408 THE PENOKEE IRON BEARING SERIES. 



51. Cherty qnartzites. Specimens 12G20 (slide 5.371) ; 12820 (slide 540.".), IfiOO N., 

 13G0 W.; 12621 (slide 5.372), 1000 K, 1300 W.; 12912 (.slide 551G), 1015 N., 1.300 W.; 

 12917 (slide .5519), lf.2S N., 1.300 W.; 12027 (.slide 5370), 17.30 N., 1480 W.; 12S12 (slide 

 5488), 1712 N., 1462 W., Sec 28, T. 47 N., jj. 42 W., Michigan. 



These qnartzites are tolerably coarse grained and massive. Some of them con- 

 tain rather large pebbles of white and cherty quartz. They all have a rough fracture, 

 due to the fat^t that when broken the large quartz fragments are torn from their 

 sockets. In color they vary from greejiish gray to grayish brown or red. Sjiecimeus 

 12620 and 12621, besides being reddened are somewhat honeycombed, as though some 

 constituent, possibly a carbonate, had been leached out. 



The sections of these rocks consist of two i)arts, a matrix and coarse material. 

 The latter is chiefly quartz in large, simple, or coarsely complex grains, which are often 

 well rounded, but also quite as often more or less angular, while part of them are 

 very angular. There are a few fragments of very finely crystalline and perliai)s partly 

 amorphous cherty silica. Many of the simple quartz grains are slightly eidarged, the 

 enlargements merging gradually into the matrix. Praginental feldsi)ar is quite plen- 

 tiful in one of the sections, and sparse in the others. Also in one section are nu- 

 merous large altered leaflets of biotite. The ])redominant constituent of tlie matrix is 

 cherty silica, mingled with which is much sericite or kaolin, in small lirilliantly polar 

 izing flakes. In one or two sections this sericite is so plentifnl as to be cnnqjarable 

 with the silica in abundance. In such sections sonu', of tli(^ complex sericite-(|nartz 

 areas have a roundish appearance as though they were comidex fragments. They 

 pei'haps represent altered feldspar detritus, as indicated by the fact that in (nie of 

 the sections many undoubted feldspars have undt^gone alteration to quart/, and seri- 

 cite. Tlie cavities seen in hand specimen are found in thin section to l)e miKstly bor- 

 dered by iron oxide. They are of very irregular foi'm, resembling in this respect 

 areas of iron oxides found in the matrix; and it seems probable that the cavities were 

 either entirely filled with iron oxide, or more ])robably ferriferous carbonate, which 

 largely went into solution, but from which the iron oxides now remaining were ])ro- 

 duced. This probability is farther strengthened by the fact that in other rocks pre- 

 cisely similar to these and associated with them is fouml a good deal of carbonate. 

 The sections difl'er from those of the slates ,'iiid graywackes in one important partic- 

 ular. The fragmental particles are all large, none nearly approaching in minuteness 

 the particles of the matrix, while in the graywackes there is every gradation IVoni 

 the coai'sely fragmental matei'ial to the fine matrix. The minutely crystalline 

 matrix in these rocks is taken to be a nontVagmental or recomposed sediment. Tlic 

 sericite in the matrix is largely arranged with its l)lades in a common direction. 



52. Cherty and ferro-dolomitic qnartzites. Specimens 12915 (slides 5517); 12916 

 (slide 5518), 1628 N., 1.360 W.; 12628 (slide 5377), 17.30 K, 1480 W.; 12922 (.ilide 5.520), 

 1744 N., 1466 W., Sec. 28, T. 47 N., R. 42 W., Michigan. 



