742 WALTER HARVEY WEED 
The analysis of the Butte rock is the mean of the four 
analyses already given. The granodiorite is the mean of five 
analyses given by Turner. The Boise rock is described by 
Lindgren.? The Banatite is a mean of the analyses quoted by 
Brogger.3 From the analyses it will be seen that the rock closely 
resembles a banatite in composition; the lime and sum total of 
the alkalis being the same, but in the Butte rock the potash 
exceeds the soda in amount. It also resembles the granodior- 
ites, but the alkalies present an inverse ratio. 
In order to furnish a basis for calculating the mineral com- 
position of this rock, fresh rock was crushed, the hornblende and 
biotite separated by Dr. H. N. Stokes by the use of Thoulet solu- 
tion, and these two minerals from each other by sliding on 
paper. The resulting material was examined under the micro- 
scope and found to be quite pure. 
The biotite is quite black in color to the eye, but dark brown 
under the microscope showing very marked pleochroism. This 
biotite is characteristic not only of the Butte granite but of the 
Boulder granite as well. In many sections examined it appears 
the same in color and pleochroism not only for the Butte rock 
but for the normal granite of the batholith, and it may be 
assumed to be the same for both these rocks. The microscopic 
examination of the biotite material showed a little apatite pres- 
ent, a very little chlorite, and very little hornblende, but these 
impurities form a very minute part of the whole. The amphibole 
is also characteristic of both the Butte granite and the normal 
granite of the region. It is black or very dark green when seen 
with a hand lens, but pleochroic in dark green tints with large 
extinction angle when seen under the microscope. The mate- 
rial analyzed was very pure. 
The following table shows the result of the analyses made 
of these minerals. Dr. Stokes was unable to obtain a higher 
+ Woccit.5 ps 150: 
? Mining Districts of the Idaho Basin and Boise Ridge, Idaho. Eighteenth Ann. 
Rept. Director U.S.G.S. 1898, p. 740. 
SWocrcit.; p. 62. 
