136 THE PENOKEE IRON-BEARING SERIES. 



A finely granular, uniformly textured, light gray rock, containing quite numer- 

 ous small graius and crystals of pyrite. The rock contains: calcium carbonate, 50-52 ; 

 magnesium carbonate, 33'41 ; iron, 1'19; insoluble ingredients, 13-85; undetermined 

 1-03=100.' 



The sections are composed almost wholly of small, closely fitting particles of 

 dolomite. Scattered through them are quite a good many blades of tremolite. Quartz 

 and pyrite are sjiarse accessories. A small portion of the dolomite exhibits the char- 

 acteristic cleavage and twin laraellation. 



7. Sericitic quartz rock. Specinien 9533 (slide 3139). From 14G0 N., 1200 W., 

 Sec. 14, T. 44 N., R. 3 W., Wisconsin. 



A rock mucli like 3, but of a coarser grain. 



The thin section is composed of line grains of quartz, ■which include many minute 

 flakes of sericite. The individuals of quartz are coarser than in 3, and wliile occa- 

 sionally showing the crystal outlines, the greater number of particles form junctions 

 which are more or less irregularly curveil. The sericite is equally distributed 

 throughout the section, being included in each of the quartz grains. Often a single 

 flake of sericite is seen to i)enetrate two or more grains of the quartz. No evidence 

 of enlargement of the quartz can be detected. 



S. Quartz rock. Specimens 4520 (slide 1112), 4520 (slide 1110). From 14(10 N., 

 1200 W., Sec. 14, T. 44 N., R. 3 W., Wisconsin. 



The tliin section difl'ers from that of 7 in being coai'ser grained, in not having 

 the quartz particles very thoroughly interlocked, and in containing relatively little 

 sericite. Chlorite also occurs as a sparse accessory. 



9. Sericitic quartz rock. Specimen 4534 (slide 14G5). From the extreme NE. J 

 of Sec. 16, T. 44 N., R. 2 W., Wisconsin. 



The thin section resembles closely that of 7, the only difference of importance 

 being the occurrence in it of magnetite in a few small crystals. 



10. Flinty cherts. Specimens 7511 (slide 20.W). From 1925 N., 1934 W. ; 9430 

 Xslide 3129), from 1900 N., 1900 W.; 9434 (slide 3131), from 1923 N., 1940 W., Sec. 14, 

 T. 47 N., R. 45 W., Michigan. 



A fine grained to aphanitic, light gray or jiinkish gray chert with a conchoidal 

 fracture. 



The sections are composed essentially of a minutely crystalline silica, the i)arti- 

 cles of which, however, vary a good deal in size; .some portions looking as though 

 the rock might be in i)art amorphous. In other parts one sees that there is a vague 

 (•(uicentric arrangement, the amorphous and more minutely crystalline silica tending 

 to lie in the middh^ portions of certain areas whose exterior portions are made uj) of 

 more coarsely crystalline ])articles. This is an arrangement which approaches to the 

 texture of true chalcedony, the resemblance to which mineral is also very evident in 



' Wiscousiu Geol. S'Lirvey, vol. iii, p. 107. 



