Valleys of CLEAftwATER country, Idaho 533 



merges with the lava plateau surface, but my impression is that in a 

 general way it does. At any rate, the mountains which were visited by 

 me are of Archean granites, gneisses, schists, etcetera, and distinctly rise 

 above the lava plateaus as they do above the old valley floors. Hence 

 the presumed peneplain represented by the crests of the higher ridges is 

 much older than the lava. The river crosses the plateau region in a 

 deep, narrow, rocky canyon wliose general appearance suggests an age 

 similar to that of the canyon in the N^ewsome Creek region. The walls 

 are generally of basalt, but in places older rocks appear under it, indi- 

 cating that the lava overlies a hilly surface. Tributary streams have 

 corresponding canyons. There is evidence that, in addition to the can- 

 yon cutting, there has been erosion on the plateau surface to a depth in 

 places of several hundred feet. 



Plains and Valleys of eastern Washington 



The eastern Washington and Idaho wheat country, as seen from an 

 isolated mountain north of Tekoa, in Whitman County, Washington, 

 appears to be a sharply undulating plain, characterized by rounded hills 

 rather than long, broad swells. The range of altitude between the main 

 hill crests and the streams in the narrow valleys may average several 

 hundred feet. In the distance the hilltops merge into an apparent plain, 

 though not an absolutely level one. It is broken by a prominent peak, 

 Steptoe Butte, and by several lesser elevations in the vicinity. At Gar- 

 field the country is more sharply Avrinkled, but the hills do not much 

 exceed 100 feet in height. The soil is dark brown in color and free 

 from fragments larger than grains of sand. The subsoil is the light 

 brown siltlike material produced by the decomposition of basalt. Fresh 

 railroad cuts near Tekoa show 20 to 40 feet depth of this material. 

 Dark hrown basalt appears in low bluffs in some of the valleys. It is 

 to the great depth of the fine and relatively homogeneous residuum of 

 the basalt that the hills owe their smooth contours. Tlie topography is 

 like that of a long-eroded, loess-covered upland region in the Mississippi 

 Basin. 



This great "plain," which is a plain by reason only of the small range 

 of altitude of its hilltops, passes into the reentrants between the spurs 

 of the Coeur d^A.lene and Clearwater groups of mountains on the east. 

 The line of demarkation l)etween the plain and the mountains, as seen 

 from such elevations as the mountain near Tekoa, is a sharp one, but at 

 closer range the undulations of the })lain merge into the greater undu- 

 lations of the mountains. The mountain north of Tekoa consists of 



