14 
THE EL PASO TIN DEPOSITS. 
[bull. 178. 
Fig. 3.— Section of north vein (6 feet across) . 
up about 75 per cent of the mass. The cassiterite grains are, along the edges, inti- 
mately intergrown with quartz. If this is a metasomatic form of the granite a silic- 
ification has taken place. The microscope affords no direct evidence, however, that 
this ore is metasomatic. 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 feldspar 
were observed. 
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 west. The surveys b}^ the 
owners of the property show 
a course N. 85^° W. for the 
middle and 65° 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 out- 
crop. 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, shattered 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 wall. A crosscut about 8 feet in length, driven from the bottom 
of the shaft, cuts the vein, but does not pass through it. The sheet- 
ing 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 
Fig. 4.— Greisen bowl- 
der in tin vein (shaft 
on middle vein). 
