102 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
by the geographer, botanist, and meteorologist. The outlines of these 
might be roughly traced by lines inclosing the stations shown in fig. 4. 
These regions may be designated as the Sonora-Nevada Desert and the 
Chihuahua Desert. 
The Sonora-Nevada Desert embraces portions of Utah, Idaho, Wash- 
ington, Oregon, Nevada, California, Arizona, Baja California, Sonora, and 
Sinaloa. The northern portion of this region is mainly comprised in the 
Great Basin and embraces the beds of a number of ancient lakes and the 
surviving Great Salt Lake. Other special physiographic features of 
interest in this connection are the areas which bear the names of Snake 
River Desert of Idaho; the Sage Plains of Washington; the Lava Beds of 
Oregon; the Ralston Desert in Nevada; Death Valley, Mohave Desert, 
Colorado Desert, Salton Desert in southern California and Arizona; 
the Painted Desert in Arizona and New Mexico; and the Sonora Desert in 
Mexico. Thesouthern portionof the region consists ofa series of extended 
slopes and terraces traversed by many ranges of hills and mountains 
with peaks of some altitude. Along the shores of the Gulf of California 
and of the Pacific Ocean proper the desert area includes the entire surface 
to within a few feet of the water’s edge, and the xerophytic vegetation of 
the plains comes.into direct contact with the mangrove and strand flora. 
The Chihuahua Desert occupies the central table-land of Mexico east 
of the Sierra Madre, extending as far south as San Luis Potosi and includ- 
ing parts of the States of Coahuila, Chihuahua, and Texas and also por- 
tions of Arizona and New Mexico. The Bad Lands of the Dakotas and 
Montana and the Red Desert of Wyoming, both included in the “‘ Great 
American Desert’’ (fig. 6), might be regarded as a northern arm of this 
region for the purposes of this paper. The arid portions of this area 
consist for the most part of great valleys inclosed by parallel ranges of 
mountains which in some instances attain such altitudes as to be timber- 
clad and even bear an alpine vegetation. 
FORMATION AND EXTENT OF DESERTS. 
The deserts of the world’s surface are not easily delimited, even in North 
America, where some attention has been given to the geographical extent 
and position of the arid areas, which may be taken to cover over 500,000 
square miles. The accompanying map (plate 61) indicates the general 
location of deserts only and not an outline of the areas to be included. 
So far as estimates may be based upon the data obtained from it, a very 
large proportion of the earth’s surface receives a rainfall much less than 
the possible evaporation, and is therefore inhabited by plants of special- 
ized form and of xerophytic and halophytic adaptations. 
These desert areas have been subject to many conditions of change in 
recent geological history, as a result of which some are more arid now than 
ever before in their history, while in other cases the weight of evidence lies 
