-114 K. H. DARTOX PALEOZOIC AXD MESOZOIC OF ^VYOMlXG 



next north. There is alwaj's a lower member of red shale about 50 feet 

 thick containing thin beds of fine-grained, white limestone. In most 

 places this red shale lies on Madison limestone, but -west of Anchor it is 

 separated by graj' sandstone. The middle and upper members are sand- 

 stones and limestones, the latter cherty, especially near the top. 



Wuid River mountains. — The Amsden outcrop is about half way up 

 the slopes of the front range of Wind Eiver mountains. The basal red 

 shales are about To feet thick, and the upper member is a yellow limy 

 shale, which is only 25 feet thick near Bull and Willow creeks. 



Rattlesnake mountains and eastward. — In Eattlesnake mountains the 

 interval between Madison limestone and Tensleep sandstone is al^out 250 

 feet. I found no good exj^osiires of the intervening beds excepting of the 

 red shale, which is 60 feet thick. ISText above are slabby sandstones and 

 cherty limestones. 



In the Shirley and Alcova uplifts and in portions of the northern ex- 

 tension of the Laramie uplift the Anasden red shale is distinct, but it is 

 overlain by gray limestone, which is not typical of the Amsden. In ex- 

 posures 9 miles west-northwest of Difficulty the red shale is 40 feet thick 

 and underlain by 400 feet of limestone, probably Madison. Fifteen miles 

 northwest of Difficiilty, where the red shale is 25 feet thick, it is overlain 

 by 80 feet of limestone and underlain by 75 feet of limestone, the latter 

 probably Madison. Southeast of Leo, where the red shale is 80 to 100 

 feet thick, there are 55 feet of limestone above and 110 feet of limestone 

 below. In the northwest corner of Albany county the red shale is sepa- 

 rated from the granite by a small amount of sandstone and overlain by 

 limestones of which the lower 100 feet are cherty. At a point 5 miles 

 north-northwest of Marshall, where the red shale is 30 feet thick, it is 

 separated from the granite by only 1 to 2 feet of sandstone. There 

 is strong suggestion here of lateral overlap of the Amsden southward 

 across the edge of the Madison limestone, for the latter is absent to the 

 south. 



Fossils and age. — Fossils are rare in the Amsden in central Wyoming, 

 but in the Bighorn uplift the upper beds yielded Pennsylvanian species, 

 while a few supposed Mississippian forms occiir in limestones in the basal 

 red shales. Accordingly it is believed that the formation comprises part 

 of both divisions of the Carboniferous. A few fossils obtained from basal 

 b^eds of the Casper formation 7 miles northwest of Marshall post-office 

 were determined by Doctor Girty as Spirifer centronatus, S. cameratus, ? 

 and StraparoUus utahensis, which are regarded as Mississippian. The 

 rocks are chertj^ limestones lying on red shales strongly suggestive in ap- 

 pearance of the Amsden formation of the Bighorn mountains. 



