Plate XVII. — From the Base of the Quartz-Slate Member. 



Fig. 1. Chert containiug fragineiital ciuartz. .Specimen 9534, slide 3140. From the NW. i of See. 

 14, T. 44 N., R. 3 W., Wisconsiu. lu polarized light, X 30. The background of the section 

 is composed of finely crystalline quartz. In it are contained larjje well rounded grains of 

 quartz, one of which has plainly received a second growth. In the lower part of the figure 

 is a complex rounded area of chert not greatly ilifi'erent from the matrix in which it is set. 

 (See pp. 157-158.) 



Fig. 3. yuartzo.se (Oiert. Specimen 95.35, slide 3141. From the same place as Fig. 1. In polarized 

 light, X 60. The center of the figure shows in the cherty background a large quantity 

 of magnetite. In the exteriors of the largo fragmental grains of quartz.are very numerous 

 minute needles which are taken to be actinolite. In certain cases these actinolite needles 

 are included only in the enlargements of the (jnartz grains, but in others appear to penetrate 

 rather deeply into the cores. This occurrence strongly suggests that actinolite may develop 

 in such a manner as to penetrate (jnartz the latter, of course, being simultaneously removed. 

 (Seepp, 157-1.5«.) 



Fig. 3. Chert-congli)ineratc. (Specimen 9410, slide 3127. From the SW. i of 8cc. 14, T. 47 N., R. 45 

 W., Michigan. In polarized light, X HO. The background of the conglomerate is a chloritic 

 quartz-slate, the quartz-grains of which at many places distinctly show thc^ <nlargen)cnts. 

 The large roundish area with the spotty appearances is a part of one of tlic smaller of the 

 chert fragments which, in the form of pebbles and liowldcrs, are very abundantly contained 

 in the rock. (See p. 169.) 



Fig. 4. fireen-schist conglomerate. Specimen 9175, slide 2994. From the SE. i of Sec. 19, T. 45 N., 

 K. 1 E., Wisconsin. In polarized light, X 6(). I'lic figure is fnun the Potato river basal 

 conglomerate. For tlie most part it consists of three large fragments of green srliist. which 

 in <me case very distinctly shows the scliislose structure. These fragments are set in a 

 matrix consisting very largely of small roundi.ih (luartz-graius, mingled with which are 

 quite numerous well ilefiued crystals of magnetite. 

 (See p. 159.) 

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