106 GUIDEBOOK OF THE WESTERN UNITED STATES, 
number of rounded forms which the granite assumes on weathering 
are due to the facts that the corners and edges are more exposed to 
the attacks of weather than broad surfaces are, and that this rock, 
which is popularly regarded as the type of stability, readily disinte- 
grates or falls to pieces on exposure to the atmosphere. At many 
places along the track the granite has been reduced by weathering 
to fine fragments called “‘sand.”’ 
The general slope leading to the summit seems to be an old sur- 
face, on which the granite is deeply decayed, indicating long exposure 
to the weather, but the ravines, which have been cut more recently, 
are jagged and irregular. The difference in the two surfaces may be 
seen near milepost 56, where the railway leaves the smooth surface 
of the upland and enters a gorge which is cut below the general level 
and which is marked by blocks of all sizes and shapes, including 
domes, towers, and pinnacles that seem to be scattered over the 
ground in hopeless confusion. 
This combination of smooth upland and rocky canyons continues 
to the top, which is remarkably free from rugged peaks. Although 
there is a short tunnel here, the real summit of the mountain is only 
about 100 or 150 feet above the railway track. 
This is the backbone of the continent, the height toward which 
the train has been climbing since leaving Missouri River at Mandan. 
The traveler may be disappointed in the Continental 
Divide, for it is no more conspicuous than many of 
the other ranges in sight. In fact, the Rocky Moun- 
ains are a great complex of mountain ranges, no one 
of which dominates the others to any extent. On the west side of 
the Homestake tunnel can be obtained a better realization of height, 
for the descent to Butte is made along a rugged mountain side, and 
one can look down to the left 1,000 feet to the valley, seemingly almost 
vertically below.! (See Pl. XVI.) 
‘The difference in the appearance of 
Homestake. 
Elevation 6,345 feet. 
St. Paul 1,118 miles. 
the two sides of the Continental Divide 
is, very striki e east side the 
slopes are gradual, so that the railway 
can follow. almost directly up the old 
smooth slope, but on the west the slope 
is so precipitous that the road must be 
graded along the rocky mountain side for 
8 or 9 miles in order to get down to the 
valley floor. So steep and regular a 
front bordering a broad, flat valley sug- 
gests some other mode of origin than 
erosion, which is the normal agency in 
the production of mountains and valleys. 
It gives the impression that this i 
7 4 ee, eee 
kh 
ig height to the 
carving out of the valley by rain and 
streams, is really separated from the low- 
land by an actual break in the rocks and 
has been lifted bodily to its present posi- 
tion, or the valley has been depressed 
with reference to the mountain. If the 
rocks along the mountain front south of 
Butte are examined, evidence will be 
found to show that there has been recent 
movement in them and a break (fault) or 
series of breaks along the west face which 
allowed the great block of strata on the 
east side of the fault to be raised at least 
1,000 feet with reference to the strata 
on the other side. The steep mountain 
front is therefore the cut edge of a great 
