126 GUIDEBOOK OF THE WESTERN UNITED STATES 
The climate at El Paso is typical of that in much of trans-Pecos 
Texas. The mean annual precipitation is slightly less than 10 
inches, with a recorded range from 2.22 to 18 inches. Most of the 
rain falls in heavy local showers, and more than half of the annual 
total comes in July, August, and September. The greatest recorded 
fall was on July 9, 1881; it amounted to 6% inches. The mean annual 
temperature is 63.5°, and the mean monthly minimum about 31° in 
January. The temperature is rarely below 20° and then only for a 
few hours. The average humidity ranges from 23.2 per cent in May 
to 47.3 per cent in January, with an average of 39 per cent for the 
year. The annual evaporation is estimated at 82 inches. Snow 
falls rarely and then only in small amount, and usually it melts in a 
few hours. The percentage of sunshine is about 81. 
Outside of the irrigated zone the vegetation is characteristic of an 
arid climate. Trees are rare, even on the mountains, but there is a 
scattered growth of mesquite and creosote bush (Covillea). Yucca, 
lechuguilla, sotol, bear vrass, ocotillo, and several species of cactus 
are abundant on the slopes. 
In New Mexico and Texas above El Paso the Rio Grande flows in a 
wide valley of alluvium, bordered by a high older terrace plain; in 
parts of its course farther north in New Mexico it is in deep rocky 
canyons. At El Paso the valley is constricted to the narrow rock- 
walled pass that gives name to the city, but the bordering high terrace 
continues far down the valley. Below the pass the alluvial plain is a 
broad flat in which the river meanders widely, often changing its 
course by cutting new channels at times of freshets. The high terrace 
plain that borders this valley terminates in bluffs and steep slopes, in 
places 200 feet high above the bottom lands. The smooth plain at 
the top of these bluffs extends far north as a wide bolson or desert flat 
between mountain ranges. Near El Paso there are several distinct 
benches, 3,800 to 3,950 feet above sea level, mostly in the form of 
mesas or projections from the base of the Franklin Mountains. 
These benches slope gently toward the river and are in part capped by 
caliche, an infiltration of calcium carbonate in the sand, which makes 
a material so hard that it helps to preserve the tabular form and sharp 
edges of the mesas. The Franklin Mountains form a high ridge on 
the southern prolongation of an axis of uplift which extends across 
central New Mexico from the Rocky Mountains. Probably this 
uplift is cut off to the south by a fault. The Tange rises abruptly 
about 3,000 feet above the adjoining plains or valleys and culminates 
in Mount Franklin (elevation 7,152 feet). The west side is mainly a 
dip slope of heavy beds of limestone with pronounced westerly dip; 
the east side shows many ridges irregul Ae 
ee : ar lower crests, and buttes, 
deeply cut by canyons. The range is a typical tilted block of 
