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Show 208 EXPLORATION OF TrJ E CA~ONS OF TilE COLORADO. have been broken up or lost, and these fragments only remain to attest to the existence of such beds in some former time, and all stages may be observed, from the beds the edges only of which have been broken up, to those that have only fragments remaining or have entirely disappeared. Another interesting fact has been observed, that these erratics or boulders are often found distributed somewhat in lines due to the undermining of lines of cliffs. Often where we have cliffs capped with a bed of lava, former and more advan~ od positions of these lines of cliffs can be recognized by the position of lines of lava. fragments which are seen in the valley or plains in front of the cliffs. It will be seen that these local accumulations of material, due to the excess of erosion over that of transportation, greatly resemble the accumulations of "the Drift." Especially is this true where I have studied the latter in the valley of the Mississippi, and I have been led to query whether it may not be possible to refer the origin of the Drift of the Valley of the Missis ippi, in part at least, to some such action as this; not that I question the evidence of extended glacial action in that region, but may it not be that this glacial action has only resulted in somewhat modifying a vast accumulation of irrogulurly bedded material, originally due to the fact that the grand base level of erosion had been reached by tho running streams of that region, and hills and mountains had been degraded by having the material of which they were composed scattered over lower lands. without being carried a·way by streams to the sea~ All the mountain forms of this region are due to erosion ; all the canons, channels of living rivers and intermittent streams, were carved by the running waters, and they represent an amount of con-asion difficult to comprehend. But the carvin0rr of the canons and mountains is insirrnificant when b ' compared with tho denudation of the whole area, as evidenced in the cliffs of erosion. Beds hundreds of feet in thickness and hundreds of thousands of square miles in extent, beds of granite and beds of schist, beds of marble and beds of sandstone, crumbling shales and adamantine lavas have slowly yielded to the silent and unseen powers of the air, and crumbled into duHt and beon washed away by the rains and carried into the sea by the rivers. 'The story we have told is a history of the war of the elements to beat back the march of the lands from ocean depths. NOT A REGION OF MOST HA.PID EROSION. 209 And yet the conditions necessary to groat erosion in the Valley of the Colorado are not found to exceed those of many other regions. In fact, the aridity of the climate is such that this may be considered a region of lesser, rather than greater, erosion. We may suppose that, had this country been favored with an amount of rain-fall similar to that of the Appa.Iachian country, and many other districts on the surface of the earth, that the base level of erosion of the entire area would have been the level of the sea; and, under such circumstances, though the erosion would have been much greater than we now find, the evidences of erosion would have been more or less obliterated. As it is, we are able to study erosion in thiR country, and find evidences of its progress and its great magnitude, from the very fact that the conditions of erosion have been imperfect. It is proper to remark here that erosion does not increase in ratio to the increase of the precipitation of moisture, cceteris paribus, as might be supposed; for, with the increase of rains there will be an increase of vegetation, which serves as a protection to the rocks, and distributes ero~iou more evenly, and it may be that a great increase of rains in this region would only produce a different series of topographic outlines, without greatly increasing the general degradation of the' Valley of the Colorado. To a mot•e thorongh discussion of this subject I hope to return at some future time. From the considerations heretofore presented, it is not thought necessary to refer the exhibition of erosion shown in the canons and cliffs to a more vigorous action of aqueous dynamics than no~v exists, for, as I have stated, a greater precipitation of moisture would have resulted in a very different class of topographic features. Instead of ca11ons, we should have had water-gaps and ravines; instead of valleys with cliff like walls, we should have had valleys bounded by hills and slopes ; and if the conclusions to which we·have anived are true, the arid conditions now existiog must have extended back for a period of time of sufficient length to produce the present canons and cliffs. But there are facts which seem to warrant the conclusion that this condition has existed for a much longer period than that necessary for tho production of the present feature~; that is, the characteristics of the present topography have existed for a long time. There are 27 OOL |