GEOLOGY OF CENTRAL WASHINGTON TERRITORY. 
479 
The upper stratum was broken into small irregular columns, of the usual character, very com¬ 
pact, and with somewhat conchoidal faces. The same appearance was afterwards noticed in 
descending the Peluse. In the Sewacksa Pass, crossing the mountains between the mouth of 
the Yakima and the camp at Wenass, the trail passes through a narrow canon or crevasse, now 
the bed of a winter torrent, which seems to have been the immediate seat of some volcanic action. 
The walls generally are composed of very perfect pentagonal columns, five or six inches through; 
but at one point, where the face has been broken down, it consisted of cellular lava, containing 
cavities filled with sublimed sulphur. Seams of sand and ashes occurred here beneath the basalt. 
In the lower part of the Yakima valley are two or three basins surrounded by walls, partially 
obliterated, which appear to be the remains of small craters. 
The boulders contained in the Pisko, the Atahnam, and other streams lying between the latter 
and the Nahchess, were of the same description as in those already noticed, being invariably 
trachytic or basaltic. On reaching the Nahchess, granite and quartzose rock for the first time 
occur, intermixed with the trachyte. Of this branch, which is the next in size to the main river, 
one fork heads in Mount Rainier, the other in a peak, having an extinct crater, to the south. As 
we found that from here northward the trachyte ceases entirely, this river may be assumed, gen¬ 
erally speaking, as the northern limit of trachyte, and the southern of the granite series. 
On the northern side of the Nahchess, not far from its mouth, the first sedimentary deposit 
was met with, in a soft, yellowish sandstone, resting on the basalt. Its dip appeared to be to the 
northwest, and at an angle of 10°. In the lower stratum, at the height of about fifty feet from 
the level of the valley, a bed of river-sand and stones, precisely similar to those in the present 
stream, and three or four feet thick, was exposed. Above this was a layer of friable, coarse¬ 
grained sandstone, and next a seam apparently of intermixed clay and volcanic ashes about a foot 
in thickness, which was in turn overlaid by sandstone. The bluffs run to the height of some two 
hundred and fifty feet, and exhibited several strata, the lower only, however, containing the river- 
stone and clay. It would appear that a former bed of the Nahchess had been covered to a con¬ 
siderable depth with detritus from the mountains and subsequently elevated to this position, the 
river meantime seeking a new channel. 
The prevalent rocks on the summit of the Cascades, at the main Yakima Pass, were breccias, 
constituted for the most part of argillaceous or magnesian rocks. These continued as far down as 
Lake Kitchelus, forming the walls, of the ravines wherever visible. The higher peaks, to the 
northward, seemed to be of basalt or conglomerate similar to that on the Nahchess, but were too 
distant to be determined with certainty. Gneiss was observed upon the borders of Lake 
Kahchess, dipping westward at an angle of 15 0, Granite was not seen in place, but is found in 
boulders in the river. Towards the foot of the mountain hornblende rocks occurred, succeeded 
by basalt. The most remarkable feature of the Yakima is the series of lakes, through some of 
which it passes; while the others communicate with it by outlets. The source of the river is 
itself a small lake, situated on the very summit of the dividing ridge, at an elevation of about 3,600 
feet, from which the D’Wamish also, emptying into Puget sound, derives its origin. It thence 
passes by a very rapid descent to another somewhat larger, and again into a third, falling, in the 
course of five miles, over 1,000 feet. 
Lower down it receives through one outlet Lake Kahchess, into which a smaller adjunct 
empties, and by another Lake Kleallum. The largest of these is about eight miles in length. 
The lower lakes, particularly the last, bear evidence of a former much greater extent; lines of 
terrace of considerable height being apparent at a distance from its present bed, and there is 
abundant proof on the river that the chain was once more extensive. The existing lakes are 
deep, excepting the small one at the summit of the pass. 
The upper valley of the main Yakima, from its exit from the mountains to the mouth of the 
Atahnam, is separated by the spurs already alluded to into several basins, of which that around 
Ketetas is the largest. In these there are some traces of pretty good soil, chiefly on the margin 
