GRANITE ROCKS OF BUITE, MONT; AND. VICINITY: 
In the western mountainous part of Montana there are sev- 
eral extensive areas of granitic rocks, which are commonly sur- 
rounded by sedimentary beds and in part covered by later 
volcanic rocks. The largest of these granite masses forms a 
mountainous area having no commanding summits, but consti- 
tuting the continental water parting separating the waters of the 
Atlantic from those tributary to the Pacific Ocean. This district 
is largely drained by the Boulder River, and as the mountains 
have no other name, they too are sometimes called by this name, 
for which reason it will be used to designate the intrusive mass of 
granite itself. Unmistakable evidences of intrusion are common 
about its borders, and as the rock cuts and metamorphoses fos- 
siliferous Carboniferous rocks and what are believed to be Creta- 
ceous rocks as well, and is overlaid by Neocene sediments, its 
age is known within these limits. 
The Boulder batholith is a body of granitic rock, in part cov- 
ered by later lavas, but continuously exposed from the Highland 
Mountains (sixteen miles south of Butte) to the vicinity of 
Helena, a distance of fifty miles in a north and south direction 
and twenty-four miles from east to west. The intrusive nature 
of the mass is very strikingly shown at the northern and southern 
limits, and also at Elkhorn on the east. At these places the 
granitic rocks have produced very marked contact metamorphism, 
and cut across the ends of the upturned sedimentary series. 
Near its border the granite also includes in its mass fragments of 
the other rocks. There is no suggestion of a laccolithic uplift- 
ing, for although near Helena, and probably elsewhere, the 
granitic rocks extend outward under the sedimentary rocks, and 
the latter in certain places form a thin cover over the intrusion, 
yet the strata dip toward the intrusion conformable to a great 
anticlinal uplift wholly independent of the batholith. 
* Published by permission of the Director of the U.S. Geological Survey. 
737 
