206 EXPLORATION OF THE CANONS OF THE COLOEADO. 
There i,s still another agency in the production of topographic features, 
viz, the eruption of molten matter from below the general surface. The 
beds formed are soon modified by erosion, and then the forms produced are 
due to that agency, and fall under the general series. But there is a time, 
immediately after the eruption, when these beds lie in forms due to igneous 
dynamics, and the most important features produced are cones. These 
cones are very conspicuous features of the landscape over much of the 
region drained by the Colorado River. 
The district of country drained by the Colorado and its tributaries is 
divided into two parts, by a well marked line of displacements. The lower 
third of the valley, which lies southward from this line, is but little above 
the level of the sea, except that here and there ranges of mountains are found. 
From this region, there is usually a bold step to a higher. 
The upper two-thirds of the area drained by the Colorado is from four 
to eight thousand feet above the level of the sea, with mountain ranges on 
-the east, north, and west, of greater altitude. The bold step from the lower 
country to the table lands is usually an escarpment in rocks of Carboniferous 
Age, marked, here and there, by beds of lava, and along its margin stand 
many volcanic cones. San Francisco Mountain is made up of a group of 
these beds of eruptive matter, covering stratified rocks. This higher region 
is the one to which we have given especial attention in the previous dis 
cussion. 
The principal condensation of moisture occurs on and about the mount 
ains standing on the rim of the basin, the region within being arid. 
Bad-lands, alcove lands, plains of naked rock, plains of drifting sands, 
mesas, plateaus, buttes, hog-backs, cliffs, volcanic cones, volcanic mountains, 
canons, canon valleys, and valleys are all found in this region and make 
up its topographic features. Mountains, hills, and small elevated valleys 
are the features of the irregular boundary belt. 
No valley is found along the course of the Colorado, from the Grand 
Wash toward the sources of the river, until we reach the head of Labyrinth 
Canon. For this entire distance the base level of erosion is below -the 
general surface level of the country adjacent to the river, but at Gunnison's 
Valley we have a local base level of erosion which has resulted in the pro- 
