WHITE. J SURFACE GEOLOGY VALLEYS. 53 



than they did when they were first established, they have cut their way 

 through many thousand feet in thickness of strata which were origiually 

 beneath their beds, and which have l)een brought up from below against 

 the wearing action of the constantly-flowing stream. 



l!^ot only have the valleys been thus carved out by the flow of the 

 perennial streams, but their minor or tributary drainage-channels ; 

 mostly those having only a periodical flow of water, have carried away 

 from the surface such an immense bulk of material, that great moun- 

 tain masses now remain in many places that are only shreds of the for- 

 mations that were once spread continuously over the region. This 

 tributary drainage has produced what are now tho most conspicuous 

 topographical features of the great Eocky Mountain region, in the 

 course of which the channels have been much influenced in their direc- 

 tion and position by the varying conditions of the strata with which 

 they have been brought in contact. Therefore the drainage of this 

 latter kind has been called consequent drainage, while that of the per- 

 manent streams has been designated as antecedent drainage. The 

 larger streams have of course had their tributaries from the beginning, 

 but it is hardly likely that the identity of any, except those which con- 

 tain perennial water, have been fully preserved j while not only the 

 identity, but the exact position of nearly or quite all the streams that 

 contain j)erennial water have remained unchanged from the beginning. 

 This district, together with that which adjoins it on the north, afl'ords 

 some remarkable examples of antecedent drainage in connection with 

 great and afbrupt displacements of the strata that are crossed by the 

 drainage-channels. 



Greeii River. — The cutting of Green Eiver through the Uinta Mount- 

 ain chain is probably one of the most extraordinary examples of this 

 kind to be found on the continent, but as this has been fully described 

 by Professor Powell in his report on the geology of the Uinta Mount- 

 ains, and as only a small proportion of the wonderful canons of that river 

 exist within the limits of this district, a full description of even those of 

 the Uinta Mountains will be omitted in this report. The portion which 

 lies within the limits of this district is most impressive and remarkable, 

 cutting, as it does, through Split Mountain from its base to its summit. 

 This mountain is a sharply-folded spur, which projects westward from 

 the body of Yampa Plateau, the summit of which is 3,000 feet above 

 the river; and is composed, like the principal part of the plateau, of 

 hard limestones and sandstones of Carboniferous age. The strata sur- 

 rounding Split Mountain to the westward are comparatively soft, and 

 the surface there has comparatively slight elevation. Viewing the re- 

 gion topographically, the most natural course for the river to have pur- 

 sued from and below Island Park would seem to be by way of a tribu- 

 tary drainage, and the main channel of Brush Creek, around the western 

 end of the mountain, instead of cutting entirely thrqugh it, as it does, 

 splitting it through its highest portion. Below Split Mountain the 

 relation of the river to the underlying strata is not especially re- 

 markable. 



Yampa River. — Although the canons of the Yampa are not so deep 

 as many of those of the Green and Colorado Bivers are, its valley is 

 remarkable for the extraordinary displacements of strata through which 

 they have been excavated. From the eastern boundary of the district, 

 nearly to Yampa Mountain, the river runs in a monoclinal valley ; that 

 is, the general course of the river is to the westward, and the general 

 dip of the strata out of which the valley has been excavated is to the 

 northward. Moreover, the bendings of the river coincide in some degree 



