03

than 1000 feet above the Hueco Bolson, coinciding with the general level of the Diablo Plateau, and viewed from the west they present an even sky line. The main mass of the Hueco Mountains is composed of limestones having low eastward dips, cross sections of which are exposed in the escarpment that faces the Hueco Bolson. The drainage of the eroded scarp is westward to the mesa through a number of short, deep arroyos. A belt of foothills, composed in general of limestone with low westward dips, lies west of the main range, from which they are separated by stretches of the waste-covered mesa.

DESCRIPTIVE GEOLOGY.

INTRODUCTION.

The rocks of the El Paso district can be divided into two main classes—older consolidated rocks and younger unconsolidated deposits—the latter covering by far the greater part of the area. The outcrops of consolidated rocks are confined to the Franklin and Hueco mountains and the outlying hills; the unconsolidated deposits occupy the Hueco Bolson and the Rio Grande valley and underlie them to considerable depths. The older rocks range in age from pre-Cambrian to Cretaceous and include both sedimentary and igneous rocks. At the base of the section there is a great thickness of pre-Cambrian rocks, which are separable into two formations. The older consists of about 1800 feet of quartzite, and this is succeeded by rhyolitic conglomerate and a mass of rhyolite porphyry. These pre-Cambrian rocks are separated from overlying sediments by a well-marked erosional unconformity and are succeeded by about 300 feet of Cambrian sandstone containing pebbles of the porphyry. This sandstone is overlain by a great mass of limestone at least 5000 feet in thickness, which lithologically can be subdivided only with difficulty, but which on paleontologic grounds is separable into earlier and later Ordovician (the latter including Middle Ordovician and Upper Ordovician forms), Silurian, and Pennsylvanian formations. The Devonian and Mississippian, so far as known, are not represented by sediments within or near the El Paso district, nor are rocks of early Mesozoic age present, but the Cretaceous system is represented in a few small areas by rocks referred to the Comanche series and the Colorado formation. No Tertiary outcrops have been found in the quadrangle, although probably sediments of that age are included in the lower part of the bolson deposits. In the upper part of these deposits Pleistocene bones have been found. Considerable areas of the El Paso district are also occupied by igneous rocks, including, besides the rhyolite porphyry already mentioned, much granite and smaller masses of syenite and andesite porphyries and a few diabase dikes.

The descriptions of the rocks occurring in the El Paso district are summarized in the following columnar section.

SEDIMENTARY ROCKS.

PRE-CAMBRIAN ROCKS.

Pre-Cambrian rocks are known to have a wide but scattered distribution in the southwestern United States, although they have been described in detail from only a few localities, notably the Grand Canyon district and the Globe, Bisbee, Bradshaw Mountain, and Clifton quadrangles, Arizona; and the central Texas area. In the El Paso district the pre-Cambrian rocks in part appear to be the southern continuation of a little-known group of ancient rocks that outcrop in the ranges in New Mexico which form the northern continuation of the Franklin Mountains. In Texas they include both igneous and sedimentary rocks; the former are described on page 6 and the latter constitute the Lanoria quartzite.

LANORIA QUARTZITE.

Definition.— The name Lanoria is here given to a mass of quartzite of pre-Cambrian age which outcrops along the eastern flank of the Franklin Range. The name is taken from an old settlement near the base of the mountains 8 miles northeast of El Paso, just east of a typical exposure of the formation.

Character and distribution.—The Lanoria quartzite consists of alternating layers of thick and thin bedded quartzite having a total thickness of about 1800 feet. Some of the rocks are almost white; others are dark-colored, the prevailing tint being gray. Both varieties are present in alternating nonpersistent bands from a few feet to several hundred feet in thickness. The quartzite is fine textured and thoroughly indurated and is composed of rounded and subangular grains of quartz in a matrix of silica, sericite, and kaolin. It is usually massive and even bedded in layers about 2 feet thick, but locally the beds become thinner and give way to narrow, slaty layers. The rocks of the Lanoria quartzite, in common with all others of the Franklin Mountains, are traversed by two well-developed sets of joints, one striking nearly parallel with and the other transverse to the range and dipping at steep angles. The strata all dip westward at angles between 20° and 45°, conforming with the general structure of the range.

The Lanoria quartzite outcrops in several detached areas along the eastern flanks of the Franklin Mountains, the most complete exposures occurring east and southeast of the highest peak in the central part of the range. The outcrop of greatest continuous extent is along the middle slope of the mountain, north of Fusselman Canyon, and is 2 miles long and half a mile wide.

Age and correlation.—No organic remains have been found in the Lanoria quartzite, and it is referred to the pre-Cambrian because of its stratigraphic relations. The formation is overlain by rhyolitic conglomerate which contains pebbles of the underlying quartzite, and the conglomerate is succeeded by a mass of rhyolite porphyry which, as presently will be shown, underlies Cambrian sandstone. The base of the formation is not exposed, being either cut off by granite or covered by Quaternary débris. Fossils found in the sandstone above the rhyolite porphyry indicate that it is either upper or middle Cambrian. It is, therefore, possible that the Lanoria quartzite may be lower Cambrian, but more probably it is pre-Cambrian.

Quartzites thought to be pre-Cambrian and similar to the Lanoria have been found in the Manzano and other ranges in the chain forming the northward continuation of the Franklin Mountains, but detailed knowledge of them is lacking. The nearest described pre-Cambrian rocks are those constituting the Pinal schist, a sedimentary formation in the Clifton and Bisbee quadrangles. About 100 miles east of El Paso, near Allamore, on the Texas and Pacific Railway, there are two pre-Cambrian formations of sedimentary origin — one consisting of fine-textured red sandstone, cherty limestone, and conglomerate; the other of quartz schist, quartzite, and clay slate. The origin and stratigraphic relations of these trans-Pecos rocks suggest that they are probably equivalent to the Llano series of central Texas, although data for exact correlation are lacking.

CAMBRIAN SYSTEM.

The basal Paleozoic rocks wherever found in the Southwestern States consist of sandstone of middle or upper Cambrian age, which was deposited in an advancing sea on an old land mass.

BLISS SANDSTONE.

Definition.—The Bliss sandstone, named from Fort Bliss, is a massive, fine-textured, brownish indurated sandstone that varies from a few feet to slightly over 300 feet in thickness and occurs principally along the eastern base of the Franklin Mountains.

Character and distribution.—The Bliss sandstone is composed of small grains of quartz embedded in a matrix of sericite and kaolin. The basal beds, contiguous to the granite, are characteristically indurated and are practically quartzites; the higher beds are generally softer and in the Hueco Mountains the formation is much less indurated than in the Franklin Mountains. The layers average between 2 and 3 feet in thick- ness, ranging from a maximum of about 5 feet down to a few inches, and locally the sandstone is cross-bedded. In color the formation is prevailingly brown, but ranges from dull brown through gray to almost white. At the base of the formation occur coarser-textured beds, which locally are conglomeratic. Where the Bliss sandstone is exposed in contact with the underlying rhyolite porphyry, rounded pebbles of the porphyry occur in the sandstone. These pebbles are usually small, few exceeding an inch in diameter, and as a rule their occurrence is limited to a zone generally not more than 3 feet thick, but in places the pebbles are disseminated through the lower 100 feet of the formation. The contact of the Bliss sandstone and the granite, on the other hand, is intrusive wherever proof was obtained. The contact of the Bliss sandstone with the lower part of the overlying El Paso limestone is in general apparently conformable, but in the central part of the range the sandstone thins out and locally disappears and the limestone, containing a basal conglomerate, lies directly upon the rhyolite porphyry.

The main occurrence of the Bliss sandstone is along the eastern slopes of the Franklin Mountains, but it also outcrops in small areas on the upper western flanks of the central part of the range. A sandstone which on lithologic and stratigraphic grounds is thought to be the Bliss outcrops near the eastern border of the El Paso quadrangle and is exposed for about 6 miles at the base of the cliff that marks the south end of the Hueco Mountains in the adjacent Cerro Alto quadrangle. In the Franklin Mountains the formation outcrops in a narrow, dark band, which is conspicuous because of its contrast in color with the light-gray overlying limestone. The considerable faulting to which the range has been subjected causes the irregular distribution of the formation shown on the map. The two longest continuous outcrops occur in the northern and southern ridges of the range, where the sandstone can be followed for about 4 miles. In the central block of the range the Bliss sandstone occurs only on the western slope.

Age and correlation.—Annelid borings both perpendicular and parallel to the bedding occur abundantly in the Bliss sandstone. Other fossils are rare, but in places in the lower strata some brachiopod shells have been found. Of these Walcott has identified Lingulepis acuminata, Obolus matinalisf, and fragments of Lingidella, which determine the Cambrian age of the sediments and indicate that either the upper or middle division of the system is here represented. The Bliss sandstone is the probable approximate equivalent of the "Tonto" sandstone of the Grand Canyon, the Bolsa quartzite of Bisbee, the Coronado quartzite of Clifton, the Reagan sandstone of Oklahoma, and the Cambrian sandstone of the central Texas Paleozoic area.

ORDOVICIAN SYSTEM.

Rocks of Ordovician age are known in but few areas in the general region of the El Paso district outside of the area here considered, but here they are well developed. They consist of more than 1000 feet of limestone which has been divided into two formations—the El Paso limestone, of Lower Ordovician age, and the Montoya limestone, which is assigned to the Middle and Upper Ordovician.

EL PASO LIMESTONE.

Definition.—The El Paso limestone as originally defined ^a included all of the limestone of Ordovician age in the Franklin Mountains. As a result of the present work, however, in spite of the fact that sharp divisional lines between the two parts of the system can not be drawn, it is considered desirable to subdivide the Ordovician rocks in tile area under consideration into two formations, and the name El Paso is retained for the lower formation which contains Lower Ordovician fossils.

Character and distribution.—The El Paso formation consists of about 1000 feet of gray limestone, usually massive but locally thin bedded. The lower 100 feet of the formation, lying with apparent conformity on the Bliss sandstone, is characteristically arenaceous and weathers brownish. In the central part of the mountains the Bliss sandstone is locally absent and the El Paso limestone rests directly upon the pre-Cambrian rocks and includes in its basal beds pebbles of rhyolite porphyry having a maximum diameter of 3 or 4 inches. In places along the central-western slopes of the Franklin Mountains there is present at the base of the limestone a bed of conglomerate of variable thickness up to 20 feet, composed of well-rounded pebbles of rhyolite porphyry in a calcareous matrix. A common and distinctive feature, especially in the middle portion of the limestone, is the presence of thin, connected nodules of brown chert arranged in irregular streaks parallel to the bedding. Near the top of the formation local bands are streaked with rounded bits of quartz, about the size of a pin head, which are probably of detrital origin. The El Paso limestone is essentially magnesian, although the amount of magnesia present varies and locally is rather small, especially in the more siliceous beds. A specimen from the Franklin Mountains .8 miles north of El Paso was