396 THE PENOKEE IRON-BEARING SERIES. 



lu the ordinaiy quartzites which have been indurated b}- the enhxrge- 

 ment of quartz-grains it is always taken for granted that the induration is 

 subsequent to the deposition of the fragmental quartz. Doubtless when 

 the grains of quartz lie close to one another and the enlargements have con- 

 tinued Until they completely interlock this is true, but in the cases of the 

 above quartzites, in which a considerable quantity of cherty silica is 

 jjresent, it is possible that the silica which now forms the mati'ix, and to 

 which the induration is due, was deposited at least in part simultaneously 

 with the fragmental material, perhaps by organic agencies. If so, it has 

 subsequently been rearranged. 



TABTTLATION OF PETROGRAPHICAL OBSERVATIONS.' 



Sections from the («ist i)art of T. 47 N., E. ii W., <ut(l the W. 2 miles of T. 47 N., R. 43 



TF., Michif/fui. 



1. Fenuginons quartzite. Specimens 9318 (slide SOIS), 0319 (slides 2972), 9320 

 (slide 2973), 1S25 N., 975 W., Sec. 14, T. 47 N., R. 44 W., Micljisaii. 



The rocks are gray, dark brown or black, medium grained, and vary from mass- 

 ive to schistose. 



In thin sections a continmms ramifying mixture of hematite and l>rown opacjue 

 oxide of iron contains numerous rather small grains of quartz and feldspar. The 

 simple particles of quartz are often enlarged. The feldspar, which is much less 

 abundant than the quartz, is mostly fresh, although a few grains are kaolinized. The 

 sections are almost exactly like 2962 and 1891 (No. 2 of fragmental. rocks soutli of 

 greenstone conglomerates, p. 371). 



2. Ferruginous and chloritic quartzites. Specimens 7445 (sUde 1898), 900 N., 

 W.; 9308 (slide 3111), 925 N., 40 W., Sec. 13, T. 47 N., R. 44 W., Michigan. 



The rocks are alternately light and dark greenish gray, fine grained, :ind com- 

 pact. 



The thin sections are mostly composed of small well rounded grains of quartz, 



with also some of feldspar. The induration of the rocks is due to the enlargement 

 of the quartz grains, and to a rather plentiful interstitial carbonate, which is prcthahly 

 ferriferous. Chlorite and iron oxide are also plentiful. 



3. Maguetitic graywacke. Specimens 7433 (slide 1888), 9309 (slide 29C9), 9310 

 (slide 3010), 4.50 N., 1350 W., Sec. 18, T. 47 N., R.^43 W., Michigan. 



The rocks are dark green, flue grained, schistose. The bedding is almost per- 

 pendicular to the plane of readiest cleavage. 



'The luirabers of specimensand slides are those of the collection of the Lake Superior Division. 

 Locations nvi- {jivcii fiimi the smithcast ccirneis of the srctioiis, in Mtejjs of 2,000 per mile. 



