June, 1896 
386 The American Geologist , 
mountain to the north bank of the Merced river, opposite the 
old Benton mill. This dike in following the contact makes an 
S-shaped curve. Some of the syenite-porphyry dikes are also 
to be seen near the vein of the Red Bank gold quartz mine on 
the north side of the Merced river, about half way between 
Benton mill and Split Rock ferry. 
The rock described by Dr. Charles Palache* as containing 
a blue soda-hornblende, called by him crossite. appears to be 
very similar in mineral and chemical composition to some of 
the. dikes described above. Dr. Palache’s material came from 
a boulder in the Coast, ranges near Berkeley, but was pre¬ 
sumed to be of local origin. It is probably from a dike in the 
neighborhood. At a future time a more exact description of 
•the soda-syenite dikes of the Sierra Nevada will probably be 
published. 
Apo-trachytes .—The fossiliferous beds of Plumas county, 
called Robinson formation by Mr. Diller, are largely made up 
of tuffs, in which porphyritic feldspars are abundant. These 
beds are considered by Prof. Walcott and Mr. Schuchert as 
of Upper Carboniferous age. Chemical analyses (Nos. 219 
and 352) appear to show these tuffs to have the composition 
of trachytes and this is confirmed by a microscopical exami¬ 
nation. for while the feldspathic material of the ground mass 
is difficult of determination, on account of the minute size of 
the grains and the presence of much secondary mica, the por¬ 
phyritic feldspars are seen to be mierocline and ort.hoclase. 
Trachytes .—In the area of the Big Trees sheet, in 1887, the 
writer collected some peculiar looking Tertiary igneous rocks 
on the ridge north of Beaver creek, about one mile northwest 
of the South grove of Big trees. The lower part of the igne¬ 
ous mass (No. 86 Tuol. Co.) is a dark, fine-grained, basaltic 
looking rock. The microscope shows the phenocrysts of 
plagioclase to be near labradorite. There is also augite pres¬ 
ent. The feldspars of the glassy microlitic ground mass are 
much more acid than the phenocrysts. Overlying this dark, 
very fine-grained lava is a somewhat coarser dark-brown glassy 
fragmental-looking lava containing black mica (No. 85 Tuol. 
Co.). The microscope shows this to be a tuff containing bio- 
*Bull. Dept. Geol. Univ. of Calif., vol. i, p. 182. 
