CHAPTER VII T. 



THE EXCAVATION OF THE CHASM. 



The excavation of the Grand Canon and the sculpture of its walls and 

 buttes are the results of two processes acting in concert — corrosion and 

 weathering. In discussing them it is necessary to take into the account 

 the peculiar conditions under which they have operated; conditions 

 which have no parallel in any other part of the world. 



In common parlance it is customary to say, for brevity's sake, that the 

 rivers have cut their canons ; but the expression states only a part ot 

 the truth. The river has in reality cut only a narrow trench no wider 

 than the river's water surface. It has been the vehicle which has car- 

 ried away to another part of the world the materials which have been 

 torn from the strata by corrasion and weathering. Opening laterally into 

 the main chasm are many amphitheaters excavated back into the plat- 

 form of the country. At the bottom of each of them is a stream-bed 

 over which in some cases a perenni d river flows, while in other cases 

 the flows are spasmodic. Like the trunk river these streams have cor- 

 raded their channels to depths varying somewhat among themselves, 

 but generally a little less than the depth of the central chasm. These 

 tributaries often fork, and the forks are in the foregoing respects quite 

 homologous to the main amphitheaters. Down the faces of the walls 

 and down the steep slopes of the taluses run thousands of rain gullies. 

 When the rain comes freely it gathers into rills which cascade down 

 the wall clefts and rush headlong through the troughs in the talus carry- 

 ing an abundance of sand and grit. These waters scour out their little 

 channels in much the same way as their united waters cut down their 

 beds in the amphitheaters of the second and iirst orders, and in the main 

 chasm itself. But the work of flowing water, whether in the main chan- 

 nel or in an amphitheater, or in a gully or cranny of the cliff', is limited 

 to two functions. The first is the cutting of a channel no wider than 

 the surface of the stream ; the second is the transportation of the debris. 

 Corrasion alone then could never have made the Grand Canon what it 

 is. Another process, acting conjointly with corrasion and dependent 

 upon it, has effected by far the greater part of the excavation. This 

 other process is weathering. In order to comprehend their combined 

 action it is necessary to study their action in detail, and to study also the 

 special conditions under which they have operated here. We shall find 

 the subject a very complicated one. 

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