weed.] TIN DEPOSITS AT EL PASO, TEX. 101 
From a thin section of the ore, examined under the microscope, the 
rock is seen to be quartz cassiterite. It is a coarsely granular rock 
consisting of anhedral quartz, with grains of slightly brownish cassit- 
erite intimately intergrown with quartz along the edges. The quartz 
is full of fluid inclusions and makes up about 75 per cent of the mass. 
One small grain of tourmaline and a few flakes of sericite were seen. 
Neither topaz nor mica occurs in the section, and no remains of feld- 
spar were observed. If this is a metasomatic form of the granite a 
silicification has taken place. The microscope affords no direct evi- 
dence, however, that this ore is metasomatic. 
The north vein has a course of N. 85|° W. magnetic, as determined 
from the openings at the east end. At the west end of the workings 
the course observed, looking back along the outcrop, appears to be 
N. 80° E. for the northern vein and N. 80° W. for the middle vein; 
so that if these observations are correct the veins must intersect toward 
the Avest. The surveys by the owners of the property show a course 
N. 85|° W. for the middle and 05° W. for the south vein. 
DEVELOPMENT. 
A shaft 35 feet deep has been sunk on the north vein at the eastern 
end of the vein outcrop. This shaft is about 5 by 10 feet across and 
shows a very well-defined vein about 5 feet wide, having a dip of 
about 70° to the north. The sides of the shaft show excellent ore, 
mostly of the greisen variety, extending down for 8 to 15 feet below 
the top. At this point a slip crosses the shaft and cuts out the ore. 
This slip, or fault, is a clay seam, but one-fourth to one-half inch in 
thickness, and seems to have thrown the upper part of the vein to the 
north. The lower half of the shaft reveals only rusty granite, shat- 
tered and showing films of quartz, but without recognizable ore. A 
crosscut south from the bottom of the shaft should reach the vein if 
the fault is a normal one. In the exposure seen in the upper part of 
the shaft the ore occurs in bunches in altered granite and lies on the 
north side of a 15-inch streak of sheeted and rusty quartz. A second 
shaft on the north vein has been sunk at a point about 300 feet west 
of the one just noted. This shaft is about 25 feet deep. The vein is 
well exposed at the top, and shows a dip northward, but the shaft 
passes out of the vein into the sheeted granite, forming the foot Avail. 
A crosscut about 8 feet in length, driven from the bottom of the shaft, 
cuts the vein, but does not pass through it. The sheeting of the 
granite seen in this shaft is very pronounced, the rock being divided 
into plates from one-fourth inch to 12 inches in thickness by planes 
dipping Gl° E. and crossing the vein at 90°. The outcrop of the vein 
is traceable westward up the slopes by its rusty quartz, and a nearly 
continuous ledge can be followed. This outcrop has been opened at 
intervals of a few yards by trenches, Avhich expose the A^ein and show 
it to haA r e a thickness of from 2 to 6 feet, Avith about half this thickness 
