



20. Chloritic graywackeslates, from middle part of belt, north of l(j. Specimens 

 7396 (slide 1859), 500 N., 1090 W.; 9270 (slide 4927); 9271 (slide 4926), 575 N., 1145 

 W., Sec. 15, T. 47 N., E. 43 W., Michigau. 



The rocks are grayish green, tine grained, and have a feebly developed cleavage 

 The thin sections show a clayey grouudmass, consistiug apparently of kaolin, 

 quartz, chlorite, and ferrite, which contain many fragments of both quartz and feld- 

 spar. The sections ditt'er from one another only in the relative abundance of these 

 larger fragments. In slide 4926 the large fragments are nearly wanting and the 

 rock approaches a clay-slate. 



Section in cast half of Sees. 11 and 23, T. 17 N., B. 13 W., Michigan. 



21. Cherty ferro-dolomite, from base of belt. Specimen 9276 (slide 30(((i), 1840 

 N., 1000 W., Sec. 23, T. 47 N., E. 43 W., Michigan. 



The rock is light gray, weathering to daik brown, tine grained, massive, aud has 

 a conchoidal fracture. 



The section consists of intimately mingled particles of brilliantly polarizing 

 ferro-dolomite and minutely crystalline interlocking quartz. 



22. Quartzites, fiiom near base of belt. Specimens 7362 (slide 1831), 1965 N., 580 

 W.; 9274 (slide 4493), 1950 N., 590 W., Sec. 23, T. 47 N., E. 43 W., Michigan. 



The rocks are gray, tine grained, vitreous, aud break with conchoidal fracture. 



The thin sections have a tine grained interlocking grouudmass comi)osed of 

 cherty silica, which contain numerous rounded grains of quartz, with fewer of ortho* 

 clase, microcline, and oligoclasc. There occur quite plentiful areas of chlorite, which 

 in most cases are heavily stained with oxide of iron. 



23. Conglomerates inter strati tied with 22. Specimens 7364 (slide 1833), 1955 N., 

 590 W.; 9273 (slide 4492), 1965 N., 520 W., Sec. 23, T. 47 N., E. 43 W., Michigan. 



The matrix of the rocks is mottled pink and gray, medium grained, and massive. 

 In the matrix are very luimerous granite pebbles, which vary from larg*' size to those 

 so minute as to be lost in the matrix. 



