The cutting power of a large, rapid volume of water, like the Colorado, charged with sand and gravel, is very great. According to Major Dutton, in the hydraulic mines of California, the escaping water has been known to cut a chasm from twelve to twenty feet deep in hard basaltic rock, in a single year. This is, of course, exceptional, but there have, no doubt, been times when the Colorado cut downward very rapidly. The enormous weathering of its side walls is to me the more wonderful, probably because the forces that have achieved this task are silent and invisible, and, so far as our experience goes, so infinitely slow in their action. 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 incredulous 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 Parunuweap 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 subaerial forces have been at work to account for it.