MUSEUM OF COMPARATIVE ZOOLOGY. 59 



niechnnically out of the materials of older rocks," yet he describes a 

 qiuirtzite that he regards as having been a fragmental rock. In fiict, 

 his lines between fragmental and non-fragmental rocks seem to have 

 been drawn from the realms of fancy, as a large proportion of his non- 

 fragmental rocks are evidently, both from field and microscopic charac- 

 ters, as truly sedimentary as those classed as fragmental ones. If we 

 understand Dr. Wichmann aright, we presume that the St. Peters 

 sandstone when indurated, as we have frequently seen it, would form a 

 quartzite, while the Potsdam sandstone, if it were indurated in like 

 manner, would form a — sandstone, unless some of it had its cement- 

 ing material entirely crystallized.* 



The reddish and grayish quai'tzite, near the northern railroad track 

 west of Ishpeming, is composed of quartz in rounded grains held by a 

 chloritic cementing material. Considerable hematite was observed as a 

 decomposition product. The quartz contains microlites, trichites, and 

 fluid cavities. The quartzite forming the fallen wall of the New York 

 mine (186) is a very dark greenish-gray rock, composed of rounded 

 grains of quartz, and crystals and fragments of magnetite cemented by a 

 chloritic material. The interstitial substance in this appears to have 

 been entirely changed to chlorite. The quartz contains microlites, tri- 

 chites, and fluid inclusions more abundantly than the rock last men- 

 tioned. A fragment of jasper was observed in this rock. Another 

 specimen of the same rock (187) is composed of macroscopically evi- 

 dent fragments of jasper and magnetite, and quartz grains. Under the 

 microscope the section was seen to be composed of quartz, jasper, magnet- 

 ite, and chlorite. The quartz has the same inclusions, but part of the 

 microlites appear to be zircon. These rocks, as we have before pointed 

 out, were evidently derived from the underlying ore-bed (page 30). 



The quartzite (197, 198) found to be older than the intrusive " Lau- 

 rentian" granite (page 55) is a dark gray rock composed of quartz 

 grains, biotite, and fibrous microlitic cementing material. The quartz 

 contains fluid inclusions and little crystals of zircon. The crystals of 

 zircon ai"e the same as those observed in the gneiss in that vicinity 

 (193), and it is not improbable that the quartzite is derived from it. In 

 this case the order of succession is, 1st, the gneiss; 2d, the quartzite; 

 3d, the eruptive red granite (gneiss of Mr. Brooks). 



The quartzite overlying the ore at the north pit of the Jackson mine 

 at Negaunee is made up of the ruins of the underlying ore. It is com- 

 posed of quartz, jasper, magnetite, hematite, and a fibrous microlitic 



* Geol. of "Wise, III. 613, 615, 6i9, 655. 



