98 GRAND CANON DISTRICT. 



may be shown that the rate of recession is substantially independent of 

 the magnitude Of the cliff, whatever its altitude. Here a momentary 

 digression is necessary. 



\\V have hitherto spoken of the recession of cliff's as if it comprised 

 the whole process of erosion, and have hardly alluded to the possible 

 degradation of the tlat surfaces of plateaus, terraces, and plains. Is it 

 meant that there is no degradation of the horizontal surfaces, and that 

 the waste of the land is wholly wrought by t lie decay of cliffs .' Approx- 

 imately that is the meaning, but some greater precision may be given to 

 the statement. 



Erosion is the result of two complex groups of processes. The first 

 group comprises those which accomplish the disintegration of the rocks, 

 reducing them to fragments, pebbles, sand, and clay. The second com 

 prises those processes which remove the dCbris and carry it away to 

 another part of the world. The first is called disintegration ; t he second, 

 transportation. We need not attempt to study these processes in all 

 their scope and relations, but we may advert only to those considerations 

 which are of immediate concern. When the debris produced by the dis- 

 integration of rocks is left to accumulate upon a tlat surface it forms a 

 protecting mantle to the rocks beneath, and the disintegration is greatly 

 retarded, or even wholly stopped. In order that disintegration may go 

 on rapidly the ddbris must be carried away as rapidly as it forms. But 

 the efficiency of transportation depends upon the declivity. The greater 

 the slope the greater the power of water to transport. When the slope 

 is greater than 30° to 33° ("the angle of repose") loose matter cannot 

 lie upon the rocks, and shoots down until it finds a resting place. Hence 

 the greater the slope the more fully are the rocks exposed to the disin- 

 tegrating forces, and the more rapidly do they decay. This relation is 

 universal, applying to all countries, and explains how it comes about 

 that the attack of erosion is highly effective against the cliffs and steep 

 slopes, and has but a trilling effect upon tlat surfaces. 



Keverting to the main argument, it now appears that erosion goes on 

 by the decay and removal of material from cliff's and slopes; that the 

 recession of high cliffs is as rapid as the recession of low ones, and that 

 the quantity of material removed in a given time increases with the al- 

 titudes of the cliffs and slopes. In other words, the thickness of the 

 strata removed in a given period of erosion should be proportional to the 

 amount of relief in the profiles of the country. Hut in the Plateau 

 country, and especially in the Grand Canon district, t hese reliefs are very 

 great. It is a region of giant cliffs and profound canons, and, as will ul- 

 timately appear, it has been so during a very long stretch of geological 

 time. The thickness of the strata removed from it is only proportional 

 to the values of those conditions which favor rapid erosion. In the fore 

 going discussion it may appear that the area of denudation in the Grand 

 Cafion district, though large, and the thickness of the strata denuded, 



