GRANITE ROCKS OF BUTTE, MONT., AND VICINITY 741 
the mass. Owing to disintegration prefectly fresh specimens 
can only be obtained from surface quarries or underground work- 
ings. The exact relations of this mass to the general area were 
not satisfactorily determined, though it appears certain from the 
exposures that it is an integral part of the batholith and not a 
separate intrusion. At several localities a sharp gradation was 
observed, with narrow transition bands between the lighter 
colored granite with its white feldspars and the darker Butte 
type. 
Orthoclase is an abundant and usually a readily recognizable 
constituent as its pinkish color is in contrast to the green tones 
of the plagioclase, and it has, moreover, a tendency to develop 
in relatively large crystals which give the rock a somewhat por- 
phyritic look. Plagioclase, black hornblende, black biotite, and 
quartz are easily distinguished by the eye. Under the micro- 
scope the rock is seen to vary between a rather basic hornblende- 
granitite and a quartz-diorite. There is usually a slight amount 
of chlorite present, but the biotite and hornblende are as a rule 
fresh. Titanite, apatite, iron ore, and zircon are present as 
accessories. It will be seen that the rock is only a somewhat 
more basic phase of the granite of the region, and that it closely 
resembles granodiorite, though in the Butte rock the plagioclase 
is more basic, being a sodic labradorite. The four analyses given 
in the general table show the rock to be remarkably uniform in 
chemical composition over the entire area. 
The following table gives partial analyses of the Butte granite 
and of the related rocks from other localities : 
PARTIAL ANALYSES OF BUTTE GRANITE AND RELATED ROCKS 
aenuse) NGpeeen ORS aes Boulder 
D1 Oseretes sitar 64.03 65.48 65.25 64.39 67.12 
ANIEN @)rvecterietsie 15.58 16.05 16.94 15.90 15. 
INS OW las Gnoe 1.96 1.47 1.60 5.63 1.62 
He Oa as 2.83 3.02 1.91 222 
Ca@ arte 25 x 4.20 4.80 3.85 4.15 3.43 
WEROeatownia ae 2eTic 2.12 1.31 1.93 gy jal 
KA Omer eae 4.11 6.8 2.43 2°02 SG) Ai52 
Nias Olensercse: 2.7.0) ae 7 3.49 By/ 3.49 7.05 2.76 
