Here may be seen rocks of the four geological periods which are among the very oldest of our earth. The rocks of later periods were here once, too, making a layer more than two miles high resting on what is to-day the top, but in some remote age they were shaved off by some great natural force, perhaps a glacier. The eating away of the rocks which formed the cañon itself is modern. Scientists say it was done, as it were, last Monday or Tuesday, for it was when the top two thirds had been "shaved off," as we have said, that the Colorado River began to cut the Grand Cañon through the rocks that formed the lower third. While the cracking of the crust, caused by internal fires, may have helped the process of cañon-making, the result of erosion is seen everywhere. Every passing shower, every desert wind, every snowfall, changes the contour of the region imperceptibly but surely. The cañon is Nature's open book in which we may read how the earth was built. With the coming of the railroad, when this century was yet a baby, tourists began to flock in, hotels were built, highways constructed, trails bettered, and other improvements made. To-day the traveler finds here every comfort. Although first glimpsed by white men in 1540, when the Spanish conquistadors appeared,--one expedition journeying from the Hopi pueblos in Tusayan across the Painted Desert,--the big cañon remained unvisited, except for Indians and trappers, until 1858, when Lieutenant Ives, of the army engineer corps, made a brief exploration of the lower reaches of the Colorado, coming out at Cataract Creek. It was not thoroughly explored until the year 1869, when Major John W. Powell made his memorable voyage from the entrance to the mouth of the great gorge, passing down the Green and Colorado Rivers. Though he lost two boats and four men, he pushed on to the end. It is fitting that the United States Government has erected to his memory a massive monument of native rock with bronze tablets on one of the points near El Tovar Hotel. Powell's outfit consisted of nine men and four rowboats. The distance traveled exceeded one thousand miles, from what is now Greenriver, Utah, through the series of cañons to the mouth of the Rio Virgin. In the spring of 1871 he again started with three boats and descended the river to the Crossing of the Fathers. The following summer Lee's Ferry was his point of departure and he went as far as the mouth of Kanab Wash. Beginning with the Russell and Monett party, in 1907, several others have essayed to duplicate Powell's achievement, and successfully, too, though without adding to our scientific knowledge of the cañon. The trips are exceedingly dangerous, for the rapids conceal rocks that would wreck any boat, and the currents are treacherous. It is safer, by far, to sit at home and read Powell's story.