PLAINS AND MOUNTAINS OF SONORA. 89 



Ranges,"* constituting a considerable part of that portion of the province 

 characterized by " Open Basins ; " but the district is so distinctive that, 

 for purposes of description at least, it may be set apart as the Sonoran 

 district. It corresponds approximate!} 7 with the region known among 

 the Mexican inhabitants as Papagueria, or land of the Papago Indians. 



The Sonoran district slopes from the high Sierra of Mexico and the 

 Mogollon escarpment at its northern extremity to tide level in the gulf 

 of California. To the casual observer traversing its expanse it seems a 

 region of mountains, for rugged buttes, mesas, and sierras are always 

 in sight and usually dominate the landscape ; but more careful observa- 

 tion shows that it is primarily a plains region, since fully four-fifths of its 

 area consists of plains, hardly one-fifth of mountains, and the elongated 

 sierras and scattered buttes are nearly always flanked by vast tracts of 

 lowland. The mountains range up to 6,000 or 6,500 feet above tide in 

 the interior (imposing Baboquivari, southwest of Tucson, measures 6,798 

 feet) and still greater altitudes toward the Sierra Madre ; the intermon- 

 tane plains rise gently from sealevel to 3,500 or 4,000 feet,t while the 

 buttes and sierras may rise anywhere from 100 to 4,000 feet above the 

 circumscribing plain. The mountains are notably rugged, abounding 

 in lofty precipices, vertical and sometimes overhanging cliffs, knife-edge 

 aretes, and leaning (apparently or actually) picachos, and they rise with 

 remarkable abruptness from the smooth plains, so that, to the casual ob- 

 server, they seem an archipelago of rocky peaks rising from an ocean of 

 desert sands. Though monotonously smooth, partly by contrast with 

 the rugged mountains, the plains slope gently away from the mountains 

 and merge in flat-bottomed valleys leading directly or deviously toward 

 the sea, so that the entire surface, with the exception perhaps of a few 

 playas near the coast, drains seaward. In the valleys occupied by washes 

 (rivers during the wet seasons, sand wastes during most of the year) the 

 grades may be as low as 20 or even 15 feet to the mile ; over the average 

 plain the slope is 50 or 75 feet to the mile, and on the great apron-like 

 foot-slopes pushing among the aretes and lateral spurs of the larger 

 sierras the grade may be 100, 200, even 300 or more feet to the mile. 



Climate. — The Sonoran district is excessively hot and arid. Yuma, noted 

 as the hottest station in the United States, is in its northwestern corner, and 

 while records are lacking, it is probable that Caborca and Hermosillo are 

 considerably hotter ; the rainfall in the Sierra may reach 15 or 20 inches, 



* National Geographic Monographs, vol. i, 1895, pp. 95-98. 



t Captain D. D. Gaillard, of the International Boundary Commission, writes : " The sacred peak 

 of the Papago, Baboquivari, is 6,798 feet above mean sea level of San Diego bay. The lowest part 

 of the valley on the east of this peak (called the " Sasabe flat") is about 3,200 feet above the same 

 datum plane, while the lowest part of the valley on the west of the peak (called the " Moreno 

 flat ") is only about 2,300 feet above the plane of reference" (official letter dated February 6, 1896). 



