674 THE POPULAR SCIENCE MONTHLY. 



mountains standing on the rim of the basin, the region within being 

 arid. 



Bad-lands, alcove-lands, plains of naked rock, plains of drifting 

 sands, mesas, plateaus, buttes, hog-backs, cliffs, volcanic cones, vol- 

 canic mountains, canons, caiion valleys, and valleys, are all found 

 in this region, and make up its topographic features. Mountains, 

 hills, and small elevated valleys, are the features of the irregular 

 boundary belt. 



No valley is found along the course of the Colorado, from the 

 Grand Wash toward the sources of the river, until we reach the head 

 of Labyiinth Caiion. For this entire distance the base-level of erosion 

 is below the general surface-level of the country adjacent to the river, 

 but at Gunnison's Valley we have a local base-level of erosion which 

 has resulted in the production of low plains and hills for a number 

 of miles back from the stream. North of the Canon of Desolation 

 and south of the Uinta Mountains, another local base-level of ero- 

 sion is found, so near to the general surface of the country that we 

 find a district of valleys and low hills stretching back from Green 

 River, up the Uinta to the west, and White River to the east, for 

 many miles. North of the Uinta Mountains a third local base-level 

 of erosion is seen, but its influence on the topographic features is con- 

 fined to a small area of 200 or 300 square miles. Going up the chief 

 lateral streams of the Colorado, we find one or more of these local base- 

 levels of erosion, where the streams course through valleys. 



Where these local base-levels of erosion exist, forming valley and 

 hill regions, the streams no longer cut their channels deeper, and the 

 waters of the streams, running at a low angle, course slowly along, 

 and are not able to carry away the products of surface-wash, and 

 these are deposited along the flood-plains, in part, and in the valleys, 

 among hills, and on the gentler slopes. This results in a redistribu- 

 tion of the material in irregular beds and aggregations. 



In this region, there are occasional local storms of great violence. 

 Such storms may occur in any particular district only at intervals of 

 many years, possibly centuries. When such a one does occur, it re- 

 opens great numbers of channels that have been filled by the ordinary 

 wash of rains, and often cuts a new channel through beds which have 

 accumulated in the manner above described. The structure of these 

 beds is well exposed, and we find beds of clay, beds of sand, and beds 

 of gravel occurring in a very irregular way, due to the vicissitudes of 

 local wash, and, where the progress of erosion has been more or less 

 by undermining, larger fragments or bowlders are found, and these 

 bowlders are sometimes mixed with clay, and sometimes with sand 

 and gravel, and where thin sheets of eruptive rocks have been torn to 

 pieces, more or less by undermining (for such is the usual way in this 

 country), the beds appear to contain erratics, and in fact some of the 

 rocks are erratics, for in the various changes in the levels produced 



