408 X. H. BARTON PALEOZOIC AND MESOZOIC OF WYOMING 



Jurassic which follows is similar to that of the Black hills and northern 

 and western Wyoming, but it thins out to the south at an unknown point 

 a few miles southeast of Medicine Bow. The next formation, the ]\Ior- 

 rison shale, with an average thickness of 200 feet, is persistent through- 

 out, together with the overlying Cloverly sandstone, the latter represent- 

 ing the "Dakota" of earlier writers. The great series of upper Cretaceous 

 shales attains a thickness of 5,000 feet or more and occupies broad basins 

 between the great uplifts. At the base is the Benton shale, which lacks 

 its medial limestone member (Greenhorn) excepting along the southeast 

 side of the Laramie mountains. The Mowry beds and upper sandstone 

 are, however, prominent throughout. West of longitude 107° the limy 

 deposits of the N'iobrara give place to shale. The Montana consists of a 

 lower division of dark shale with all the characteristics of the Pierre, 

 and an upper member of sandstone similar to the Pox hills of other 

 regions. This sandstone is followed by coal measures and shales of 

 Montana age, overlapped by sandstones which are regarded as Laramie. 

 The Tertiary occixpies wide areas on some of the tabular divides and 

 attains considerable thickness in the Wind Eiver basin. The formations 

 are mostly Wasatch, White River, and Arikaree. They lie across up- 

 turned edges of the earlier formations. The Quaternary is represented 

 by old terraces, which are of great extent in the Laramie basin, and by 

 alluvial flats in the present valleys. 



Cambrian System 

 deadwood formation 



General relations. — The sandstone of the Deadwood formation is at the 

 base of the sedimentary series in northern and central Wyoming. It 

 thins out to the southeast and apparently ceases at no great distance east 

 of longitude 107°, so that in Converse, Albany, and Laramie counties the 

 Carboniferous rocks lie directly on pre-Cambrian granites and schists. 

 In the Shirley hills a hard, massive sandstone and conglomerate iinder- 

 lying Madison limestone may be Deadwood. On the West fork of Trou- 

 blesome creek, 9 miles northwest of Difficulty post-office, and about Leo, 

 this sandstone, 30 feet or more thick, is hard and in part conglomeratic. 

 Near North Platte river, north of the mouth of the Medicine Bow, it is 

 about 100 feet thick. Whether in these areas it is a shore deposit of 

 Madison age or the Deadwood formation was not ascertained. In the 

 basal sandstone in the gorge of North Platte river, 3 miles northeast of 

 Pathfinder, Mr Walcott found a few fragmentary fossils which he regards 

 as Cambrian. The hard sandstone at the base of the sedimentary series 



[ 



