6 A Study of the Vegetation of 
the valleys, and, to a large extent, bury the old topography. An 
average thickness of at least 4,200 feet has been estimated for 
the Columbia basalt in Washington. For the most part, the Co- 
lumbia River lava completely covered all of the older rocks over 
which it flowed. Exceptions'to this appear in peaks which were 
too high to be overwhelmed, such as Kamiak Mountain and Step- 
toe Butte in eastern Washington. Likewise, old divides extend 
into the basaltic fields as capes or promontories against which the 
lava came to rest. These older rocks, like those of the foothills 
of the Bitterroot Mountains, are mainly granites, gneisses, schists, 
and quartzites. Canyon walls, notably those of Snake River, not 
only reveal the older rock, but also expose the successive lava 
flows with their beds of dust and lake deposits. Snake River 
has cut down to a distance of over 1,600 feet and has uncovered 
in places the tops of buried mountains. Its canyon shows at 
least ten overflows of the lava. Between some of these over- 
flows sufficient time elapsed for soil to form and forests to grow. 
The evidences of the latter appear either as charcoal or as silici- 
fied tree trunks. Geological investigations indicate that the great 
outpourings of lava were antecedent to the main uplift of the 
Cascade Mountains. The appearance of these mountains so ef- 
fectually modified the climate by cutting off the cool, moist, south- 
west winds as to make much of the region eastward quite too 
arid for forest growth. 
SOILS 
The basalt, although hard and resistant to forces which abrade 
it, yields with comparative rapidity to agencies which tend to dis- 
integration and decay. The weathered basalt has decomposed 
to form a rich residual soil which mantles the surface and gives 
its slopes characteristically soft, rounded, flowing outlines. It 
consists of a friable, dark-brown silt loam, with a depth ranging 
from a few inches to several feet. The subsoil consists of a light- 
brown silt loam. It is heavier and more compact and plastic 
than the surface soil. Both soil and subsoil are extremely re- 
tentive of moisture. They form a mantle usually many feet 
deep, and only along the canyons of streams, or in the scab-rock 
6 
