338 THE PENOKEE IRON-BEARING SERIES. 



tonned, the result of the aUeratioii is to make the rock a finer gramed one. 



The rather sjiarse matrix of the rock does not (htfer materially from 

 that of (2), except that it contains more mica. The induration is caused 

 by the same processes as in (2). 



(4) Mmcovitic hiotite-slatc (PI. xxxiii, Fig-s. 3 and 4). — Macroscopically, 

 this phase of rock is mottled dark gray and black, (piite massive. The 

 mottling is due to more or less distinct roundish areas, which show in a 

 greater or less degree cleavage. In some of the specimens the roundish 

 areas are distinctly outlined, and in these cases the cleavage is eminent. 

 In others, these areas are indistinctly outlined, and in such the cleavage 

 is less plain. Under the microscope the grains showing cleavage macro- 

 scopically are found to be well rounded, partly altered feldspai-s, mostly 

 of the species orthoclase. These feldspars are set in a groundraass 

 which consists of intimately mingled minute grains of (|uartz and brown 

 folia of biotite with a consideraljle quantity of ferrite. In some specimens 

 the quantity of matrix is considerable, even equal or gi'eater than the 

 known feldspar fragments and the material coming from them. In 

 other cases, however, the feldspar areas were set very close together 

 with room for little other material. The partial decomposition of the frag- 

 mental feldspar has resulted in the formation of very numerous small folia 

 of biotite, fewer larger ones of muscovite, and also quartz. In the fresher 

 feldspar areas the secondary mica is foinid most plentifully at or ne'ar the 

 exteriors of the grains, although in almost every case the alteration has 

 proceeded to a greater or less degree quite to the centers. In some speci- 

 mens most (^tf the original feldspar grains are yet chiefly unaltered, and in 

 these the mottling and cleavage, seen macroscopically, is most distinct. In 

 other specimens nearl}' eveiy feldspar grain has almost wholly altered to 

 mica and quartz, and in such specimens, those which are most completely 

 altered and now therefore consist of a complex interlocking aggregate of 

 quartz and mica, with little or no remaining feldspar, are with great diffi- 

 culty separated from interstitial material. Many also are doubtless so 

 completely cbanged as to ])e indistinguishable from the matrix. In the 

 matrix of the rock it is usuallv (juite impossilde to determine if any of the 

 quartz is fragmental. In some specimens some grains are certainly so, as 



