76 WARREN N. THAYER 



of 8o° F., and it is safe to assume that the mean daily variation is 

 not less than 50 F. 



These sudden and excessive changes in temperature produce 

 expansion and contraction stresses which even the hardest rocks 

 are unable to withstand, and consequently they crack, splinter, and 

 chip off. Then by gravity these particles are dragged down the 

 slopes, comminuting each other as they go, until when they reach 

 a certain minuteness of size they are seized by the strong, ever- 

 blowing desert wind, which, like a great winnowing machine, fans 

 them away. Much of this rock dust is carried out to sea, some of 

 it perhaps is carried hundreds of miles to the northeast to add to the 

 accumulation of loess on our Great Plains. Sheet floods following 

 storms play an important part in erosion, but they are quite sub- 

 ordinate to the process just described. In the words of Hill, "the 

 Mexican Plateau is being literally blown away." 1 



THE SONORAN DESERT PROMNCE 



Definition and boundaries. — -This province is the most degraded 

 portion of the old Mexican Plateau. It is 1,700 miles long and lies 

 between the Sierra Madre Occidental on the east and the Pacific 

 Ocean on the west. It includes parts of Nevada, California, and 

 Arizona in the United States, and the Mexican states of Baja 

 California, Sonora, Sinaloa, and Tepic. 



The northern boundary line runs from the edge of the Colorado 

 Plateau near the great bend of the Colorado River westward to the 

 southern terminus of the Sierra Nevada Mountains. It has been 

 drawn to agree with the line marking the southern edge of the 

 Great Basin as described by Fenneman 2 and Bowman 3 and requires 

 no further justification. 



The western boundary extends southward from the southern 

 terminus of the Sierra Nevadas, between the Tehachapi Mountains 

 of the Coast Ranges and a group of mountains which Ransome has 

 referred to as the Sierra de Los Angeles, and reaches the coast 

 between Los Angeles and San Diego. 4 From this point southward 

 it coincides with the Pacific coast line to Cape San Lucas, and then 



1 Hill, Science, N.S., XXV, 710. 



2 Op. cit. 3 Op. cit. 4 J. S. Diller, Science, N.S., XLI, 1045. 



