G. K. Gilbert—The Colorado Plateau Province. 93 
absence of soil. Whena shower falls, nearly all the water runs 
off from the bare rock, and the little that is absorbed is rapidly 
reduced by evaporation. Solution becomes a slow process for 
lack of a continuous supply of water, and frost accomplishes 
its work only when it closely follows the infrequent rain. 
Thus weathering is retarded, and transportation has its work so 
concentrated by the quick gathering of showers into floods, as 
compensate, in part at least, for the smallness of the total 
rainfall from which they derive their power. 
Hence in regions of small rainfall, surface degradation is 
usually limited by the slow rate of disintegration ; while in re- 
gions of great rainfall it is limited by the rate of transporta- 
tion. There is probably an intermediate condition, with mod- 
erate rainfall, in which a rate of disintegration greater than that 
of an arid climate is balanced by a more rapid transportation 
than consists with a very moist climate, and in which the rate 
of degradation attains its maximum. 
Having examined the conditions of erosion separately, let us 
now group them in such combination as will help to an under- 
standing of the cafions. 
Over nearly the whole of the earth’s surface there is a soil, 
and wherever this exists we know that the conditions are more 
favorable to weathering than to transportation. Hence it is 
true in general that the conditions which limit transportation 
are those which limit the general degradaticn of the surface. 
_ To understand the manner in which this limit is reached, it 
intea to look at the process by which the work is accom- 
pushed. 
Transportation and Comminution.—.A stream of water flowing 
down its bed expends an amount of energy that is measured b 
the quantity of water and the vertical distance through whic 
it descends. If there were no friction of the water upon its 
channel the velocity of the current would continually imcrease ; 
but if, as is the usual case, there is no increase of velocity, then 
the whole of the energy is consumed in friction. The friction 
produces inequalities in the motion of the water, and especially 
Induces subsidiary currents more or less oblique to the general 
and are dropped at once. "Still larger are only half lifted; that 
1s, they are lifted on the side mh current and rolled over, 
