CARBOXIFEROl S SYSTEM 419 



SO that the lowest beds included in the northern part of the area are not 

 present at Laramie and southward. The Casper formation caps Casper 

 mountain and the series of high ridges extending from east of Freeland 

 nearl}' to Douglas. It extends along both slopes of Laramie mountains, 

 but in places it is hidden by Tertiary deposits, especially in the north- 

 western part of Laramie county. It extends around the south end of 

 Laramie basin and along the west side of that basin as far north as Jelm 

 mountain, beyond which it is dropped far beneath the surface by the great 

 fault which follows the east side of j\Iedicine Bow mountains. It occurs 

 in the syncline of Centennial valley and appears in the uplifts of Freeze- 

 out and Shirley hills and south of Alcova, but in the latter region and in 

 the Eattlesnake mountains and westward the Carl3oniferous can be sub- 

 divided into Madison, Amsden, and Tensleep formations. 



For the greater part of its course the Casper formation consists mainly 

 of 300 to 400 feet of massive limestone and dolomite of light color, with 

 gray sandstone at its top and base. To the north of latitude 42° there is 

 often a red shale member near its base. Excepting near Laramie, it is 

 sharply separated above from the red shales of the Chugwater formation. 

 The basal sandstone lies on the pre-Cambrian. Near the latitude of Lar- 

 amie the formation includes many beds of sandstone, and these increase 

 in thickness to the south and west, until near the Colorado line very little 

 limestone remains. Much of this sandstone also is of red color — a feat- 

 ure which becomes even more prominent in Colorado, where along the 

 east side of the Front range the formation consists mostly of red beds 

 and becomes the lower Wyoming, or Fountain formation, of previous 

 publications. 



Casper to Lc Bonfe creel. — The limestones and sandstones of Casper 

 mountain and the ranges extending east are from 350 to 700 feet thick 

 and lie imconformably on the pre-Cambrian. There is usually a basal 

 sandstone or quartzite, often conglomeratic, generally from 60 to 100 feet 

 thick to the west, but thinning to the east, so that in the slopes west and 

 south of Douglas the limestones lie very near or directly on the schists 

 and granite. The limestone is from 200 to 500 feet thick, and at the top 

 of the formation is a hard, gra}', massive sandstone from 50 to 60 feet 

 thick, which doubtless represents the Tensleep sandstone of the Bighorn 

 and "Wind Eiver ranges. This tripartite character is shown in lower 

 cut of plate 25. The limestones are mostly of light gray color, mas- 

 sively bedded and hard, and some of them contain chert. T?ed shales 

 and thin sandstone members are included locally. Extensive exposures 

 occur in Muddy, Deer, Boxelder, and La Prele canyons, the latter show- 

 ing a fine arch and the upper sandstone member constituting the well 



Xr. — Bull. Geol. Soc. Am., Vol. 10, 1007 



