Page:Popular Science Monthly Volume 74.djvu/29

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westward and then northward again until it passes into the British possessions. The simile is as striking as it is apt.

To the average mountaineer, one of the strangest characteristics of the desert ranges is the absence of foot-hills. The main eminences of the mountains rise directly from the plains. The line where plain and mountain meet is as sharply defined as if drawn with a pen. Often this line is represented by high mural faces that can be scaled at but few points. For this reason it is, chiefly, that the mountains appear to be half buried by the drifting sands. It is on this account largely that the plains areas appear to be leveled by the waters of former seas, of which the mountains formed the coastal cliffs. The illusion is all but completed by the fact that the phenomenon is perfectly independent of geologic structure. This surprising feature is really one of the most novel peculiarities of eolianÆolian [sic] action under conditions of aridity.

That the substructure of the intermont plains is made up of the softer or non-resistant rocks is an observation the full significance of which has been only lately appreciated. Under conditions of a moist or wet climate it is not an unlooked-for fact that the belts of weak rocks coincide with the valleys or lowlands. It is unexpected, however, that this is also true in an arid country, especially since the intermont plains are commonly considered as areas of extensive accumulations of mountain waste. On the whole they are now thought to be areas of