02

CLIMATE.

Local conditions are largely controlled by the climate, the general nature of which is indicated by the following tables, compiled from data collected by the Weather Bureau:

These tables show that the climate of El Paso is characterized by low precipitation and high temperature. The mean annual precipitation is only 9.85 inches, ranging between a maximum of 18.30 in 1884 and a minimum of 2.22 in 1891. In the five years succeeding 1901 the rainfall averaged 3.32 inches above the normal, the amount for 1905 being the third largest ever recorded, but in 1907 the precipitation was 1.44 inches below the normal. Most of the precipitation falls in heavy local showers, and more than half the annual amount occurs during July, August, and September. The mean annual temperature at El Paso is 63.4° F., and the mean monthly minimum varies between 31° in January and 69° in July. The extreme dryness of the atmosphere is indicated by the mean relative humidity, which ranges from 23.2 per cent in May to 47.3 per cent in January, an average of 38.8 per cent for the year, and by the estimated annual evaporation of 82 inches.

The arid nature of the climate is emphasized by the character of the vegetation. Although the river valley has been converted into a garden where water for irrigation is available, desert conditions prevail over the greater part of the district. Trees are normally absent, and the Franklin and Hueco mountains are almost bare of vegetation. Scattered mesquite and greasewood bushes dot the Hueco Bolson, and its upper slopes are occupied by desert growths, among which yucca, lechuguilla, sotol, ocatillo, and a variety of cacti are prominent.

TOPOGRAPHY.

Topographically the El Paso district possesses the characteristic features of the trans-Pecos region. The Hueco Bolson, a broad, waste-filled lowland, is bordered on the west by the narrow north-south Franklin Range and on the east by the Hueco Mountains. The Rio Grande, constituting the western and southern boundary of the area, flows through a gorge near El Paso, but above and below the city its valley is broad.

HUECO BOLSON.

The Hueco Bolson is one of the largest of the intermontane, waste-filled plains of the trans-Pecos region. Together with its northward and southward continuations it is somewhat over 200 miles in length, about half of it lying each side of the Texas-New Mexico boundary. It varies in width, averaging possibly 25 miles. The greater part of the bolson has an elevation approximating 4000 feet, and it is bordered by mountains which rise from 2000 to 5000 feet higher. On the west are the San Andreas, Organ, and Franklin ranges and others in Mexico; on the east are the Sacramento, Hueco, Finlay, and Quitman mountains. In a large way this intermontane lowland is a unit, but it is divided into two distinct parts by a low transverse divide a few miles north of the State boundary. The northern part, known as the Tularosa Desert, trends north and south, and is a closed basin with no drainage outlet. A large part of its surface is characterized by salt marshes and dunes of gypsiferous white sands. The southern part of the lowland trends northwest and southeast, contains no salt or gypsum, and is traversed by the Rio Grande.

In the El Paso quadrangle the Hueco Bolson, which is known locally as "the mesa," is a structural trough deeply filled with detritus. Viewed from a distance its surface appears to be practically flat, but the elevation between Fort Bliss and the State boundary increases at the rate of about 7 feet to the mile, and at the eastern and western margins the surface ascends more steeply in alluvial slopes toward the adjacent highlands. Near the Franklin Mountains the alluvial slopes are locally much dissected by arroyos. The mouths of the mountain arroyos are marked by detrital cones, the outer margins of which coalesce with the "wash" from the intervening slopes. The eastern and western borders of the bolson next to the highlands are much dissected where occasional torrential floods cut deeply into the alluvial accumulations; but the precipitation on the mountain slopes is soon lost by evaporation and absorption, and even shortly after heavy rains streams do not flow far from the mountains. An ill-defined valley is followed by the El Paso and Southwestern Railroad across the bolson, but in general its surface is scarcely indented by erosion channels. The small rainfall is the direct cause of the present conditions; with adequate precipitation the wash from the mountains would be carried down the Rio Grande and the lowland would become a normal stream valley.

RIO GRANDE VALLEY.

The Rio Grande rises in the Rocky Mountains of southern Colorado and flows southward across New Mexico. Near the southern boundary of the Territory the river turns southeastward, and it forms the boundary between Texas and Mexico for almost a thousand miles, finally emptying into the Gulf of Mexico. Within the Cordillera, after leaving the Rocky Mountains, the river receives few tributaries, and its flow is dependent mainly on two factors—the melting snows tributary to its headwater drainage and the torrential storms characteristic of the region through which it passes. As shown by the discharge table on page 10, the flow of the river is subject to extreme variation; for months at a time the river bed may be dry, but during floods the stream becomes much swollen and frequently shifts its channel.

Throughout the greater part of its course in New Mexico the Rio Grande flows in a succession of open waste-filled valleys separated by narrow rock-walled gorges. About 30 miles north of the Texas-New Mexico boundary, above the town of Las Cruces, the river leaves one of these narrows and in the Mesilla Valley cuts across a broad upland wash-covered plain known as the Jornada del Muerto northeast of the river and as La Mesa southwest of it. The floor of the Mesilla Valley is about 300 feet beneath the general elevation of the upland plain, and the valley is about 45 miles long and 5 miles wide. South of the Texas-New Mexico boundary the valley contracts, and for several miles above El Paso the river flows in a narrow gorge. Below this pass the valley widens again to a lowland, known as the El Paso Valley, in which the river meanders in a broad flood plain trenched about 250 feet below the general level of the Hueco Bolson. The stream flows parallel to the trend of the bolson to its south end near the Quitman Mountains, about 100 miles southeast of El Paso, where it enters another gorge.

Terraces are well developed along the Rio Grande, the most prominent in the El Paso district being marked by the eroded bluff which separates the flood plain of the river in the El Paso Valley from the floor of the Hueco Bolson. This bluff extends across the quadrangle and averages about 250 feet in height above the flood plain, the rise in elevation being accomplished in about a mile. The bluff affords a minimum measure of the amount of down cutting done by the river on its way across the bolson. In the Mesilla Valley a similar escarpment extends along the eastern edge of La Mesa west of the Rio Grande, but east of the stream, owing to the erosion of the flanks of the Franklin Mountains, only remnants of this level are preserved in a fringing terrace. The interval between the flood plain and the western base of the mountains is occupied by an outwash alluvial slope having a gradient of approximately 200 feet to the mile. This slope is broken in places by low bluffs, the most conspicuous of which extends along the 4250-foot level at the approximate elevation of La Mesa. In the Rio Grande valley in New Mexico there are well-marked remains of a terrace several hundred feet higher, but in the El Paso district they are not recognizable, and if ever present they have been removed by erosion. In the immediate vicinity of El Paso there are several distinct benches at elevations of 3800 to 3950 feet above sea level, consisting of disconnected even-topped areas bordered riverward by escarpments from 10 to 50 feet in height. These benches slope toward the river with inclinations between 1° and 3°, and most of them are capped by beds of caliche or cemented gravel which preserve and accentuate them.

FRANKLIN MOUNTAINS.

The Franklin Mountains are the southern extremity of the long, narrow chain that extends from the termination of the main mass of the Rocky Mountains, in northern New Mexico, southward as far as El Paso. This chain occupies a belt about 10 miles wide and 250 miles long across central New Mexico immediately east of the Rio Grande valley. Its continuity is broken in places, causing a separation into several units known as the Sandia, Manzano, Oscura, San Andreas, and Franklin ranges, named in order from north to south.

The Franklin Range trends slightly wrest of north and extends from El Paso to a point a few miles north of the New Mexico-Texas boundary, where it is separated by a low wash-filled pass from the Organ Mountains, which form the southern extremity of the San Andreas Range. (See fig. 1.) The main part of the Franklin Range lies entirely within Texas and is 15 miles long and about 3 miles wide, but low outlying hills extend the range 8 miles beyond the State boundary. The mountains rise abruptly more than 3000 feet above the Rio Grande valley on the west and the Hueco Bolson on the east, culminating in a peak 7152 feet above sea level. (See figs. 9, 10, and 11, illustration sheet.) The western face of the range is relatively little eroded and in the main constitutes a dip slope; the eastern face, on the contrary, is more dissected and exposes cross sections of the rocks, deep valleys that extend back almost to the rim of the range separating several transverse ridges. Individuality is given to the topography by the varying character of the formations. The crest of the range, capped for the greater part of its length by westward-dipping limestone, presents a rugged scarp; the lower slopes and transverse ridges have characteristic irregular surfaces due to the varying resistance to weathering of the component rocks. The mountains are practically bare of vegetation save for a scanty desert growth on the lower slopes, so that the rocks are plainly exposed except where they are covered by accumulations of débris. As a whole, the Franklin Range resembles an eroded block mountain of the Basin Range type.

HUECO MOUNTAINS.

As has been stated, the Hueco Mountains constitute the dissected northwestern scarp of the Diablo Plateau. They trend north and south and occupy a belt 6 or 8 miles in width and about 25 miles in length, but only a part of the mountans are included in the El Paso quadrangle. The summits rise more