Southeastern Washington and Adjacent Idaho. 5 



provinces. The Okanogan Highlands occupy the northeastern 

 part of the state, while the Blue Mountains cover a relatively 

 small area in the southeastern part. The remainder and larger 

 part of the area makes up the Columbia Plateau. 



The Okanogan Highlands consist mainly of gently rounded 

 hills rising into peaks 4,900 to 6,600 feet high. Geologically, they 

 are similar to the northern part of the Cascades and are com- 

 posed largely of granite. They form, with the adjacent moun- 

 tains of British Columbia, a connecting link between the Cascades 

 and the Rocky Mountain system in Idaho. They are important 

 in connection with these studies mainly for the ameliorating effect 

 they exert upon the climate, and for the role they have played in 

 plant distribution in the foothills of the Bitterroot Mountains. 



The portion of the Blue Mountains occurring in Washington 

 are composed wholly of basalt, and represent a great uplift of 

 this rock surrounding a central mass of granite peaks. In Wash- 

 ington they reach an elevation of more than 6,500 feet, while 

 in Oregon they rise about 3,300 feet higher. 



The Columbia Plateau is thus bounded in Washington on three 

 sides by high mountain ranges, while in Idaho, on the east, it 

 merges into the Bitterroot Mountains.. The plateau forms the 

 greater portion of eastern Washington, and is made up of an 

 immense mass of basalt, known geologically as the Columbia River 

 basalt. This basalt is the result of a series of lava overflows 

 which involved not only Washington, but also large areas in 

 Oregon and Idaho, and even northern California. In Washing- 

 ton it covered all of the region south of the Okanogan Highlands 

 and extended westward from the Bitterroot Mountains to the 

 Cascades. These basaltic lavas were extruded in a highly fluid 

 condition from numerous vents thickly strewn over the floor of 

 the region, and were spread over the surface in great flows (i, 

 12, 13, 14). The apparently flat basaltic plains stand out in 

 sharp contrast to the mountain borders. The floor upon which 

 the lava was extruded had considerable relief and was not greatly 

 unlike the present Okanogan Highlands, with canyons, gorges, 

 and mountain peaks ; the whole showing a state of vigorous dis- 

 section (14). The effect of the basaltic inundations was to fill 



