Plate XVIII. — From the Qitartz-Slate Member. 



Fig. 1. Giaywacke-slatp. Specimen 9442, slide 3071. From the SW. i of .Sec. 10, T 47 N., R. 45 

 W., Michigan. In orilinary light, X 60. The figure represents a typical average grained 

 graywacke-slate. The rounded fragments are in about equal quantity quartz and feldspar. 

 Chlorite is the chief insterstitial mineral. (.See pp. 165-166.) 



Fig. 2. The same in polarized light. The orthocla.se is in most cases separable from the quartz, 

 In that it lacks the perfect clearness and uniformity of color which each grain of that 

 mineral shows. The striated feldspars are nicely shown. (See pp. 165-166.) 



Fig. 3. Cherty slate. .Specimen 9641, slide 3310. From the NW. i of Sec. 13, T. 47 N., K. 46 W., 

 Michigan. In polarized light, x 60. The background of the section i.* an ordinary chlo- 

 ritic slate. It contains, however, in certain laye»s numerous large well rounded frag- 

 ments of (juartz and chert, both of which are well shown in the figure. (See pp. 164-165.) 



Fig. 4. .Serlcitic and cliloritic slate. Specimen 9523, slide 3091. From the SE. i of .Se<'. 16, T. 47 N., 

 R. 46 W., Michigan. In polarized light, X 60. The section illu.strates one of the rather uni- 

 form fine grained feldspathic quartz-slates. The small roundish areas are in part quaPtz 

 and in part feldsjiar. The dark nuitcrial is mostly chlorite and grains of ([uartz and feld- 

 spar, which chance to be near the point of extinguishment. (See p. 164.) 



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