- 
456 Hague and Iddings—Rocks of the Great Basin. 
solution of the double iodide of mercury and potassium and 
were found upon analysis to be andesine or oligoclase. Again, 
. . 
in examining the feldspars in these so-called trachytes applica- 
~ tion was made of Dr. Szabd’s method of determining feldspar 
they possess many of the superficial aspects which formerly” 
pylite and trachyte in the Great Basin we classify all the 
_ volcanic rocks of the region under the following types, arrang- 
ing them for the purposes of the present paper according to 
their basicity rather than according to their geological relations: 
asalt, pyroxene-andesite, hornblende andesite, hornblende- 
miea-andesite, dacite and rhyolite. Within the limits of the 
present article it is only designed to point out some of the more 
important mineralogical and structural features, leaving all 
questions of their mode of occurrence, order of succession and 
chemical relations till the final report. 
_ Basalt.—These rocks may be divided into two general types: 
(a) the porphyritic, consisting of a glassy and microlitic or 
‘microcrystalline groundmass, bearing relatively large crystals 
of olivine, feldspar, and occasionally augite, a structure show- 
ing close relations to that of many andesites; (6) the granular 
(granular in the sense used by Rosenbusch,*) an aggregate 0 
quite uniform grains composed of well-developed plagioclase 
and olivine crystals with ill-defined patches of augite and mag- 
netite, and frequently with considerable glass base. 
The porphyritic variety is the type most frequently observed 
in the collection of the Fortieth Parallel Exploration and 18 
probably by far the most abundant in the Great Basin. It 18 
well described by Zirkel in his report. It is not, however; 
_ always holocrystalline, often dst ying aeatderabls glass base. 
* Neues Jahrbuch fiir Mineralogie, etc., 1882, Band 2. 
