largely due to the difference in the kinds of rocks which the river encounters in its journey from the mountains to the sea.
Leaving the immediate vicinity of the cañon and going to the north, the tourist would come upon various younger geological formations which at one time buried the whole region. Although from ten to fifteen thousand feet thick, they no longer remain in the vicinity of the canon, having been washed away by flowing water after the elevation of the land in early Eocene times. When that was accomplished and the country for miles around was comparatively level, the land was again gradually raised and the river began in Neocene times to cut the outer cañon. When it had cut down to the Tonto terrace—' The
Esplanades'—elevation ceased and the river wandered back and forth in a lazy manner, widening instead of deepening its channel. Since in that arid region there was not enough rainfall to wash down the side walls, they have remained precipitous, while the main stream has had time to push them back until they are a dozen miles apart. Then, in the Pliocene, slow elevation of the plateau began again, causing the river to run swiftly and renew its carving. This action is proceeding at the present time with the result of deepening the inner cañon.
Standing on the cañon's rim and looking down upon the immense gulf, one marvels that so small a river could have accomplished so gigantic a work. But if he laboriously descends to the brink of the roaring river below and puts his hand into the rushing water and feels
