The Basin-and-Range physiographic province, as outlined by a committee of the Association of American Geographers and the Physiographic Committee of the U. S. Geological Survey,2 extends in the United States from southern Oregon to trans-Pecos Texas, and contains not less than 125 closed drainage basins—that is, basins which have no surface outlets for the water that falls upon them. The mountainous rims of these basins, with their great relief and remarkable exposures of geologic formations and structures, have, from the beginning of explorations in the region, compelled the attention of geologists. In contrast, the interior depressions of the basins, mantled by recent sediments and with surface features that are hardly discernible except by an experienced observer, have received relatively little attention and are commonly regarded as lacking in geologic interest. However, the serious work that has been done in these interior depressions, or basin valleys, has shown that they . . .