. Elementary physical geography . ills of circumdenudation, becauseall around the elevated portion the rocks have been cut away(Figs. 218 and 257). Next in prominence to the elevations of the mountains arethe depressions. Between the ridges, systems, and peaks,there are valleys; and these have quite distinct character-istics. Between systems, and really forming a natural partof cordilleras, there are often great valleys, sometimes hun-dreds of miles in width and length, to which the name inte-rior basin is generally given. They are great plateau areasbetween mountain walls, and they are usuall
. Elementary physical geography . ills of circumdenudation, becauseall around the elevated portion the rocks have been cut away(Figs. 218 and 257). Next in prominence to the elevations of the mountains arethe depressions. Between the ridges, systems, and peaks,there are valleys; and these have quite distinct character-istics. Between systems, and really forming a natural partof cordilleras, there are often great valleys, sometimes hun-dreds of miles in width and length, to which the name inte-rior basin is generally given. They are great plateau areasbetween mountain walls, and they are usually more or less PLATEAUS AND MOUNTAINS. 357 broken by mountain ridges. Sometimes, in part of theirarea, there is drainage to the sea ; but very often, and asa characteristic feature, a part of the drainage finds its wayinto these great troughs, and does not escape to the sea, butis returned to the air by evaporation. The Great Basin of the United States has an area of over200,000 square miles; but notwithstanding the great size of. Fig. mountain park (Bakers). the basins of interior drainage on this continent, these formbut percent of the total continental area. In Australianearly 52 per cent of the area is in the condition of interiordrainage, while 31 per cent of Africa is in the same con«dition, and 28 per cent of the continental mass of Eurasiais an enclosed basin. The Sahara interior basin is 16 timesas large as our Great Basin, and the interior basin region ofAsia occupies an area 23 times as great as that of the west. 358 PHYSICAL GEOGRAPHY. Between mountain ridges and chains, there are oftenlongitudinal valleys of considerable size, extending par-allel to the chains between which they occur. These areamong the striking features of mountains, and they aregenerally occupied by streams which are evidently too smallto have carved such immense valleys. When the rockstructure is studied, it is evident that these valleys repre-sent either down-folded portions of the
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