PEALE.} GEOLOGY DIVIDE WEST OF ELK MOUNTAINS. 93 



the peaks and separating the various streams. This portion of the 

 divide, however, was in the district of 1873, and we have to do here with 

 the portion west of the Elk Mountains, where it presents characters en- 

 tirely different, being mostly a plateau, from the fact that the beds un- 

 derlying it are almost horizontal and covered with a flow of basalt. The 

 plateau character is best shown toward the west, where the basaltic 

 capping is for the most part intact. 



West of Eock Creek, at the head of the ISTorth Fork of the Gunnison, 

 the plateau is broken into low rolling hills, of which the general level 

 is very nearly the same. The basaltic capping here is very irregular 

 and difficult to define, as a great portion of it has been removed by ero- 

 sion. The hills and also the beds of the streams are covered with 

 round masses of the rock. At station 45 there is more basalt in place. 

 The streams rising near have their origin in small lakes. All over the 

 plateau these lakelets may be found, and along the creeks are beautiful 

 meadows. The timber on the plateau is pine and quaking aspens {Popu- 

 his tremuloides). There is good grass in most of the valleys. The 

 western edge of the plateau is somewhat irregular but sharply defined, 

 forming a bluff edge of from two hundred to four hundred feet in height. 

 It is underlaid with sandstones, as will be shown in the sections given 

 in a subsequent portion of the report. 



