The southwestern part of the United States rose above sea-level and sank below it many times during the many thousands of centuries preceding its present state, which is that of a sandy and generally desert plateau, five to ten thousand feet in altitude. How many times it repeated the cycle is not fully known. Some portions of it doubtless were submerged oftener than others. Some were lifting while others were lowering. And, meantime, mountains rose and were carried away by erosion to give place to other mountains which also wore away; river systems formed and disappeared, lakes and inland seas existed and ceased to exist. The history of our southwest would have been tempestuous indeed had it been compassed within say the life of one man; but, spread over a period of time inconceivable to man, there may have been no time when it might have seemed to be more active in change than its still hot deserts seem to-day to the traveller in passing trains. Other parts of the continent, no doubt, have undergone as many changes; our southwest is not singular in that. But nowhere else, perhaps, has the change left evidences so plain and so interesting to the unscientific observer. The page of earth's history is more easily read upon the bare deserts of our southwest than on the grass-concealed prairies of the Mississippi Valley or the eroded and forested ranges of the Appalachians. Before the Rockies and the Sierra even existed, in the shallow sea which covered this part of the continent were deposited the ooze which later, when this region rose above the sea, became the magnificent limestones of the Grand Canyon. Muds accumulated which to-day are seen in many highly colored shales. Long ages of erosion from outlying mountain regions spread it thick with gravels and sands which now appear in rocky walls of deep canyons. A vast plain was built up and graded by these deposits. The trunks of trees washed down by the floods from far distant uplands were buried in these muds and sands, where, in the course of unnumbered centuries, they turned to stone. They are the petrified forests of to-day. Mountains, predecessors of our modern Sierra, lifted in the south and west, squeezed the moisture from the Pacific winds, and turned the region into desert. This was in the Jurassic Period. Sands thousands of feet deep were accumulated by the desert winds which are to-day the sandstones of the giant walls of Zion Canyon. But this was not the last desert, for again the region sank below the sea. Again for half a million years or more ooze settled upon the sands to turn to limestone millions of years later. In this Jurassic sea sported enormous marine monsters whose bones settled to the bottom to be unearthed in our times, and great flying reptiles crossed its water.