PHYSICAL FEATURES OF COLORADO VALLEY. 675 



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 have been broken up or lost, and these fragments only remain 

 to attest 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 bowlders 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 advanced posi^ 

 tions 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 Mississippi, in part at least, to some 

 such action as this ; not that I question the evidence of extended gla- 

 cial action in that region, but may it not be that this glacial action 

 has only resulted in somewhat modifying a vast accumulation of 

 irregularly-bedded material, originally due to the fact that the grand 

 base-level of erosion had been reached by the running streams of that 

 region, and hills and mountains had been degraded by having the ma- 

 terial of which they were composed scattered over lower lands, with- 

 out being carried away by streams to the sea ? 



All the mountain-forms of this region are due to erosion ; all the 

 caSons, channels of living rivers and intermittent streams, were carved 

 by the running waters, and they represent an amount of corrasion diffi- 

 cult to comprehend. But the carving of the canons and mountains 

 is insignificant, when compared with the 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 dust and been 

 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. 



And yet the conditions necessary to great 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 rainfall similar 

 to that of the Appalachian country, and many other districts on 



