2L\3 THE PENOKEB IRON-BEARING SERIES. 



From the Penokee range in See. 6', T. 45 JV., B. 2 E., Wisconsin. 



26. Ferrugiuous chert-schist, from the base of the Iroii-benring member. Speci- 

 men 0190 (slide 2931); from 1825 N., 325 W., Sec. «, T. 45 N., li. 2 E., Wiscousiu. 



The rock is a brown stained i)orous chert. 



The greater part of the section is made uj) of a concentrically arranged silica, 

 quite analogous in its appearance to the concretionary cherts already described as 

 lielonging to the Limestone member of the Penokee series. This silica is in the main 

 divided oft' into minute interlocking quartz individuals, which vary somewhat in 

 degree of minuteness, the finest material at times passing over into chalcedony. 

 Here and there through tlie section are seen what appear in the ordinary light to be 

 areas of some different material from the rest of the section. These are stained a 

 light brown color, ami liave smooth, curved outlines, concentric with which there is at 

 times near the edges of them a curvilinear arrangement of the particles composing 

 the iron oxide stain. In the polarized light these areas are seen to be made up of a 

 minutely divided silica analogous to that forming the bulk of the rock, but usually 

 detined from that portion t>f the nuitrix witli which it is directly in contact by being 

 much finer grained. Some of the areas, however, lack deflniteness of outline, and 

 appear as if invaded and dissolved away by the surrounding matrix. In one or two 

 cases remnants of iron carbouiites were detected within them. Scattered through 

 the chert matrix in irregularly outlined aggregates and curving lines is a brown 

 iron oxide in which may be seen at times remnants of iron carbonate. The cavities 

 seen macroscopically in the specimen appear in the thin secti(m as liolcs with ragged 

 outlines, which are brought into i)articular prominence by a coating of iron oxide. 



27. Flint or chert, from near the base of the Iron-bearing member. Specimen 

 9144 (slide 2799); from 11.50 N., 1(525 W., Sec. 6, T. 45 N., R. 2 E., Wisconsin. 



The rock is an ai)hanitic, white, chalcedony-like flint or cliert, translucent on 

 the edges, and i)ossessed of a conchoidal fracture. Irregular bands of a nearly black 

 color also occur. Sp. gr. of white portion, 2-07; of Ijlack bands, 2-(i9. 



The thin section is composed almost wholly of a colorless silica, which for the 

 most i)art i)olarizes only as an aggregate and very feebly. High powers, however, 

 bring out distinctly minute polarizing i>articles, which are mingled with much mate- 

 rial which seems to be quite amorphous. Portions of the section show, running 

 througli this silica, cloudy bands and streaks arranged in general i)arallelism to one 

 another; a number of these streaks appearing as if once continuous biinds which 

 have been separated by the intrusion of silica. These bands owe their cloudiness, in 

 ])art at least, to the jiresence of a minutely divided brown iron oxide. In tlie main, 

 however, their substance seems now to be made u]) of silica more thoroughly amor- 

 phous than is that of tlic l)ulk of the rock. In some of these streaks are apparent 

 remnants of a carbonate, in aggregates of minute individuals. This section is of 



