THE EASTEKN AREA. 397 



The thin sections show that the rocks in their oiij^inal cuiKlition were mostly 

 composed of rather small Iragmcntal particles of ([iiartz and fchlspar. Tlie srains of 

 quartz have received secondary euhirf,'cment. Thosc'of fchlspar liavc largely altered 

 to aggregates of green chlorite, tiiis mineral now beini; one ol' the most jdentifnl in 

 tlie section. Uniformly distributed are numerous crystals ot maguiitite. 



1. Uematitic graywaclies and licnuititic schists. Specimens '.I.JDl'A (slide 3015) 

 140 N., 1000 W.; 7438 (slide ISiKJ), 150 N., 1000 W.; !).!0;i (slide 1220) ; !t:504 (slide 4221 )• 

 9305 (slide 3109), 175 N., 975 VV.; 7439 (slide 1894), 180 X., 1000 VV.; 7440 (slide 

 1895), 190 N., 1000 W., Sec. 18, T. 47 N., R. 43 W., Mic^higan. 



The rocks vary from rather line grained, banded, dark greenish gray gray wackes 

 to a dark brown hematitie schist. The belts of heavily ferruginous material vary in 

 thickness from mere films to layers several feet across. 



In thin sections, the least ferruginous phases arc mostly composed of small frag- 

 mental particles of quartz and feldspar. The enlargements of the (piart/, are often 

 relatively wide. The feldspar is iiltered to a considerable extent to kaolin and chlorite. 

 In the interstices are found, in varying proportions, finely taystallineipiartz, chlorite, 

 hematite, crystals of nmguetite and Hakes of white mica. In tlie more ferruginous 

 phases, the hematite, with some magnetite, constitutes a continuous ramifying sheet 

 which contains fragmental material. 



5. Ferrugiuuous graywacke. Specimen 9302 (slide 3014), 1850 N., 325 VV., Sec. 

 19, T. 47 N., R. 43 W., Michigan. 



The rock is dark green, flue grained, massive, and includes a few crystals of ]iyrite. 

 The thin section differs from the least ferruginous jdiases of 4 oidy in that in 

 the interstices a considerable quantity of ferro-dolomite is present. 



6. Chloritic graywacke-slate. Specimen 9238 (slide 4478), 1750 N., 12.50 W., 

 Sec. 20, T. 47 N., 43 W., Michigan. 



The rock is greenish gray, of a medium uniform grain and feebly schistose. 



Fragmental quartz and feldspar in about equal (piantity, tlie latter being a 

 mixture of orthoclase, microcline, and plagioclase, comi5o.se nine-tenths of the thin 

 section. The grains of quartz are enlarged and the interstices are tilled with finely 

 crystalline quartz and chlorite. Iron oxide is an accessory. The rock is a ty|)ical 

 gray w acke-sl a te. 



7. Chloritic and magnetitie gray wacke-slates, from south imrtof belt. Specinu-ns 

 9237 (slide 4477), 1625 N., 750 W.; 923G (slide 4476), 1750 N., 750 W., Sec. 20, T. 47 

 N., R, 43 W., Michigan. 



The rocks are like 6. 



The thin sections differ tj-om that of 6 in that the feldsi)ars have altered more 

 extensively to chlorite and kaolin; in that there was originally present a considerable 

 anu>unt of clayey material, aiul in that they contain some quantity of magnetite 

 which is mostly in crystals. Sericite is an accessory. 



