1883.] on the Canons of the Far West. 269 



coincide with those of older date. Hence the present river-channels 

 in the glaciated regions of Europe cannot have a higher antiquity 

 than the later part of the Glacial jDeriod. The European rivers 

 therefore do not offer illustrations of river action in its most active 

 phase. 



Probably the region of the earth's surface, where the erosive action 

 Df rivers can be witnessed on the grandest scale is the basin of the 

 Colorado river between southern Wyoming and the desert plateaux of 

 Arizona. Throughout that region the strata are generally so slightly 

 inclined that to the eye they appear to be horizontal. They form 

 vast plains and elevated plateaux, the higher portions being densely 

 clothed with pine-forests, while the lower tracts are in large measure 

 barren desert. The strangest feature in this strange region is the way 

 in which the surface has been trenched by running water into a 

 system of profound gorges or caiions. Through the main canons flows 

 the rushing turbid Colorado river, but save where its few tributaries 

 join, the side canons have no permanent streams, and are only occa- 

 sionally the beds of torrents when a heavy rain-shower falls. Nearly 

 the whole of the water of the Colorado comes from distant high 

 grounds, and the caiion region through which the river winds is iu 

 great part dry and desert. The Grand Caiion of the Colorado, ac- 

 cording to the recent measurements of Captain Button, is 220 miles 

 long, and in places about 6000 feet deep. There are really two 

 canons — a wide outer valley five or six miles broad, with precipitous 

 sides descending 2000 or 3000 feet to a platform through which winds 

 the inner gorge 3000 feet deep and from 3500 to 4000 feet from crest 

 to crest. These colossal excavations are entirely the work of running 

 water. No trace of any fracture has been found to account for them ; 

 on the contrary, they have been eroded across the line of large folds 

 and dislocations of the rocks. 



Eiver-erosion is here at its maximum rate. This appears to depend 

 partly on the high altitude of the region and the consequent great 

 declivity and rapidity of the rivers, partly on the aridity of the 

 climate, which permits the rivers to erode their beds unimpeded by 

 the various causes that hinder them in Europe ; and partly on the 

 large amount of detritus washed down into the rivers by the occa- 

 sional heavy rain-showers. 



But there has likewise been simultaneously an enormous denuda- 

 tion of the surface of the plains and plateaux through which the Colo- 

 rado flows. In the Grand Caiion district a mass of strata not less 

 than 10,000 feet thick is comjDuted to have been removed. From the 

 broad fold of the Uinta Mountains the amount of rock swept away is 

 estimated at 3^ miles in vertical thickness. This vast denudation has 

 been entirely the work of sub-aerial agents, and appears to have been 

 in progi'ess ever since the floor of the Cretaceous sea was gently raised 

 into land in early Eocene times. 



There may have been variations in the meteorological conditions 

 of the past, so that the rate of denudation has possibly been 



