Plate XVI. — From the Chebty Limestone Member. 



Fig. 1. Tremolitic dolomite. Specimen 9678, slide 3165. From the NW. i of Sec. 22, T. 44 N., R. 5 W., 

 Wisconsin. In ordinary light, X 60. The finer grained part of the section shows the evenly 

 granular appearance characteristic of the massive limestones and dolomites. In one part 

 of the figure is coarsely crystalline carbonate and a broad blade of tromolite. The sec- 

 tion chances to be so cut that tliis mineral appears in part as a mere film, below which the 

 vague outlines of the particles of dolomite are seen. (See p. 135.) 



Fig. 2. Cherty limestone. Spoaimen 7485 A, slide 1934. From the SE. Jof Sec. 18, T. 47N., R. 44 W,, 

 Michigan. In polarized light, X 60. The central band of the figure is largely granular 

 dolomite, in which is, however, a considerable quantity of chert. This dolomite is inter- 

 laminated with two layers of nearly iiure chert, one of which is much more finely <ry8- 

 tallinc than the other. (See j). 137.) 



Fig. 3. Concretionary chert. Specimen 9434, slide 3131. From the NW. i of Sec. 41, T. 47 N., R. 

 45 W., Michigan. In polarized light, x 60. A fine grained chert. The minute mosaic char- 

 acteristic of chert when viewed in polarized light is nicely shown*. The material is 

 arranged in a vague concretionary fashion, areas composed of very finely crystalline silica 

 . being surrounded by borders of more coarsely crystalline quartz. Although imperfectly 

 shown in the figure, the latter has to some extent the radial fibrous arraugement of chal- 

 cedony. (See p. 137.) 



Fig. 4. Chert. Specimen 9424, slide 3064. From the NW. i of Sec. 14, T. 47 N., R. 45 W., Mich- 

 igan. In )>olarized light, X 60. The section shows very well the variations in fieneuess of 

 grain of the mon^ coarsely cryslallinc varieties of chert. It is wholly coiiiposi'd of com- 

 pletely individualized quartz, tlie inti'icate interlocking of which is well shown and which 

 is in strong coutnist to the ajipearaucc presented by quartzites in which the interspaces 

 have l)eeu tilled by the enlargements of fragmental quartz. (See p. 137.) 

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