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 fect, 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 (1, 
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 
5 
