The American Deserts 



163 



Oregon, the Ralston Desert in Nevada, 

 Death Valley, Mohave Desert, Colorado 

 Desert, Salton Desert, in southern Cali- 

 fornia and Arizona ; the Painted Desert 

 in Arizona and New Mexico, and the 

 Sonora Desert in Mexico. The southern 

 portion of the region consists of a series 

 of extended slopes and terraces trav- 

 ersed by many ranges of hills and moun- 

 tains with peaks of some altitude. 

 Along the shores of the Gulf of Cali- 

 fornia and of the Pacific Ocean proper, 

 the desert area includes the entire sur- 

 face 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 tableland of Mexico east of the 

 Sierra Madre, extending as far south as 

 San Luis Potosi, and including parts of 

 the states of Coahuila, Chihuahua, and 

 Texas, and also portions of Arizona and 

 New Mexico. The Bad Lands of the 

 Dakotas and Montana and the Red 

 Desert of Wyoming might be regarded 

 as a northern arm of this region for the 

 purposes of this paper. The arid por- 

 tions of this area consist, for the most 

 part, of great valleys inclosed by parallel 

 ranges of mountains, which in some in- 

 stances attain such altitudes as to be 

 timber-clad and even bear an alpine 

 vegetation. 



One of the striking features of the 

 Chihuahua Desert consists of the great 

 sand dunes, or " medanos," more than 

 a hundred feet high, in some instances 

 forming great ridges that have almost 

 the imposing appearance of mountain 

 ranges ; these move across the floor of 

 the desert plains with a sweep that ob- 

 literates minor features of topography. 

 These moving dunes bear a characteris- 

 tic vegetation adapted to the unusual 

 physical conditions offered by the sand 

 and lack of water and nutrient material. 

 The gypsum deposits forming the White 

 Sands in theTularosa Desert north of the 



Rio Grande possess unusual chemical 

 properties. The Jornada del Muerto 

 (Journey of Death) of the ancient Span- 

 ish explorers lies in the western portion 

 of the Chihuahua Desert in New Mexico 

 separated from the Tularosa Desert by 

 the Organ Mountains. Farther north- 

 ward the great stretches of malpais, or 

 black volcanic rock, form a desert dis- 

 trict of the extreme type, while numer- 

 ous areas are impregnated with alkali, 

 and are either almost wholly free from 

 vegetation or support only halophytic 

 species. The Bad Dands of Dakota owe 

 their desert character to the peculiar 

 composition of the soil, which is clayey, 

 poor in nutrient substances, and subject 

 to great erosion, so that extensive areas 

 are destitute of vegetation of any kind. 



THE INTENSITY OF DESERTS 



The most interesting feature of- the 

 report is a table showing the intensity 

 of the deserts — the ratio of rainfall to 

 evaporation — in the arid regions in the 

 United States. 



From this table it appears that Fort 

 Yuma is the driest spot in the country, 

 its rate of evaporation being 35.2 times 

 greater than its normal rainfall. 





Annual precipitation 



u 5 

 C *j 



< 





Place. 





as 



e 



a 



a 



V 



.2 



« 



El Paso 



24 



I 

 I 



39 

 26 

 18 

 25 

 26 

 14 

 22 



13 

 IS 

 6 



7 



13 

 1 



In. 



1S.30 



In. 

 2.22 



In. 



9-32 



10.41 



10.86 



14.00 



2.84 



7.06 



11.74* 



4-97 



4-50 



8.31 



6. 4 6t 



649 



9.01 



8.47 



9.81 



16.79 



In. 

 80 



8.7 



San Luis Potosi 













25.00 

 5-S6 

 12.83 

 18.37 

 21.38 

 8.3s 

 11.91 

 9.81 



1 1-43 

 ir. 64 

 11.86 

 34 42 



6-37 

 0.60 

 3-77 

 5.26 

 2.20 

 1.89 

 4.89 

 3-55 

 4-36 

 7-49 

 6.22 



5.56 



80 

 100 

 90 

 90 



^5 

 80 

 80 

 90 



75 

 70 

 7° 

 70 

 100 



5-7 





35-2 

 12 7 







7-7 





19. 1 



17-5 







06 





13-9 





TT 6 





78 





8 ^ 





7- 1 





60 







* From 15 years' observations. 

 t From 22 years' observations. 



