metas Se: 
Weed and Pirsson—Igneous Locks of Montana. 309 
Art. XXXIV.—On the Igneous Rocks of the Sweet Grass 
Hills, Montana; by W. H. Weep and L. V. Pirsson. 
In northern Montana, the broad expanse of the great plains 
that lie east of the Rocky Mountain Cordillera is interrupted 
by a group of peaks, rising abruptly from the general level 
near the Canadian boundary line. These peaks, forming three 
isolated mountain masses, are known as the Sweet Grass Hills, 
or Three Buttes. Their sharp outlines and isolated position 
attract attention from every point of view, while recent dis- 
coveries of copper leads and ores of the precious metals, 
together with the excellence of the coal beds upon their flanks, 
have caused an influx of prospectors from the mining regions 
of the state. : 
The locality is now easily reached, as a tri-weekly stage runs 
from Shelby junction, where the Great Falls and Canada road 
crosses the Great Northern Railway, to the settlements on 
Birch Creek. The East and West Buttes are twenty miles 
apart, the Middle Butte lying to the south about midway 
between. The summits reach a height of 3,000 feet above 
the plains, and each butte is the center of a small mountain 
area whose verdure-clad slopes, copious springs and flowing 
streams are in strong and pleasing contrast to the desolate 
monotony of “ Lonesome” prairie to the south. Through the 
kindness of Dr. G. M. Dawson, Assistant Director of the 
Canadian Geological Survey, who is the only geologist that has 
explored the Hills, the authors have been allowed to examine 
a number of specimens of the volcanic rocks constituting the 
central cores of these interesting mountain masses. The gen- 
eral geology of the Sweet Grass Hills has been described by 
Dr. Dawson,* from whose report the following notes have been 
taken. 
The three buttes constituting the Sweet Grass Hills occur in 
the center of a broad and low anticlinal uplift that stretches 
for many miles parallel to the Rocky Mountain front, from 
which it is separated by a broad, shallow, synclinal basin. 
Each mountain mass consists of a core of igneous rock sur- 
rounded by Cretaceous beds uptilted at high angles, whose 
inclination gradually becomes less and less away from the erup- 
tive rock, merging into the horizontal strata of the plains. 
The igneous rocks forming the central masses of these 
mountains, though very dense and compact, are seldom seen 
in solid masses, since they break readily into irregular, angular 
fragments from a few inches to two feet across, forming debris 
* Report Canadian Geol. Survey, 1882-4, C, pp.16, 45. “ Report upon country 
in vicinity of the Bow and Belly rivers, Northwest Territory.” 
Am. Jour. Sct.—Tuirp Series, Vou. L, No. 298.—OcrToBER, 1895. 
