GEOLOGICAL STRUCTURE. 627 
a flat country, which continues for sixty miles to the Grand Rapids. 
Ledges of basaltic rocks crop out at intervals along these low shores ; 
and at the Grand Rapids, they stand in high broken walls, peaks and 
needles, on both sides of the river. ‘These rocks cease again at Walla- 
walla, twenty miles beyond. 
The south fork of the Columbia was examined by Dr. Pickering as 
far as the Kooskooski; basalt continued to be the rock of the shores. 
The reports of travellers state that the whole Snake River region is 
characterized by the same basalt, either cellular or compact, and 
basaltic conglomerate, with intervals of basaltic lavas. Although the 
conclusion may be too comprehensive, we may safely infer from it 
that basalt is the prevailing rock throughout this region. 
With relation to the north branch of the Columbia, and the region 
between Nisqually and Fort Colville, around by Wallawalla, I extract 
the following from the journal of my friend Dr. C. Pickering. The 
party left Nisqually, and crossed the Cascade Range, twenty miles to 
the north of Mount Rainier. On the ascent, pebbles of granite and 
porphyry were found in the beds of the streams, the first signs of 
granite being observed about twenty miles from Nisqually. Near the 
summit of the range the rocks were trachytic, and contained black 
crystals (probably hornblende). After descending and passing a 
rolling country to the northeast for three days, they crossed a ridge 
still higher by a thousand feet, and then made the descent to the bed 
of the Columbia. The rocks on this route were seldom in view; 
where exposed, they consisted of basalt, much of which was very cel- 
lular. They reached the Columbia at the mouth of the Pescaos 
River, and leaving a fine-grained granite on the west side, they 
crossed over and found basalt at the summit on the east side. Gra- 
nitic rocks appeared to characterize the country north of Okanagan, 
and so on east to Fort Colville. The northern entrance of the Grande 
Coulée consists of granite, although the country in this region to the 
south of the Columbia is mostly basaltic. Many hills of granite occur 
in this part of the gorge, and some, whose tops reach the general 
height of the plains above, have the upper three or four hundred feet 
basaltic, while the base is granitic. 
At the Kettle Falls, near Fort Colville, the rock is a quartz rock 
containing a little mica, and breaks out in large slabs. No granular 
limestone was met with, but it is said to occur near the mouth of the 
Spokane River. 
From Fort Colville the party traversed the dreary plains which lie 
