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Show 202 EXPJ.OHATION OF TDE CANONS OF THE COLORADO. Since these vermilion beds were stripped from the adjacent country, the few showers of this arid region condense chiefly about the summits of the mountains, and the waters, gathering into streams, and running down into the lower region, have cut deep gulches through the sheets of basalt, in many places tevealing the structure of the mountains themselves. The last puff in these eruptive vents tossed high into the air scoria and ashes; the lighter materials were carried away by the winds, the heavier fragments fell, and thus cinder-cones were piled up; and in many of these cindercones the outlines of the craters are still preserved. The beds of lava are of various ages. The first were poured out in that ancient time before the sandstones had been carried away. From time to time new beds were formed, and the latest beds have been poured out in a time so recent., that the very waves of the congealed floods are still preserved, and there is no reason to suppose that this action is completed. In time another vent may be opened, and another river of red hot rock gush from the earth. Nor are all the cones of late origin; each outflow of molten matter seems to have ended in the formation of a cone. In the elder beds the cones have been washed away, but their sites are marked by scattered cinders. In the very latest cones tho craters are still preserved, and their cinders are angular fragments of slag, that show that many storms have not fallen upon them since they broke in cooling. So, even these eruptive mountains were hewn from the rock, and only the cinder-cones, scattered hero and there, small in comparison to the great mountain masses, were piled up in their present forms. It is probable that the cones have cores which extend to great depths, and perhaps connect the sheets of basalt above with masses of like material below, and thus the more enduring and protecting beds to which these mountains owe their preservation are anchored to the heart of the earth. METHODS OF EROSION. In this and the foregoing chapter I have attempted to describe tho ao-encies and.conditions which have produced the more important topogra;hic features in the Valley of the Colorado. These features are mountains, hills, hog-backs, bad-lands, alcove lands, cliffs ' buttes ' and canons . The pl'·l mary METIIODS OF EHOSION. 203 agency in the production of these features is upheaval, i. e., uphea' al in relation to the level of the sea, though it may possibly be down-throw in relation to the center of the earth. This movement in portions of the crust of the earth may be by great folds, with anticlinal or synclinal axes, and by monoclinal folds and faults. The second great agency is erosion, and the action of this agency is conditioned on the character of the displacements above mentioned, the texture and constitution of the rocks, and the amount and. relative distribution of the rains . . In a district of country, the different portions of which lie at different altitudes above the sea, the higher the region the greater the amount of rainfall, and hence the eroding agency increases in some well observed, but not accurately uefined, ratio, from the low to the high lands. The power of running water, · in corrading channels and transporting the products of erosion, increases with the velocity of the stream in geometric ratio, and hence the degradation of the rocks increases with the inclination of the slopes. Thus altitude and inclination both are important elements in the problem. Let me state this in another way. We may consider the level of the • sea to be a grand base level, below which the dry lands cannot be eroded; but we may also have, for local and temporary purposes, other base levels of erosion, which are the levels of the beds of the principal streams which carry away the products of erosion. (I take some liberty in using the term level in this connection, as the action of a running stream in wearing its channel ceases, for all practical purposes, before its bed has quite reached the lev.el of the lower end of the stream. What I have called the base level would, in fact, be an imaginary surface, inclining slightly in all its parts toward the lower end of the principal stream draining the area through which the level is supposed to extend, or having the inclination of its parts varied in direction as determined by tributary streams.) Where such a stream crosses a series of rocks in its course, some of which are hard, and others soft, the harder beds form a serieB of temporary dams, above which the corrasion of the channel through the softer beds is checked, and thus we may have a series of base levels of erosion, below which the rocks on |