18 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
COMPARATIVE COMPOSITION, SAMPLES I anp II. 
Direct comparison of results for composition of the two sands is made in the 
appended summary of average analytic data for substance dried to constant weight 
atxzro° to: 1260°.C.* 
(Sand dried at 110° to 120°C.) 



Sampje I. | Sample IT. 
Pret. tee 
CER, oot eR me ace arate ky wreec tees ae ah ee aed an ee ar 355)5 trace 
) O Fiera ie ari ee cera iis ee a i ee gat trace 
wg ak eee Sew wi ailsce nem, Sonaetae RVGES hg ig ie re eae a4 86.0 
A1,0, ) 
Fe,0, f G Sevinca ve hl te xAb Mann lacecs Kies Bon, ap eRe ee Cae eee 0.5 8.2 
EE Nas ee eames ade eal oi ee aide ean ere Gr too w5 
O in silicate and traces of other elements (by difference)... .4 5.3 
Chief constituents: Calcium sulphate. (0°... ©. -..p ee ae) eee ate 
ALICA AIG SENCATESL 5 Fas w aoorkses oe ne a nec 3-9+ | 94.2+ 



Sample I, from Tularosa Desert, consists mainly of gypsum. 
Sample II, from Samalayuca, is almost entirely silicious. 
NOGALES AND THE INTERNATIONAL BOUNDARY REGION. 
The region immediately accessible from Nogales and to the westward 
along the international boundary comprises a series of valleys at altitudes 
between 3,000 and 4,000 feet, separated by ranges of mountains with a 
general trend of north and south. 
The arid slopes are in the main characterized by yucca-like plants, 
including Yucca, Nolina, and Dasylirion, while agaves arealso inevidence. 
Two species of oak are abundant. Arboreous opuntias are numerous, 
and a few species of Echinocactus and Echinocereus arefound. One of the 
characteristic plants of this region is Cactus heydert, the flattened globose 
body of which sets so deeply that its upper surface is flush with that 
of the ground. Spinose shrubs are a prominent feature, but along the 
streamways Sambucus mexicana, Juglans, and Platanus are fairly abun- 
dant. Much detailed work upon the natural history of the region has 
been accomplished by the members of the various boundary surveys. 
(See E. A. Mearns, Mammals of the Mexican Boundary Survey, U. S. 
National Museum Bulletin No. 56, 1907.) 
The dome-shaped hills have very steep slopes, which permit the 
rapid descent of precipitated water, with the result that the streamways 
often show currents of great volume which last for a few hours only, 
the channel then becoming dry for long periods, perhaps for months 
(plate 7). The main mountain ridges offer some fine examples of long 
