10 REPORT UNITED STATES GEOLOGICAL SURVEY. 



■wbich are surrounded by such hills or high lauds as usually border the 

 valleys. The term " basiu" I shall use for similarly excavated or hollowed 

 spaces in the general surface of the country through which no river or 

 perennial stream flows by which its lands may be irrigated. The basin 

 has been produced by the erosion of the dry drainage-channels, while 

 the park has been excavated mainly through the agency of a perennial 

 stream. Parks, then, being really portions of the valleys, will be de- 

 scribed in that connection, but the basins of the district will be sepa- 

 rately considered, mainly with reference to their relation to the struct- 

 ural geology of the district. All the parks of this district are small, and 

 their importance lies only in the facts that their relation to the struct- 

 ural geology is similar to that which the basins hold, and that they 

 contain far the greater part of the tillable land of the region. 



Midland Basin. — This basin is comparatively small, but a separate de- 

 scription of it is given because of the important relation ic bears to the 

 structural geology of its neighborhood. It lies near the middle of the 

 district, and nearer to the southern than to the northern border. Its 

 boundaries may be designated in general terms as follows : PiSon 

 Kidge borders it upon the east ; the hogbacks of the Midland Flexure 

 upon the south ; the eastern end of the Midland Eidge, in part, on the 

 west ; and its northern boundary is indistinctly defined by the broad 

 ridge which forms the water-shed between the Yam pa and White Kiv- 

 ers, separating Midland Basin from Lil.^'s Park, and is continuous with 

 the western portion of the Danforth Hills. It occupies a part of the 

 broad anticlinal axis of the flexure that has brought up the great mass 

 of Triassic strata which constitutes Midland Ridge and also the Fox 

 Hills, and Laramie strata that constitute Piiion Eidge. The same flex- 

 ure has, of course, brought up the sandy shales and clays of the Colorado 

 Group, out of which the basin has been mostly excavated. Southwest- 

 wardly this basin is continuous, with a broad dry valley extending far, 

 but gradually narrowing, to the westward, which valley, like the basin, 

 is mostly excavated out of the shales of the Colorado Group. The sur- 

 face of the basin has considerable irregularity, but is free from hills of 

 any considerable height. Its drainage all passes into White Eiver 

 through a gap in tbe hogbacks of Midland Flexure, just west of the east- 

 ern end of Piiion Eidge. 



Coyote Basin. — Unlike the other basins of this district. Coyote Basin 

 occupies a broad synclinal instead of an anticlinal ; also, it is mostly 

 excavated out of tbe bad-land strata of the Wasatch Group instead of 

 out of the soft strata of the Colorado Group as the others are. With per- 

 haps the exception of the Bridger and Uinta groups, the Wasatch and 

 Colorado groups are constituted of softer material than any others in this 

 region, although in a portion of the district the Wasatch Group is in part 

 a firm sandstone. Wlien the conditions were made favorable by the flex- 

 ures that all the strata of the region have suffered, those of the two last- 

 named groups have yielded more readily to erosion than those with which 

 they are associated. Hence the basins have been excavated out of these 

 yielding strata only. Coyote Basin is bounded easterly by the concave 

 sweep formed by the Gray Hills and Citadel Plateau; on the north, in 

 part by Citadel Plateau and in part by the broad ridge that forms the 

 water-shed between the White and Yampa Elvers and separates Coyote 

 Basin from Axial Basin ; on the west by Piiion Eidge and the high 

 land that connects it with the water-shed lidge before referred to. On 

 the south this basin is, in part, continuous with the valley of Wbite 

 Eiver, with wbich it communicates and into which river it is drained. 

 Its surface is an irregular, desolate bad-laud area, traversed by numer- 



