Weed and Pirsson—Igneous Rocks of Montana. 467 
Art. LIil.—Ilgneous Rocks of Yogo Peak, Montana ;* by 
W.H. Weep and L. V. Pirsson. 
THE two great geographic provinces of Montana, the Plains 
and the Cordilleran region, which form the eastern and west- 
ern parts of the State respectively, are quite as sharply delim- 
ited as is usually the case along the Rocky Mountain front. In 
-the central part of the State, however, the broad level expanse 
to which the Cretaceous rocks have been reduced is broken by 
several mountainous tracts, rising abruptly above the plains 
and suggesting the Indian designation of “ Island Mountains.” 
To the westward the continuous but irregular front of the 
Rocky Mountain Cordillera stretches in a sinuous, indented 
line, in a generally northwest and southeast direction. 
In the northern part of the State the Rocky Mountains pre- 
sent an abrupt chain of rugged, serrated peaks, which are visi- 
ble for a hundred miles eastward. ‘To the southward, where 
the Missouri emerges from the mountains and enters the open 
plains, the main chain or continental watershed is flanked by 
the low and broad mountainous area of the Belt ranges, and 
still farther south, where the waters of the Yellowstone Lake 
feed the mountain drainage, the front of the Cordillera is com- 
posed of several lesser and detached mountain systems. 
The Belt Mountains, although a part of the Rocky Mountain 
Cordillera, constitute an area between the western limits of the 
Plains and the mountain valley of the Missouri river. This 
broad and relatively low mountain district is separated by the 
wide, intermontane valley of Smith river, a tributary of the 
Missouri, into two ranges, known as the Big Belt and the Lit- 
tle Belt mountains. Yogo Peak, whose rocks form the subject 
of the present paper, is a conspicuous summit of the Little 
Belt Mountains, the easternmost of these two ranges. 
The Little Belt Mountains thus form a distinct geographic 
unit in the topography of the State. They are flanked by the 
valley of the Musselshell river on the south, and extend, from 
the Judith Gap on the east, westward to their union with the 
Big Belt Range in the canyon of the Missouri river. They 
are embraced between the meridians of 109° 45’ and 112° west 
longitude, and the parallels 45° 30’ and 47° 15’ of latitude. 
The Belt ranges together constitute a broad anticlinal upiift 
formed by the union of the lesser anticlinal axes of the two ranges. 
The Little Belt uplift has a southeast and northwest trend and 
dies out in the eastern point of the range at Judith Gap. In 
the central portion of the mountains the anticlinal uplift is 
* By permission of the Director of the U. S. Geological Survey. 
Am. Jour. Sc1.—TsIrD SERIES, Vou. L, No. 300.—DECEMBER, 1895. 
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