CHAPTER XIII. 
THE EXCAVATION OF THE GRAND CANON. 
The cafion is the result of two processes acting in concert — corrasion and weathering. — G. K. Gilbert on 
the subject of corrasion. — Nature and extent of the water supply of the Colorado. — The tribu- 
taries. — Springs in the canons. — Evaporation. — Quantity of water flowing in the river. — Origin 
and character of the load to be transported. — Character of the tributary streams. — Extremely 
turbid condition of their waters. — Rainfall of the region. — Character of the sediment. — The 
declivity of the river bed in the Grand and Marble Canons. — The rapids. — Relation of the bed- 
rock to declivity. — Rapids at the openings of lateral gorges. — General nature of the corrasive 
action. — Scouring action of the sand. — Weathering. — Conditions essential to the development of 
the topographical forms of the region. — The elevation of the country. — The horizontality of the 
strata. — Vertical heterogeneity and horizontal homogeneity. — Effects of an arid cRmate. 
The excavation of the Grand Canon and the sculpture of its walls and 
buttes are the results of two processes acting in concert — corrasion and weath- 
ering. In discussing these processes it is necessary to take into the account 
the peculiar conditions under which they have operated, and these are 
chiefly the climate and the elevation of the country. 
In common parlance it is customary to say that the river has cut its 
canon. This expression states but a small jDortion of the truth. The river 
has in reality cut only a narrow trench, of which the width is equal to the 
width of the water surface. It has also been the vehicle which has carried 
away to another part of the world the materials which have been cut away 
by both processes. Opening laterally into the main chasm are many am- 
phitheaters excavated back into the main platform of the country. At the 
bottom of each is a sti’eam-bed, over which in some cases a perennial river 
flows, while in other cases the water runs only during the rains. Like the 
trunk- river itself, these streams, whether permanent or spasmodic, have cut 
down their channels to depths varying somewhat among themselves, but 
generally a little less than the depth of the central chasm. These tributa- 
ries often fork, and the forks are quite homologous to the tributaries in the 
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