Plate XXXII.— THIN SECTIONS FROM CLARKSBURG FORMATION AND REPUBLIC GREEN- 

 STONE. 



Fig. 1. Thin section of sedimentary bed from Clarksburg formation, showing secondary hornblende 

 crystals. No. 17640, from 200 steps N., 1,800 steps W., of SE. corner of sec. 17, T. 47 N., R. 28 

 W. From one of the beds constituting the Clarksburg formation. The large anhedra are of 

 yellowish-green amphibole, idiomorphic in oross-.section, but very irregular in outline in 

 longitudinal section. The groundmass consists of biotite flakes between vhich is a crypto- 

 crystalline aggregate of quartz. The black mineral is magnetite. Natural light. X 50. 



Fig. 2. Thin section of fragmental rock from near base of Clarksburg formation. No. 14785, from 

 SW. i^sec. 32, T. 48N.,R. 29 W. In ordinary light the fragmental structure is plainly revealed. 

 Between crossed nicols the quartz grains (one of which is indicated by the dotted line in 

 the figure) break up into differently orientated portions, so that the section appears like that 

 of a very quartzose gneiss. Polarized light. X 22. 



Fig. 3. Thin section of banded tuff froui Clarksburg formation, SE. i sec. 4, T. 47 N., R. 29 W. The 

 figure shows a large Carlsbad twin of plagioclase in a fine-grained crystalline groundmass 

 composed mainly of quartz, biotite, and magnetite, with a little green hornblende. The 

 "streaming" of the biotite around the upper end of the feldspar crystal may be seen upon 

 close inspection. In polarized light. X 22. 



Fig. 4. Thin section of greenstone from Republic, showing secondary hornblende crystals. No. 16485, 

 from SE. comer of Republic Mountain, SE. i of SE. i sec. 8, T. 46 N., R. 29 W. The rock is 

 associated with the members of the iron formation. The section shows the typical struc- 

 ture of the secondary amphibole in many of the western greenstones. To the right is the 

 cross-section of a small idiomorphic grain. The greater portion of the hornblende exhibits 

 the cellular structure, which has been regarded as the characteristic structure of contact 

 minerals. The clear white areas represent quartz grains and the cloudy areas altered 

 plagioclase. The large light areas at the top and toward the lower right-hand edge of 

 the figure represent spaces in the section. Natural light. X 55. 

 470 



