THE IRON BEARING MEMBER. 21 5 



In the detailed description of the actinolitic and magnetitic scliist.s 

 below, the iron oxide is spoken of as if wholl}' magnetite, nnless the hematite 

 can be microscopically discriminated. However, as shown 1)\- the analvses, 

 page 197, the magnetite is always accompanied b}' hematite. The actiiio- 

 lite is so named becanse it has the microscopical properties of that mineral. 

 No separations of it have been attempted, but the universal presence of con- 

 siderable quantities of magnesium and calcium in these rocks, as sho^'Mi by 

 the analyses referred to, renders it prol)able tliat the mineral is actinolite 

 rather than griinerite; since no other silic;ite is common and the only 

 remaining important minerals are ([uartz and iron oxide. 



TABULATION OF PETROGBAPHIf'AL OBSERVATIONS. 



From the exjMsiirrs in the ShJ. ^ of Sec. 26, T. 14 N., R. li W., Wixcuii.siii. 



1. Magiietitic actiuolite-seliist. Specimens 2(12 Wr.,' 2(».'> W'r., from .'!."><i N., <m(» 

 W., Sec. 20, T. 44 N., K. W., ^^'isconsin. 



A (lark colored slaty rock, in whicli bands about onc-fouitli of an inch wide are 

 mainly coni])osed respectively of inajinetitc and actinolitr, tlif latter arranged in (|nite 

 hu'jj;e, dark green radiating blades. In some cases tlie inagnetitic bauds are wider, 

 however, and show the minute aiitinolites miugled with the magnetite. 



The thin sections are both cut from the nH)re actinolitic seams. In both sections 

 the actinolite is in very large and interlocking blades, which are often altcic<l to 

 chloiite and have in places given rise to a secoiulary biotite. Included within the 

 actinolite blades are numerous small particles of magnetite, often witii crystal 

 outlines. 



2. Magnetitic actinoliteschist. Specimen !»(i!t8 (slide 3171) ; from 480 N., r>2(> \\'., 

 Sec. 26, T. 44 N., R. t; W., Wisconsin. 



A dark gray, nearly ai>lianitic rock, with faint lamination lines, and narrow 

 seams parallel to the lamination, which are (jiiite rich in magnetite. The magnifying 

 glass reveals the presence in tlic body of the r(K'k of numeioiis minute actinolile 

 blades, mingled witli (pnirt/. ])artic]es and some magnetite. Sp. gr., .'V."i(t. 



In thin section interlaced blades and neetlles of actinolite compose the larger 

 l>art of the rock. This actinolite is ](artly altered to chlorite and includes numerous 

 small particles of magnetite. 



'The muiibers of specimens auil slides are usually tho.se of tlie collection of the laUe Su)>eruir 

 division. Specimens with Wr. after the niiinliers :ire Iriiiii the eciUeetiiin iii'thi- hite .Mr. Clmries K. 

 Wright. SpiM'iinens with Wis. alter the MUiiil>ei's are t'rcmi the colU'eticm iil' thi' Wiseuiisiii (ie<ihi;;ical 

 Survey. Locations are given from the southeast corner of the .sections, in steps of 2,000 pur mile. 



