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beveled tables of flexed strata, lava-sheets, or cemented regolith, are of quite different elevations even in the same district. In New Mexico, for instance, these plains attain all altitudes above the general plains-surface, from a few feet in the case of the very recently formed Malagro malpais, in the Hueco bolson northeast of El Paso, to the broad Mesa de Maya which is 3,500 feet above the general plains-surface, and 9,000 feet above the sea-level. The Sierra del Datil in western New Mexico has a magnificent northward-facing escarpment 1,000 feet high; and in sight of it is the Acoma Mesa 500 feet above the plains floor (Fig. 7).

Toyalané is a conspicuous flat-topped mountain situated just over the continental divide. The region is the largest, highest and driest desert plain in this country. Structurally and topographically it constitutes an essential section of the great Colorado dome, arching from the Rio Grande to the Rio Colorado. Save in one place—the Zuni Swell—its surface is unbroken by tectonic features. In this mesa-land