THE lEON-BEAKING MEMBER. 239 



The tuialteiftl port ions of the thin sections are made up mainly of grayish aggre- 

 gates of siderite individuals mingled with more or less amori)hous or minutely crys- 

 talline silica, and some darkening material in certain bands wliicli is perhaps in the 

 nature of carbonaceous matter. Numerous minute black crystals of magnetite are 

 scattered through the siderite. P^om these less altered forms tliere are grades of 

 transition to an opaciue mass of red irou oxide, which is aggregated particulaily along 

 certain seams. In slide 2290 there is mingled with the other ingredients a consider- 

 able ])roportion of greenish chlorite and a few small i)articles of fragmental quartz. 



From the vximsures in Sees. 10 and 11, T. 47 N., R. 45 W., Michigan. 



59. Actinolitic aiul magnetitic quartz-schist, from alow horizon. S])ccimeTi 7512 

 (slide 2294), from 570 X., .S7;5 W., Sec. 10, T. 47 X., R. 45 W., Michigan. 



The thin section of this rock shows magnetite and hematite predominating, and 

 for the most part aggregated into opaque masses. Within tlie masses, however, are 

 irregular areas and streaks of lighter colored materials wliich arc in ])art minutely 

 divided crystalline (juartz, and in part a greenish material which includes both actin- 

 olite and chlorite, the latter apparently derived from the actinolite. Quite often the 

 sumll detached particles of the irou oxide are seen to have a rhc)nil)ic outline. In the 

 section are a number of small rounded grains of fragmental ([uartz. These quartz 

 grains are often enlarged, and are all readily distinguishable from the nonfragmental 

 finely crystalline interlocking quartzose background. 



60. Cherty iron carbonate, from a middle horizon and just beneatli a greenstone 

 of the Keweenaw series. Specimen 7516 (slide 2295), from 815 N., 147(t VV., Sec. 10, 

 T. 47 N., E. 45 W., Michigan. 



The thin section is composed uuiinly of a compact grayish mass of siderite, 

 slightly stained along certain lines with iron oxide and containing here and there 

 particles of nuignetite. On one side of the section is a band of very minutely crystal- 

 line to amorphous silica containing numenms particles of iron carlionate, which are 

 separated from the main mass by the silica. 



61. Ferruginous chert and jasper, from a middle horizon, and only a short dis- 

 tance beneath a greenstone of the Keweenaw series. Specimens 12791 (slide 5477), 

 12794 (sUde 5478), 12794rt. (slide 5479). All from the SW. i of the SE. {, Sec 11, T. 47 

 N., K. 45 W., Michigan. 



These spe(!imens represent the several laminations seen in the crosscut at the 

 bottom of a test pit. Specimen 12791 shows a thin lamination of bright red jasper 

 with black seams. Specimen 12794 shows nearly the same, except that the jas])cry 

 portions take on a more cherty ai)pearance and whitisli color, and tlic daik colored 

 bands now and then present the metallic luster of magnetite. Specimen 12794rt is 

 mainly composed of a thinly laminated dark red hematite, being rich enough in iron 

 to constitute an iron ore. 



