2iy2 The American Geologist. JnnG, 1904. 
ning to the southeast. As the latter gradually climbs the west 
side of the range to its crest and low pass at Homestake sta- 
tion, about ten miles distant from Butte, we everywhere saw 
countless boulders, variously angular and in part rounded, 
from a few feet in diameter up to 20 or occasionally 30 feet, 
spread generally almost as plentiful as they can lie on the 
mountain slope. Descending from Homestake eastward to 
the Jefferson river, the same decomposing granite, with many 
boulders, continues ten miles or more, to the base of the range. 
The localities thus described, in the vicinity of Butte, prob- 
ably equalling or surpassing any others in the world by their 
development of bculders in the processes of weathering and 
erosion, are a hundred miles southwest of the limit of the 
glacial drift on the upper Missouri river, as mapped by Cham- 
berhn and Salisbury. The topographic features of the country 
surrounding Butte are doubtless nearly the same now as just 
before the Ice age ; and we may well believe that multitudes of 
weathering boulders were conspicuous there during the long 
Tertiary era of deep erosion of all that region, and especially 
in the closing part of the Pliocene period, immediately pre- 
ceding the Ice age, perhaps about 100,000 years ago. 
If at that time such areas of plentiful boulders, formed by 
rock decay and denudation, lay within the limits of tHe ensuing 
continental glaciation, they would have yielded a great pro- 
portion of the rock masses in the drift for many miles onward 
in the course of the ice currents. Hence the larger share of 
boulders supplied to the drift from some rock areas than 
from others seems to me partly or chiefly due to differences in 
their capacity for preglacial origination of boulders ready for 
transportation in the ice-sheet. 
The diverse conditions of the rock formations in this re- 
spect, however, were dependent in a high degree on the di- 
verse joint structure of the rocks, favoring or hindering their 
separation into residual masses while being subaerially sculp- 
tured and worn sway. Likewise, when glacial erosion had 
removed all the preglacial boulders and decaying sin-face of 
the bed rock, its jointing determined whether it should supply 
many or few additional boulder masses plucked away in the 
grasp of the overriding ice. 
