STRUCTURE OF THE PLATEAU PLAIN 715 



to be the direct result of circumdenudation ; but, as will be seen later on, 

 of a somewhat different kind. In marked contrast to the humid-land 

 effects the remnantal plains of the desert, whether their surfaces be on 

 stratum planes, beveled tables of flexed strata, lava-sheets, or cemented 

 regolith, are of quite different elevations, even in the same district. In 

 New Mexico, for example, these plains attain all altitudes above the gen- 

 eral plains surface, from a few feet in the instance 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 sealevel (plate 42, figure 1). 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. 



TOYALANE 



Toyalane is a conspicuous flat-topped mountain (plate 42, figure 2), 

 situated in the Zuni Basin just over the continental divide in western 

 New Mexico. The region is the largest, highest, and driest desert plain 

 in this country. For the most part it lies 7,000 feet above tide level. 

 Structurally and topographically it constitutes an essential section of the 

 great Colorado dome, arching regularly from the Rio Grande to the Rio 

 Colorado. Save in one place — the Zuni swell — its broad surface is un- 

 broken by tectonic features. The region is preeminentlly one of plateau 

 plains standing at all heights above the general plains surface. To the 

 significance of this particular feature chief attention is here directed. 



Around the southern slope of the dome there is, as Gilbert observes, 

 one of the great lava tracts 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 Mount Taylor on one 

 horn and the San Francisco Mountain 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 limited lava-sheets, which spread out over the 

 soft sedimentaries constituting the substructure of the plains in this part 

 of New Mexico. In Arizona the hard Carboniferous limestone is the 

 main surface rock, the shales once overlying it having been recently 

 stripped off. Other volcanic evidences are the denuded necks and dikes. 

 Lava-sheets form the foundation of many plateau plains. 



Outside the limits of the lava fields the massive and more indurated 

 L — Bull. Geol. Soc. Am., Vol. 28, 1911 



