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 lymg 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. heir 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 carion of the Snake river extends down to its mouth. The country on the Columbia 
