42 GEOLOGY. 
That portion of the central plateau which lies west of the Rocky mountains varies in elevation 
from 5,000 to 8,600 feet; the smaller number representing the altitude of its surface where 
deeply eroded. Its average altitude, in the vicinity of our route, may be estimated at 
6,000 feet. 
Over this plateau the Colorado formerly flowed for at least 500 miles of its course, but, in 
the lapse of ages, its rapid current has cut its bed down through all the sedimentary strata and 
several hundred feet into the granite base on which they rest. 
For 300 miles the cut edges of the table-lands rise abruptly, often perpendicularly, from the 
water’s edge, forming walls from 3,000 to 6,000 feet in height. This is the ‘‘Great Cafion of 
the Colerado,’’ the most magnificent gorge, as well as the grandest geological section, of which 
we have any knowledge. 
Above Upper Carboniferous bove 
Colorado. lim i e fhe sea level. 
5,500 feet. : 6,800 feet. 
Cross-stratified sand- S 
stones. 
Red calc. sandstones 
with gypsum. 
4,300 feet. Se ea 
Lower Carboniferous ? iT Las esd 
limestone. a oae™ caper rae yee 
3,300 feet, E nn! 4,600 feet. 
Limestones, shales,and 
grits. 
Devonian. ? 
Limestones, mud,rocks 
and sandstones. 
Silurian, ? 
1 i i 
coms Potsdam sandstone. 
ert whys . . 
1,000 feet. a 34448 aS" a 2,300 feet. 
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i Fig. 12.—SEOTION OF THE CARON OF THE COLORADO ON HIGH MESA WEST OF THE LITTLE COLORADO. 
That portion of the table-lands lying between the mouth of the Virgen and the Little Colo- 
ia 18 COL ea : over — feet of sedimentary rocks, representing the Silurian, Devonian, 
_ The Silurian and Devonian strata are entirely conformable among themselves and with the 
