160 GRAND CANON DISTRICT. 



time of the water, however, had been equivalent to about 145 days ot 

 twenty-four hours each. This case is indeed a most extreme one, and 

 no natural river can show any such rapid corrasion of any considerable 

 length of its bed. It is not cited to support an inference of phenomenal 

 rapidity in the corrasion of the Grand Canon, but rather to illustrate 

 the efficiency of corrasive action when all the attendant conditions are 

 extremely favorable and no countervailing condition is present. But 

 although the Colorado is far from beiug such an extreme case as the 

 one just mentioned, it is still a very strong one. Yet there are some 

 stretches in the river where the corrasion must be proceeding at a very 

 rapid rate — at a rate not very many times slower than in the Bloom field 

 mine. These portions are in the hardest rocks, and they illustrate well 

 the law which Mr. Gilbert has so clearly enunciated (p. 157, line 44), 



The course of the Colorado in the Grand Canon is a succession of 

 headlong rapids or cataracts and of smooth but swiftly-flowing reaches. 

 In the Kaibab division the rapids are very numerous, very long, and 

 very frequent, while the still reaches are short. In the Kanab division 

 the rapids are fewer and less formidable, while the still reaches are 

 longer. In the Sheavwits, the condition is intermediate between those 

 of the Kaibab and Kanab divisions. The rapids, however, are of two 

 kinds, and are the results of two wholly independent causes. (1.) 

 When the stream lies in the hard rocks, the declivity is much greater, 

 and the rapid is then due to the greater slope of the bed. (2.) At the 

 opening of every side-gorge, a pile of large bowlders and rubble is 

 pushed out into the stream. Most of the side-gorges are dry through- 

 out the greater part of the year. But when the rains do come, their 

 effects are prodigious.* In the vast amphitheaters the water is quickly 

 shot down into the channel and rushes with frightful velocity along the 

 bed, which has a slope of 2()0 feet or more to the mile. Nothing which 

 is loose and which lies in the way of it can resist its terrible rush. 

 Bowlders of many tons' weight are swept along like chaff, and go than- 

 ding down the side gorges into the main river. When the torrents reach 

 the river the large fragments are dropped; for the maximum slope of 

 the main stream (reckoned throughout any stretch exceeding four or five 

 miles) never exceeds 25 feet to the mile; and the water, though great in 

 volume at flood-time, has much less velocity than the torrents of the 

 side chasms. The river has, however, abundant power to sweep along 

 fragments of considerable size, which are ground up as they move on- 

 ward. The coarse material, the large bowlders and rubble washed out 

 of a lateral chasm, form a dam where the river becomes a cataract. 

 They are also strung out for considerable distances below the dam, and 

 thus the tendency is to build up and increase the grade of the river 



* It is well to remember here the grand scale on which these lateral features of the 

 chasm are laid out. The watersheds of these amphitheaters cover each from 10 to 

 50 square miles! And when a heavy rain comes, whatever water is not soaked up 

 by the rocks and soil is in the bottom of the amphitheater in less than ten minutes!! 



