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I
CARBONIFEROUS
By G. BE. ichardson.
a term of Spanish origin, current in Mexico, which is being
introduced into the southwestern part of the United States.
Bolsons are aggradation plains that commonly occupy struc-
tural basins and have been built up by wash derived from
the disintegration of the rocks of adjacent highlands. Their
central -ports- are almost level, but their margins slope up
toward the highlands. Some bolsons are closed basins, being
entirely surrounded by a rim, but many have outlets, which,
however, in this arid climate, are practically free from sur-
face drainage, unless the bolson is crossed by one of the
few perennial streams of the region. If the climate were
more humid bolson plains would not be formed, for the
debris, instead of accumulating, would be carried away by
streams.
Climate.-Many of the characteristic features of the trans-
Pecos region are due to its arid climate. The annual precipi-
tation on the greater part of the area is only about 15 inches
and in places is less than 10 inches. The common type of
rainfall is the occasional heavy local summer shower of short
duration. Such showers give rise to local torrential floods but
yield no permanent run-off, for the short-lived streams that
gather in the highlands disappear by absorption and evapora-
tion shortly after reaching the lowlands.
VThe aridity of the climate is emphasized by the character of
the vegetation`;.which is sparse and of the desert type, so that
the general appearance of the country is barren. (See Pls. I to
IX, illustration sheet.) Desert growths like yucca, lechuguilla,
cacti, sotol, ocotillo, creosote bush, cat's-claw, mesquite, and a
variety of bunch grasses are common. Except a few stunted
junipers and pinons on some of the highlands there are no
trees in the Van Horn quadrangle. Only the highest moun-
tains of the trans-Pecos region, like the Sacramento Mountains
in New Mexico, support a forest growth.
NORTHERN TRANS-PECOS TEXAS.
Across the northern part of trans-Pecos Texas, in the midst
of which the Van Horn quadrangle is situated, run three belts
of highland separated by parallel belts of lowland, all having
a northwest-southeast trend. (See fig. 2.) Named in order
from west to east they are the Franklin Mountains, the
Hueco Bolson, the Diablo Plateau, Salt Flat, the Guadalupe-
Delaware-Apache Mountains, and Toyah Basin (Pecos Valley).
An outline geologic map of this strip of country is shown in
figure 3.
Franklin Mountains.-The Franklin Mountains are the
southern extremity of a broken chain about 10 miles wide and
250 miles long, lying east of the Rio Grande valley and
extending from the termination of the main mass of the Rocky
Mountains in northern New Mexico southward to El Paso.
The main part of the Franklin Range lies entirely in Texas
and is 15 miles long and about 3 miles wide, but low outlying
hills extend from the range northward a few miles beyond the
State boundary. The mountains rise 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. The western face of the range is comparatively little
eroded and in the main constitutes a dip slope; the eastern
face, on the contrary, is much dissected and exposes cross
sections of the strata.
The Franklin Mountains are composed chiefly of pre-Cam-
brian and Paleozoic rocks that strike in general parallel to
the trend of the range and dip westward at steep angles. The
pre-Cambrian rocks consist of quartzite overlain by rhyolite
porphyry. The Paleozoic strata consist of Cambrian sand-
stone overlain by limestone containing Ordovician, Silurian,
and upper Carboniferous (Pennsylvanian) fossils. Devonian
time, so far as known, is
1o0 not represented by sedi-
EW ME-- - mente, although some
-N-A I relatively thin bedded
FIGURE 1.-Index map of part of trans-Pecos region, Texas and New
Mexico.
The location of the Van Horn quadrangle is shown-by the darker ruling. The El Paso folio
(No. 166), previously published, is indicated by lighter ruling.
TRANS-PECOS TEXAS.
Characteristic features.-Trans-Pecos Texas, which lies west
of Pecos River, is distinctly different from the eastern part of
the State in topography, climate, and geology. The surface
of the greater part of Texas consists of plains, but trans-
Pecos Texas is mountainous, the boundary between the two
parts being marked by the northern course of the Pecos.
The part of the Cordillera that is included in trans-Pecos Texas
is the southern continuation of the central mountainous area of
New Mexico, and is characterized by an assemblage of diverse
topographic forms which individually resemble features of the
Rocky Mountain province on the north, the Basin Range
province on the west, and the Mexican Plateau province on the
southwest. Topographically the trans-Pecos region forms a
transition between these provinces.
Relief.-The trans-Pecos region lies in a belt of compara-
tively low country that extends across the interior of the
continent. Paisano, the highest pass through the Cordilleran
range on the Sunset Route of the Southern Pacific system,
has an altitude of 5082 feet, and the summit of the Texas &
Pacific Railway, which is in the Van Horn quadrangle, has an
altitude of 4603 feet. Only two peaks rise higher than 8000
feet above sea level and the lowlands commonly range in
height from 3500 to 4500 feet.
limestone that overlies
the massive Silurian beds
may be of Devonian age.
The Mississippian and
Permianseriesareabsent.
Granite of post-Paleozoic
age outcrops along the
eastern base of the moun-
tains. Faults border the
eastern and western mar-
gins of the range and
other faults cut it inter-
nally. The range is
a westward-tilted fault
block in a mature stage^.,
of erosion.
kueco Bolson.-One
of the largest of the
intermontane plains -of
the trans-Pecos region
is the Hueco Bolson,
which, with its north-
ward and southward con-
tinuations, is more than
PENNSYLVANIAN
A- I
QUATERNA
RY CRETACEOUS ' PERMIAN 1 _
Upland Comanche Red beds of Guadalupe
wash series PecosValley group
ORDOVICIAN CAMBRIAN PRE-CAMBRIAN
Montoya Van Horn" Sedimentary I
andEI Paso and Bliss and igneous i
limestones sandstones rocks
FiouRS 8.-Generalized geologic map of northern trans-Peeos Texas.
Bolson
deposits
SILURIAN
Fusselman
limestone
Hueco
limestone
Meteorologic records that have been kept for more than 30
years at El Paso indicate the general features of the climate of
trans-Pecos Texas. The mean annual precipitation is only 9.8
inches, most of which occurs in heavy local showers and more
than half of which falls during July, August, and September.
The mean annual temperature is 63.4 F., the mean monthly
maximum ranging from 57 in January to 96 in June, and the
mean monthly minimum from 31 in January to 69 in July.
The average daily range of temperature for the year in a ther-
mometer shelter is about 26 and on exposed rock surfaces is
probably more than twice that amount. The mean annual
relative humidity is 38.8 per cent and the annual evaporation
is about 82 inches. The annual average wind velocity is 10
miles an hour, the maximum figure on record being 78 miles
an hour. Velocities of 50 to 60 miles an hour for short
periods are not uncommon.
Geology.-The sedimentary rocks of trans-Pecos Texas
include representatives of almost all the systems from the
Algonkian to the Quaternary and are intruded or overlain, in
parts of the area, by a variety of igneous rocks, as stated
under the heading " Descriptive geology" (p. 3).
200 miles long, about half of it lying on each side of the
Texas-New Mexico boundary. Its width is irregular, averag-
ing possibly 25 miles. Its greater part lies about 4000 feet
above sea level and is bordered by mountains that rise 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 Sierra Blanca, Sacramento, Hueco, Finlay, and Quitman
mountains. As a whole the lowland is a unit, but it is divided
into two distinct parts by a low transverse debris-covered
divide a few miles north of the State boundary. The northern
part, known as the Tularosa Desert, is a closed basin with no
drainage outlet, a large part of its surface being occupied by
salt flats and dunes of white gypsiferous sand. The southern
part contains practically no salt or gypsum and is crossed by
the Rio G-rande, which has cut its valley more than 200 feet
beneath the general level. /Structurally the bolson is a trough
occupied by more than 2000 feet of uncon'soidated deposits,
the upper part of which is proved by Ssil bones to be of
Pleistocene age, though the lower part may be Tertiary.
Diablo Plateau.-Northeast of the Hueeo Bolson is the
Diablo Plateau, a flattih-toppd upland having an aea of
FiuRE 28.-Relief map of part of trans-Pecos region, Texas and New
Mexico.
The Van Horn quadrangle lies in the south-central part of the area, including the southeast
end of the Sierra Diablo and the west end of the Apache Mountains.
The region is characterized by mountains and intermontane
plains having a northwest trend. (See fig. 2.) In general
the highlands lack continuity, consisting of isolated peaks,
groups of peaks, plateaus, narrow ridges, and broad mono-
clinal slopes. The intermontane plains are called bolsons,
DESCRIPTION
OF
THE
VAIAIOR
QUADRANGLE.
INTRODUCTION.
GENERAL RELATIONS.
IThe Van Horn quadrangle is situated in El Paso and
Culbersoii counties, Tex., about 100 miles southeast of El
Paso. It is bounded by parallels 31 and 31 30' and meri-
dians 104 30' and 105 and includes 1019 square miles. This
area lies within the Cordilleran region, about midway between
Pecos River and the Rio Grttnde, and forms a part of what is
known as trans-Pecos Texas. (See index map, fig. 1.)
intrusive
igneous
rocks
