24 



THE POPULAR SCIENCE MONTHLY 



abruptly the numerous mountain ranges to like heights above the 

 plains-surface. Of this region four fifths are plain; one fifth high- 

 land. The vastness and evenness of the intermont plains is a matter 

 of much speculation with all who travel the region, scientist and lay- 

 man alike. Extensive desiccated lake-bottoms they are usually re- 

 garded. 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 tlie 

 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 moun- 

 tains, 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 



Great I'oso ^■EKDE Tlain. Isolation of the mouutaius, sharp meeting of plain and 

 mountain, and absence of foothills are noteworthy characteristics. 



(W J McGee, photo.) 



