TIME AND CHANGE 
The river is a tremendous machine for grinding 
and sawing and transporting, but the rains and the 
frost and the air and the sunbeams smite the rocks 
as with weapons of down, and one is naturally in- 
credulous as to their destructive effects. 
Some of the smaller rivers in the plateau region 
flow in very deep but very narrow cafions. The 
rocks being harder and more homogeneous, the 
weathering has been slight. The meteoric forces 
have not taken a hand in the game. Thus the Parun- 
uweap Cafion is only twenty to thirty feet wide, but 
from six hundred to fifteen hundred feet deep. 
I suppose the slow, inappreciable erosion to which 
the old guide alluded would have cut the cafion since 
Middle Tertiary times. The river, eating downward 
at the rate of one sixteenth of an inch a year, would 
do it in about one million years. At half that rate it 
would do it in double that time. In the earlier part 
of its history, when the rainfall was doubtless 
greater, and the river fuller, the erosion must have 
been much more rapid than it is at present. The 
widening of the cafion was doubtless a slower process 
than the downward cutting. But, as I have said, 
the downward cutting would tend to check itself 
from age to age, while the widening process would 
go steadily forward. Hence, when we look into 
the great abyss, we have only to remember the 
enormous length of time that the aerial and sub- 
aerial forces have been at work to account for it. 
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