12 LAKAMIE BASIN, WYOMING. 
The granites and other crystalline rocks present considerable 
variety, which it is not the purpose of this report to map or describe. 
They constitute a floor upon which lie sediments of Carboniferous 
age, but in their extension to other regions they are overlain by 
Cambrian rocks, so that they are known to belong to the Algonkian 
or Archean. J No Cambrian, Ordovician, Silurian, or Devonian sedi- 
ments appear in the Laramie Basin and apparently the Mississippian 
series of the Carboniferous is absent also, except in small amount to 
the north.. The upper Carboniferous is extensively represented by a 
formation consisting mostly of limestones to the north, and of coarse 
red beds to the southwest. This formation is overlain by finer 
grained red beds which, in part at least, may include a representa- 
tive of the Triassic. The marine Jurassic, which follows, is similar 
to that of the Black Hills and of central and western Wyoming, but 
it thins out and disappears near Rock Creek. The next formation 
is the Morrison shale, about 200 feet thick, which presents its charac- 
teristic features. Representatives of the Cloverly formation appear 
throughout the basin; the hard, coarse sandstone at its base (sup- 
posed to represent the Lakota sandstone of the Black Hills region) 
is its most conspicuous feature. The great series of Upper Creta- 
ceous shales has a thickness of 5,000 feet or more and constitutes the 
principal material at the surface or under the Quaternary in the 
Laramie Basin. At the base is the Benton formation, in which, 
however, the middle limestone member (Greenhorn) appears not to be 
present. The Mowry shale member and upper sandstone (upper Car- 
lile) are prominent features in the group. The Niobrara formation, 
with its calcareous sediments, is conspicuous throughout the area. 
The Montana group occupies a broad basin west and north of Lara- 
mie and a smaller area in the basin of Little Medicine Bow Creek. 
Its lower part comprises a thick succession of gray shales such as 
are characteristic of the Pierre, and its upper part consists largely of 
sandstones, possibly overlain by Laramie. The Tertiary deposits 
occur in irregular areas and probably include several formations, 
but the only one that can be correlated is in the Little Medicine Bow 
basin and consists of White River deposits of early Oligocene age 
(Chadron formation). The Quaternary comprises numerous terrace 
deposits of wide extent, the lowest of which are the broad alluvial 
flats along the rivers. 
