TERTIARY VOLCANIC ROCKS-LATITE-PHONOLITE. 
81 
This specimen from Bull Cliff is a good representative of the latite-phonolite 
type, both in composition and appearance. Masses of the same variety occur 
south of Altman, in the vicinity of the Last Dollar mine, on the north slope of Big 
Bull Mountain, and probably on the north and west slopes of Bull Hill, though 
decomposition at those places is too great to justify a conclusion. 
V. This specimen is described on page 71. It represents masses on Battle 
Mountain and on Mineral Hill. 
VI. This is a dark-gray rock, rather granular in appearance, with large and 
small phenocrysts of feldspar, and only little pyroxene visible. The microscope 
shows that the larger phenocrysts are orthoclase, probably rich in soda; the smaller 
ones are a soda-rich plagioclase, some being albite, some oligoclase-albite. The 
pyroxene crystals, which are well developed crystallographically, are segirine-augite. 
Original hornblende is almost wholly resorbed. Nosean is comparatively abundant 
in clouded crystals. Sodalite, which is of lighter color, is less common as pheno¬ 
crysts, but is plentiful in small grains and crystals in the groundmass. Analcite 
can not be identified with certainty, but is probably preserft as inclusions in some of 
the feldspar phenocrysts. Nepheline is absent. Magnetite, apatite, and titanite 
are present in irregular grains and in sharp cr 3 r stals. The groundmass is beautifully 
trachytoid, consisting principally of tiny laths of orthoclase, with microlites of 
pyroxene, sodalite, and magnetite. This rock is closely related in chemical composi¬ 
tion to the phonolites. The specimen is from the same mass as No. IV and thus 
affords a good illustration of variation in a single mass. 
The characteristic chemical features of these rocks are high alkalies, with 
plentiful alumina and moderate amounts of the bivalvent bases. The amounts of 
phosphorus pentoxide (from apatite) and of titania (mainly from sphene) are note¬ 
worthy. The main distinctions from the phonolites are greater variability in com¬ 
position, lower alkalies, and higher lime, magnesia, and iron. 
Of the specimens analyzed, Nos. Ill and IV best represent the type and their 
agreement in composition is very close. It is interesting to note, in passing, that 
No. IV has a predominant groundmass, while in No. Ill the groundmass is subor¬ 
dinate^—a good example of different textural development with constant composi¬ 
tion. A calculation from the analyses of these two specimens, in connection with 
a study of the thin sections, gives an idea of the proportionate amounts of their 
mineral constituents. The appended list is approximate only, because of the num¬ 
ber of independent factors entering into the calculation: 
Mineral molecules in latite-phonolite. 
Orthoclase. 29. 75 
Albite. 26. 10 
Anorthite. 3. 08 
Sodalite. 2.74 
Nosean.-. 3. 53 
Analcite. 4. 60 
.iEgirine.-. 1.02 
Augite. 10.23 
Hornblende. 5.63 
Apatite... 2. 37 
Titanite. 2. 36 
Magnetite. 1.96 
Other minerals, hygroscopic water, water of 
hydration, etc... 6. 63 
100.00 
