Page:Popular Science Monthly Volume 81.djvu/237

Rh the plateau-plains stand at many different heights above the general plains-surface. To this particular great genetic significance is attached.

Several notable peculiarities distinguish the Colorado dome. Around its southern slope there is, as Gilbert observes, one of the great lavatracts of the world, second in magnitude in our country only to the great northwestern lava-field, and fifteen times as large as the classical district of extinct volcanoes in central France. Sweeping in a broad crescent, 250 miles long, with the lofty cone of San Mateo on one horn and the towering San Francisco volcano on the other, the main body of lava-flows superposed in countless numbers, covers an area half the size of New York state. Beyond the borders of the crescent are numberless cinder-cones, coulees, and minor lava-sheets which spread out over the soft sedimentaries constituting the chief substructure of the plains in this part of the country. Farther west, in Arizona, the hard carboniferous limestone is the principal surface-rock, the shales once overlying it having been recently stripped off. Other volcanic evidences