PRACTICAL STUDY OF PL AN 'T FORMATIONS. 645 



Columbia and the Malheur and Harney plains; in California it encircles 

 the Sacramento and San Joaquin valleys and forms a narrow belt along 

 the eastern boundary of the Colorado and Mohave deserts; in Utah it 

 covers the Salt Lake and Sevier deserts, in Idaho the Snake Plains, and 

 in Nevada and Arizona irregular areas of suitable elevation. Except in 

 California the most conspicuous vegetation of the Upper Sonoran areas 

 is the true sage-brush (Artemisia tridentata), which, however, is equally 

 abundant in the Transition zone. Several of the so-called ' grease woods ' 

 ( Atriplex confertifolia, A. canescens, A. nuttallii, Tetradymia canescens, 

 Sarcobatus vermiculatus, and Grayia spinosa) are on characteristic soils; 

 and nut-pines (pinon) and junipers occur here and there, mostly on the 

 mountain slopes. 



"The Upper Sonoran area, notwithstanding its aridity, is of consider- 

 able agricultural importance. Fruits and cereals succeed wherever water 

 may be had for irrigation, and in the less arid parts wheat, corn, barley, 

 and rye yield their heaviest returns. Kaffir corn (a kind of millet) thrives 

 without irrigation, particularly on the great plains, and alfalfa with irri- 

 gation matures several crops a year, though not so many as in the Lower 

 Sonoran." 



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 California. It is interrupted by the continental divide 

 in eastern Arizona and western New Mexico, and is divided into an eastern 

 or Austroriparian, and a western or Lower Sonoran, area." 



a. The Lower Sonoran 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 Valley, in which it sends an arm northwest to 

 a point a little north of Albuquerque, New Mexico. Another arm reaches 

 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 

 plateau, covering southern and western Arizona, the deserts of southern 

 Nevada and eastern California, and the San Joaquin and Sacramento val- 

 leys. Followed more in detail, the Lower Sonoran in western Arizona 

 sends a narrow, tortuous arm eastward in the Grand Canon of the Colo- 

 rado, 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 Val- 

 ley, 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 Carson rivers, fills the whole of Death, Pana- 

 mint, and Saline valleys and part of Owens Valley, and thence curving 



