12 THE EL PASO TIN DEPOSITS. [bull. 178. 
were observed in the drift seen in arroyos, so that the granite is 
probably intruded between the base of the Cambrian 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 
masses capping the hillocks. These relations are shown in the diagram, 
fig. 1, which is a rough sketch of the range, representing a cross sec- 
tion at the tin mines. North of the place where this section was made 
a transverse ridge 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. No fresh material was observed anywhere on 
the surface, but fairly good material was obtained from the dump heap 
of the shaft on the north vein. 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 
Fig. 1.— Cross section of Franklin Mountains 10 miles north of El Paso, Texas. 
being N. 20 E, and the dip T<> SE. A thin section of this granite 
has been examined under the microscope by Mr. Lindgren, who fur- 
nishes the following notes: 
The rock is a coarse-grained normal granite. It shows much anhedral quartz with 
anhedral feldspar, largely microperthite, with sonic few grains of microcline. A few 
small flakes of brownish-green hornblende and some small grains of magnetite were 
also seen. The rock is a soda granite. 
White aplite-granite occurs in veinlets and irregular masses intru- 
sive in the granite, but none was observed close to the veins. The 
mesa is underlain by cemented gravels, which form also the lower 
slopes of the foothills. 
ORES AND VEINS. 
The ores consist of cassiterite, or oxide of tin, with wolframite 
(tungstate of iron and manganese) in a gangue of quartz. Specimens 
of nearly pure cassiterite weighing several pounds have been found on 
the surface, and this mineral occurs in the quartz, either alone or asso- 
ciated with wolframite. The most abundant ore is a granular mixture 
