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Show . 206 BXPLORATION OF TDE CANONS OF THE COLORADO . There i~ 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 <h·ained 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 ~hove 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 th~ one to which we have given especial attention in the . previous discussiOn. The principal condensation of moisture occurs on and about the mountains 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 fcatnres of the irregular boundary belt. No valley is found along the course of the Colorado, from the Grand ~ash toward the sources of the river, until we reach the head of Labyrinth Canon. For this entire distance the base level of erosi·O n I·S b e1 o w t h e 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- BASE LEVELS OF EROSION. 207 duction of low plains and hills for a number of miles back from the stream. North of the Canon of Desolation and south of the Uinta Mountains, another local base level of erosion is found, so 'ncar to the general surface of the country that we find a district of valleys and low hills stretching back from Green River, up the Uinta to the west, and White River to the cast, for many miles. North of the Uinta Mountains a third local base level of erosion is seen, but its influence on the topographic features is confined to a small are.a of two or three hundred square miles. Going up the chief lateral streams of the Colorado, we find one or more of these local base levels of erosion, where the ~treams course through valleys. Where these local base levels of erosion exist, forming valley and hill regions, the streams no longer cut their channels deeper, and the waters of the streams, running at a low angle, course slowly along and are not able to carry away the proaucts of smface wash, and these are deposited along the flood-plains, in part, and in the valleys, among hills, and on tho gentler slopes. This results in a redistribution of the material in iiTegular beds and aggregations. In this region, there are occasional local storms of great violence. Such storms may occur in any particular district only at intervals of many years, possibly centuries. When such a one does occur, it reopens great numbers of channels that have been filled by the ordinary wash of rains, and often cuts a new channel through beds which have accumulated in the manner above described. The structure of these beds is well exposed, and we find beds of clay, beds of sand, and beds of gravel occurring in a very irregular way, due to the vicissitudes of local wash, and, where the progress of erosion has been more or less by ltndermining, larger fragments or boulders are found, and these boulders are sometimes mixed with clay, and sometimes with sand and gravel, and where thin sheets of eruptive rocks havo been torn to piece8, more or less by undermining, (for such is the usual way in this country,) the beds appear to contain erratics, and in fact some of the rocks are erratics, for in the various changes in the levels produced they have often been transported many miles, not by sudden and rapid excursions, but moved a little from time to time. Again, the beds from which they were derived, doubtless, in many cases |