The Austin weekly statesman. (Austin, Tex.) 1883-1898, 1890-11-20
PILOT KNOB.
AN EMINENT GEOLOGIST SAYS 1T IS
AN ANCIENT VOLCANO
CRATER.
Will 1t Again Belch Forth, and Making
the Heavens Lurid, Shake up
the Population?

teen miles southeast from Austin anc
is known to all the old settlers.
Prof. Robt. T. Hill, an eminent
geologist, has been giving the Knob
with those adjacant, some study and
he says:
"The knobs were probably the cen
ter of disturbance and the basaltic
hills of today represent the neck Of
an ancient volcano whose crater has
long since been denuded Proof oil
this hypothesis is the increasing oc-
currence of debris as we recede from
the knobs vfor four or five miles
around. At Kunz station, four miles
south of Austin, railroad cut gives
fine exposure of this volcanic de
bris in close cnntact with unmeta-
morphosed chalk. II is yellow and
brown. loose material, hardening on
exposure and contains occasional
bombs of limestone (metamorphosed
chalk) from pea to.a foot in diam.
eter. About twenty feet of ash are
here exposed and the bank is cut in
two by great fault which will be ex-
planned. Other beds of debris occur
two miles west Of Manchaca station
in Bouldin's creek, and in Fairview
Park, South Austin. These exposures
are made by the denudation of the
cretaceous chalks which covered them
after their deposition. Superficial de
composition and the formation of zeo-
lites followed.
Accompanying the exposure of de-
bries are occasional strata of yellow
or black. yellow crystalline limestones
full of the fossils of the chalk. Thin
slices of this limestone made by Mr..
R. l.. Ziller show Divine specks in the
calcareous mixture, hence l incline to
think it mixture of chalk material
and debris.
The historic geology revealed in the
sedimentary strata of the adjacent re-
gion is complete and by its aid the
period of the activity OF Pilot Knob
can be determined. The interming-
ling Of the earliest debris with the o.
leviuscula beds and Inoceramus hori-
zon of the Austin chalk shows that
Pilot Knob was active during the lat
ter half of the Austin chalk sub-epoch
of the upper Cretaceous period (about
the Niobrara sub epoch of the North
wesD. The alterations of chalk and
debris show long continuance of &C
tivity, while the great bed OF chalk
above shows succeeding period OF
quiescence and subsistence.
The great faulting which the whole
region mas undergone since Pilot
Knobs activity gives another proof of
the age of the volanic eruptions. The
system of faults extend approximately
north 20 degree east; several of them
cut through the igneous material, as
at Kunz Station, Onion creek and
Fairview Park. The age of these
great faults is later than the volcanic
activity-is post upper Cretaceous, not
Cretaceous, for the very latest Gre
taceous strata are involved in their
disturbance. These faults belong to
the great BALCONIES system described
by me in this journal and whose line
is followed by the 1. 4 G. N. railroad
on map
From its structure it is shown that
Pilot Knob is the neck of an ancient
volcano which rose out of and depos.
tied its debris in the deep water of the
upper Cretaceous sea. From its
isolated position remote from any con-
temporaneous short line it must have
been an island eruption.