AUTHORS' ABSTRACTS 247 
turns northeast and enters the broad, open valley beyond the frontal 
ranges of the Rocky Mountains. 
The rocks forming the surface of the country are partly crystalline 
schists, including gneiss schist, with granite and other granular 
rocks; partly sedimentary formations, including limestone, sandstone, 
and shales; and partly lavas and other igneous rocks. ‘The crystalline 
schists are mainly Archean, and constitute a large part of the southern 
half of the region. They form the high mountains and plateau 
drained by Boulder River, and those from Emigrant Peak south. A 
small area of sandstones, conglomerates, slates, and arenaceous lime- 
stones occurring in the Bridger Range have been referred to the 
Algonkian. They lie unconformably upon the crystalline schists, and 
are overlain unconformably by the Paleozoic series. 
The sedimentary formations cover one-half the area, and present a 
total thickness of 20,000 feet, embracing all the grand divisions of 
geologic time since the Archean. The chief feature is the great devel- 
opment of the latest Cretaceous strata, which are 12,000 feet thick above 
the Laramie, the total thickness of the Paleozoic being only 3500 feet. 
The series from the basal (Flathead) quartzite to and including the 
Laramie coal beds, is conformable throughout. The Palceozvic strata 
occur upturned at steep angles against the crystalline schists, or in 
steep anticlines. The lowest bed is the Flathead quartzite. Above it 
are shales and limestones of Cambrian age. ‘The Silurian is repre- 
sented by only a few feet of formation, whose precise age is doubtful. 
Four hundred and fifty feet of shales and limestones represent the 
Devonian. The Carboniferous strata are 2000 feet thick. They are 
here, as elsewhere, the mountain limestones, and form the crest of the 
Bridger Range and the summits of some peaks of the Snowy Range. 
The Trias is recognized only in the southern part of the region, as 
thin belts of red sandstone. The Jura varies considerably in charac- 
ter, being mostly shales and fissile limestones. These two formations 
are 500 feet thick. 
The Cretaceous constitutes more than one-half of the total thick- 
ness of strata. Its lowest member is the Dakota conglomerate, with 
sandstone andsome shale. Over this is the Colorado group, including 
Benton shales and Niobrara limestone, aggregating 1800 feet in thick- 
ness. Over this is the Montana group, 1800 feet thick, consisting of 
Pierre shales and Fox Hills limestone. The Laramie sandstone, with 
some intercalated clays and beds of coal, is 1000 feet thick. Above 
