24 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



THE OKANOGAN HIGHLANDS. 



These mountains occupy the northeast portion of the State, includ- 

 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 thoy 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. 



I HE COLUMBIA PLAINS. 



The greater portion of eastern Washington is covered by an im- 

 mense mass of basalt, of an average thickness of at least 1,300 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- 

 mately level, but subsequent to the last great overflow a large lake or 

 else a series of lq,kes 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. 



