LONE MOUNTAIN FOOTHILLS. 
zircon. Quartz shows rather strong undulatory extinction. The 
rock is somewhat altered, biotite changing to chlorite, orthoclase 
and microcline to muscovite, and oligoclase to zoisite. 
The granite is cut by aplite and grades into pegmatite. The 
aplite is a white, fine-grained rock composed of white feldspar and 
some quartz. It forms dikes from one-half inch to 2 inches wide, 
which weather in relief. The constituent minerals of the pegmatite 
reach a maximum diameter of 1 inch. Some veins of pegmatitic 
quartz are also present, as is graphic granite, with characters one- 
fourth inch in length. 
At the contact the Cambrian rocks are rather highly metamor- 
phosed. The impure limestone is altered to a quartz-epidote rock 
in which brown garnet occurs locally. The microscope shows that 
magnetite and pyrite are also present. The shale is metamorphosed 
to a silvery knotted schist. In thin section it is found to be a 
chlorite-epidote schist in which chlorite, quartz, and orthoclase form 
m interlocking mosaic. Epidote with a little zoisite is equally 
abundant, although less evenly distributed. Magnetite grains and a 
few fibrous sea polite aggregates are also present. Veinlets and lenses 
of coarser quartz, epidote, and zoisite appear to form the " knots." 
The granite cuts Cambrian rocks and is itself cut by dikes of 
diorite porphyry. It is presumably one of the post-Jurassic granites. 
Diorite and diorite porphyry. — A small intrusive mass of diorite 
cuts the granite and another the Cambrian rocks northwest of Gen- 
eral Thomas Camp. The diorite appears to be a granitoid rock com- 
posed of feldspar and hornblende. The microscope shows that the 
feldspar is a rather basic labradorite in crystals, with some ortho- 
clase. The hornblende is rudely idiomorphic. Some quartz and 
magnetite are present, the rock approaching a granodiorite. Epidote 
ind chlorite are secondary to plagioclase and hornblende. 
The granite of Lone Mountain is also cut by dikes of a greenish - 
*ray diorite porphyry, which breaks into, angular joint blocks with 
slight tendency to spheroidal weathering. The dikes, which are 
Prom 2 to 20 feet wide, weather in troughs beneath the surface of the 
granite. 
Under the microscope the groundmass appears as small plagioclase 
aths, between which are some orthoclase grains. The phenocrysts 
nclude hornblende, plagioclase (labradorite-andesine), and titanite. 
^lany of the simple plagioclase crystals, which are more or less 
ltered to sericite, calcite, and chlorite, are twinned according to the 
barlsbad law. The brown hornblende phenocrvsts are partially 
dtered to calcite, chlorite, and iron ores. Apatite and magnetite are 
iccessories. 
I Sheets and dikes of diorite porphyry, with abundant phenocrysts 
»f striated feldspar and fewer of hornblende, occur in the vicinity 
