Notice 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, 69% 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 
phenoc^sts 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 are 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 sulphuret-s. 
There were some dikes in the greenstone itself. One of 
these show T s the same pale hornblende needles found in No. 
1639 and in a fresher condition. 
Between the serpentine body above noted and the area of 
siliceous Paleozoic rocks is a w T hite dike fifty feet in width at 
one point, w T here it is crossed by the road from Buckhorn 
*Etude sur la determination des feldspaths, 1891, p. 33. 
