THE QUARTZ-SLATE MEMBER. 157 



interstitial mineral be ainpliilx)!^. Most of the quartz individuals have plaiidy been 

 enlarged, and Mie rock is certainly an ordinary trai^tnental ijuartzite. In the ease of 

 a few of the large (juartz individuals the outlines of the original fraginental cores 

 are not visible; but these areas are so coHi])letely like the otin-rs, in wiiieli the cores 

 are iierceiitil>Ie, that all must b(^ taken as of the same nature. Both enlargements and 

 oi'iginal cores are composed of a siugularly pure limiiid (piartz; whence the occasional 

 invisibility of the outlines of the cores. Tiiis section, then, furnishes us abundant 

 proof ol' the i)ossibi]ity, under favorable circumstances, of the development, from an 

 (H'dinary samlstone by enlargement of the qiuirtz fragments, of a (juartzitt! whose 

 fragiin'iital cliaracter would never be suspected from the thin section. In this same 

 connection reference should be made to 2 and ."5, above described, whose fragmental 

 (wigin is less clear than in the section now under consideration. 



From thv section in tScc. 1(1, T. II N.. h'. :i 11'., Wisconsin. 



<). Hornblendic anil chioritic (juartzite, from tiie up])ermost layers and imme 

 diately beneath the Iron-bearing juembcr. Specimen 9iH:i (slide 3153). From 1S5() 

 N., 10r,0 W., Sec. l(i, T. U N., R. 3 W.. Wisconsin. 



A rock similar to l', both macroscopically and microscopically. 



From tlir rciiiihcc i/dj) .section. 



7. Magnetitic chert-breccias, from the base of tin' (i>uartz slate menil)er. Speci- 

 mens 9.")34 (slide 3110), 953.") (slide 3141), 1421, Wis. (slide 252). From 1470 N., 1200 

 W.; 1455 Wis. (slide 205). From 1500 N., 1U35 W., Sec. 14, T. 41N., R.3 VV., Wisconsiu. 



In these peculiar rocks a greenish to black schistose matrix contains numerous 

 angular pieces of white chert, and fewer smaller ones of clear (juartz. 



The groundmass of the, thin sections (;onsists mainly of a minnti'ly crystalline 

 silica of nonfragmental appearance. This is explained by the fact that this rock 

 rests directly upon the friable quartz rock (Nos. 7 and 8, p. 13(>) of tlic ciicrty lime- 

 stone and has derived nn)st of its material from this underlying forination. It 

 approaches closely to a rccomi)osed <|uartz rock, the i)articlcs of which are separate 

 grains. With the (juartz arc mingled more or less chlorite and actinolite, some 

 brown and red iron oxide, and a large <]uantity of magnetite. In 3140 and .iUl this 

 magnetite plays a very subordinate part. In 252, liow^'ver, it is very alimidant, 

 while ill 2(15 it |ireilominates over the silica of the cement, occurring in most Jieauti- 

 fully outlined crystals, often of a very considerable size. There is little ai first sight in 

 the matrix of any of these rocks to suggest a fragmental origin. IIoucNcr, there are 

 contained in it, in quantity sutlicient to compose from one-third to three fourths of the 

 area of the thin sections, fragments of (|uartz and of a minutely crystalline silica or 

 chert. riie(|uaitz fragments are well rounded, and the jiic'es of chert are only jiartly 

 so, being often quite angular. The cUert jiieces reach as much as a quarter of an inch 



