THE QUARTZ-SLATE MEMBER. 171 



An olive green rock of nearly aphanitic texture and slaty structure. 



Tiie tliin section sliows a i)redominatiug fine grained hackgrouiul coinpo.sed of 

 flakes of chlorite, sericite, and kaolin, witli some minutely crystalline (|nart/., tlirough 

 whicii are scattered rather pliMitifully small fragments of quartz and feldspar. The 

 section is cut across the lamination of the slate, and shows flakes of chlorite, sericite, 

 etc., arranged with their longer axes in a common direction. 



.")0. Quartzites, from a high horizon. Specimens 12(iS0 (slide 5402), 12081 (slide 

 5403). Prom 477 N., 795 to 825 W., Sec. 17, T. 47 N., R. 44 W., Michigan. 



These siiecimens, taken from the same pit, differ in character; 12680 is a massive, 

 medium grained, ordinary quartzite; 12081, a massive, strongly magnetitic quartzite. 



The thin section 5402 is that of a tyjiical quartzite, whose induration and 

 vitreous appearance are due to the wide enlargement of each grain. Broad lilms of 

 chlorite and ferrite .separate cores and enlargements and occur again along the coji- 

 tacts of the enlargements themselves. There are some irregular areas com])osed of 

 hematite, magnetite, and a minutely crystalline (piartz. The (piartz areas are all 

 arrangetl with their longer axes in a common direction jiarallel to the structure, which 

 perhaps indicates subjection to pressure. The thin section 540;i differs fro7n'5402 in 

 that the fragmental ((uartz grains compose only about half of the mass of the rock, 

 there being a very plentifnl interstitial material consisting mainly of magnetite and 

 C(mtaining also a considerable (|uantityof hematite, chhu'ite, and minutely crystalline 

 quartz. The magnetite in this inatrix is for the most part in veiy regular outlined 

 cry.stals. These crystals occur also within the enlargements of the ipiartz fragments, 

 and occasionally ap]iear to be even within the cores themselves, altiu)ugh this appear- 

 ance may be due to the positions at which these grains are cut. Tliey also occur 

 somewhat jilentifully along the bonmling liue between fragmental cores and enlarge- 

 ments. That portion of the magnetite which occurs within the cores themselves 

 appears to be very distinctly more plentiful in the neighborhood of their (tutline-s. 

 As to this singular occurrence, see the description of 7. 



Contacts with the Chrrfi/ Vimrstour iiiemhrr. — A.s ii, rule tlie eojitact of 

 the Quartz-slate member witli tlie nu)re southerly rocks is concealed. At 

 several points, however, it may be seen, either in contact with or very close 

 indeed to t^xposures of the white chert or of the limestone itself (»f the 

 underlying formation, and in orher places again with the granite, gneiss, or 

 schist of the Southern Complex. The contacts with white chert are at 

 Penokee gap, Sec. 14, T. 44 N., R. 3 W., Wisconsin; in the NW. \ Sec. 16, 

 T. 44 N., R. 2 W., Wisconsin ; in the northern j)art of Sec. 14, T. 47 N., R. 

 4h W., Michigan; and in the SW. \ Sec. 17, T. 47 N., R. 44 W., Michigan. 

 It has alread)- been intimated that in these places there are found in tlie 



