342 THE PENOKEE IRON-BEARiNa SERIES. 



black mica-slates oi' tlie Penokee series. Macroscopically, these slates 

 are all exceed! ii<^ly iiiie jj;raiued and finely laminated, cleaving readily 

 along the planes of lamination. In color they vary from dark gray to 

 black. A lens shows many of them to contain numerous small particles 

 of pyrite. Very numerous roundish black areas are contained in the fine 

 grained, gray material in many of the specimens. These areas in some 

 cases show distinct cleavage surfaces and are taken to be large fragmental 

 particles. In other cases tliey are dull and break without giving cleavage 

 siu'taces, wliile in yet other specimens these darker spots ai-e not found 

 at all. 



Under the microscope the rocks whicli liave the mottled appearance 

 described above consist of two parts, a finely crystalline matrix and 

 coarse, well i-ounded fragmental feldspars, which are always altered to a 

 greater or lesser extent. This alteration is to biotite and (juartz, many small 

 folia of biotite and grains of (piartz always being found in a single feld- 

 spar individual. Every gradation of tliis change is seen, from grains of 

 feldspar which i-ontain but little l)iotite and (piartz to those in which the 

 remaining feldspar is just sufficient in quantit}' to enable one to perceive 

 that the detached particles are parts of a common individual. Doubtless 

 also the alteration to biotite and quartz has often been carried out com- 

 pletely. Accompanying the biotite, thus secondary to the felilspar, is much 

 black, opacpie, yjartly altered pyrite in niinute particles. The black, 

 roundish spots seen macrosco})ically are evidently the partly altered feld- 

 spar fragments. Tlie degree of this alteration as determined 1>\' micro- 

 scopical study corresponds exactly with the appearance of the rock as seen in 

 the various hand specimens. In the specimens in which the feldspar areas 

 are well outlined and sliow clearly marked cleavage surfaces, the biotitic 

 and quartzose alteration has taken place to but a small extent. In the 

 sj)ecimens in which the feldspars are ojjscurely outlined and which lack 

 clfeavage the alteration has extended very far. The matrices of these rocks 

 and the sections of tliose not containing large grains of felds])ar are coni- 

 ])osed of intinKiteh' miiigled ([uai'tz, feldspar, biotite, and j)yrite, with 

 perhaps a little organic matter.' A portion of the quartz is certainly frag- 



'Geol. Wis., vol. in, p. 136. 



