24 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM, 
THE OKANOGAN HIGHLANDS. 
These mountains occupy the northeast portion of the State, inelud- 
ing most of Stevens and Ferry counties. To the southward they pass 
gradually into the Columbia Plains. To the westward they are 
naturally limited by the Okanogan River. 
The Okanogan Highlands consist mainly of gently rounded hills, 
rising into peaks 1,500 to 2,000 meters high. Geologically they are 
similar to the northern part of the Cascades, and are composed largely 
of granite. The vegetation is nearly identical with that of the eastern 
slope of the Cascades. These mountains and those of British 
Columbia connect the Cascade system with the Rocky Mountain 
system in Idaho. In consonance with the markedly similar conditions 
of soil and climate it is not surprising that a number of species of the 
coast region occur through these mountains and in north Idaho. 
THE COLUMBIA PLAINS. 
The greater portion of eastern Washington is covered by an im- 
mense mass of basalt, of an average thickness of at least 1,800 meters. 
This mass is the result of a series of lava overflows which involved 
not only eastern Washington but also great portions of eastern Oregon 
and Idaho, covering in all an area of over 500,000 square kilometers 
(200,000 square miles). Geologically this is known as the Columbia 
River basalt. It covered in Washington all of the region south of 
the Okanogan Highlands and extended westward from the Bitter- 
roots nearly to the present crest of the Cascade Mountains and 
beyond. this at least in Clarke and Cowlitz counties. The canyon 
walls of Snake River and other streams indicate a number of succes- 
sive overflows, at least ten, between some of which sufficient time 
elapsed for soil to form and forests to grow. The remains of the 
latter appear either as charcoal embedded in the soil of old lake bot- 
toms, or else as silicified trunks, these often remaining in their origi- 
nal vertical positions. . 
Originally the surface of the lava appears to have been approxi- 
nately level, but subsequent to the last great overflow a large lake or 
else a series of lakes existed in Yakima, Douglas, Klickitat, and 
Franklin counties as well as in adjacent Oregon. This was Lake 
John Day. Whether this lake merely filled a basin formed beyond 
the heads of the last lava flows, or whether it was formed through 
the sinking of the surface concomitantly with the commencement of 
the uplifting of the Cascade and Blue mountains is not clear. 
The deposits formed in the bottoms of Lake John Day consist. of 
soft sandstone and conglomerates which have largely been removed 
by subsequent erosion. The light, ashy soils formed from these de- 
posits are very different from the heavy clay loam formed by the 
disintegration of basalt. 
