30 THE POPULAR SCIENCE MONTHLY 



so situated as to be periodically renewed for a part of each year the 

 areas become vast mud-flats, forming the playas of the Mexicans. Not- 

 withstanding the fact that most of these lakes are ephemeral in charac- 

 ter, they are sometimes long-lived. In the valley of the Eio Conchos, 

 in the state of Chihuahua, the rainfall was once so heavy that in a 

 single night a great lake was formed that lasted for more than eighty 

 years. 



A most singular phenomenon resulting directly from water-action 

 in the desert is sheet-flood erosion. The vigor with which it often 

 acts makes it locally an important plains-forming agent. Instead of 

 the water from the rainfall tending to concentrate along certain lines 

 of lowest level, as in humid regions, it spreads out over the sloping 

 plains- surface. The railroads crossing the desert have yet to discover 

 an effective method to overcome the disastrous work of the flood-sheet. 



In the desert, however, extensive water-action is exceptional and 

 relatively unimportant. 



Not the least striking characteristic of desert regions is the marked 

 absence of rock-weathering as we ordinarily know it. All exposed rock 

 surfaces present an aspect of wonderful freshness. Chemical decom- 

 position of rock-masses is all but unknown under the influences of a 

 dry climate. The breaking down of the rocks is mainly mechanical, 

 assisted somewhat by sand-blast action. 



During the next decade the features of the arid portions of our 

 earth promise to give us many novel ideas concerning the workings of 

 certain of the geologic processes. To the scientist the desert will 

 become one of the most interesting fields of research. 



