‘ at 
Hague and lddings—Rocks of the Great Basin. . 457 
In this variety of basalt there is great variation not only in the 
relation of groundmass to porphyritic crystals but also in the 
size of individual erystals. No sanidins could be detected. In 
some basalts the larger secretions are confined to olivine, in 
others to small feldspars, with olivine in minute grains, while 
augite only occurs occasionally among the larger crystals, and 
only in exceptional cases are all three minerals found associated | 
together as porphyritic secretions. 
Other varieties show less and _ less olivine, and with the 
gradual increase of silica and the coming in of more and more 
hypersthene, rocks occur intermediate between normal 
basalts and those with the typical andesitic composition and 
structure. The granular variety of basalt occurs far less fre- 
of the rock which forms the top of the lava plain at Shoshone 
Falls, and there is reason to believe is well developed on the 
great table-land of the Snake Plains. 
roxene-andesite.—For the purposes of the present paper this 
provisional designation may be used to include both hypers- 
thene-andesite and hypersthene-augite-andesite, two varieties of 
andesitic rocks not always easily distinguished. | Whether 
there are any extrusions in the Great Basin which should be 
classed as augite-andesite is a matter of some doubt, and presents 
& question which can only be answered by the investigation of 
similar rocks in various quarters of the globe, and by litholo- 
pee coming to some conelusion as to where the line should be 
microscopical erystals of plagioclase, ene and augite. 
ine. 
not till Mr. Whitman Cross* published his description of the 
rock from Buffalo Peaks, Colorado, was the —. hypersthene- 
* Bulletin of the U. 8. Geological Survey, No. 1, 1883. ee 
+ Niedzwiedzki described a hypersthene-andesite from St. Egidi in Sonth- 
Steiermark in 1872. Tschermak’s Mineralogische Mittheilungen, 1872, iv, p. 253, 
_ } This Journal, Sept., 1883. ‘ ee 
