404 WEED AND PIRSSON 
the rocks and the structure of the uplift. The accompanying 
geological map (Fig. 2) therefore represents merely a sketch of 
the surface distribution of the various rocks and their general 
relations to one another, boundaries being drawn without any 
pretensions to detailed accuracy. The great areal extent of 
the porphyry is at once seen on the map, and the fact that where 
the streams have trenched through it the underlying crystalline 
schists are exposed. 
Archean-Algonkian.—The nucleal core of the mountains is 
formed of crystalline schists. These rocks are exposed in the 
headwater gorges of all the larger streams and in the deeply cut 
saddles of the main crest or ridge of the mountains. The type 
most usually seen is a black, glistening amphibole schist, or 
amphibolite, fine-grained, dense, and compact, splitting into 
flat, bright-surfaced fragments. In the saddle west of Shellrock 
Mountain the rocks of the series are schistose, occurring in beds 
but a few feet thick and of rapidly alternating character, show- 
ing garnetiferous amphibole and mica schists, pink gneiss 
with sheared and elongated feldspear crystals, and white quartz- 
ites that are clearly altered sandstones. The presence of this 
latter rock makes it certain that the series is of sedimentary 
origin, as the original grains are well rounded though now firmly 
cemented. This would place the rocks in the Algonkian series, 
but similar schists occurring in Montana have been generally 
classed as Archean, and these beds are metamorphosed and 
- quite unlike the slightly altered Belt Mountain Algonkian series. 
SEDIMENTARY ROCKS. 
The sedimentary rocks form the dominant feature of the 
region as seen from the surrounding plains. The geological 
section embraces rocks of the Cambrian epoch, together with a 
considerable thickness of rocks whose age is not definitely 
known, occurring beneath limestones carrying characteristic 
lower Carboniferous fossils. The Mesozoic is represented 
by Jurassic limestones, and a thickness of several thousand feet 
of Cretaceous beds, which are found, however, only in the hills 
