Mica-Slates of the Penokee- Gogebic Iron Series. 459 



The micaceous alteration of feldspars has also been very im- 

 portant in the production of various black mica-slates of the 

 Penokee series. Macroscopically these slates are all exceed- 

 ingly fine-grained 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 they are dull and break without giving cleavage 

 surfaces, while in yet other specimens these darker spots are 

 not found at all. 



Under the microscope the rocks which have the mot- 

 tled appearance described above consist of two parts, a finely 

 crystalline matrix, and coarse, well rounded fragmental feld- 

 spars, which are always altered to a greater or less extent. 

 This alteration is to biotite and quartz, many small folia of 

 biotite and grains of quartz always being found in a single feld- 

 spar individual. Every gradation of this change is seen, from 

 grains of feldspar which contain but little biotite and quartz to 

 those in which the remaining feldspar is just sufficient in quan- 

 tity 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 completely. Ac- 

 companying the biotite thus secondary to the feldspar, is much 

 black, opaque, partly-altered pyrite in minute particles. The 

 black, roundish spots seen macroscopically are evidently the 

 partly-altered feldspar fragments. The degree of this alteration 

 as determined by microscopic study corresponds exactly with 

 the appearance of the rock as seen in the various hand-speci- 

 mens. In the specimens in which the feldspar areas are well 

 outlined and show clearly marked cleavage surfaces, the biotitic 

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

 In the specimens in which the feldspars are obscurely outlined 

 and which lack cleavage the alteration has extended very far. 

 The matrices of these rocks and the sections of those not con- 

 taining large grains of feldspar are composed of intimately 

 mingled quartz, feldspar, biotite and pyrite, with perhaps a 

 little organic matter.* A portion of the quartz is certainly 

 fragmental, as is evidenced by a secondary enlargement. The 

 biotite is all believed to be due to the alteration of feldspar, 

 much of it certainly being of this nature. The matrices of the 

 different sections vary in coarseness and in the relative propor- 

 tions of the various mineral constituents, but are alike in all 

 essential points. 



* Geol. Wis., vol. iii, p. 1S6. 



