TERTIARY VOLCANIC ROCKS-SYENITE. 
85 
hand, their divergencies from these two types are of such a character as to relate 
these syenites to the more basic rocks whose descriptions follow. They thus form 
another step in the increasingly basic series which begins with phonolite and ends 
with basic dike rocks. 
In appearance they are medium to dark gray rocks, with prominent prismatic 
crystals of pyroxene in a feldspathic matrix. They are distinguished in the field 
with difficulty and not always with certainty from the coarser-grained types of 
latite-phonolite. Weathering or other alteration produces changes that are iden¬ 
tical with those of the related porphyritic rocks, the resulting mass being usually of 
lighter color and more or less porous. 
These granular rocks are not very liberally distributed. The Vindicator mine 
is near the center of the largest area, which is about 2,000 feet in diameter. Another 
mass of some size occurs near the Last Dollar mine, and smaller bodies are found near 
the Rose Nicol mine on the northeast side of Battle Mountain, west of the Logan 
mine on Bull Hill, and in the vicinity of the Pointer mine on Gold Hill. Granular 
rocks referable to this class are also encountered in masses which do not reach the 
surface in the Mollie Kathleen, Granite, and Portland mines and in the Ophelia 
tunnel. 
The gradation to latite-phonolite is well shown on the surface both east and 
west of the Vindicator mine and near the Last Dollar mine and is strikingly seen 
in the underground workings of the Portland, Last Dollar, and Vindicator mines. 
Sometimes, however, there is a sharp contact of intrusion between the two, as shown 
in the Vindicator mine. The nature of the two groups of rocks and their manner 
of occurrence make it probable that only a slight difference in conditions, such as 
change of temperature or pressure, or minor variations in composition, were needed 
to cause the one or the other to crystallize from the magma. There is little or no 
evidence available to show that the granular rock is the deep-seated equivalent of 
the porphyritic.- The transition from one to the other is apparently regardless of 
distance from the surface or from contacts. It is probable that these granular 
rocks have formed much nearer the surface than is believed usually to be the case, 
for erosion since the volcanic period does not appear to have been very great. It 
seems likely, in fact, that some of these rocks solidified within a few hundred feet of 
the surface. It is possible that the rocks into which they were intruded were then 
at a high temperature and that the cooling was consequently slower than it would 
otherwise have been. 
MINERALOGICAL CHARACTER. 
These syenites are in general made up of the same minerals as are the latite- 
phonolites. Analcite, however, is much less common, biotite is of more general dis¬ 
tribution, and pyroxene and hornblende are on the whole more abundant, though 
in the case of the latter mineral the difference may be due to the fact that prac¬ 
tically no resorption has taken place in these granular rocks. Nosean and sodalite 
are often present, but, as in the latite-phonolites, vary in amount and frequently 
can not be detected. 
Orthoclase is the most abundant constituent. It is usually present in individ¬ 
uals of irregular outline, but occasionally gives a subporphyritic texture to the rock 
