TIN DEPOSITS AT EL PASO, TEX. 
By W. H. Weed. 
The El Paso tin deposits lie on the east flank of the Franklin Moun- 
tains, the southern extension of the Oregon or San Andreas Range, 
about 10 miles north of El Paso. The ores were discovered in 1899 
and have been prospected by several open cuts and pits, the deepest 
of which is about 50 feet below the surface. The place is distant 
about 14 miles by wagon road from El Paso. The Rock Island Rail- 
road crosses the flat 3 or 4 miles east of the property, and the main 
line of the Southern Pacific lies 10 miles to the south. There is a 
good spring one-fourth of a mile from the ledges, but there is no large 
supply of water nearer than the Rio Grande. The mesa is under- 
lain by water, the city of El Paso being supplied from driven wells 
sunk in the mesa gravels. 
GEOLOGICAL STRUCTURE AND FORMATION. 
The geological structure is simple and easily made out. The moun- 
tain range consists of Cambrian and other Paleozoic limestones, 
upturned by and resting upon an intrusive mass of coarse-grained 
granite that forms the central core of the range. This granite is well 
exposed for a distance of 4 or 5 miles along the eastern side of the 
mountains, forming the lower half of the mountains proper, and in 
places extending out to the foothills. The crest of the range consists 
of steeply tilted, heavily bedded, dark-gray limestones dipping west- 
ward. The basal quartzites were observed in the drift seen in arroyos, 
so that the granite is probably intruded between the base of the Cam- 
brian rocks and the underlying Archean complex. 
The eastern foothills consist mainly of limestones, but near the tin 
deposits these bedded rocks have been cut through and granite now 
forms the surface, remnants of the limestone cover showing as isolated 
j masses capping the hillocks. North of the tin mines a transverse 
iridge of the range shows the granite to be sheeted by well-marked 
planes, dipping eastward at an angle of about 45° to 50°. The granite 
is very much altered by surface decomposition, and crumbles readily 
to a coarse sand. The granite is sheeted near the veins, the planes 
of sheeting being parallel to the veins themselves. The general 
sheeting, however, is in a different direction, the average strike being 
|N. 20° E., and the dip 70° SE. A thin section of this granite, exam- 
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