07

An analysis of a fresh specimen of granite obtained from one of the tin prospects 12 miles north of El Paso was made by E. C. Sullivan with the following results:

SiO2	73.76
CaO	.81
K2O	5.66
Na2O	3.64

The texture of the rock is allotriomorphic granular, feldspar and quartz being the dominant minerals. The quartz contains fluid inclusions. The feldspars are chiefly orthoclase and albite, commonly intergrown as microperthite, and microcline and acidic oligoclase are also sparingly present. As calculated from the analysis the orthoclase molecules constitute 33.36 per cent of the rock and the albite molecules 30.39 per cent. Brown biotite, altering to chlorite, and less abundant greenish-brown hornblende occur sparingly, and small grains of magnetite are also present.

Mode of occurrence and age.—The granite occurs in stocklike masses and is usually associated with faults. It is in contact with all the formations from the Lanoria to the Hueco inclusive. The contact is generally concealed by débris, but where well exposed it is smooth, and locally stringers of granite a few inches wide intrude much-indurated sandstone. In the transverse ridge 1½ miles south of the State boundary a narrow faulted mass of Bliss sandstone and El Paso limestone is almost surrounded by the granite. Where the granite crosses the range the rock is in contact with several formations, including the Lanoria quartzite, the rhyolite porphyry, the Bliss sandstone, and the El Paso, Montoya, and Fusselman limestones. The contact with the limestone, however, where locally exposed, shows little evidence of metamorphism; the limestone is not noticeably altered and contact minerals have not been observed. At the eastern base of the main range immediately north of El Paso the granite occurs along a fault of about 2000 feet displacement. There the lower limit of the granite is covered by alluvium except about midway along the outcrop, where it is in contact with the Fusselman limestone in the downthrown block. Throughout this outcrop the upper margin of the granite is in contact with the Bliss sandstone, the lower part of which is so indurated as to constitute quartzite. Small apophyses of granite extend into the sandstone, and one tongue of granite 12 feet thick and 50 feet long was observed. Locally veinlike aggregates of quartz are developed at the contact, and masses of quartzite up to a foot in diameter are included in the granite.

The age of the granite is not definitely known. Its proximity to the Bliss sandstone and Lanoria quartzite suggest the possibility of pre-Cambrian intrusion, but although part of the granite may have such an ancient origin no proof of it has been found. On the contrary, as already stated, there is direct evidence of the intrusion by granite of the several Paleozoic formations. The intrusion of the granite may possibly have accompanied the post-Carboniferous-pre-Cretaceous uplift of this region, but the preponderant evidence connects the intrusion of the granite with the tilting of the rocks of the range. The occurrence of the granite adjacent to the faults in the Franklin Mountains, the association of the faulting with the general deformation of the range, and the fact that tilting of the strata would probably accompany the intrusion of so large a mass of granite suggest the general contemporaneity of these events. The inclined Cretaceous strata which apparently dip conformable with the Paleozoic rocks on the southwestern flanks of the range 7 miles north of El Paso imply a relatively late date for the tilting of the range. It seems plausible, therefore, that some of the granite may be as young as post-Cretaceous, but proof is not available. All that is definitely known is that, in part at least, the granite is post-Carboniferous.

SYENITE PORPHYRY.

Distribution.—There are several outcrops of syenite porphyry at the base of the Hueco Mountains, near Hueco Tanks. The two largest masses form hills of elliptical outline, trending north and south, each occupying less than 2 square miles and rising several hundred feet above the surrounding wash-covered slopes. The other bodies of igneous rock in this vicinity are considerably smaller, and they also are flanked by wash, except a small mass east of Hueco Tanks, which is in direct contact with the Hueco limestone. (See fig. 13, illustration sheet.)

Character and composition.—The rocks are almost bare of vegetation and are much broken by joints. The principal trend of these joints is north and south; another transverse set is less conspicuous. Parting planes rudely parallel with the surface are well developed and weathering has rounded the outcrops into spheroidal masses. The syenite porphyry is readily acted upon by subaerial influences and a number of erosion hollows have been formed, across some of which dams have been constructed to impound rain water. These are known as the Hueco Tanks. (See fig. 13, illustration sheet.) The weathered surfaces are brownish, but fresh specimens are light colored. The fresh syenite porphyry is a light-gray, El Paso. holocrystalline, fine-textured, slightly porphyritic rock composed of preponderating white feldspars and subordinate biotite and augite.

An analysis of a fresh sample collected where blasting has been done at one of the Hueco Tanks was made by George Steiger, with the following result:

SiO2	64.51
Al2O3	16.75
Fe2O3	2.05
FeO	1.00
MgO	.60
CaO	1.38
Na2O	6.08
K2O	5.74
H2O +	.31
H2O—	.46
TiO2	.75
P2O5	. 14
Cl	.04
MnO	.21
	______
	100. 02
Less O	.01
	______
	100. 01

The rock is composed of phenocrysts of feldspar and biotite up to 5 millimeters in length, in a granular groundmass. Feldspars consisting of preponderating albite, slightly less abundant orthoclase, and a little acidic oligoclase constitute the mass of the rock. Brown biotite is about twice as abundant as light augite, and quartz occurs in minor quantities. Ilmenite, magnetite, and apatite are present in minute crystals as accessories. The mineral composition of the rock analyzed can not be stated with precision because of the unknown composition of the biotite and augite, but the following is approximate:

Orthoclase molecule	34
Albite molecule	51
Anorthite molecule	1
Biotite and augite	8
Quartz	4
Ilmenite	1
Magnetite and apatite	1
	___
	100

In the quantitative classification this rock is a phlegrose near nordmarkose.

Mode of occurrence and age.—The syenite porphyry occurs as dikelike stocks along the line of structural weakness at the western base of the Hueco Mountains. The outcrops are almost entirely surrounded by wash, but at the base of the mountains a dike intrudes the Hueco limestone, thus proving that the porphyry is post-Carboniferous. Similar rocks are intrusive in Comanche sediments in the Cornudas Mountains, about 30 miles northeast of Hueco Tanks, and it may be presumed that the syenite porphyry in the El Paso quadrangle is post-Cretaceous.

ANDESITE PORPHYRY.

A few small masses of andesite porphyry form hills in the Rio Grande valley southwest of the Franklin Mountains, the largest of which occupies an area of about 1 square mile between El Paso and the smelter.

The outcrops are almost destitute of vegetation and the exposures have the usual bare appearance. The porphyry is traversed by many joints, whose principal trend is north and south, and there is a tendency toward spheroidal weathering. The rock is grayish in color and on casual inspection appears to be granular, but closer study shows dominant phenocrysts of feldspar and biotite in a dull-gray groundmass. The rocks in all the exposures are rather homogeneous, but at the south end of the main outcrop in the city of El Paso a finer-textured and less porphyritic facies occupies a small area.

A partial analysis of this rock by E. C. Sullivan gave the following result:

SiO2	63.73
CaO	3.53
K2O	2.74
Na2O	5.94

The porphyritic texture is well marked under the microscope. Phenocrysts of feldspar and biotite up to 5 millimeters in cross section predominate over a microcrystalline groundmass consisting chiefly of feldspar, some biotite, and a little quartz. The feldspar is chiefly oligoclase with subordinate orthoclase. In composition this rock falls between the monzonite-latite and diorite-andesite families, but on the whole the specimens examined have closer affiliations with the latter and the rock is so classified. The name andesite porphyry was chosen because of the fine groundmass observed under the microscope, although from the general appearance of the rock diorite porphyry would be more appropriate.

The andesite porphyry occurs in small stocks and is either entirely surrounded by Quaternary débris or in contact with Comanche strata. Immediately east of the smelter, northwest of El Paso, the intrusion of the igneous rock has tilted the adjacent Cretaceous sediments so that they stand almost perpendicular. The age of these rocks, therefore, is post- Comanche, and their intrusion may be associated with the disturbance accompanying the uplift of the region in Tertiary time.

DIABASE.

Small masses of diabase occur in the Franklin Mountains as sills irregularly intruded parallel to the bedding of the Lanoria quartzite, and also as dikes cutting across the bedding. Diabase dikes occur at a few localities in the granite and also here and there near the contact of the granite and the Bliss sandstone. The dikes and sills range in thickness from a few inches to a maximum of about 25 feet and are too small to be shown on the map. They are much weathered and little fresh rock is exposed. The rock is massive, fine textured and dark colored. Thin sections show under the microscope the characteristic ophitic texture and the presence of basic plagioclase, augite, magnetite, and some olivine.

RELATIONS.

Comparison of the igneous rocks of the El Paso quadrangle brings out well-marked relations. Except the diabase, which is only feebly developed, the rocks are all acidic, with a silica content ranging from 63.7 to 76.3 per cent. They are notably rich in alkalies and contain from 8.6 to 11.8 per cent of soda and potash. Lime is low, varying from 0.77 to 3.53 per cent. The rhyolite porphyry and the granite are similar in composition, and so are the syenite and andesite porphyries. The chief difference between the two groups is that the former are more acidic than the latter. This fact is illustrated by the mineral composition. Quartz is abundant in the rhyolite porphyry and granite, but is only sparingly present if at all in the syenite and andesite porphyries. Alkali feldspars are the prevailing minerals in all these rocks, the sum of the calculated orthoclase and albite molecules ranging from 58 to 85 per cent; they are more abundant in the syenite-andesite group than in the rhyo-lite-granite group. Ferromagnesian minerals are not plentiful, especially in the earlier rocks. Biotite is the most common and is present in all except the rhyolite porphyry, in which it has not been detected.

The abundance of alkalies is characteristic of many igneous rocks of the trans-Pecos region. For instance, the quartz pantellerite from the Vieja Mountains described by Lord and the paisanite and related rocks from the Davis Mountains described by Osann contain more than 10 per cent of potash and soda.

STRUCTURE.

GENERAL STATEMENT.

The structure of the region in the vicinity of the El Paso quadrangle finds expression in northwestward- to northward-trending highlands and intervening lowlands. In general the highlands are areas of relative uplift and the lowlands are areas of corresponding depression. (See figs. 1 and 2.) The rocks of the highlands are plainly exposed and the strata either lie nearly flat, in the position in which they were laid down, underlying plateaus, or are inclined at greater or less angles as the result of deformation, constituting narrow mountain ridges or broad monoclinal slopes. The larger masses of igneous rocks commonly form isolated peaks or groups of peaks. Bed rock beneath the lowlands is generally concealed by the unconsolidated and undisturbed deposits that underlie the bolson plains.

Several of the trans-Pecos lowlands, for instance the Jornada del Muerto, are underlain by broad synclines some of which are bordered by unsymmetrical or faulted anticlines. The folds are relatively inconspicuous, however, and the dominant structural features are normal faults that strike in general with the Cordilleran trend. Igneous intrusions have also been important in tilting and doming the rocks with which they are associated.

STRUCTRE OF THE EL PASO DISTRICT.

GENERAL OUTLINE.

The main structural features of the El Paso district may be summarized as follows: The long, narrow Franklin Range, rising 3000 feet above broad lowlands, resembles a "basin range" fault block of westward-dipping rocks, but it differs from the type by being part of a long chain of ranges and by being complexly faulted internally. The Hueco Mountains in the main form a monocline of low eastward dip along the western border of which the rocks have been disturbed. In the northern part of the quadrangle the strata in the belt of low outlying hills west of the Hueco Mountains dip westward, marking an unsymmetrical anticline; farther south more complex conditions are indicated by dips in various directions. In the Hueco Bolson the deep cover of unconsolidated material conceals the structure of the underlying rocks. Possibly a large part of the area is underlain by practically flat-lying beds which are faulted near the western margin of the bolson along the eastern base of the Franklin Mountains. (See fig. 6.)

FRANKLIN MOUNTAINS.

The structure of the Franklin Mountains viewed from a distance appears simple. The strata strike parallel to the trend of the range and dip westward at steep angles. But the simplicity