468 Weed and Pirsson—Igneous Rocks of Montana. 
broken by profound faults, while the outer flanks of the range 
show many subsidiary folds further modified by laccolitic and 
other forms of igneous intrusions. Metamorphic rocks, which 
belong to the Archean complex of the region, are exposed in 
the central portion of the range and are covered by a series of 
sediments known as the Belt Mountain formation, belonging 
to the Algonquin (or possibly lower Cambrian) age. The over- 
lying Paleozoic series is well developed, characterized by abun- 
dant fossil faunas, and includes all the subdivisions thus far 
recognized in the northern Rocky Mountains. The Mesozoic 
does not occur within the mountain region, but is everywhere 
upturned upon the flanks of the range. 
The topography of the area is a consequence of its geologic 
structure. The heavily-bedded Paleozoic limestones, which 
are nearly horizontal in the central portions of the mountains, 
form broad and level, pine-clad plateaus whose average eleva- 
tion is 8,000 feet above the sea and 4,000 feet above the adja- 
cent plains. These plateaus are trenched by the streams whose 
valleys vary in character with the nature of the rocks, being 
especially bold and rugged where cut through the massive, 
heavily-bedded Carboniferous limestones. This is especially 
noticeable along the course of belt.Creek and in the canyons of 
the Judith river, the two principal streams which drain the moun- 
tains. The former exposes excellent sections along its course, 
the stream cutting through the Archean area, flowing in a high- 
walled canyon across the Cambrian rocks and emerging from 
the mountains through Sluice Box Canyon, a narrow gorge cut 
in the heavy Carboniferous beds. A branch of the Great 
Northern railroad, built along the course of the stream, runs to 
Neihart, a well-known mining town situated near the head of 
Belt Creek. 
While the range as a whole consists of uplifted sedimentary 
strata, this structure is greatly modified by igneous intrusions 
in the northern part of the mountains. Several large typical 
laccolites, only partly stripped of their sedimentary cover, 
form the dominant peaks of this part of the range, and two 
centers of igneous intrusion have been found where massive, 
granular, igneous rocks occur. One of these is found at 
Barker, a town at the head of the dry fork of Belt Creek; the 
other at Yogo Peak, the most striking and nearly the highest 
peak of the mountains. | 
Yogo Peak.—Yogo Peak is from many points of view the 
most conspicuous elevation of the Little Belt range. Although 
not snow-capped, it projects above the timber line, and its som- 
bre crown of crags, formed of massive igneous rock, is in 
sharp contrast to the rounded summits and level plateaus adja- 
cent. It is situated 8 miles east of Neihart and on the high 
