642 IDAHO MINING DISTRICTS. 



SiO 2 5.85 



.ALA- - 3.85 



Fe 2 3 - - -- i' 19 



FeO.... - 1-71 



MgO 1.31 



K 3 O --- -- 1-20 



H 2 0.. -88 



Biotite..- --- 15.99 



Hygroscopic water . -.- .18 



Total - 99.78 



The biotite would have the following composition: 



Si0. 2 --- - 36.59 



A1 2 0, -- --- 24.08 



Fe,O 3 ---- 7.44 



FeO 10.69 



MgO... - 8.19 



K;O._.- - '--- 7.51 



H 2 O.. 5.50 



100. 00 



The water is somewhat too high, no doubt due to the fact that chlo- 

 rite and other decomposition products are present. 



According to mineralogical composition, habit, and chemical com- 

 position, this rock corresponds closely to the granodiorite of the Sierra 

 Nevada, the only difference being that it contains no hornblende, 

 which in the granodiorite is as a rule abundant. It thus occupies a 

 position between a quartz-mica-diorite and a granite. However, as it 

 grades imperceptibly over into more normal granites, it has not been 

 thought worth while to segregate it from that rock. The plagioclase 

 in the Willow Creek rock is, according to the ratio between albite and 

 anorthite, Ab 2 An, or a basic oligoclase. 



The second rock (80 Boise collection) is grayish white, granular, 

 with a clearly apparent granitic habit. It consists of quartz grains of 

 about the same size as in the fresh rock, while a greenish-gray com- 

 pact mass replaces the feldspar and has a hardness of about 3. The 

 biotite is replaced by a dull- white micaceous mineral. Small crystals 

 of pyrite abound, and the rock is also traversed by a few small quartz 

 veins. In a few places small grains of zinc blende are seen. 



Under the microscope, large grains of quartz with undulous extinc- 

 tion are noted. Between them lies a fine-felted mass of sericite 

 fibers, calcite grains, and in places a little fine-grained quartz. No < 

 feldspar remains, though occasionally the outlines of the grains may 

 still be noted. In places larger muscovite foils appear, evidently 

 representing the original biotite. The quartz is in many places 

 attacked by sericitization, fibers and tufts of sericite and calcite 

 developing in it or projecting into it from the surrounding sericite 

 mass. Titanite, extremely abundant in the unaltered rock, is con- 



