Wotice of Syenitic Rocks from California. — Turner. 385 
needles of a pale brown hornblende, and numerous idiomorphic 
phenocrysts of plagioclase in a microcrystalline granular 
ground mass of feldspar, muscovite and probably quartz, for 
the silica contents (71.88% ) is too high for all the silica to be 
in the constituents already mentioned. In the table of albite 
analyses given in Dana's Manual of Mineralogy, <i ( .K/ r is the. 
highest amount given, and it is therefore evident that if no 
more basic element than albite were present, the silica per- 
centage could scarcely reach the amount found by Mr. Steiger 
in the rock. Nevertheless no quartz was certainly micros- 
copically determined in the rock. The feldspar phenocrysts 
are many of them twinned on the albite law giving symmetri- 
cal extinctions on the trace of the twinning plane (010) of 
from 6.5° to 21°, the average of nine crystals determined be- 
ing 13°; none were detected showing albite and carlsbad 
twinning combined, so that the new method of Michel Levy* 
for determining feldspars could not be applied. From the 
lime percentage of the rock (2.03%) and from the general 
tendency of the more basic feldspars to crystallize first, and 
the apparent formation of epidote directly from the feldspar 
phenocrysts it is presumed that they are near andesine in 
composition. The rock may be provisionally called a soda- 
granite porphyry. 
Along the borders of the serpentine area from six to seven 
miles southeast of Coulterville arc several soda-syenite dikes. 
Some of them follow quite closely the contact of the serpen- 
tine and the adjoining rock, which, to the east of the serpen- 
tine, is a greenstone (apo-augite-andesite) tuff and to the 
south is the same belt of siliceous Paleozoic schist or quartz- 
ite before noted. One of these dikes apparently forms the 
lode of a gold deposit (No. 1555) as it had evidently been 
mined. It is much altered in places, containing abundant 
quartz, calcite or dolomite, and sulphurets. 
There were some dikes in the greenstone itself. One of 
these shows the same pale hornblende needles found in No. 
L639 and in a fresher condition. 
Between the serpentine body above noted and the area of 
siliceous Paleozoic rocks is a white dike fifty feel in width at 
one point, where it is crossed by the road from Buckhom 
*Etude sur la determination des Eeldspaths, lsiii. p. 33. 
