340 



GEOLOGY AND MINING INDUSTltY OF LEADVILLE. 



(lecnmpospd, chlorite aud calcite being the chief products. The plagiochise is still 

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

 rather rare iu this rock. 



Analysis II was made upou a compact biotite i ock, Type VI of the table, from 

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

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

 greenish flakes. Quartz is abundant in clusters of small grains iu 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. 



SiOs 56.62 



TiOj -- I 



AI2O3 



Fe203 



FeO 



MnO 



CaO 



SrO '. I Trace 



MgO 



K2O 



NiiiiO 



H;0. 



16.74 

 4.94 

 3.27 

 0.15 

 7.39 



CO2.. 

 P2O6. 



Total 



Specific gravity, 10° C. 



1.97 

 3.50 

 0.92 

 1.15 

 Trace 



0.08 

 4.22 

 Trace 

 2.82 

 1 43 

 3.98 

 0.62 

 1.08 

 0.23 

 0.04 

 0.90 = 



100. 73 

 2.768 



100. 61 

 2.740 



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 diftereuce lies 

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

 lime-bearing varieties to a very large extent. 



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

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

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

 that iiirnishing Analy.sis I, but occurring as dikes iu the Arehean, one iu Ten-Mile 

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

 tested, aud yielded, respectively, 57.76 per cent, and 57.33 per cent. SiO^, figures 

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

 rock from North Jlosquito amphitheater [132], which occurs side by side with the cor- 

 responding biotite rock (Analysis II), was found to contain 54.54 per cent. S1O2. Thirdly, 

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

 hornblende in the grouudmass and had a number of the rounded quaitz grains macro- 



