THE IRON-BE AEING MEMBEE. 243 



68. Magnetitic actinnlite schist, fioiii a vury liigli horizon immediately beneath 

 the Keweenawan greenstone. Specimen 10402 (slide o321); from ir>40 X., IGOO W., 

 Sec. 17, T. 47 X., R. 44 W., Mieiiigan. 



A felted mass of aetinolite needles composes the background of the section. 

 This mass is commonly stained red and brown by iron oxides, aud .onlains besides, 

 opaque aggregations of magnetite crystals whicli make up as much as half the section. 



Fnnn tilt' f'.rpo.s II ris iiiiil tist-pif.s in ,Srcs. I'j, Id mid i'l, T. 17 K., li. 1 J W'.. Mich 



11)1111 . 



(!!t. Actinolitic magnetite-schist, from a low horizon. tSpecimeiis 1278() (slide 

 5474), 12787 (slide 5475); from 1625 N., 650 W., Sec. 21, T. 47 \., R. 44 \V., Michigan. 



An aphaiiitic, laminated, very dark gray rock, analogous to tliose described 

 under (i8. Sp. gr. of 12787, 3.53. 



The thin sections are typical actinolitic magnetite schists, sucli as have l>een 

 described already as occurring at Penokee gap, etc. Tin; groundmass is a finely crys- 

 talline (piartz, tliroughout whicli are contained minute blades of aetinolite. The mag- 

 netite occurs as usual in bunchy aggregations of sharply outlined crystals, and also 

 in single crystals scattered throughout the groundmass. In slide 5475 there is a gen- 

 eral tendency towards a concretionary arrangement, which in ]iortions of the section 

 is very strongly developed; and after having seen the various concretioMary develop- 

 ments of the foregoing rocks one has no hesitation in saying that these are of the 

 same origin with all the rest. 



70. Actinolitic magnetite-schist, from a low horizon. Specimen 12781 (slide 

 5470); from 70 N., 1790 W., Sec. 15, T. 47 N., E. 44 W., Michigan. 



A very heavy, .slaty, dark gray rock, atuilogous to that last described, except 

 that certain vaguely defined bands have a pale reddish or Jaspery appearance. The 

 content of magnetite is evidently great, as large sized pieces an^ easily lilted by 

 the magnet. 



The section is again one of the typical ;ictiuolitic magnetite-schists. The back- 

 ground, as usual, is a minutely crystalline ipiartz. In tliis are cont;iined Ian-like ag- 

 gregates of unusually large aetinolite blades, and crystals of magnetite, partly aggre- 

 gated in certain bauds, in which are contained a large proportion of red aud brown 

 iron oxides. 



71. Ferruginous cherts, from a middle horizon. Specimens 121)71 (slide 5395), 

 12672 (slide 539()), 12675 (slide 5398); all from 450 N., 675 to 729 W., Sec, 16, T. 47 F., 

 K. 44 W., Michigan. 



The rocks are brown and red stained, highly ferruginous ciierty schists, made 

 uj) of alternating lamina^ of black, red, and brown colors. 



The thin sections are com])osed essentially of a minutely crystalline to amor- 

 phous silica with the red and brown and magnetic oxides of iron. These oxides are 

 aggregated more especially into certain lamina', other lamina' being almost pure 



