FOEMATION OF RAPIDS. 
241 
the canon is a succession alternately of smooth reaches, with very small 
declivity, and swift rapids where the declivity is very great. In the Kaibab 
the smooth reaches are short, while the rapids are long and of great descent. 
In the Kanab and Uinkaret divisions the reverse is the case — the smooth 
stretches are long, the rapids short and much less frequent and powerful. 
In the Sheavwits division the proportion of the smooth to the rapid portions 
is between those prevailing in the Kaibab and Kanab divisions respectively. 
By comparing the distribution of fall with the nature of the rocks through 
which the river runs, it appears very clearly that the greater declivities 
occur in the Archaean rocks. The river enters the Archaean in two great 
bends, one in the Kaibab, the other in the Sheavwits. In the Kanab and 
Uinkaret divisions the river flows in the Silurian and lower Carboniferous. 
The Archman rocks are, as a rule, much harder and are corraded with more 
difficulty than the others, and to this more obdurate character the greater 
declivities may be traced. 
The rapids are, however, the results of two independent causes. (1) 
When the course of the stream lies in the hard rocks the rate of declivity is 
greater. The explanation is obvious. (2) The second cause is of a totally 
distinct nature. At the opening of every lateral chasm or side gorge a pile 
of rocks and rubble is thrown out into the main stream. Most of these 
side gorges are dry throughout the greater part of the year. But when 
the rains come their narrow beds are occupied with floods of muddy 
water, rushing downward with great velocity and often in great volume, 
bowling along fragments of all sizes from a few pounds to many tons. Thus 
an obstruction like a low dam is built across the river. The declivity of the 
side gorges is always much greater than that of the main stream. Their 
slopes are rarely less than 200 feet to the mile, except in those tributaries 
which, like Kanab Creek and Cataract Canon, are of great antiquity. The 
minimum slopes of the beds of the great amphitheaters in the Kaibab are 
seldom so small as 200 feet to the mile. The power of a great flood rush- 
ing down such slopes is indeed formidable. When the torrents reach the 
river the larger fragments are dropped ; for the maximum slope of the main 
stream (reckoned throughout any stretch exceeding four or five miles in 
length) never exceeds 25 feet to the mile, and the water, though enormous 
16 G c 
