Geological Notes on the Sierra Nevada. — Turner. 313 
but as 'may be noted below has an unusually high percentage of silica 
for a basalt. It is also characterized macroscopically by containing 
numerous cavities and tabular porphyritic feldspars, which arc some- 
times half an inch long. Microscopically the table mountain basalt 
shows large, tabular feldspars, smaller augites and numerous olivine 
grains in a nearly opaque groundmass, in which are very abundant 
feldspar microlites. This groundmass doubtless contains a large amount 
of magnetite and strongly resembles some of the sections of the older 
basalt. There is a good deal of brownish serpent inoid substance in the 
groundmass. The porphyritic feldspars are polysynthetically twinned 
and have a corroded appearance, due to the abundant inclusions of ag- 
gregates of opaque particles, with some glass inclusions. There is also 
some serpentine in the large feldspars. The relative" age of this basalt 
has not been determined. Pebbles of hornblende-pyroxene-andesite (No. 
165 ( 'alaveras county) occur under a basalt table mountain on the nort h 
side of the Stanislaus river, near the Byrne's Perry bridge. This ba- 
salt is doubtless part of the Tuolumne table mountain How. This would 
suggest that t he Tuolumne table mountain basalt is later in aye than 
the hornblende-pyroxene-andesile-breccia. For convenience, however, 
it is included here. 
Dolerite. — This lava was described under the name of •"late basalt"* 
in a, previous paper. It occurs at numerous points in Plumas and Sierra 
counties. Through the kindness of Mr. .1. S. Dillerl was permitted to ex- 
amine a series id' basalt slides of the Cascade range collect ion and found 
that a con si d era Id e number of the basalts to t he north of Plumas county 
are of the same type. The "hypersthene basalt*' mentioned by Mr. 
Diller in his paper on "Fulgurite" from Mount Thielson, Oregon (No. 
1S1 Cascade range collection), is practically identical with some of the 
dolerite of Mount In-alls. 
Almost all the areas examined l>\ the writer overlie the hornblende- 
pyroxene-aiidesite-breccia. I n t he ravine 1 hat heads about one and a 
half miles south by east from the summit of Mount [ngalls the dolerite 
is marked by glacial striae, and the pond indicated on the map near 
this point has been formed by a terminal moraine. It is. therefore, 
earlier in age than at least the latter part of the glacial period. On Mt. 
[ngalls il presents the appearance of having issued after the presenl 
drainage system had been partiallj forming, suggesting the aye to be 
Pleistocene. 
The rock is macroscopicallj a. medium to coarse grained, lighl graj 
rock, sometimes pinkish, with usually large scattered olivines. Under 
the microscope it is seen to be nearly or quite holocrystalline and to he 
composed of lath-like plagioclase, augite and magnetite, with occa- 
sional olivines and frequently hypersthenes. < >ne of the slides shows a 
hypocrystalline base containing abundant grains of magnetite, with nu- 
merous phenocrysts of plagioclase and augite, and a few hypersthenes 
and olivines. In general, however, no glass is to be seen. 
A in. .lour. Sci.. vol. I I. p. I.V>. 
