(54 
THE GRAND GANON DISTRICT. 
rate of recession is substantially independent of the magnitude of the cliff', 
whatever its altitude. Here a momentary digression is necessary. 
We have hitherto spoken of the recession of the cliffs as if it comprised 
the whole process of erosion, and have hardly alluded to the possible deg- 
radation of the flat 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 the decay of cliff's! Approximately 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 debris and carry it away to another 
part of the world. The fii’st is called disintegration; the second, transpor- 
tation. 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 disintegration 
of rocks is left to accumulate upon a flat 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 
debris 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 disintegrating 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 trifling 
effect upon flat surfaces. 
Reverting to the main argument, it now appears that erosion goes on 
by the deca}'^and removal of material from cliffs 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 altitudes of the cliffs 
and slopes. In other words, the thickness of the strata removed in a given 
