1 
460 Hague and Iddings—Rocks of the Great Basin. 
oughly characteristic cleavages, extinctions, etc. Others are 
partially converted to chlorite, and yet others are wholly 
replaced the uniaxial dichroitic green mineral.” In the 
light of the present investigation it is now perfectly evident 
that the decomposed mineral is mainly referable to hypersthene. 
_ The relationship already pointed out by us as existing between 
olivine and hypersthene in the rocks of the Pacific Coast 
volcanoes* holds equally good for the Great Basin, the olivine 
increasing and replacing the hypersthene as the rock becomes 
more and more basic 
Hornblende-andesite-—This rock forms a well characterized 
andesite, showing transitions into pyroxene-andesite, while, on 
.the other hand, mica gradually comes in as the rock becomes 
more and more acidic. 
Hornblende-mica-andesite—Under this head may be classed 
a large number of andesitic extrusions scattered throughou 
the Great Basin from the Sierras to the Wahsatch in which 
rs as a characteristic and essential ingredient. 
Indeed a large proportion of the rocks formerly regarded as 
trachytes properly fall under this division. In these rocks the 
feldspars have a decidedly vitreous appearance, while the 
texture of groundmass possesses a rough porous character 
presenting what is known as the “trachytic habit,” but, as 
already shown, the entire absence of orthoclase among the 
porphyritic crystals prevents their being considered in any 
other light than as andesites. While, as already stated, sanidin 
is regarded by most lithologists as the prevailing feldspar 
found in trachytes, it should be remembered that von Richt- 
hofen recognized an “ oligoclase-trachyte” in which sanidin 
was not even an essential ingredient; this rock agreeing with 
the hornblende-mica-andesites of the Great Basin.t 
Dacite.—Following the development of the mica, quartz 
secretions begin to appear as the rock passes more and more — 
into acidic varieties, and with the appearance of quartz, horn- 
blende and pyroxene rapidly diminish. This gives a well- 
efined rock composed mainly of plagioclase, quartz and mica, 
* This Journal, Sept., 1883. 
+ Natural System of Voleanic Rocks, San Francisco, 1867, p. 36. 
“ 
