240 ' THE PENOKEE lEON-BEAKING SKKIES. 



In thin sections the black .seams above mentioned appear to owe their dark color 



mainly to the presence of magnetite, which is in sharply outlined crystals, 11 utlincs 



being nearly always rhombic. In some cases the magnetite ajipears to coiistitule flie 

 main ingredient of the dark colored bands, while in others it is mingled with more 

 or less hematite and actiuolite. The red bands are an admixture of liiiely divided 

 bright red hematite and very minutely crystalline to amorphous silica. These two 

 minerals are arranged in such a fashion as to produce a general laminated apjiearancc, 

 _ the heniatite being aggregated in thin belts. There are portions of these jasper bands 

 whi(;h show relatively little hematite, being made up mostly of sjlica. The belts of 

 silica follow the lamination in a general way, now and then breaking across it, while 

 a single seam of the silica often branches. In some of the black bands a considera- 

 ble quantity of actiuolite arranged in aggregates of dark green blades is visible. The 

 red iron ore seams above refeiTed to are small portions richer than usual in the sesqui- 

 oxide of iron. (ri. xxvm, Fig. 3.) 



From the test-pit near the center of Sec. IS, T. 17 N'., H. 11 W., Mkhigan. 



02. Hematitic siderite, from a low horizon. Specimen 127SS (slide 5638) ; from 

 near the center of Sec. 18, T. 47 N., li. 44 W., Michigan. 



An aphanitic, earthy, light reddish rock. Sp. gr. 2.8t). 



Iron carbonate, in pai't well crystallized, makes up the larger jjart of the thin 

 section. It is as usual mingled with some minutely crystalUue to amorphous quartz 

 and particles of red and brown iron oxides. 



From the test-jnt in Sec. 17, T. 17 N., R. 11 W., Michigan. 



63. Cherty siderite., from a low horizon. Specimens 12783 (slide 5471), 12784 

 (slide .5472); from 550 N., 1100 W., Sec. 17, T. 47 N., R. 44 W., Michigan. 



The rocks are composed of alternate bands of an aphanitic, earthy looking, dark 

 gray material and of reddish cherty and jaspery looking material, in which are per- 

 ceptible numerous pieces of a translucent quartz, red jasper, and black (lint. Sp. 

 gr. of gray bands, 3.20; of cherty bands, 2.34. 



In thin sections the dark colored bands are composed of a mixture of grayish 

 siderite and finely crystalline, with perhaps amorphous, silica. Irregular dark colored 

 streaks traverse these bands in a direction i)arallel to the lamination of the rock. 

 These streaks are made up of minute diirk colored ))articles, which are taken to be 

 mainly the oxides of iron, but may be partly of carbonaceous material. Crystals of 

 magnetite are seen scattered here and there, and the relations of the silica and carbon- 

 ate are such as have been repeatedly described above. In the cherty belts a very 

 finely crystalline quartzose background, including little oxide; with i)crhaits some 

 amorphous silica, contains many well rounded large simple grains of fragmental 

 quartz which have undergone a second growth, and very many more lounded areas 

 of ferruginous chert. The chert areas comjirise those with little iron oxide, with 



