684 O. H. HERSHEY TERTIARY AND QUATERNARY GEOLOGY 



hard gray quartzite, apparently Cataldo. Other western spurs of the 

 Coeur d'Alene Mountains are evidently of the metamorphic rocks, at 

 least in their higher parts. Without a detailed study, I can not say how 

 high the basalt rises on the flanks of the quartzite mountains, but I think 

 it is true that in a general way the undulating plain of the wheat coun- 

 try represents the original lava plain and the mountains which rise above 

 its eastern border are of older rocks that were^ never lava-covered. 



The new Oregon- Washington Kailway and Navigation Company's cut- 

 off between Spokane and Harrison leaves Coeur d'Alene Lake in a canyon 

 cut in lava, which it presently leaves and traverses the gently rolling 

 plateau surface. Then it ascends through a range of low hills in which 

 the cuttings are in granitic rock, and comes out on a broad, gently rolling 

 plain which seems to merge into the great lava plain of eastern Wash- 

 ington. The Mica Eange rises abruptly on the northern side of the 

 plain; the railroad descends to the broad valley of the Spokane Eiver 

 through a narrow, crooked valley cut in light gray micaceous rock. 



Between Harrison and Tekoa the railroad ascends one of the canyons 

 cut into the lava near Coeur d'Alene Lake to a low ridge on the eastern 

 border of the great lava plain, and thence descends into a shallow valley 

 excavated in the lava on the western slope from the divide. I have no 

 doubt that the lava about the south end of Coeur d'Alene Lake consists 

 of the same sheets as underlie the eastern Washington wheat country; 

 in other words, that the uppermost gravel of the 600-foot terrace of the 

 Coeur d'Alene Valley may be practically the same stratigraphic horizon 

 as the highest basalt sheet in the Tekoa region. 



The Great Northern E ail way west of Spokane climbs out of the valley 

 on to the lava plateau on the south, and traverses its gently undulating 

 surface at altitudes mostly between 2,300 and 2,500 feet above sealevel. 

 From Harrington (altitude, 2,167 feet) the rolling plain descends 

 westward to about 1,300 feet at Wilson Creek, and thence for over 40 

 miles there is no material descent. The railroad is generally below the 

 level of the rolling plain, in shallow valleys and small canyons which 

 increase in number and depth as the Columbia Eiver bluff is approached. 

 Many of these small valleys are bordered by basalt bluffs, but the higher 

 divides are smooth and must have a deep soil, for they are largely under 

 cultivation. Finally the railroad descends into the canyon-like valley 

 of the Columbia Eiver, crossing the stream at an altitude of 588 feet. 

 The Columbia Eiver lava appears to be upturned on the west side of the 

 river, but soon gives place to older rocks. 



All over the Columbia Basin the essential features are: A pro- 

 nouncedly undulating surface, with broad smooth ridges or small 



