666 OREGON AND NORTHERN CALIFORNIA. 
to one thousand feet high; and there are two or three successive 
ranges of precipices, at nearly the same height, on both sides of the 
river. There are no alluvial flats along the shores. 
Alluvial Islands and lower Prairie of the Columbia.—T he Columbia, 
below Vancouver, abounds in alluvial islands as well as submerged 
flats. The islands have generally the same height as the lower 
prairie, but are usually depressed at centre. From the broad line of 
cottonwood and willows which borders them, the surface gradually de- 
clines inward to a low flat, sometimes below low water level, though 
generally somewhat above it. A few of them consequently contain a 
permanent lagoon ; and as remarked by Mr. Douglass, of the Hudson’s 
Bay Company, they resemble in form the coral islands of the Pacific. 
All are filled during the freshets, and only a narrow rim of land re- 
mains above the surface. 
The island of Multnomah, at the mouth of the Willammet, is mostly 
alluvial, but in the western part it is basaltic. 
The lower prairies of the river, like the alluvial islands, are generally 
highest along the shores, and covered with a forest line, (fig. 1,) while 
the surface back is much depressed, and a bare meadow. In sinkinga 
well near Vancouver, two or three feet of soil were first passed through ; 
then nearly thirty feet of gravel; below this, a light quicksand, too 
loose and mobile to be penetrated without much difficulty, on account 
of its falling in. We observed here, as we had elsewhere, that these 
alluvial deposits were not minutely divided into thin layers of deposi- 
tion, but were in thick, compact layers, unlike the gradual depositions 
over most alluvial flats. 
During freshets, the water usually commences to flood the prairies 
by rising through the soil, instead of overflowing the more elevated 
banks. The turf is seen to swell in small spots over the surface ; and 
if these swellings of the turf at this time are pierced with a cane, the 
waters spout out inastream. They break as the pressure increases, 
and the ponds then begin to fill, and the flats to be overflowed. It 
was the opinion of Dr. McLaughlin that the water penetrated laterally 
along the quicksand layer, and thence rose, by the pressure, to the 
surface. At the usual floods in June, the water of the Columbia rises 
from sixteen to nineteen feet. 
The less height of the land back from the shores may be owing to 
the following causes: the fact that the waters flowing along tend to 
heap up material on the banks, from the counter-currents produced 
along the shores; that the shores are covered with shrubbery and 
