The daily silver state. [volume] (Winnemucca, Nev.) 1903-1907, 1903-06-04
GEOLOGY OF
TONOPAH
Article in Salt Lake Mining
Review That is Worthy
of Preservation
INTERESTING TO EVERYONE
Written by j. e. Spur in q Bull
letin of the Geological
Survey
INTERESTING TO EVERYONE

AChcPbhs NN'oNHt oANuo'Nh ~ q
volcanic region. During a long per-
iod of time this region has been the
eat of vulcanism. There was not
continuous eruption, however. Again
and again the volcanoes became APP
parently extinct. and the wearing
down of the surface carved out hills
and valleys. But after each period of
rest, the subterannean forces again
broke loose; new volcanoes were
formed and the new streams of lava
and showers of ashes were thrown out.
Although this volcanic activity last
ed over a long period of years (many
thousand), yet nearly the whole is
comprised in one of the comparatively
recent divisions of geologic time
the Tertiary. The eruptions began
in the early Tertiary. OF these earl
lest volcanoes no trace has been rec
ognized, although the lavas which
they poured out are found. The lat
est eruptions were post Tertiary, and
reached we'll down toward the present
day. The latest lava erupted in the
Tonopah district was basalt, and at
Silver Peak a basalt cone belonging
to the same period is so perfect in form
that it is ~porbably not more than a
few hundred years old.
1t is thanks to this volcanic action
that the ores have been formed.
The geologic history is somewhat as
follows:
In the early Tertiary period (before
the appearance of man on the earth)
the country consisted of ancient lime
stones and GRANTEES, and had been for
millions of years free from vulcanism.
The commissions of nature began, and
great masses of hornblende andesite
were poured out. This is the early
andesite or lode porphyry. After
the eruption the hot waters which
usually follow volcanic eruptions rose
through the lava, following fractures.
The waters contained SILICA and the
precious metals, which they had de-
rived from the heated region below.
They deposited these along the frac-
lures. and the present quartz veins
we're formed. At the same time the
andesite was decomposed and altered
to a remarkable extent by the action
OF the waters and their contained
gases.
A long period Of volcanic rest fol-
lowed. The hornblende andesite was
deeply worn, being stripped in most
cases down to the rich portion of its
contained veins.
This period of rest was brought to
a close by another andesite eruption.
Two closely successive flows were
poured out. They were of pyroxene
andesite and biotite andesite. respec-
lively; but are hard to distinguish
from each other without the micro
scope. and may be grouped under the
name of the later andesite. The later
andesite was not unlike the early an
desite in composition. and many va
rieties Of the two can be distinguish
ed only with difficulty. Moreover,
the later andesite has also been con-
siderably altered in many places. In
general, however. the later andesite
is somewhat coarser than the early
andesite, and presents a more mottled
appearance, it's crystals OF biotite
(mica) are frequently large, and it is
frequently, but by no means always,
colored a characteristic purple at the
outcrop, which may give way to blue,
Of green, or even white. below. AS
the early andesite is called the lode
porphyry. the later andesite may be
conveniently called the purple por-
phyry.
The eruption of the later andesite
was followed by another period OF
volcanic rest probably not quite so
long as the previous one. which occur
red between the appearance of the two
andesites). Then the pentup energy