THE KASTEIIN AUEA. 867 



Actinolite, chlorito, quartz, iiiafiiu'tite, liematitc, and l)r(t\v)i iron oxide arc all 

 found in the thin sections, but iron stained actinolite is predominant. The alteration 

 of actinolite has t'oiined tlie chlorite. The sections resemble closely those of 73 in the 

 iron member west of the l*rcs(iuo Isle, p. 244. 



4. Ilcmatitic and magiietitic actiuolite-schist. Specimens 0291 (slide, .iflll), 

 9292 (slide ;?1()7), 127.0 N., 1775 W., Sec. .W, T. 47 N., R. 43 W., Michigan. 



Tlie.sc rocks are finer ,<irainc<l than those of 3, but arc otherwise like them. 



Tiie minerals present and their relations are the same as in the previous number. 

 The predominant actinolite, which is largely arranged in sheaf-like forms, is every- 

 where heavily stained with brilliant red hematite. Magnetite in crystals is abundant. 



."). Hematitic cherts. Specimens 12581 (slide 5341), 500 N., 1000 W.; 12582 

 (slide 5.342), 5.33 N., 810 W., Sec. 20, T. 47 N., It. 43 W., Michigan. 



The rocks are composed of alteruate belts of white or gray chert and f)f red 

 or brown iron oxide. 



In thin sections a groundmass of finely crystalline, with perhai)S sonui ainor])hous 

 silica, contains both hematite and limonite, which are largely concentrated in nmghly 

 parallel belts. 



(i. Ankerite. Specimen 9264 (slide 4487), 75 N.,600 W., Sec. 20, T. 47 N., R. 43 

 W., Michigan. 



The rock is light gray, flue grained, ruassive, and breaks with conchoidal frac- 

 ture. It si)aringly contains pyrite in small crystals antl thin layers of almost pure 

 magnetite. The weathered surface is brown, from the presence of peroxide of iron. 

 The following is an analysis made by Mr. W. F. Ilillebraud: SiOj, 3'l(i, TiOz, none; 

 Al,O;„0-08; IW);,, 0-93, FeO, 15-18; MuO, 1-15; CaO, 2G-G5; MgO, 11-01; H2O, 0-.-)4; 

 CO., 41-10; P,,O5 0-0(>, 01, trace; FeS, 0-34. Total, 100-20. 



The thin section consists almost wholly of an interlocking mass of finely but 

 perfectly crystalline ankerite. Contained as accessories in this are small crystals of 

 pyrite, magnetite, and small clusters of radiating needles of.nuu-h altered actinolite. 



7. Actinolitic chert. Specimen 92ti5 (slide 4488), 75 N., (iOO W., Sec. 20, T. 47 

 N., R. 43 W., Michigan. 



The rock is light olive green, line grained, and massive. 



The thin section consists chieHy of the two minerals, quartz and actinolite. 

 The (piartz ntiikes a finely crystalline interlocking^roundmass, in which the actinolite is 

 containcMl in clusters of small radiating blades. A little brown tcrrite and a few 

 crystals of magnetite are seen. The section is cut by a vein which is composed mostly 

 of comparatively (coarsely crystalline ({uartz, including almost indefinitely numerous 

 minute long needles of actinolite, which are arranged parallel to one another. 



8. (^lartzose siderite. Specimens 92(i(i (slide 4489), 92(J7 (.slide 4490), 100 N., 

 600 W., Sec. 20, T. 47 N., R. 43 W., .^lichigan. 



The rocks are dark green, rather fine grained and finely laminar. 



