THE LOWER AUSTRAL ZONE. 41 



6. THE LOWER AUSTRAL ZONE. 



The Lower Austral zone occupies the southern part of the United 

 States, from Chesapeake Bay to the great interior valley of Califor- 

 nia. It is interrupted by the continental divide in eastern Arizona 

 and western New Mexico, and is divided, according to conditions of 

 humidity, into an eastern or Ausiroriparian, and a western or Lotoer 

 Sonoran, area. 



(a) The Lov^er Sonoran Faunal Area. 



The Lower Sonoran area begins with the arid region of Texas in the 

 neighborhood of latitude 98°, and stretches westerly to the Rio Grande 

 Vallej^ in which it sends an arm northwest to a iDoint a little north 

 of Albuquerque, N. Mex. Another arm readies up the valley of the 

 Pecos. West of the Rio Grande Valley in New Mexico the Lower 

 Sonoran is interrupted by the continental divide. It begins again in 

 eastern Arizona and sweeps broadly westward below the high i3lateau, 

 covering southern and western Arizona, the deserts of southern 

 Nevada and eastern California, and the San Joaquin and Sacramento 

 valleys. Followed more in detail, the Lower Sonoran in western 

 Arizona sends a narrow tortuous arm eastward in the Grand Canyon 

 of tlie Colorado, which expands to cover the lower levels of the 

 Painted Desert, and another arm northward, which enters the extreme 

 southwestern corner of Utah, where it is restricted to the St. George 

 or lower Santa Clara Valley, and is of much agricultural importance.^ 

 From western Arizona it spreads over southern Nevada, pushes 

 northerly into Pahranagat Valley, sends an arm by way of Oasis and 

 Sarcobatus valleys all the way to the sink of the Humboldt and Car- 

 son rivers, fills the whole of Death, Panamint, and Saline valleys and 

 part of Owens Valley, and thence curving southwesterly follows the 

 eastern base of the Sierra Nevada, Tehachapi, and Tejon mountains, 

 and covers the whole of the Mohave and Colorado deserts and all the 

 rest of southern California except the mountains. It sends an arm 

 southward over most of the peninsula of Lower California, and 

 another northward over the San Joaquin and Sacramento valleys. 



The Lower Sonoran area comprises the most arid deserts of North 

 Amei'ica, and is characterized by a fauna and flora of extreme inter- 

 est. Among the commoner plants are the creosote bush, mesquites, 

 acacias, cactuses, yuccas, and agaves. Among the characteristic 

 birds are the mockingbird, road runner, cactus wren, canyon wren, 

 desert thrashers, hooded oriole, black-throated desert sparrow, Texas 

 nighthawk, and Gambel's quail. Among the distinctive mammals 

 are the four-toed kangaroo rats, Sonoran pocket mice, long-eared 

 desert fox, and the big-eared and tiny white-haired bats. The 

 region, wherever water may be had for irrigation, is of great agricul- 

 tural importance, particularly for fruit. 



' Cotton, tobacco, raisin grapes, almonds, olives, figs, and other Lower Sonoran 

 crops are raised successfully in the St. George Valley. 



