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GEOLOGY AND MINING INDUSTRY OP LBADVILLE. 



decomposed, chlorite and calcite being the chief products. The plagioclase is still 

 quite fresh, but some filmy calcite is scattered through the groundmass. Apatite is 

 rather rare in this rock. 



Analysis II was made upon a compact biotite lock, Type VI of the table, from 

 North Mosquito amphitheater, where it occurs as a dike in gneiss [260]. There is uo 

 hornblende present in this rock and biotite appears mainly in numberless minute, 

 greenish flakes. Quartz is abundant in clusters of small grains in the grouudmass, 

 but seldom reaches macroscopic dimensions. Apatite is quite abundant. Pyrite is 

 the chief ore of the rock, accompanied by some magnetite. Except for some calcite 

 and chlorite the rock seems to be very fresh. 



Discussion of analyses There is far more difference between the two rocks than 

 one would surmise from the microscopical study, but it is after all not so remarkable 

 when one considers how little substance is actually represented by the minute flakes 

 of biotite in contrast to that in the numerous prisms of hornblende. The difference lies 

 chiefly in the large amount of hornblende, while the feldspars in both are plainly soda- 

 linie-hearing varieties to a very large extent. 



In order to test the influence of the amount of hornblende present, as indicated 

 by the macroscopical appearance of the rock, special silica determinations were made 

 by Mr. Hillebrand upon various types. First, two rocks having closely the habit of 

 that furnishing Analysis I, but occurring as dikes in the Archenu, one in Ten-Mile 

 amphitheater [125], the other on the east wall of Arkansas amphitheater [131], were 

 tested, and yielded, respectively, 57.76 per cent, and 57.33 per cent. SiO*, ligim-s 

 agreeing quite closely with those of the analysis. Secondly, the compact horubleudic 

 rock from North Mosquito amphitheater [132], which occurs side by side witli the cor- 

 responding biotite rock (Analysis II), was found to contain 54.54 per cent. SiO 2 . Thirdly, 

 a rock from the extreme head of Buckskin gulch [121], which contained very little 

 hornblende in the groundmass and had a number of the rounded quartz grains macro- 



