Plate XXVIII. — Magnetitio and AfTiNOLiTic Slates from the Iron- 



Bearino Member. 



Fig. 1. Actinolitic slate. Specimen 9020, slide 3147. From Tylers fork, NE. i ol' Sec. 33, T. 45 N., 

 R. 1 W., Wisconsin. In ordinary light, X 16,5. The figure shows the rhombohedral shape 

 of a complex area of actinolite and ma]a;n<tite and the close association of these minerals. 

 The exterior of the area in the center of the figure is mostly magnetite, mingled, however, 

 with some actinolite, while the interior is pun- actinolite. Tliat this area rejiresents an 

 original rhomliohedron of iron carbonate is vi-ry prolialile. (.See pp. 220-221.) 



Fici. 2. Magnetitic concretionary chert. Specimen 962.'5, slide 31.50. From Tylers fork, NE. i of Se<-. 

 33, T. 45 N., R. 1 W., Wisconsin. In ordinary light, x GO. A (^herty background contains 

 roundish, oval, and roughly rhombic outlined areas. The iron oxide in these areas is mostly 

 magnetite, in the form of crystals. The magnetitic is often concentrateil upon the exteriors 

 of the areas, projecting somewhat into the eherty backgrounil. Tlie forms of the areas at 

 once suggest the carbonate areas of PI. xxi, Figs. 1 and 2, and PI. XX VII, Fig. 1, and each 

 doubtless now occupies the place once taken by sideriti^. In the alteration to magnetite the 

 individuals grew beyond tlie outer borders of the sidcritc areas, but the forms as a whole 

 were nuiintained. A considerable amount of silica must ha\e entered, as the magnetite 

 occupies but a small part of 'the space once taken by tlie carbonate. (See ])p. 222-223.) 



Fig. 3. Banded magnetitic Jasper. Specimen 12791, slide 5477. From Sec. 11, T. 47 N., R. 45 W.. 

 Michigan. In ordinary light, X 25. The white liackground is a completely individualizeil 

 but tinely crystalline chert. The part coutaiiiiug the abundant red hematite is in hand 

 specimen a brilli.aut red. jasper. These jaspery bands alternate witli the black ones, which, 

 instead of hematite, consist of magnetite, mostly in well defined crystals. The change from 

 magnetite to hematite is gradual. (See pp 239-240.) 



Fig 4. Actinolitic slate. .Specimen 9555, slide 3190. From Penokee gap, NAV. J of Sec. 11, T. 44 N., 

 R. 3 W., Wisconsin. In polaiizcil light, x 165. The section is a typical actinolitic slate. 

 The quartz is comjiletely crystiillized. The magnetite has mostly well detined crystal out- 

 lines and is manifestly the lirst mineral to crystallize, being scattered uniformly through 

 the section without any regard to the actinolite and (|uartz, and therefore included by both 

 of them. The actinolite is in its characteristic blades and sheaf-like forms, having a radial 

 arrangement of its fibers. It is as plainly the second mineral to crystallize, as needles of 

 actinolite everywhere penetrate the (|uartz, but never the magnetite. The quartz constit- 

 utes a background for the magnetite and ai'tinolite, .and includes them in such a manner 

 as to make the conclusion certain that it must in the main have crystallized subsequently 

 to the formation of the magnetite and actinolite. (See pp. 218-219). 



5(H 



