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of the low outlying hills which are separated from the main mountain range by débris, the rocks dip westward at an angle of about 25°, thus marking an unsymmetrical northward pitching anticline. A small synclinal area lies north of this anticline, about 8 miles south of Hueco Tanks, where the dips are low to the northwest and southeast. Immediately northwest of this area, 6 miles west of south of Hueco Tanks, a local disturbance has caused the Fusselman limestone overlain by Hueco limestone to outcrop, dipping 40° SW., but the relations with adjacent rocks are concealed by débris. North of this, in the main mountain mass, the dips are eastward and northeastward at angles between 5° and 15°, and in the outlying hills the dip is about 3° W. These hills are separated from the main range by a débris-filled area containing, along the line of structural weakness, stocks of syenite porphyry, the intrusion of which doubtless disturbed the adjacent strata also,.

BOLSON FAULTS.

Besides the faults of relatively ancient date, which are revealed by the distribution of the strata, there are indications of later displacements involving the bolson deposits. A disconnected line of high-level benches extends along the eastern base of the Franklin Range and is well exposed west and northwest of Fort Bliss. At the southeast end of the range these benches lie at an elevation of about 3900 feet; east of the central part of the mountains they extend approximately along the 4250-foot level. They are much dissected by the many arroyos which head in the mountains and in places are inconspicuous. These benches are the upper parts of broken alluvial slopes which in places fringe the base of the range in an uneven eastward-facing scarp varying from 10 to 50 feet in height. West of the scarp the alluvial débris slopes up to the mountains, and east of the scarp the alluvium gradually descends in an even grade to the general level of the Hueco Bolson. These interrupted alluvial slopes strongly suggest Quaternary faulting that may represent renewed uplift along the old hypothetical fault which delimits the Franklin Range on the east.

That faulting in this region has actually occurred in the Quaternary is shown in a sand pit at the head of North Virginia street, El Paso, where displacements similar to those in the mountains but on a much smaller scale are well exposed. Beds of sand intercalated with layers of clay dipping 10° SW. are cut by two normal faults whose hade is east, the upthrow being on the west. The displacements amount to only about 2 feet. (See fig. 13, illustration sheet.)

AGE OF STRUCTURAL DISTURBANCES.

The structural conformity of the rocks from the pre-Cambrian through the Carboniferous system implies that the region was not subjected to marked folding or faulting until, at the earliest, the close of the Carboniferous. Probably, however, during that great length of time there were general crustal movements that did not much disturb the relative position of the strata. It seems likely that the intrusion of the great mass of granite now exposed along the eastern base of the Franklin Mountains was associated with the deformation of the rocks, but, as already stated, the age of the granite is not known, save that it is, in part at least, post-Carboniferous. The Cretaceous sediments that appear to dip conformably with the Paleozoic strata on the southwestern flanks of the range, 7 miles north of El Paso, indicate that the tilting is of post-Cretaceous age. The topographic relations of the faults suggest a rather ancient date of origin, for the original fault scarp along the east base of the Franklin Mountains has been effaced and the well-developed drainage of the mountains, especially on the east side of the range, is in pronounced contrast with the asymmetric drainage of a young block range. In the interior of the range there is no physiographic expression of the displacements, and in several areas the surfaces of the down-thrown blocks are higher than those of the adjacent relatively up-thrown ones. In the Cerro de Muleros the intrusion of the porphyry into Comanche sediments and the folded and faulted Cretaceous rocks clearly indicate post-Cretaceous disturbance. Direct evidence is not present in the Hueco Mountains as to the age of the development of the structure there, more than that it is post-Carboniferous, but the presence of post-Cretaceous igneous rocks in the Cornudas Mountains, about 25 miles east of the El Paso quadrangle, also points to post-Cretaceous deformation. Though data for the close determination of the age of the development of the structure are not available, it seems probable that the major deformation of the district was associated with the Tertiary continental uplift. The present elevation of the country implies a comparatively late regional uplift, and the transverse fault which terminates the Franklin Mountains on the south and the dislocated unconsolidated bolson deposits show that the structural disturbances have been long continued.

HISTORICAL GEOLOGY.

PRE-CAMBRIAN TIME.

The earliest recorded event in the history of the El Paso district was the sedimentation of the Lanoria quartzite, in pre-El Paso. Cambrian time. The composition of the sediments indicates an origin from crystalline rocks, but the source of the material is not known. The Franklin Mountain area was apparently occupied by a sea in which almost 2000 feet of fine sand was deposited. These sands were buried, metamorphosed to quartzite (but not altered dynamically), uplifted, and eroded in pre-Cambrian time, as indicated by pebbles of quartzite in the overlying agglomerate. The rocks were not folded nor much tilted, for the dip of the Lanoria quartzite is practically conformable with that of the overlying strata. The succeeding period of volcanism, introduced by the deposition of agglomerate, was followed by the accumulation of a mass of rhyolitic lava, the considerable thickness and limited areas of which indicate a near-by source and a rather small original extent. After the cessation of igneous activity there was apparently no rock making in the El Paso district until middle or upper Cambrian time.

PALEOZOIC TIME.

During the early part of the Cambrian period the district was apparently a land mass, upon which, in the latter part of the period, the sea transgressed. The area was finally covered by the advancing sea, in which 300 feet of sand was deposited. This formation, the Bliss sandstone, thins out and is absent in a narrow zone in the central Franklin Mountains where, as stated above, the El Paso limestone rests directly upon the rhyolite porphyry, the contact being marked by a basal conglomerate consisting of porphyry pebbles up to a foot in diameter, embedded in a calcareous matrix. These conditions indicate either pre-Ordovician erosion or the nondeposition of Cambrian sediments in a narrow zone which was not submerged until early in the Ordovician period.

Paleozoic time as a whole in the El Paso district was characterized by subsidence and, after the deposition of the Cambrian sandstone, by the accumulation of several thousand feet of limestone. Only five distinct groups of fossils, representing the Lower, Middle, and Upper Ordovician, the Niagaran stage of the Silurian, and the Pennsylvanian series of the Carboniferous, are present in this great mass of limestone. The several hiatuses implied by the absence of the intervening faunas, which appear in the complete Paleozoic section, may be due to uplift and erosion or to nondeposition. It is noteworthy that in the Franklin Mountains upper Carboniferous (Hueco) limestone lies with apparent conformity on Silurian (Fusselman) limestone. The absence of the intermediate series between the Ordovician and the Pennsylvanian in both the El Paso and the Van Horn quadrangles may point to several regional uplifts which in general did not appreciably deform the rocks, and apparently the emergences were so slight that there is little record of erosion. Devonian and Mississippian strata are present in southwestern New Mexico and in adjacent parts of Arizona, and Mississippian limestone also occurs in the Sacramento Mountains, 75 miles northeast of El Paso. In the Van Horn quadrangle, wherever the base of the Carboniferous is exposed the Hueco limestone with a well-developed basal conglomerate rests on rocks of pre-Cambrian, possible Cambrian, and Ordovician age. The contact is at about the same elevation over large areas, a fact which indicates that pre-Pennsylvanian erosion was of long duration and the surface was well reduced. On the other hand, in the El Paso district the great sequence of limestone, ranging in age from Ordovician through Pennsylvanian, indicates relatively uniform conditions during a long period of geologic time. Apparently little terrigenous material was deposited in this area during the Paleozoic era after the Cambrian period, the sediments consisting only of shells and other secretions of marine life. These facts illustrate the diverse Paleozoic conditions that existed in this general region.

MESOZOIC TIME.

There are no outcrops of Permian or of early Mesozoic rocks in the El Paso region and it is not known whether strata of these ages were never deposited or whether they have been removed by erosion. The Van Horn quadrangle shows more than 5000 feet of strata younger than the Hueco limestone, which are probably Permian. In the Malone Mountains, 65 miles southeast of El Paso, there is a small area of marine Jurassic rocks whose relations to the adjacent formations are concealed by Quaternary deposits. In the Finlay Mountains, about 10 miles northwest of the Malone Mountains, the Hueco limestone is unconformably overlain by the Fredericksburg group of the Cretaceous Comanche series. The unconformity is marked by a slight difference in dip and by the presence of a conglomerate consisting of pebbles of Pennsylvanian limestone at the base of the Fredericksburg. Near Van Horn also Cretaceous sediments lie on an eroded surface of Hueco limestone. This region, therefore, at least in part, was uplifted and eroded between the Carboniferous and Cretaceous periods.

The few small and isolated outcrops of Cretaceous strata in the El Paso district are outlying areas of the great bodies of these rocks in Texas and Mexico, and in themselves throw little light on the late Mesozoic history of this region. East of the El Paso quadrangle, however, the distribution of Cretaceous strata implies that the Comanche sediments of western Texas were deposited in a sea which progressively encroached from the south upon a pre-Cretaceous land. The Trinity, the lowermost group of the Comanche, is well developed south of the Texas and Pacific Railway, though none but the upper groups, the Fredericksburg and Washita, are known in trans-Pecos Texas north of that road. In this district the Upper Cretaceous is represented only in the city of El Paso by small areas of shale belonging to the Colorado formation. These areas are isolated and the surrounding unconsolidated deposits conceal the relationships as well as the possible greater development of Cretaceous rocks in this region. The character of the deposits does not imply the immediate vicinity of a shore. Moreover, the wide occurrence of Cretaceous strata on the Diablo Plateau almost compels the belief that they also extended over the Hueco Mountains; and the remnants of Comanche sediments on the southwestern flank of the Franklin Mountains which have shared in the general tilting suggest that the Cretaceous sea occupied the site of that range also. The El Paso district, therefore, probably was entirely covered by the Cretaceous sea.

TERTIARY AND QUATERNARY TIME.

At the close of the Cretaceous period or early in Tertiary time continental uplift and associated orogenic disturbances occurred throughout the Cordilleran region. The major deformation of the El Paso district probably developed during this period, when the mountain blocks and intervening basins were outlined. What little is known of the Tertiary history of the district implies that erosion of the recently uplifted land mass was the dominant process and was accompanied by local igneous intrusions and probably by continued uplift, both regional and differential. A great mass of Cretaceous and underlying rocks was removed from the highlands and at least part of the débris accumulated in the adjacent trough. The differential movement resulting in the uplift of the highlands above the basin was probably of long duration, progressing with the erosion of the uplands.

The Quaternary record of the district is one of continued erosion and deposition, accompanied by relatively minor uplift. Although the salient masses, the Franklin and Hueco mountains and the Hueco Bolson, are primarily of structural origin, they have been much modified by erosion and deposition, which have formed the present mature topography. The highlands have been considerably reduced from their original forms, as shown in part by the well-developed drainage of the Franklin Mountains contrasted with the unsymmetrical drainage of tilted block mountains in a youthful stage; and the Hueco Bolson trough has been deeply filled to the present almost level plain by débris derived from the disintegration of the rocks of the highlands. Although many of the earlier deposits were probably laid down in water, the later material, constituting the uppermost bolson deposits, accumulated in large part under arid subaerial conditions. Detritus collects in the lowlands because the rainfall is insufficient to maintain streams that can convey the material to the river. The ultimate result of these conditions, if unchecked, will be the reduction of the area to a plain.

The complex history of the Rio Grande remains to be determined, but it has been suggested by Robert T. Hill and Willis T. Lee, who have studied long stretches of the river, that its ancient course was west of the Cerro de Muleros, into the lowlands of northern Mexico, where it had no outlet to the sea; and that the change to its present course was due to capture by a stream tributary to the Gulf of Mexico. The river formerly was at an elevation considerably higher than the present channel, and in deepening its way through the unconsolidated deposits in which it flowed it encountered hard rock at the narrows above El Paso. During the time occupied by the stream in cutting the gorge at this place broad lowlands— the Mesilla Valley above and the El Paso Valley below the narrows—were excavated from 200 to 300 feet beneath the general level of the adjacent bolson plains. In the El Paso district the river is now building up its flood plain. A measure of the filling is afforded by the drillings in the gorge above El Paso, which show a thickness averaging 55 feet of river deposits lying on bed rock.

ECONOMIC GEOLOGY.

The mineral wealth of the El Paso district is varied. Water is the most valuable resource; the soils support thriving farms in the valley and grass for thousands of cattle on the mesa; stone, clay, sand, and gravel are of considerable importance; and tin has been found in the Franklin Mountains.

TIN.

Tin ore was discovered in 1899 in the Franklin Mountains about 12 miles north of El Paso. The ore is cassiterite associated with quartz veins in the granite at the eastern base of the range. The granite here is characteristically jointed and