270 Mr. Geihie [April 6, 



accelerated or diminished from time to time. At present, part of 

 the disintegration is due to the su])erficial strain upon the surfaces of 

 rock from the rapid expansion and contraction caused by the great 

 daily range of temperature. The surface of the bare rocky wastes 

 criunbles down. The wind brushes off the lighter particles, and makes 

 use of them as a kind of sand-blast to wear down the rocks and stones 

 over which they are driven. Eain, though infrequent, falls in occa- 

 sional heavy showers, which, sweeping off the loosened materials, 

 rush in torrents of mud and sand down the side canons into the main 

 gorge. ^ ^ ^ 



Much that is most characteristic m the scenery of the canon 

 region arises from the horizontality of the strata. The same beds of 

 rock, with the same forms of weathering and the same peculiar colours, 

 may be traced from cliff to cliff for many miles. A strangely regular 

 and almost architectural symmetry thus arises, and is traceable even 

 through the most rugged and broken parts of the scenery. The 

 array of spires, towers, buttresses, alcoves, pediments, mouldings, 

 cornices, along the escarpment of any stratum or group of strata is in 

 continual decay and renewal. Slice after slice is cut away from the 

 face of a cliff by the disintegration and removal of the softer beds 

 underneath, and thus the escarpments are creeping backward across 

 the plateaux. Old valleys now devoid of any living stream are no 

 longer deepened, but the sapping of their boundary walls continues, 

 so that they are becoming slowly wider. There are traces of an older 

 system of drainage on the plateaux, but the channels are now dry and 

 are even in some instances truncated by the walls of the main canon, 

 These deserted channels probably indicate a former period of greater 

 moisture. The glacial period undoubtedly filled up the basin of the 

 Great Salt Lake nearly a thousand feet above its present level, until, 

 as a sheet of fresh water, it sent its drainage northwards into the Snake 

 river, and thence into the Pacific Ocean. The author had searched 

 in vain for evidence that the glaciers of the mountains to the west, 

 north, and east of the caiion region had advanced into the plains, but 

 they probably supplied a larger volume of water to the rivers of that 

 region than these now possess. 



The history of the erosion of the canons, as shown by Powell and 

 Button, throws light on the slow rate at which some of the grander 

 movements of the earth's crust may take place. The Green river, as 

 the northern part of the Colorado is called, instead of turning round 

 the flank of the Uinta Mountains, strikes boldly into them, and has 

 cut a series of deep caiions across the end of the chain. It can 

 be shown that this operation cannot have been effected after the 

 uplift but must have been contemporaneous with it. The upheaval 

 of the vast fold of these mountains, from which some 24,000 feet of 

 rock have been removed, must have been so slow as not to dislodge the 

 river from the winding channel it had previously selected As fast as 

 the mountains rose, the river sawed its way down through them. In the 

 Grand Canon region, faults of sometimes 6000 feet in displacement 



