90 W J MC GEE — SHEETFLOOD EROSION. 



but the meager records in even the most fertile valleys in the foothills 

 seldom rise above 10 inches, while the average over the interior probably 

 falls below 5, and may be no more than 2 or 3 inches during the year. 

 There are two nominally wet seasons, occurring respectively about mid- 

 winter and midsummer. The midwinter precipitation is generally the 

 heavier and the more widely distributed, but both in summer and in 

 winter the greater part of the rainfall occurs in local storms. Snow 

 falls on the Mogollon and the Sierra, remaining half the year on a few 

 of the highest points ; to this fact the perennial character of some of the 

 northern tributaries of the Gila and the stronger branches of the Yaki 

 may be ascribed. During the occasional drizzles of the wet seasons the 

 scanty moisture is chiefly absorbed by the sun-baked earth, so that 

 floods do not ensue, though the spring-fed streams may rise by reason 

 of the diminished evaporation ; during the moderate storms that occur 

 here and there from year to year torrents .are produced which rush 

 tumultuously down the slopes and become potent geologic agents, and 

 during the great storms occurring from decade to decade or from cen- 

 tury to century whole plains are flooded ; yet so dry are air and earth 

 that the deluge is absorbed within a few miles or scores of miles. Thus, 

 although the entire surface slopes seaward, no living water reaches the 

 sea between the Colorado and the Yaki, 700 miles away. 



By reason of heat and aridity the Sonoran district is desert or subdesert 

 throughout ; the vegetation is too scant, stunted, and scattered to protect 

 the surface from storms; the meager flora forms little. or no humus, and 

 thus there is no soil and little of that chemic action initiated by vegetal 

 growth and decay. Through a combination of biotic conditions of great 

 significance the vegetal life and the sedentary animal life are concen- 

 trated in scattered colonies with bare earth between, and the colonies 

 collectively form but a small fraction of the total area. Thus the region 

 is one in which physical agency operates directly, with little aid or ob- 

 struction from the biotic agency always present and often predominant 

 in humid districts. 



Since the waters of the Sonoran district never reach the sea the terri- 

 tory is complete in itself as a geologic province ; the storm waters gather 

 detritus in the mountains and transport it into the valleys, but their 

 agency is limited to shifting the rock matter from one point to another 

 in the same vicinity, and thus degradation and aggradation go hand in 

 hand, and gradation is completed within the district. 



Topography. — At first sight the Sonoran district appears to be one of 

 half-buried mountains, with broad alluvial plains rising far up their 

 flanks, and so strong is this impression on one fresh from humid lands 

 that he finds it difficult to trust his senses when he perceives that much 



