EOLIAN GEOLOGY 
169 
wide shelf running entirely around the central elevation much in 
the same way that the continental shelf borders the large land 
areas of the globe. 
Whether composed of modern volcanics, ancient crystallines, 
or old, indurated elastics, the desert mountain masses are always 
the very resistant rocks. In marked contrast, even more so than 
in humid regions, the intermontane plains, or lowlands, are nota¬ 
bly the belts of the weak rocks. There is this difference in the 
relief contrasts of the arid lands: A considerable part of the hard 
mountain-block also belongs to the lowland plain. The ancient 
fault-line, if it ever existed, along which the orographic block was 
supposed to be tilted is, when it can be located, found ofttimes to 
be several miles out on the plains. This lowland belt of mountain- 
block has somewhat higher gradient than the general plains-sur- 
face and is distinguished by the special name of the Bajada Belt. 
The plains-belt of the orographic-block which surrounds the moun¬ 
tain is frequently wider than the width of the mountain base. Its 
even surface is manifestly produced by the same erosive powers 
as the plains themselves. Whatever the process the general plains- 
surface now extends far back into the orographic block. The 
mountain is completely girdled. Its slopes are rendered unex¬ 
pectedly rugged. Its mass becomes perfectly enisled. It stands 
out in boldest cameo on its orographic base. 
The girdling may not be at the same height on diiferent sides of 
the mountain. This feature is especially well displayed on the 
Sierra de los Caballos in central New Mexico (Fig. 12). 
