San Francisco Mountain. 293 
is in places replaced by Yucca baccata. Both the pihion and the cedar occupy 
elevations of corresponding temperature in the arid lands from western Texas 
through New Mexico, Arizona and north to central Colorado, and the cedar 
extends westward to southern California. Closely related and strictly represent- 
ative forms extend northward through the Great Basin to the plains of the 
Columbia; some other species push northward over the Great Plains as well 
as the interior basin. 
The desert flora‘), the origin of which has been previously discussed (alti- 
tude 4000—6000 feet; 1200—ı1800 m), is scattered and scanty, and consists of 
such genera, as Airiplex, Sarcobatus, Oxytaenia, Tetradymia, Yucca, and cac- 
tuses of several genera, while in the rainy season Boxteloua, Lupinus capitatus, 
Mirabilis multiflora, Riddellia tagetinae and Zinnia grandiflora are conspicuous. 
South of the mountain ranges, which embrace San Francisco Mountain, 
a practically pure Mexican flora may be said to occur. Over against the 
valley, of the Gila River, Crreus giganteus 30—50 feet high is found. Also 
Fouguiera, species of Agave, Dasylirion, Canotia, Mimosa, Acacia, Calliandra, 
Bactharis and Tessaria borealis. Near Tucson, Arizona, the previousiy abundant 
Chenopodiaceae disappear to be replaced by numerous Cactaceae and Larrea 
mexicana. The northern boundary of many Mexican forms is found in the 
ranges of mountains which extend northwest and southeast, while between the 
Gila River and the Rio Grande, the Mexican types are predominant ?). 
The time element enters into a consideration of the origin of the different 
elements of the Great Basin and desert floras of the southwestern states. 
Topographically the Great Basin is an enclosed surface diversified by hundreds 
of basins suitable for holding broad lakes, but the present aridity of the region 
precludes their being occupied by perennial water bodies. In many instances, 
the annual rainfall in these desert basins is so meager, that it is at once ab- 
sorbed by the thirsty soil, or returned to the atmosphere as vapor, and not 
even ephemeral lakes are formed. Should the Great Basin and contiguous 
tegions experience a change of climate of such a nature that the rainfall would 
be increased, or evaporation diminished, the most obvious result would be the 
appearance of lakes in the valleys that are now either dry throughout the 
year, or hold playa lakes, and the expansion of perennial lakes°). The study 
of the surface geology of the Great Basin has shown that a more humid 
Climate of the nature just suggested was prevalent in the region at a time not 
emote. The streams from the mountains increased in volume many channels 
were occupied by flowing water, that are now dry throughout the year, and 
large lakes denominated respectively Lahontan and Bonneville covered a large 
nn 
!) See ante page 285—291. 
2) Enczer, A.: Versuch einer Entwicklungsgeschichte der Pflanzenwelt, II. Theil, 1882: 219. 
on 3) Since the above was written, the Colorado River broke through its banks and we! u 
er part of the Colorado Desert, during the winter of 1904—1905, producing a large anc 
ä en ‚the Salton Sea. See MAcDovsAL, D. T. The Desert Basins of the Colorado Delta. 
er "alletin American Geographical Society. Dec. 1907. 
