438 THE MAKQUETTE IKON-BEARING DISTKICT. 



In tliin section the recomposed ore has a hematite and magnetite back- 

 gronnd. In this are included many flakes of mnscovite, broken grains of 

 coarse quartz, and simple and complex particles from the jasper. The 

 matrix of the less ferruginous conglomerate is essentially the same as the 

 quartzites, except that a considerable quantity of feldspar, including both 

 orthoclase and ])lagioclase, is contained in it. At some places this feldspar 

 has partly altered into quartz, rauscovite, and biotite, many of the roundish 

 grains being now an interlocking mass of these minerals and the resid- 

 ual feld.spar. The quartzites show the effects of extreme pressure; all of 

 the grains show undulatory extinction or fracturing, or even granulation. 

 While many of the grains have a general roundish appearance, they rarely 

 show the original cores, and are minutely angular upon their exteriors. 

 These larger grains intricately interlock, and in part finely crystalline 

 quartz has developed between them. In the rocks in which the dynamic 

 effects are least the coarse-grained original quartz is discriminated from 

 the finer-grained secondary quartz, but in the more mashed phases of the 

 rock the two are similar and the quartz grains interlock in an intricate 

 way, giving no positive evidence of their original fragmental character. 

 Between the grains considerable muscovite has de-s'eloped, making the rocks 

 sericitic quartzites, and where most mashed sericitic quartz -schists. At 

 one place on the east side of the trough, in a coarsely crystalline quartzite, 

 epidote and amphiljole are so abimdaut as to become chief constituents. 

 The quartzite is coarsely crystalline, and the roundish forms of the quartz 

 grains still show the fragmental origin of the rock. The epidote occurs in 

 small and large, distinctly pleochroic, irregular grains, some of which show 

 a tendency toward crystal outlines. In the upper part of the Ishpeming 

 formation the quartzite is fine-grained, and here the background consists of 

 small granules of quartz, similar in appearance to those making up the 

 Negaunee jasper, and this is doubtless their source. The rock, however, 

 diflPers from the Negaunee jasper in that, everywhere between the grains, 

 flakes of biotite, chlorite, and sericite have developed. In some sections 

 the biotite is predominant, in others the sericite. In the conglomerates, 

 quartzites, and quartz-schists alike, numerous crystals of magnetite are seen, 

 which are included indiscriminately in all of the other minerals present. ^ 



