GEOLOGY OF CENTRAL WASHINGTON TERTITORY. 485 



The hills between the Chemakane and the forks of the Spokane are of syenite, decomposing freely 

 on exposure to the air. South of the Spokane, basalt resumed its place as the prevailing rock, 

 forming the high table-land known as the Great Plain of the Spokanes and Nez Perces, and 

 extending to the Columbia on the west and across the Snake river to the south. Near the Spo 

 kane, however, the syenite occasionally protrudes through it, and even as far down as the camp 

 of October 30th and 31st, appears on the borders of a small lake lying to the east of the trail. 

 The soil of these plains is generally thin and sterile, and covered with oxyde of iron from the 

 decomposed basalt ; but in the swales, along the margin of the small streams, it is a rich, black 

 mould. Bunch-grass grows plentifully upon them, and they afford a good range for the horses 

 of the Indian tribes to which they belong. It is, however, to be considered that, in all the stock- 

 ranges of this country, the scattered growth of the wild grasses renders necessary a large com 

 parative extent of country. These plains, except on the northern skirts, are destitute of timber, 

 and are swept by high and piercing winds. Their surface is generally undulating, with occa 

 sional hills rising to the height of 100 or 200 feet, some of which seem to indicate, in their tabular 

 summits, the former existence of a general covering of detritus since swept away. Lines of 

 low terraces are also observable in places. Quite a number of small lakes are scattered over this 

 field, many of which appear to have been craters. On the line pursued by the expedition, a 

 series of these continued for half a day s journey, apparently connected by a crevasse. They 

 were of circular shape, and surrounded by walls of basalt. The water contained in them was 

 generally saline and nauseous. 



Approaching the Snake river the plain becomes more broken, though the general level is pur 

 sued, and the beds of the streams deepen as they descend. The basalt exhibits the usual dif 

 ferent forms, and in structure ranges through every variety, from scoriaceous to compact. In the 

 canons of the Peluse the strata were often curved, the individual columns retaining their perpen 

 dicularity to the base, radiated from or towards it according to circumstance. The lowest stratum 

 in several places exhibited the same separation into columns, separated by horizontal cleavage 

 into plates resembling stratification, as those previously mentioned in the Atahnam valley. The 

 next above it was sometimes in massive blocks, constituting a sort of compound column, while 

 the superior ones were slender, of pentagonal or hexagonal forms, and convex at the top. Many 

 of the larger blocks separate with conchoidal faces. We noticed on these plains numerous 

 small mounds left by the water, which had washed away the surrounding soil, in the same man 

 ner as those on the mound prairies near Olympia. The descent of the Peluse into the Snake 

 river is one of the most remarkable scenes in the whole of this region. The waters have hol 

 lowed out a large basin, in the midst of which stand pinnacles and needles of every form and size; 

 while black and broken precipices rise in fantastic irregularity around it to the height of three 

 or four hundred feet. 



The country south of the lake, from opposite the mouth of the Peluse to the head of the 

 Wallah-Wallah, consists of high rolling hills, rising to about the same level with the Spokane pla 

 teau, but composed of sand resting on the basalt, which shows only in the ravines. They are cut 

 with ridges and spurs by the action of water, are destitute of wood, but produce bunch-grass in 

 greater abundance than the northern plains. Much of the land in the bottoms upon the Wallah- 

 Wallah and its branches is covered with alkaline efflorescences, but there are tracts of very rich 

 soil. Indian corn can, it is said, be considered a sure crop ; melons, tomatoes, and the finer 

 vegetables grow well, and it is believed that wheat would thrive on the hills; but the country is 

 better adapted to grazing and gardening than farming. The same remarks will, it is believed, 

 apply to the heads of the Umatilla and other streams rising in the Blue mountains. The lower 

 part of the Wallah- Wallah valley is a waste of sand and sage. Large boulders of granite are 

 found in the banks of this stream, but from whence transported, sufficient is not yet known of the 

 nearest cascades to determine. 



The canon of the Snake river extends down to its mouth. The country on the Columbia 



