478 
GEOLOGY OF CENTRAL WASHINGTON TERRITORY. 
from the coast, and this appears substantially the case; nor is the fact much altered by the differ¬ 
ence of elevation. Emerging from the forest, the ground was literally covered with fragments of 
basalt, varying in size from large blocks to pieces resembling macadamized pavement—some 
compact, some cellular or scoriaceous. Interspersed were beds of tenacious blue clay, which 
had retained the footprints of horses since the spring. The source of such a deposite in this 
situation could not readily be traced. 
The two streams last mentioned unite and form one fork of the Yakima, to the lower part of 
which the Indians give the name of Pisko. The aspect of the country, on descending to its valley, 
is one of extreme desolation. Narrow ravines or cations, bordered by walls of basalt, having 
a rude columnar structure, converge from the mountains, the strata dipping towards the east 
at a small but varying angle. The foot-hills are clothed with purshia and artemisia, and strewn 
with silicious pebbles. The bottoms are barren and stony, and intersected with arroyos. A 
narrow and broken line of dwarf oaks and willows indicates the windings of the stream. The 
Indians have indeed, here and there, on the margin of the water-courses, small garden patches, 
where they raise potatoes, a little corn, and other vegetables; but the country generally is a desert. 
The elevation of this valley, at the forks, is 1,519 feet above Vancouver. From that point the 
course of the expedition was nearly north, crossing the several branches of the Yakima. The 
general direction of these is from northwest to southeast, very nearly the reverse of that followed 
by the rivers on the western side of the mountains. The main stream, which heads not far from 
latitude 47° 30' north, is, on reaching the valley, deflected by a heavy range which breaks off 
from the Cascades and curves to the south, separating the waters of the Yakima from the Pisquouse 
and the Columbia, and terminating near the mouth of the first. It thus forms a basin, in which 
the upper waters of the Yakima are enclosed. Spurs from this range run towards the Cascades, 
sinking into the general level of the intermediate plateau, or uniting with corresponding offsets 
of the latter. The main Yakima cuts through four of these spurs, at the very base of the range, 
before again turning to the eastward, and its various branches have eroded from the plateau the 
valleys through which they run to meet it. Long horizontal lines of basalt bordering these 
valleys indicate the original level of the basin, though it would appear that elevation has at some 
points disturbed it. Traces of lacustrine terraces are also observable, formed probably before 
the river had completely broken through its barriers. 
The basalt is continuous, along the base of the Cascades, on the line from the Dalles north¬ 
ward to the Columbia at its bend near the mouth of the Pisquouse. To the eastward it stretches 
across the great Spokane plains. Upon the divides between the Wenass and the main Yakima, 
and between that and the Columbia, it is interstratified with coarse basaltic conglomerate. Its 
western limit upon the Cascade mountains is not determined. Upon our route from Chequoss, 
it was, as has already been mentioned, interrupted by hornblende rock upon the Ivlikatat river, and 
in ascending the main Yakima pass; it ceased on ascending the lower hills, where the same rock 
succeeded it. The rocky bluffs which border the Nahchess, from its source down, consist of 
basalt and basaltic conglomerate, and the latter crosses the range and is visible upon the sides 
of La Tete, near the western gate of the mountains. Captain Wilkes mentions that on the 
summit of this pass the rocks are trachytic, and contain black crystals, probably hornblende. The 
structure of the basalt varies greatly, occurring both compact, cellular and scoriaceous; and its 
forms of fracture or separation are as numerous. In the Atahnam valley, which is hemmed in 
by low mural precipices, two very widely-distinct strata are noticeable; the lower is divided 
into large columns, from three to five feet in diameter, which, at a little distance, present the 
appearance of horizontal lamination. On closer inspection, they are found to consist of thin 
wedge-shaped fragments, a few inches across, and of which they seem to have been built up. 
One of these columns had been split through the centre, and it was perceivable that the structure 
extended throughout, the joints being just visible. The columns were cylindrical, and well 
relieved, both from each other and the wall, and stood ten or twelve feet high above the soil. 
