18] 
The University of New Mexico 
form the scanty grass vegetation. This district is one of the 
truest deserts in northern Mexico, the annual precipitation 
averaging less than 200 mm. 
2. Mesquite-grassland: West and south of the above 
described district is a higher, somewhat less arid region of 
steppe climate whose vegetation is of a type transitional be¬ 
tween desert shrub and mesothermal woodland. Grasses, 
mesquite, creosote bush, and hojasen (Flourensia sp.) dom¬ 
inate the landscape. Mesquite supplants the creosote bush 
as the dominant member of the vegetation, and the latter is 
restricted to high benches, outwash fans, and upper pedi¬ 
ment slopes where the soil is coarse textured, permeable, 
and well drained. The mesquite attains its best growth in 
the lowland areas of fine textured soils and high water table, 
but also extends up washes and onto areas quite deficient in 
water supply. In some of the better watered valleys thick¬ 
ets of arborescent mesquite, frequently associated with 
screw bean (Prosopis pubescens), Acacias , Koeberlinia, 
chamiso, and Mimosas, occupy the river plains. Hojasen 
(Flourensia cemua) ranks only after mesquite and creosote, 
among the shrubs. It usually occurs on lower slopes, mixed 
with mesquite and creosote. The sequence of the shrub 
association is quite marked where slopes are well developed. 
Mesquite normally occupies the middle of the valleys, fol¬ 
lowed by a mixture of mesquite and hojasen in which 
hojasen becomes more and more abundant until creosote 
bush appears as the marked slope begins. The creosote bush 
increases rapidly, and is nearly pure on the upper slopes and 
ridges. 
Grasses are well developed throughout, and great ex¬ 
panses of grass country are frequent in the western basins 
between 4,000 and 5,000 feet elevation. The chief grasses 
are the gramas, various needle grasses, curly mesquite 
(Hilaria belangeri), tobosa grass (HUaria mutica) , several 
bunch grasses (SporoboVus) , and false buffalo grass (Mun- 
roa squarrosa). Black grama, curly mesquite, and false 
buffalo grass predominate on the upper, well-drained slopes, 
