The excavation of the Grand Cañon and the sculpture of its walls and buttes are the results of two processes acting in concert— Corrasion and weathering. In discussion 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 cañons; but the expression states only a part of 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 carried 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 platform of the country. At the bottom of each of them is a stream-bed over which in some cases a perennial river flows, while in other cases the flows are spasmodic. Like the trunk river these streams have corraded 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 forgoing respect quite homologous to the main amphiteaters. 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 carrying 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 first orders, and in the main chasm itself. But the work of flowing water, whether in the main channel 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 débris. Corrasion alone then could never have maade the Grand Cañon what it is. Another process, acting conjointly wuth 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 actionn 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.