Boulders Due to Rock Decay. — Upham. 371 
Survey, published in 1897, 'i^s a width of 40 miles with a 
len.c^tli of 70 miles from north to south, and has been sculp- 
tured by subaerial erosion into mountains and valleys. "The 
granite," writes Weed, "is traversed by varying, well-defined 
systems of joint planes, is extensively decomposed in some 
localities, and weathers into rugged crags and monoliths and 
great boulder masses which form the picturesque scenery so 
frequently typical of granite areas." 
From its relations to the contiguous stratified rocks, the 
Butte granite is shown to be not older than the Carbonifer- 
ous period, and even to be possibly younger than the Laramie, 
after which, in the early part of the Tertiary era, the general 
mountain folding of the sedimentary formations of the region 
is known to have occurred. Since the time of intrusion of 
the granite, which is considered, in view of its prevailing 
coarseness and evenness of granular texture, to have cooled 
under a cover of elder rocks, that cover and much of the gran- 
ite itself have been eroded. 
About a mile tiorth of Butte, at the highest part of the hill 
tract on whose southern slope the mines are situated and the 
city is built, the granite bears many boulders of all sizes up to 
15 feet in diameter. This hill top, strown and crowned with its 
weathered boulders, is 6,500 feet above the sea, as shown by 
the Butte Folio maps, being nearly 200 feet above the top of 
the rather sharply conical hill called Big Butte (or oftener 
simply "the Butte"), close west of the city, which thence re- 
ceived its name, and nearly 1,100 feet above the Silver Bow 
creek on the south. Specimens broken from these boulders 
are entirely undecayed at the depth of only a few inches, often 
no more than two or three inches, from their surface. The 
effect of weathering extends no deeper than is usually observ- 
able in granite boulders of glacial transportation and deposi- 
tion, which were probably, as I now conclude, gathered in 
large numbers or even mostly from preglacial weathered tracts 
of this kind. 
On the mountain range running from north to south a few 
miles east of Butte, the amazing profusion of the boulders 
formed in the progress of decay of the granite is conspicuously 
seen from the Great Northern railway by the traveler going 
northeastward, and from the Northern Pacific railway run- 
