10

the tin veins occur along joints which strike across the trend of the range. The veins dip perpendicularly or at high angles either northward or southward.

In places the walls of the veins are plane and the contact with the country rock is smooth; at others the veins grade off into the granite. The mineralized zone, at the greatest width known, is not more than 6 feet thick and ranges from this down to almost nothing. Sections across this zone show irregular veins of quartz in the granite along the joint cracks. The quartz is massive, white, and of variable thickness, ranging from a fraction of an inch to 2 feet, irregularly thickening and pinching out. Cassiterite occurs in the vein quartz but is more abundant immediately contiguous to it. In the vein quartz the oxide of tin occurs both in bunches and irregularly disseminated, intergrown with the quartz. More concentrated deposits of cassiterite occur intimately intergrown with the quartz and feldspar of the country rock adjacent to the veins. Locally one or the other of these minerals has been completely replaced and the ore consists practically of cassiterite and quartz or of cassiterite and feldspar. The absence of mica is notable. Mineralization has apparently taken place only next to the veins and fades out a few inches from them. The cassiterite is present in crystals less than a tenth of an inch in size, both twinned and in simple tetragonal forms, and it also occurs massive. Specimens of nearly pure cassiterite, weighing several pounds, are reported to have been obtained from this locality.

The minerals usually associated with tin are not abundant here, although a few crystals of wolframite, topaz, and tourmaline have been found. A little fluorspar also is associated with the cassiterite, which here and there includes it. Some pyrite is present and limonite occurs superficially, locally forming a selvage between the veins and the granite. The presence of fluorspar, tourmaline, and topaz and the mode of occurrence of the ore suggest that it was formed, as in Cornwall, England, during a late stage of plutonic activity soon after the consolidation of the granite.

The principal development of this property, as recorded in 1901, ^a was on three veins which had been exposed for several hundred feet along their length; a few pits had been sunk, the deepest being 50 feet. In 1904 two small deposits of tin ore associated with quartz were found not far from the old workings. These new deposits are similar to but smaller than those first found. Development has not gone much further than scraping the surface along the veins for a few hundred feet and sinking a few shallow pits. The work reveals irregular streaks of quartz and cassiterite, varying up to 2 inches in thickness, in veins parallel to joints transverse to the trend of the range and cutting much-decomposed and broken granite. Here and there slickensided surfaces of granite occur. In a pit about 8 feet deep on the northernmost vein the quartz was found to fade away at a depth of 4 feet below the surface. In 1906 and 1907 some superficial prospecting was done, but no important results were reported further than the occasional finding of some tin ore within a radius of a mile or two from the original discovery. Present developments do not warrant a prediction as to the future of this field; it may or may not prove to be of considerable value. The chief question concerns the abundance of the ore, which can be determined only by further work. Conditions, however, appear to warrant intelligently directed development, and the entire granite outcrop might well be prospected for new occurrences of tin ore.

STONE.

Brick is almost exclusively used for building in El Paso. A few notable structures are built of limestone, however, and this rock is extensively used for foundations. The granite and syenite porphyry are much jointed and easily weather, so that they are not in demand, but the rhyolite porphyry which outcrops in the central part of the Franklin Mountains takes a fine polish, and selected slabs would make excellent ornamental building stone. Crushed limestone and andesite porphyry are much used for road making.

The abundance of limestone has been noted in the description of the formations, the El Paso, Fusselman, Montoya, and Hueco limestones and the limestone of the Comanche series constituting a large part of the consolidated rocks which outcrop in the district. Those which are conveniently located to El Paso are extensively quarried both for local use and for shipment. The percentage of lime and magnesia contained in these rocks is indicated by the following table:

	El Paso.	Montoya.	Fusselman.	Hueco.	Comanche.
CaO	32. 12	30.82	28.77	53.52	52.36
MgO	16.00	18.01	18.56	.58	1.01

From the above analysis it will be observed that the three older formations—the El Paso, Montoya, and Fusselman limestones—contain abundant magnesia, and that the younger Hueco and Comanche rocks contain very little. Both the magnesian and nonmagnesian limestones are burned for lime in the vicinity of El Paso, and for this purpose the El Paso and Montoya limestones are quarried at the south end of the Franklin Range and the Comanche at the pass above the city. Large quantities of the limestone of the Comanche series are also quarried and crushed for use as furnace flux by the smelter in the valley 4 miles above the city.

A probable future use of limestone near El Paso will be in the manufacture of Portland cement. Although the high magnesia content of the older limestones causes them to be unfit for cement making, the Hueco and Comanche rocks are well adapted to this purpose and the necessary clay is available both in the flood-plain deposits and in the shale of the Comanche series. Analyses of limestone and shale of the Comanche series are given in the table under the next heading. The analyses show considerable variation in the shale, although in general it is suitable for cement making. In No. 1 the silica is rather low; in No. 3 it is rather high and this contains relatively too little aluminum and iron for ideal Portland cement material.

CLAY.

Important deposits of clay, which can be classed as flood-plain clay, upland clay, and shale, occur in this district. The material is used extensively for brickmaking and is also available for many other purposes.

Flood-plain clay occurs at several localities in the Rio Grande valley. It is derived from the decomposition or disintegration of rocks that outcrop higher up in the drainage area of the river and has been brought down in suspension by the stream and deposited on the flood plain. In this manner deposits of clay intercalated with sand and gravel have accumulated, the mode of origin causing the deposits to be of irregular extent and composition. The beds range in thickness from a few inches to many feet and in character from a rather pure clay to one containing large admixtures of sand. More or less organic matter also is usually present. The analysis of clay from Whites Spur shows the composition of what is perhaps a typical sample of flood-plain clay.

Flood-plain clay is manufactured into common wire-cut brick at several plants in the valley—at Vinton and Whites Spur above El Paso and at others below the city. The product is a brick of fairly good grade and several millions from this source are made yearly. Adobe bricks, made of sun-dried flood-plain clays, are manufactured extensively by the Mexican inhabitants of the Rio Grande valley and are used in the construction of their picturesque buildings.

The upland clays are locally exposed in the terraces above the river and numerous beds of clay have been found in the wells that have been sunk in the Hueco Bolson. As yet none of these deposits have been developed.

The shale of the Comanche series which outcrop in relatively small exposures in the valley above El Paso are important sources of clay. These deposits are conveniently located to beds of limestone suitable for the manufacture of Portland cement and the shales are worked at the largest brick plant in this region. Bricks of excellent quality are made from crushed shale of the Comanche series on the west side of the Rio Grande above El Paso, immediately south of the Southern Pacific Railroad bridge, where many thousands of pressed brick and common wire-cut brick are made daily.

The subjoined table contains analyses of clay, shale, and limestone from the vicinity of El Paso, made by P. H. Bates, of the United States Geological Survey. The figures show a considerable variation, in the composition of the shale, the silica ranging from 49.08 to 75.15 per cent, the alumina from 10.90 to 20.71 per cent, and the lime from 0.66 to 13.56 per cent. Shale No. 3 in the following list was reported to be a fire clay, but its composition indicates only moderate refractoriness.

SAND AND GRAVEL.

Sand and gravel occur in abundance in the bolson deposits near El Paso and are well exposed for cheap excavation. The deposits are in places relatively free from admixture, but generally they are associated with varying amounts of clay. The sand usually contains appreciable quantities of lime and therefore has a relatively low fusing point which makes it unfit for some purposes. In addition to the common uses made of such material, two special uses may be noted—the manufacture of sand-lime brick and of cement blocks. The bricks are made of a mixture of sand and lime, which is subjected to the action of steam under pressure. Cement blocks are made from a mixture of sand and Portland cement. The manufacture of each of these products is a comparatively new industry, but the results are giving satisfaction

SOILS.

The flood plain of the Rio Grande valley, enriched by additions of alluvium deposited by the annual floods, is very fertile. Practically no trouble is caused by alkali and a variety of crops, including garden products, alfalfa, and different fruits, are successfully grown. The soils of the Hueco Bolson are derived from the disintegration of the highlands and are also fertile, but there is little chance of economically obtaining sufficient water for irrigating considerable areas. Bunch grasses thrive, however, and with wells at convenient intervals the mesa is a valuable cattle range.

WATER RESOURCES.

SURFACE WATER.

For a few hours after heavy storms small streams issue from the Franklin and Hueco mountains, but the flow soon ceases and practically the only surface water in the quadrangle is that in the Rio Grande. The flow of the river a few miles above El Paso is indicated by the following table:

This table shows that the discharge is very irregular. For several months the bed of the river may be dry, but during floods the flow is enormous. The highest water usually occurs during May and June, when the chief supply is derived from melting snow in the Rocky Mountains. Floods at other times of the year follow heavy precipitation on the drainage area, thus the exceptional discharge in October, 1904, succeeded unusually heavy rains. The irregularity of discharge is shown by the fact that the total flow of the river in 1907 was more than 66 times greater than in 1902.

Irrigation has been practiced in the Rio Grande valley, near El Paso, possibly as long as in any other part of the United States, for when the Spaniards conquered the region they found the Indian inhabitants familiar with the art. The soil is excellent and with ample water the valley would be very rich. During recent years plans have been perfected to construct a reservoir to impound the storm waters of the river. One available site is in the narrows above El Paso, but, considering all the conditions, the most desirable location for a dam is near Engle, N. Mex., about 100 miles above El Paso. There the United States Reclamation Service will construct a reservoir in which water can be stored sufficient to irrigate many thousand acres.