ROCK CLASSIFICATION ON THREE CO-ORDINATES 215 
plagioclase feldspars in lath-shaped forms before the solidification 
of the ferromagnesian minerals, is unknown in siliceous and ultra- 
basic rocks, and it is found in rocks crystallizing at moderate depth 
as well as in volcanic rocks. Independent of all other considera- 
tions, rocks having the ophitic texture are called diabase; they may 
occur in dikes or sills crystallizing at moderate depths, or they may 
be found in surface flows. Diabase proper has the mineral compo- 
sition of augite andesite (or auganite); olivine diabase has the com- 
position of basalt containing monoclinic pyroxene; quartz diabase 
and hypersthene diabase are also known. 
The nomenclature employed is for the most part entirely 
familiar to petrographers. No distinction based on geological age 
is recognized; therefore such terms as melaphyre and quartz 
porphyrite are excluded. In designating the aschistic hypabyssal 
rocks the term porphyry is used (even for plagioclastic types) in 
preference to porphyrite. For the alkalcic plutonic and volcanic 
rocks it is believed that compound names like quartz diorite and 
augite andesite are objectionable, and should be replaced by simpler 
designations. Therefore, Spurr’s' proposal to use tonalite in place 
of quartz diorite is adopted; and it is proposed to abbreviate augite 
andesite to the simple and almost self-explanatory form auganite.’ 
Auganite proper is a volcanic rock consisting essentially of calcic 
plagioclase and augite; other varieties include hornblende auganite, 
related to andesite, and hypersthene auganite, which contains 
plagioclase and orthorhombic pyroxene with or without augite. 
A true augite andesite is a volcanic rock consisting essentially of 
sodic plagioclase (andesine or oligoclase) and augite. 
That auganite deserves a distinctive name and is not merely a 
variety of andesite, as suggested by the name augite andesite, is 
well shown by field relations such as those existing at National, 
Nevada, where the important rocks are rhyolite, andesite, and 
auganite. At this locality the auganite and andesite are of wholly 
different age and wholly different appearance. The auganite 
might be confused with basalt without close examination, since it 
tJ. E. Spurr, Amer. Geol., XXV (1900), 210, 232; U.S. Geol. Surv. 20th Ann. 
Rpt., VII (1900), 188, 190. 
2A.N. Winchell, Mg. and Sci. Press, November 22, 1912. 
