TERTIARY VOLCANIC ROCKS-LATITE-PHONOLITE. 
71 
Appearance. —The latite-phonolites are dark-gray to black rocks, usually of 
rather fine grain. They are porphyritic in texture and contain small prismatic 
phenocrysts of pyroxene and tablets of feldspar up to 2 cm. diameter. Small scales 
of brown biotite are observable in some varieties, and fresh, smoky apatites in short 
hexagonal crystals up to 2 mm. diameter in cross section are frequently seen. 
Occasional small, resin-yellow crystals of titanite are apparent in hand specimens. 
The groundmass is compact and dark in color. Some of the rocks, however, 
appear to be granular, and only with the aid of the microscope are found to contain 
a microlitic groundmass. This phase marks the gradation into the granidar sye¬ 
nites. Small included fragments of syenite appear in the zone of transition to the 
granular rock. In some specimens these inclusions appear to be partially dissolved 
and absorbed by the surrounding rock, which there contains more hornblende— 
a prominent mineral of the inclusion—than elsewhere. 
Upon weathering the feldspars and the groundmass become dull and lighter 
colored, giving a gray color to the rock. More complete change usually produces a 
light yellow color, as in the phonolites, and at such times the pyroxene phenocrysts 
may be entirely removed, giving the characteristic porous or pitted appearance 
seen near the Blue Bird mine. Even in the most decomposed portions apatite 
remains fresh and glassy and often serves to identify the rock. 
MINERALOGICAL CHARACTER. 
General statement .—The essential minerals ofi these latite-phonolites are alkali 
and soda-lime feldspars, analcite or sodalite or nosean, pyroxene, both augite and 
aegirine-augite, and a brown hornblende. Biotite sometimes becomes important 
and produces a type somewhat different from the common one. The close relation 
to the true phonolites is shown among other ways, by the presence of nosean, 
sodalite, and analcite. Titanite, apatite, and black iron ore are noteworthy acces¬ 
sories and a few minerals present in smaller amount will be mentioned and briefly 
described later. Among the decomposition products are kaolin, sericite, chlorite, 
serpentine—and possibly talc—epidote, green mica, rutile, quartz, and carbonates. 
Variations in. the mineralogical character of this group make it more complex 
than the phonolites; but as will be seen later, the chemical composition is notably 
constant, and therefore recpiires a nice equilibrium between the amounts of the 
alkali-rich and the iron, magnesia, and lime-bearing minerals. The presence of 
nosean, sodalite, or analcite is offset by an increase in pyroxenes or hornblende 
and a decrease in feldspar. The groundmass feldspars are almost without excep¬ 
tion alkali-feldspar. When the feldspar phenocrysts are abundant, they are usually 
rather alkalic; when decidedly calcic, they are never numerous. 
A decrease in the amount of bivalent bases and consequent increase in silica 
causes the ferric iron to enter largely into the formation of a brown mica, which 
must therefore correspond to the variety lepidomelane. The rock thus produced 
is rather different from the pyroxene-rich type and while showing close chemical 
affinities is distinguished by the name biotite trachyte. It occurs on the northern 
slope of Battle Mountain and the northwestern slope of Mineral Hill. Varying 
amounts of botite in the more typical latite-phonolites show the close relation of 
the two varieties. 
