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 canons. 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 Canon 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 canon 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 canon 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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