Weed and Pirsson—Igneous Rocks of Montana. 469 
divide between the waters of Belt Creek and the Judith river 
and reaches an elevation of 9,000 feet above tide. The ridge 
above which the summit stands has an open and park-like char- 
acter. The adjacent streams have cut deeply through the hori- 
zontal sedimentary rocks, and the broad valley bottom of Dry 
Wolf Creek on the north and the placer bars of Yogo on the 
south are 3,000 feet below the crags of the mountain summit. 
The region about Yogo Peak is formed of Paleozoic rocks in 
which there are numerous intrusive sheets and occasional dikes 
of igneous rocks. The mountain mass itself is due to the resist- 
ant nature of the massive igneous rock, which formsa stock break- 
ing up through the horizontal sedimentary rocks. The flanks 
of the mountain show these bedded rocks well exposed and form- 
ing benches which mark the more resistant beds. The stock 
itself is a huge chimney of massive rock that has produced 
considerable contact metamorphism near its junction with the 
sedimentary rocks. On the southwest side of the peak an 
uplifted block of the limestones has been broken off, tilted 
and injected with a number of intrusive sheets thrust in from 
the adjacent voleanic neck. The northern slopes are obscured 
by debris and talus slides which hide the exact contact between 
the stock and the sedimentary rocks, but on the south face of 
the mountain a branch of Yogo Creek has cut an amphitheatre, 
whose massive walls of naked rock rise hundreds of feet above 
the basin whose deep blue lakelets and perennial snow banks 
form a pleasing feature of the mountain scenery. The marble- 
ized and altered limestones are here seen in actual contact with 
the massive rock. In general, the alteration of the sedimenta- 
ries adjacent to the stock rock has been accompanied by miner- 
alization, producing ore deposits that have been prospected at a 
great number of points, although they have thus far proved of 
too little value to warrant working. The summit of the peak 
is composed of three knobs or hills, separated by small saddles 
and grassy Alpine meadows. These summits lie in a nearly 
east and west line, the two outer knobs being a mile and a half 
apart. They are covered with projecting crags or piles of 
debris, with the rock in place below them. To the east the 
peak ends in a shoulder covered by a heavy mantle of platy 
slide rock, which extends down to a low saddle separating it 
from the low mountain ridge which extends eastward to Wood- 
hurst Mountain. The mountain summit shows no evidence of 
glacial action, and a small moraine in the amphitheatre is the 
only record of former glaciation. The Yogo Peak core is thus 
a huge chimney of massive granular rock, having a length of 
two miles in an east and west direction, and a width of one 
mile. The study of the region shows that this stock occupies 
the southwestern and expanded end of a great fracture which 
