THEIR DEPOSITS AND DRAINAGE. 645 
East of the Sierra Nevada we find a high and broad pla- 
teau, five hundred miles in width, and from four thousand to 
eight thousand feet in altitude, which stretches eastward to 
the base of the Rocky Mountains, and reaches southward 
far into Mexico. Of this interior elevated area the Sierra 
Nevada forms the western margin, on which it rises like a 
wall. It is evident that this mountain belt once formed the 
Pacific coast; and it would seem that then this lofty wall 
was raised upon the edge of the continent to defend it from 
the action of the ocean waves. In tracing the sinuous out- 
line of the Sierra Nevada, it will be seen that its crest is 
crowned by a series of lofty volcanic cones, and that one of 
these is placed at each conspicuous angle in its line of bear- 
ing, so that it has the appearance of a gigantic fortification, 
of which each salient and reéntering angle is defended by a 
massive and lofty tower. 
The central portion of the high table lands, to which I 
have referred, was called by Fremont the Great Basin, from 
the fact that it 4s a hydrographic basin, its waters having no 
outlet to the ocean. The northern part of this area is 
drained by the Columbia, the southern by the Colorado. Of 
these the Columbia makes its way into the ocean by the 
gorge it has cut in the Cascade Mountains, through which it 
flows nearly at the sea level; while the Colorado reaches. 
the Gulf of California through a series of caiions, of which the 
most important is nearly one thousand miles in length, and 
from three thousand to six thousand feet in depth. In vol- 
ume VI. of the Pacific Railroad Reports, I have described a 
portion of the country drained by the Columbia, and have 
given the facts that led me to assert that the gorge through 
Which it passes the Cascade Mountains has been excavated 
by its waters; and that previous to the cutting down of this 
barrier these waters accumulated to form great fresh-water 
lakes, which left deposits at an elevation of more than 
two thousand feet above the present bed of the Columbia. 
Similar faets were observed in the country drained by the 
