Page:Popular Science Monthly Volume 39.djvu/687

Rh the Archæan, era, is to-day represented by the Adirondacks, the Laurentide highlands, and the mountains of Labrador, Baffin Land, and Greenland. Asking, then, how the mountain-building forces of eastern North America have been manifested, we see that the upper part of the earth's crust here has been folded by pressure from the Atlantic toward the central area of the continent, exerted during certain epochs of mountain formation, which have alternated with long intervals of repose and of base-leveling by stream erosion. Three chief epochs of orogenic upheaval have produced the intimately blended Laurentian, Atlantic, and Appalachian mountain systems, which geologists distinguish because of their diversity in age and in many of their physical features, but which geographers unite as the eastern mountainous belt of our continent. As a whole, it may perhaps properly be called the Appalachian, or, better, the Appalachian-Laurentide belt.

Other examples of this structure are developed on the grandest scale in the Old World, comprising the Atlas Mountains, the Pyrenees, the Alps, the Apennines, the Carpathians, the Balkans, the Caucasus, the Elburz, the Hindoo Koosh, and the Himalayas, together reaching from the Pillars of Hercules to the China Sea. These complex mountain systems may collectively be called the Alp-Himalayan belt. During the Miocene, Pliocene, and Glacial periods to the present time, compression has been exerted on each side, upbuilding its mountain chains, which cover a length of about eight thousand miles, occupying a third part of a great circle. In North America the Laurentian mountain system belongs to the remote beginning of the geologic record, and the Atlantic and Appalachian systems are very old, having repeatedly been almost base-leveled; but these principal mountains of northern Africa and of Europe and Asia are geologically very new, the highest being still in the growth of infancy and youth. When upward growth ceases, erosion triumphs and by slow degrees sweeps the mountain mass into the sea. The perpetuation of ancient mountain systems has depended on repeated upheavals, and in their present condition they are remnants spared from the erosion of areas lately elevated. Portions of this great Eurasian mountain belt began to be plicated and thrust up long before the Tertiary era, and doubtless some of its mountain systems were comparatively undisturbed during the Tertiary and Quaternary folding and upbuilding of the Alps and Himalayas; but mainly the prominence of the belt is due to the lateness of the plication, and in part to its being now in progress.

Nearly all the principal mountain systems of the world have a folded structure, but in many instances they retain no semblance of their primal undulations and earliest contour. In the mountainous plateau of Scandinavia and its outlier, the Scottish