Who can safely declare that the day will not come when a new Yellowstone, hurled from reopened volcanoes, shall found itself upon the buried ruin of the present Yellowstone; when the present Sierra shall have disappeared into the Pacific and the deserts of the Great Basin become the gardens of the hemisphere; when a new Rocky Mountain system shall have grown upon the eroded and dissipated granites of the present; when shallow seas shall join anew Hudson Bay with the Gulf of Mexico; when a new and lofty Appalachian Range shall replace the rounded summits of to-day; when a race of beings as superior to man, intellectually and spiritually, as man is superior to the ape, shall endeavor to reconstruct a picture of man from the occasional remnants which floods may wash into view? NOTE EXPLANATORY OF THE ESTIMATE OF GEOLOGIC TIME IN THE TABLE ON THE OPPOSITE PAGE The general assumption of modern geologists is that a hundred million years have elapsed since the close of the Archean period; at least this is a round number, convenient for thinking and discussion. The recent tendency has been greatly to increase conceptions of geologic time over the highly conservative estimates of a few years ago, and a strong disposition is shown to regard the Algonkian period as one of very great length, extremists even suggesting that it may have equalled all time since. For the purposes of this popular book, then, let us conceive that the earth has existed for a hundred million years since Archean times, and that one-third of this was Algonkian; and let us apportion the two-thirds remaining among succeeding eras in the average of the proportions adopted by Professor Joseph Barrell of Yale University, whose recent speculations upon geologic time have attracted wide attention. Fantastic, you may say. It is fantastic. So far as I know there exists not one fact upon which definite predictions such as these may be based. But also there exists not one fact which warrants specific denial of predictions such as these. And if any inference whatever may be made from earth's history it is the inevitable inference that the period in which man lives is merely one step in an evolution of matter, mind and spirit which looks forward to changes as mighty or mightier than those I have suggested. With so inspiring an outline, the study to which I invite you can be nothing but pleasurable. Space does not permit the development of the theme in the pages which follow, but the book will have failed if it does not, incidental to its main purposes, entangle the reader in the charm of America's adventurous past. I THE NATIONAL PARKS OF THE UNITED STATES