abruptly the numerous mountain ranges to like heights above the plains-surface. Of this region four fifths are plain; one fifth highland. The vastness and evenness of the intermont plains is a matter of much speculation with all who travel the region, scientist and layman alike. Extensive desiccated lake-bottoms they are usually regarded. They are sometimes considered to be intermont basins deeply filled with wash from the peripheral highlands. At the present time the only adequate explanation of their physiognomy is that they are fashioned mainly by eolian agencies with some slight modification by water.
The complete isolation of the different mountains is one of the most remarkable facts concerning the desert features. In most parts of the world there is some more or less close structural relationship between neighboring mountains which are often united to one another by foot-hills. In the desert there is no such tectonic connection. The mountain ranges are all independent individuals without relationship of any kind to one another. Structural mountains, volcanic mountains, laccolithic mountains, fault-block mountains and high residual plateau mountains are neighbors distinctly separated by stretches of level plain.
Mountain ranges throughout the dry regions of western America are completely and evenly surrounded by level plains as if by the sea. They are numerous, short and narrow. Upon the map Button has likened them to an army of caterpillars crawling northward out of Mexico, dividing as it enters the United States, the main body turning
