224 THE PENOKEE IRON-BEARING SERIES. 



quartz groundmass, but occurs very plentifully also auiou^; tlie magnetite aggrega- 

 tions. As compared with tlic last rocks described, and others like it, this rock lacks 

 the distinct separation into a relatively pure quartzose matrix and dark colored areas 

 in which the iron oxiiles are particularly alnindant, and which are outlined so dis- 

 tinctly as to suggest tht«ir having been tVagnients. However, after liaving seen the 

 sections in which this arrangement is very pronounced, one realizes tlntt even in a 

 case like the present one there is a faint indication of the same structure. More or 

 less of the iron oxide occurs in the hematitic and hydrated forms. 



21. Magnetitic quartz-schist. Specimen ttlcS;{ (slide 4213); from 1050 N., L'70 W., 

 Sec. 19, T. 45 N., R. 1 E., Wisconsin. 



The rock is a flue grained, diirk gray magnetitic schist, closely resembling 19. 

 Sp. gr., -Mi'X 



The thin section resembles the la.st described as to its constituents, but ditters 

 from it in having the concretionary and pseudofragmental structure more largely 

 developed. Since the two come from tln^ same rockmass, it is evident thiit the faint 

 indication of this peculiar structure seen in slide 4214 (20) points to a similar origin for 

 the two rocks, Quite a little quantity of carbonate remains in some of the apparent 

 fragments. Minute actinolik' blades occur here, as in other similar sections, aggre- 

 gated in'miiiute needles about the clusters of magnetite. 



From the Fenokee range in Sec. 16, T. 45 N., R. 1 E., Wisconsin. 



22. Ferruginous quartz-schist. Specimen 12831 (slide .5495); from 513 N.. 1647 

 W., Sec. 16, T. 45 N., R. 1 E., Wisconsin. 



The rock is fine grained, dark gray schist, banded with seams of black iron 

 oxide. 



The thin section of this rock resembles those of the several peculiar concretion- 

 ary or itseudofragmental rocks above described, particularly 20. As seen in the ordi- 

 nary light, it shows a colorless background, which is thickly studded with distinctly 

 outlined areas that ditt'er from the background merely in containing numerous minute 

 dark colored particles. Some of these areas, however, are surrounded by ii rim of 

 the several iron oxides and occasionally these iron oxides, piirticuhirly the red and 

 brown oxides, cover the whole of one of these areas. In the polarized light the spotted 

 areas lose miicli of their detinitencss and nre seen to be mainly made up, like the matrix, 

 of a very minutely ca-ystalline cherty quartz. Some of the areas are quite angular in 

 outline, but most of them are more or less rounded, while at times a number of the 

 areas are so related to one another as to seem to have been once continuous, having 

 been severed liy insertion of the matrix niateriiil along irregidar rifts. Here and 

 there these areas are seen to have not merely a single ring of iron oxide, but to have 

 also one within nnd coiicentiic to the outer one. 



