244 THE PENOKEE IRON-BEARING SERIES. 



cbert. On the edges of the iron oxide laiuiiiie the individual particles of hematite 

 and brown oxide are seen veiy frequently to have siuirp rlionibie outlines. The 

 magnetite is (juite sul)ordinate in (luantity to tlie other oxides, being, however, more 

 plentiful in some bands than in others. It oeeurs in sharply outlined crystals. The 

 hematite ])artieles that occur in these bands, which are mainly made up of flinty silica, 

 are often arranged in a radiate manner. 



72. Ferruginous chert -schists, from a high liorizon. Specimens iL'tlOl (slide 

 5412), l'2G!t2 (slide r>ii:i), 12693 (slide 5414); all from 1380 N., 1960 W., Sec. L"), T. 47 

 K, R. 41 W..:\Iichigan. 



These specimens re|)resciit alternating himime of gray and led stained chert and. 

 hematite irjon ore. The iron oxiilc seams show a more distinct subordinate lamina- 

 tion than is perceptible in the cherty portions. Sp. gr. of irony layers (12093), 3.2o. 



The cherty layers in the thin section are seen to be made up almost entirely of a 

 minutely crystalline t<^ nearly amorphous silica. Theie is also throughout this silica 

 a general tendency to a concretionary structure, which is brought out by a vague 

 concentric arrangement of tlie more or less completely crystalline particles. The 

 whole apjiearance of this chert is very strikingly like the cherts which have been 

 above described and Hgured as characteristic of the limestone member of thePenokee 

 series. In these cherts occur irregularbnnches of henuitite and brown iron oxide, the 

 jiarticlcs of wiiicli often sliow most distinctly rliouiliie outlines of the carbonate crys- 

 tals, from whose alterations tliey are taken to have originated. Some of these rhombic 

 crystals are of unusually large size; and in one or two places apjiear to still retain 

 p(U'tioiis of tlie original <'arbonate. The iiioic higlil>' ferruginous lamina' differ from 

 the cherty phases mainly in ilie large proportion of the oxides of iron, which now 

 preponderate greatly over the siliceous matrix. The i)articles of iron oxide are here 

 arranged in regular limss, w liich, without much doubt, mark the original hiiniuatioii of 

 the rock. The ilionibic outlines to the hematite i)arti<'les are IVeciuent, and .some of 

 the silica shows the radiating structure characteristic of chalcedony. 



73. Actinolitic ferruginous schist, from a high horizon and immediately in 

 contac'; with overlying Keweenawan greenstone. Specimens 93.S1 (slide 3269), 9382 

 (slide 303!t). From 070 X., 1(»20 W., Sec. 16. T. 47 N., R. 44 W., Michigan. 



The specimens present an alternatioi ol' liiown cherty, red jaspery, dark green 

 and black aphanitic laniina-, the whole rock ha\iiig a distinct slaty or ])arallel struc- 

 ture. Sp. gr. of the black laniiii;e (9382), 3.90. 



The thin sections of these rocks are i)articulaiiy interesting and instructive, in 

 that they generally sho\\ in a single section all ol the eliaractcristic minerals of this 

 class of rocks; i. c, brown and red oxides ot' iron, magnetite, actinolite, and cherty or 

 flinty silica. These difl'eieni minerals occur more or less intermingled, but the vary- 

 ing ajiiiearance of the lamiine as seen macroscoi>ically is due to the preiionderance of 

 ditt'ereiit minerals in the dilTcrent bands. The black bands are particularly rich 



