06

plain at the base of the Franklin Mountains. (See fig. 12, illustration sheet.) The elevations are approximately 3790 and 3820 feet above the sea, or about 100 feet above the river and 100 feet below the top of the mesa at this locality. The bones include teeth of a mammoth and of a horse and the jawbones and teeth of a tapir, which have been determined

by J. W. Gidley, of the United States National Museum, to represent Elephas columbi, Equus complicatus, and Tapirus (haysii?).. (See fig. 14, illustration sheet.) These fossils determine the Pleistocene age of at least part of the bolson deposits. The Tertiary age of the basal beds beneath the lowlands, however, while probable, remains to be proved.

RIVER ALLUVIUM.

The floor of both the Mesilla and El Paso valleys, the broad lowlands which the Rio Grande has trenched several hundred feet beneath the general level of the bolson plains, is covered with river alluvium. A measure of the maximum amount of downcutting beneath the present valley floor and of the subsequent filling with alluvium is afforded by borings made by the International [Water] Boundary Commission at the dam site in the gorge above El Paso. These borings indicate a maximum depth to bed rock of 8 6feet. The different sections show that conditions are not uniform across the narrow valley, but that the deposits dovetail. In the gorge much sand and gravel are present and apparently there is little clay. The following section is typical:

	Feet.
Sand	27
Gravel	12
Five layers of alternating sand and gravel	2
Gravel	8
Ten layers of alternating sand and gravel	5
Sand	1
Bed rock.	__
	55

This shallow depth to bed rock prevails only at the pass. In 1896 a well was sunk in the valley about a mile and a half below El Paso, to a reported depth of 1693 feet, apparently without encountering bed rock, and a number of wells several hundred feet deep have been sunk in the valley deposits in the vicinity of El Paso. Most of the valley wells are about 60 feet deep. A few feet of silt is commonly encountered at the surface, below which sand and gravel are reported to occur down to a depth of about 60 feet, where the gravel is underlain by clay. In some places no covering of silt is found and elsewhere streaks of clay are interbedded in the sand, the materials varying as would be expected from the conditions of deposition. During high stages of the Rio Grande a considerable part of the valley is flooded and enormous quantities of sand and clay are brought down by the river and deposited on the flood plain. Accumulations of this material frequently cause the river locally to shift its course, leaving oxbow lakes and swampy areas along the abandoned channels. These are well developed in the lower Mesilla Valley south of the Texas-New Mexico boundary, and also in the upper El Paso Valley near the city, where levees are maintained. A characteristic change in the course of the river occurred during the summer of 1907, when the brick plant at Whites Spur was inundated. But the destructive effect of floods is counterbalanced by the enrichment of the agricultural lands through the supply of new material.

IGNEOUS ROCKS.

Igneous rocks are of subordinate occurrence in the El Paso quadrangle, although they are locally prominent. They form the culminating point of the Franklin Mountains and extend along the eastern base of the range; they also occur in several areas northwest of El Paso, and in the northeast corner of the quadrangle at the foot of the Hueco Mountains. Four main types are present—rhyolite porphyry, granite, syenite porphyry, and andesite porphyry; diabase also occurs in small amount. These rocks are of intrusive and extrusive origin and in age are probably pre-Cambrian, post-Carboniferous, and Tertiary.

RHYOLITE PORPHYRY AND ASSOCIATED RHYOLITIC AGGLOMERATE.

Distribution.—The oldest igneous rocks of the El Paso quadrangle, except possibly some of the diabase intruded in the Lanoria quartzite, are rhyolite porphyry and associated pyroclastic rocks of probable pre-Cambrian age. They are limited in occurrence to the central part of the Franklin Mountains, where the main outcrop occupies the summit and flanks of the highest peak of the range. Another mass, separated from this one by faulting, lies a few miles to the southeast, along the eastern middle slopes. The outcrops are commonly bare of vegetation, and the surface is strewn with blocks broken along well-developed sets of joints which extend parallel and transverse to the range.

General character and composition. —The formation consists of porphyritic lava, having a maximum thickness of about 1500 feet, with a variable basal conglomeratic member ranging in thickness, where present, from a few inches to about 400 feet. The conglomerate, or, more properly, agglomerate, is in places clearly stratified, and is composed of angular and semi-rounded pebbles of rhyolite and quartzite up to 1 foot in diameter, embedded in a fine-textured matrix of rhyolitic fragments, the whole being cemented into an indurated mass. The quartzite pebbles are similar to the underlying Lanoria quartzite, from which they were doubtless derived. Here and there thin sheets of rhyolite porphyry are interbedded with the agglomerate.

The rhyolite porphyry in general is a massive red rock. The prevailing type consists of phenocrysts of quartz and feldspar up to 15 millimeters in width embedded in a dense red (or locally black) aphanitic groundmass. Commonly the phenocrysts constitute about half of the rock, and the quartz and feldspar are usually present in about equal proportions. Varieties are produced by differences in size and relative abundance of the quartz and feldspar crystals. In places the quartz phenocrysts are absent and the rock is more basic, consisting of feldspar crystals in a black groundmass, but it is not practicable to map the different facies. In contrast with the granite the rhyolite porphyry withstands weathering remarkably well, and fresh samples are easily obtained. A partial analysis of a typical specimen, by E. C. Sullivan, gave the following results:

SiO2	76.34
CaO	.77
K2O	5.76
Na2O	2.88

Quartz and feldspar compose almost the entire rock, the feldspar being somewhat more abundant than the quartz. Calculation from the analysis shows that the orthoclase molecule composes 33.92 per cent of the rock and the albite molecule 24.10 per cent. Other minerals, consisting of unidentifiable decomposed remnants of ferromagnesian minerals are rarely present. Under the microscope the groundmass is seen to be minutely crystalline and to be composed of quartz and feldspar, and small bits of magnetite are rather common.

Mode of occurrence and age.— These rhyolitic rocks do not occur elsewhere, so far as known, than in the areas mapped in the central Franklin Mountains, and the formation thins out both to the north and to the south. The rocks appear as a whole to lie parallel to the underlying and overlying beds. At the lower contact an erosional hiatus is indicated by the basal rhyolitic conglomerate containing rounded pebbles of the underlying Lanoria quartzite. The character of the lower contact varies, however, as the conglomerate locally thins out and disappears, and in places massive rhyolite porphyry immediately overlies the quartzite. The upper contact also is variable, although in general it is marked by a pronounced erosional unconformity. Usually the porphyry is overlain by the Bliss sandstone, which contains rounded porphyry pebbles in its lower part. In a narrow zone about 7 miles north of El Paso the Bliss sandstone thins out and disappears, and the rhyolite porphyry is immediately overlain by the El Paso limestone. The greater part of this contact is marked, as stated in the description of the limestone, by a well-developed basal conglomerate up to 20 feet in thickness, composed of rounded pebbles of rhyolite porphyry in a calcareous matrix. This conglomerate is not persistent, and in places the porphyry lies directly beneath the limestone, the contact being locally suggestive of an intrusion. No evidence of metamorphism or of the presence of apophyses was observed, however, and the apparent intrusion may be accounted for by faulting, or possibly intrusive bodies of similar composition to the general body of rhyolite porphyry occur locally in it. The stratigraphic position of the main masses of rhyolitic rocks and the presence of the overlying conglomerate indicate that they are of pre-Cambrian age.

GRANITE.

Distribution.—Granite occurs in several detached areas at the eastern base of the Franklin Mountains. It usually forms the lowermost outcrop above the detrital slopes at the base of the range and extends from a few feet to 1000 feet or more up the flanks of the mountain. In one place only, at the pass immediately north of the highest peak, granite crosses the crest of the mountains in a narrow belt and is exposed on the western slope. The main outcrop occupies an irregular area about 4 miles long and varying in width from a few feet to a little more than a mile, at the northeastern base of the range. Another large mass of about the same length but averaging only half a mile in width occurs along the southeastern base of the range. In the main, although there are local variations, specimens from the several outcrops are similar in lithologic character, and in the absence of evidence to the contrary the several occurrences may tentatively be assumed to have been parts of the same magma, although it is possible that some of the granite is considerably older than the main mass.

General character and composition .—The outcrops are strewn with broken blocks caused by the joints which traverse the range. Locally, as is well exposed 2 miles south of the State boundary, the joints produce a prominent sheeting in the granite. There the north-south set dips eastward at an angle of about 45°, parting the rocks into broad parallel blocks. The common appearance of the outcrops is typically massive.

In general the granite is rather uniform in composition and texture, though there are some variations. The rock in all the exposures is much decomposed and disintegrates to a coarse arkosic sand, so that fresh specimens can be obtained only by blasting. The common type is red in color and medium to coarse grained. It is composed of quartz, alkali feldspar, and ferromagnesian minerals in very subordinate amounts. Small flakes of biotite and bits of hornblende altering to chlorite can be recognized in some hand specimens, and in places decomposition has proceeded so far as to cause the weathered surface of the rock to be pitted. Northwest of Fort Bliss and along the eastern base of the central part of the range a porphyritic type is developed, consisting of crystals of red or gray orthoclase up to 1 centimeter in width in a granular base. In a porphyritic phase of the granite about 5 miles north of Fort Bliss a vein of common garnet occurs parallel to the east-west set of joints, with no associated vein minerals. The vein varies from an inch to a foot in width and is at least 200 feet long. In the northern part of the range, in the vicinity of the tin prospects, the granite is cut by dikes of aplite and pegmatite.