20 LARAMIE BASIN, WYOMING. 
200 to 300 feet of limestones about 200 feet above the granite; the 
second was at about the same horizon ; and the third was in the lime- 
stone which lies on a thin mass of basal red sandstones. 
A few fossils obtained from basal beds of the Casper formation 
10 miles southeast of Little Medicine post-office were determined by 
Doctor Girty as Spirifer centronatus, S. cameratusf, and Straparollus 
utahensis, which are regarded as Mississippian in age. The rocks 
are cherty limestones lying upon red shales strongly suggestive in 
appearance of the Amsden formation of the Bighorn Mountains. 
Correlation. — From the general composition and stratigraphic 
relations of the Casper formation it is believed to represent the 
Amsden and Tensleep formations of the Bighorn region, the Min- 
nelusa formation of the Black Hills, or the Hartville limestone of the 
Hartville uplift. The presence of a small amount of basal cherty 
limestone and red shales with Mississippian fossils to the north is in 
accord with this correlation, although these rocks probably do not 
extend southward to Laramie. In the greater part of the region the 
upper beds of the formation are massive buff to white sandstone 
closely similar to the Tensleep sandstone. 
FOft.ELLE LIMESTONE. 
The name Forelle is proposed for the limestone which outcrops along 
the west slope of the Laramie Mountains to a point beyond Red Buttes 
through Tps. 14 to 16 N., and which also appears at Boswell Spring 
and near the head of Sybille Creek. In this outcrop zone the lime- 
stone, which ranges in thickness from 4 to 20 feet, gives rise to a low 
ridge lying a short distance west of the main slope. The shallow 
valley intervening is due to the presence of the Satanka shale, 
200 feet or more thick, which underlies the limestone. The out- 
crop is interrupted by detritus-covered slopes in Tps. 17, 18, 19, 
20, 21, and 22 N., but it is almost continuous from the north side of 
T. 16 N. (sec. 2) to Sportsman Lake. It passes 2 miles east of Lara- 
mie a short distance west of the great springs and crosses the railroad 
at Forelle and Red Buttes. In sec. 11, T. 16 N., R. 73 W., the lime- 
stone is fossiliferous and in part highly gypsiferous. A bed of pure 
gypsum occurs a short distance below; in a near-by pit in sec. 2 
it was found to be 10 feet thick. In the cut just south of Forelle, 
where the limestone is well exposed, it is heavily bedded and nearly 
20 feet thick. To the south, near Red Buttes and beyond to Sports- 
man Lake, the rock becomes gypsiferous and ranges from massive to 
thinly laminated in structure. In places it is brecciated. Several 
beds occur, some of which give place to shale, but there is always one 
bed or more in the section. At the plaster mill a mile south of Red 
Buttes the underlying red shales contain two beds of gypsum about 
25 feet apart. The lower bed, 15 feet thick, is now worked and the 
other, 10 feet thick, was formerly worked. 
