744 WALTER HARVEY WEED 
All the soda is calculated as albite. Microscopic examina- 
tion shows the plagioclase to bea calcic andesine Ab,, An,, and, 
an estimate being made of the proportion of hornblende present, 
an equivalent amount of lime is deducted and the remainder 
calculated with the albite molecule to form plagioclase. 
The result gives the following composition for the rock: 
I Il 
Quartz - - - - BETO) 20.8 
Orthoclase - - - - 19.88 18. 
Albite - - - - 22.98 28. 
Anorthite - - - - 11.48 TQ 
Hornblende - - - 15.26 16.6 
Biotite = - - - - a Ano — 
Magnetite - - : - 1.18 1.5 
Titanite - - - - - .97 1.4 
Apatite = = 3 : +33 3 
100.00 98.7 
This adds up to 1.51 molecular weight against 1.50 for the 
bulk analysis of the rock, showing a very close agreement. As 
the microscope shows the plagioclase to be andesine of about 
the composition of Ab,, An, (or Sodic labradorite), and albite is 
not seen in the section, the orthoclase must contain the albite 
molecule. In the second column the percentages given are 
those calculated by Lindgren* for the granodiorite of Grass 
Valley—a rock whose chemical analysis, as already shown, closely 
resembles that of the Butte granite. 
The rocks in the vicinity of the Frohner mine are identical 
in composition with the Butte granite, as shown by the analysis. 
The region is about thirty miles north of Butte, and the rock 
part of the general granite batholith. 
THE BLUEBIRD APLITE 
Associated with the Butte granite there is an unusual devel- 
opment of aplite. So far as known to the writer it is the most 
extensive occurrence of a granite aplite yet discovered. The 
* Gold Quartz Veins of Grass Valley, Cal., Seventeenth Ann. Rep. Dir. U. S. Geol. 
Surv., p. 42. 
