THE LAWS OF COERASIOK 
231 
foregoing respects. They too have cut narrow gashes no wider than their 
water surfaces. Down tlie faces of the walls and down the steep slopes of 
the taluses run myriads of rain gullies. When the rain conies it gathers 
into rills, which cascade down the wall-clefts and rush headlons: through 
the troughs in the talus. Carrying an abundance of sand and grit, the 
waters scour out these little channels in much the same way as their united 
streams and rills cut down their beds in the anijihi theaters and in the main 
chasm itself But the work of con-asion by running water is limited to the 
cutting of very narrow grooves in the rocks, the width of the cutting at 
any given time and place being equal to the width of the water-surface of 
the stream. Corrasion alone, then, could never have made the Grand 
Canon what it is. The amount of material removed by that process is but 
a very small fraction of the total excavation. Another process acting con- 
jointly with the coi’rasion, and in an important sense dependent upon it, has 
effected by far the greater part of the destruction. This additional process 
is weathering. In order to comprehend the combined results of the two, it 
is necessary to study their action in detail. 
Mr. G. K. Gilbert, in his excellent monograph on the Henry Mountains, 
has embodied a chapter on “ Land Sculpture,” which sets forth, in most 
logical and condensed form, the mechanical principles which enter into the 
problems of erosion. In his comprehensive analysis may be found a dis- 
cussion of the conditions under which the sculjDturing forces and processes 
achieve such abnormal results as we observe in the Plateau Country. The 
perusal of that chapter will give a delightful definiteness to the geologist’s 
comprehension of the subject, and the reader, however learned he may be, 
will take great satisfaction in finding a subject so complex made so intel- 
ligible. The principles laid down by Mr. Gilbert will be adopted here and 
applied. I quote from his woi’k the following : 
“The mechanical wear of streams is performed by the aid of hard min- 
eral fragments carried along by the current. The effective force is that of 
the current; the tools are mud, sand, and bowlders. The most important of 
them is sand; it is chiefly by the impact and friction of grains of sand that 
the rocky beds of streams are disintegrated. 
