60 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
White quartz-monzonite porphyry. — In the vicinity of Lida dikes 
and fewer sheets of white quartz-monzonite porphyry are common. 
The three areas shown on the map include both sheets and complex 
intrusive masses filled with inclusions of Cambrian rocks, while 
many other areas are too small to be mapped on the scale employed 
here. The dike which cuts the gray quartz-monzonite porphyry 
4 miles wesl of south of Alkali Spring has already been mentioned. 
This quartz-monzonite porphyry is a dense, white or locally green- 
ish rock, with rather abundant medium-sized phenocrysts, consisting 
of somewhat altered whitish feldspars, some striated and others tin- 
striated, silvery mica, and rarely a quartz crystal. The central por- 
tions of the dikes and intrusive masses are more coarsely crystalline 
than the borders, and in certain instances approach a granitoid 
texture. 
The rocks break into sharp joint block-, which in many places are 
stained by limonite. Limestone in the vicinity of this rock is here and 
there silicified and indurated, although the metamorphism is slight. 
Microscopic examination proves the medium-grained micro- 
granitic groundmass to consisl of orthoclase grains, plagioclase 
laths, and a few quartz anhedra. The phenocrysts of orthoclase 
slightly predominate over those of plagioclase (oligoclase-andesine). 
Kaolin and sericite arc the alteration products. Biotite phenocrysts 
altered to muscovite or chlorite, with an iron ore and rutile in rods 
or sagenitic web-, arc constantly present, while quartz phenocrysts, 
much corroded, appear in some thin sections. 
The white quartz-monzonite porphyry i^ younger than the gran- 
ite which it cuts in the area 4 miles west of south of Alkali Spring. 
Pebbles of the porphyry arc included in the Siebert lake beds. It is 
believed that it is genetically related to the granite and is a later 
intrusion of the same magma. 
Diorite porphyry. — Dikes of quartz -diorite porphyry cut the gray 
quartz-monzonite porphyry 4 miles west of south of Alkali Spring, 
and many dikes of diorite porphyry, probably of contemporaneous 
age. intrude Cambrian rocks near Lida. The quartz-diorite por- 
phyry is a dark, rattier fine-grained noncrystalline rock. Under the 
microscope the presence of biotite i> revealed. Ophitic texture is 
well developed, plagioclase laths (some andesine and more basic) 
lying in a mesostasis of hornblende, biotite, and a little quartz. The. 
rock weathers into sharply jointed blocks of dark color. The diorite 
porphyry is a greenish-gray rock' of porphyritic habit, both horn- 
blende and striated feldspars occurring as phenocrysts. Epidote is 
developed in the rock and forms crystalline felts along joint planes. 
Spheroidal weathering is characteristic. 
The contact of the white quartz-monzonite porphyry and the 
diorite porphyry is poorly exposed 1 mile northeast of Lida. and the 
