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*See*M AWtL Rept To" Go61. sdtva~ la m 1i a. -
the ition is broken by iulteg au& y in f"
Mom.; XTher ar two major antlinat out
thoug ffthe eer of the moohifs Jus west d the.
other 67-teirditig fronm P~lakiiddl M64ital14 orthewiitw to
a Poift wt of.BAyhoad..: fi w ein lies ai mjor syn-
clinal ax} which passes a short ditunc t of Lln6.: . Tis
boad synclinal belt is oaupied Ailly by the Pksaddl
;schist7 the antielifiAl as are marked by a of the Vip
Spring gnfis. The Pacsddle tebist dvsdies the gqiii and
therefre whee the rlaion is not disturbed by gnitmass, j
is fund on the erded flanks of thfe great blds. The m *dor
a of folding do not repent a simple structure, b minor o
folds, some of which have b recognized, ae supmp d on
the major folds, and local complexities of sahre ar numer-
ous. The thickens of the gnes and chists has not bn
determined.
LLANO SERIES.
Llano series the name aped to the, metmorphose&,seiies
of shists marbles, and gnss which resent he pre-
Cambrian sedimentary rcks of this region. . The series, which
is tentatively regarded as of Algonkian age,, is divided into two
formations, the Valley Spring gneiss and the ksaddle hst.
VALLEY SPRING GNEISS.
Definition.-The name Valley Spring gneiss, 'which is given
to the lighter-colored series of metamorphosed prerCambruian
sedimentary rocks, was first applied by T. B. Comstock" to one
of his pre-Cambrian subdivisions. The term is redefined 'by
the author of this folio, and its use strictly limited by 'the
geologic interpretation herein set forth. The type locality is
Valley Spring, in Llano County. .
The mapping of this formation was atnded locally by many
difficulties, partly in separating it from the Packsaddle cist
but chiefly in distinguishing it from granitic intrusions, which,
especially if slightly schistose, in many plasclosely rsemble
the light schists of the Valley Spring gniss and income p s
have been mapped with this formation, though not nsidere
to be a part of it. As the contacts are exceedingly irregular,
the boundaries shown on the map do not, in all places,' deft-
nitely separate distinct formations but rather indicate hp
in the dominance of rock type. In a region where here are
all gradations from pure grlite to pure -st and- whe
molten magmas have intricately interleaved' and in many
plac have fairly impregnated a mass, generalizedboun
darie such as have been used are necry to exp the
geologic facts. In many other places, however, the boundaries
are sharp and definite, but it is not practiable to diimi
thee on the map from the less definite ones. In addition to
the difficulty above noted there is evidence that older gneisses
of igneous origin are asoiatad with the schist and gneisses of
sedimentary origin.
Distribution.-The Valley Spring gneiss, a has Vn said,
underlies the Packsaddle chist and structurally wUpieS the
major anticlinal axis.
In the Llano quadmne, therefore, two broad bands more
or less interrupted by kniti intrusions are wesldefined. One
extends from Pontotoc, near the northwest corner of lthe quad
angle, southstwad to Sandy Creek, a few mile south of
Oxford. This band, with its inclusions of grnite is about 16
mil wide in its north n portion, but narrows to 3 or 4
miles at its south end and at the nose of a steeply pitchmig
anticline sinks beneth a band of Paksaddle chi whieh is
broken by a granite mas.
The ond are, about 7 mil wide at its north end, extendt
from the gion east of Magill Mountain in a narrowing iad
to a point just' north of Packldle Mountain, wher t a
pitching anticline car the formation below the Pik
schist.
A third small area whose structure is ill'-defined$ though
probably synclinal, liesouth of Castell, nr the western border
of the quadrangle.
The formation is exposed in the Burnet quadngle nort
east of Long Mountain. H the aspet of riddle a +
suggest granitic gnei and here also, the Euea ration
are obscure.
Caraeter.-The Valley Spnng gneiss ; dominaftlylit
co rd and pinkish tond and cmprs fdldpt*it and
quartkitic schistsy quartzites virllatonite ands, pgmfia addw"
gneisse(see PI. II) and rare anphibolc po ns.
The light-colored portions are a a whole or ten
histseare suI nulr or ihim'6 in toit, anda m-
in many place distingihed k withi di&uAt fm rok wia
may be :ular granite gei T granua s
in p intetsslYor contorted. a:nd upp ndy hi
pad theug s prur ande Ne q ztes
llghtcloredJ fino tanied }tocryoa be 6quii tas of tevo
quartiose siimnotik and th6 imphl-iteolb awot bniwaill
diffbikit Nin th in the 6*siyin lilk chh he
iwdlstoiiit buds nire ne~tatlirphw Imes o lii3no
and pesnt the _~ti stuctur rek"& u
.. rst A m RS ... ..6f .t . ia,,
3
2. Though he seemed to rcgn;z the pweenN of uling}
he did not use this knowledge to explain the pen of con-
tempoimueous beds in apparently illogical positions.
3. He connected granitic intrusion (of peCambran Se)
with movements due to faulting (of post-Paleozoic a).
4. He did not recognize the der of correlating pre-Cam-
brian whist over distances gr as from Canada to Texas.
5. He recogized "trends" and unconformities in the pre-
Cambrian but failed to delineate any such divisions.
6. He was misled into making statements regarding the iron
ores of the region because of his misconceptions regarding the
pre-Cambrian structure and stratigraphy.
Each of thes points will be very briefly discussed.
1. Comstock divided the strta which he assigned to the
Cambrian into three serie-the Katemcy (Upper Cambrian), X
the Riley (Middle Cambrian), and the Hickory (Lower Cam-
brian). He says in describing his Hickory series:
Whenever I have seen good contacts of the Cambrian with the
Texan strata [one of his pre-Cambrian divisions] and in many a-sea
where the granites directly underlie the Cambrian, there is a set of
beds which differ from the typical Potsdam sandstone. . In every case
in our region in which the upper contact of the terrane can be deter-
mined there is an unconformity, although this is not always detected
by casual observations. * * * Probably the best outcrops of the
lowest member of the series are those in the neighborhood of House
Mountain in the valleys of Hickory Creek and its tributaries, * * *
in many other places, as at the summits of Smoothingiron, Fox, Town,
Sandstone, Sharp, Packsaddle mountains, etc.
These basal beds which Comstock describes are geologically
contemporaneous with all the basal Cambrian beds of the region.
Their varying altitude is due partly to faulting and partly to
folding, as in Smoothingiron Mountain, or to an original
unevenness in the pre-Cambrian floor. The unconformity at
the top of his Hickory series to which he calls attention has
never been observed by the writer.
2. In describing his Riley series he fails to recognize the
structure at the east end of Packsaddle Mountain, where faulting
has dropped the western portion of the mountain with respect
to the smaller eastern portion. He does not recognize the fact
that Smoothingiron Mountain owes its relative elevation to a
downthrown block to the east and that the beds exposed on its
summit are equivalent, where faulting has not cut them out, to
those forming the base of the scarp to the northeast.
3. On page 286 he says:
On the top of Sandy Mountain * * * this Hickory layer has
been considerably altered by heat so as to exhibit in different parts
a gradual transition from above downward, between the compact
massive sand rock and a rock which most lithologists would call a
granite. Similar conditions exist on Sharp Mountain, House Moun-
tain, Smoothingiron Mountain, and elsewhere, but not on Packsaddle
Mountain. * * * It seems clear that the pulsations of the granite
magma produced several broad folds in the Hickory strata, leaving
certainly two great synclinal basins to be afterward partly filled by
the later Cambrian sediments.
* He could not therefore have observed accurately the nature
of the granite intrusions, as is also shown by the following
statement on page 259:
Most of the exposures of granitic rocks in direct contact with the
Potedam sandstone and later strata are of different character from
the Burnetan gneisses * * * and their relations to the overlying
beds show that their eruption has been later than the deposition of
the capping material.
The writer found no instance of contact metamorphism in
any post-Cambrian beds,, nor was any intrusive into these beds
noted. Moreover, the basal Cambrian beds in many places
contain fragments of the underlying igneous rocks.
4 and 5. Comstock divided the rocks he called pre-Cambrian
into an Archean and an Eparchean system; the former contains
two unconformable groups and the latter a third group. Each
of his Archean groups he divided into three series and his
Eparchean group he also divided into three series-nine series
in all. It is impossible to follow his distinctions, nor were such
structures observed by the writer as would warrant any such
detailed division. Though the danger of making long-distance
correlations has been recognized for many years, nearly all his
divisions are compared or correlated with Canadian or other
distant occurrences.
6. In directing attention to prospeting for iron ores he lays
too much stress on the presence of bands of red soil as favor-
able indications and in a mapa he shows a series of straight
lines, or "trends," of economic importance, which do not
represent any structu observed by the writer.
DESRIPION OF THE ROCKS
The Llano-Burnet rion lies within and is naly surrounded
by the lowermost Cretwlos ocks. Within the depreion
which forms this region ancet cystallina schist and granis
are xposd and upon them ar .0 depted unconebitably Cai-
brian, Ordovician, and C ni&oqs strat. The Lower Cre
tacous rocks lie as a banked over these older strat exct
whe the l e been removed by emeion.
COMPAROtN WITH OTHMM RGbIONS.
As the rs exposed -in ihi region rpent 1lic time
intervals extending from possibly the Splint rifled situei-
tation to a relatively gent ologi date (the Cratioeons) it
may be well btefly to compare the natur of the pr~aCsbran
complex and the brad charteristics of the Paleozoi sction
with those of the rocks in similarly denuded ara in neigh-
boring region. Such area are found in the Ozark, Arbuckle,
and Wichita regions to the northeast, in the El Pro and Van
Horn quadrangles to the wet, and at .many localities in
Arizona and New Mexico.
On comparing the central-Texas reion with the areas to the
northeast marked, differences, both in the pre-Cambrian and
Paleozoic rocks, are at once noted. In central Texas the pr
Cambrian complex is made up largely of schist and gneiss,
much of which is of undoubted sedimentary origin. * In the
Ozark region and in the Arbuckle and Wichita mountains the
pre-Cambrian rocks are wholly * igneous, granite, granite por-
phyry, and some gabbroic rocks forming the floor on which the
basal Paleozoic beds were deposited. An examination of the
Paleozoic column reveals also wide differences in the sedimentary
record, that of central Texas showing great gaps that represent
intervals of time during which deposition was not taking place,
or area where erosional unconformities have been produced.
In the Arbuckle Mountains the Upper Cambrian, Ordovician,
Silurian, Devonian, and Carboniferous (Mississippian, Penn-
sylvanian, and Permian) are represented. In the central
Texas region the late Cambrian and early Ordovician rocks are
decidedly thinner than to the northeast, and part. of the
Ordovician, the Silurian and Devonian systems, and the Miss-
issippian and Permian series are lacking. In the Wichita
Mountains of Oklahoma much the same sequence as in the
Arbuckle region is believed to exist beneath the overlapping
Permian red beds, which cover a part of the Ordovician, the
Silurian, the Devonian, and a part of the Carboniferous strata.
The sections in southwestern Texas also show striking differ-
ences from those of central Texas. The pre-Cambrian rocks
of the El Paso quadrangle have been divided into a lower for-
mation composed of 1800 feet of quartzite and a rhyolite
porphyry flow 1500 feet thick resting on the quartzite. On
this rhyolite flow were deposited the basal beds of the Cam-
brian system. In this region, moreover, post-Carboniferous
granite intrudes the Paleozoic formations. In central Texas
there is no post-Cambrian granite. In the Van Horn quad-
rangle, 60 miles east-southeast of the El Paso, metamorphic
rocks probably similar in type to those of central Texas are
reported to occur in the pre-Cambrian complex. In the Paleo-
zoic section the most marked differences lie in the presence in
the El Paso quadrangle of Upper Ordovician and Silurian
rocks. As in the central Texas region, however, Devonian
and. Mississippian strata are absent. The sequence represented
in the Van Horn quadrangle is perhaps, of all those under
comparison, most similar in a broad way to that of central
Texas, for Silurian, Devonian, and Mississippian rocks are
absent and the Pennsylvanian serie is represented. It is true
that the Van Horn area contains Upper Ordovician rocks,
which are not found in central Texas, but it is very probable
that the upper part of the Ellenburger limestone of central
Texas is equivalent to a portion of .the El Paso limestone of
the Van Horn quadrangle. The Van Horn area differs dis-
tinctly, however, from the central Texas region in containing
a thick series of rocks which carrie a fauna not found else-
where in America, having- Permian affiliations, and named
Guadalupiau by G. R. Girty.
AIONWKIAN (R) OCKS.
SUJBDIVI8IONS A2XD GENERAL DISTRIBUTION.
Four principal subdivisions of the pre-Cambrian rocks have
been recognized in mapping this ar-() the Valley Spring
gneiss, which includes acidic gnei, quartzite and its deriva-
tives, light-colored mica schist, and bands of wollastonite, with
some bands of dark basic schist; (2) a'seri of dark-colored,
predominantly basic rocks called the Packsaddle schist com-
prising amphibolite, graphite and mia schist, limestone, and
basic intrusive (some intrusive granite is mapped with e of
these formations) (3) a very ce grained pink gmnite which
could not be separately mappe in some parts of the a and
(4) all the remaining granitic , including a number of
aieties. These -lre all rrded as probably of Algonkian
ae. The Valley Spring iss ad the Packsaddle shist
together compose the Llano sers. In addition to tes nor
distinctions, bads of crystalline Limene and wollastmit
have been separtely mapped wherever piblo. The out
cops of a quartz porphyry of pculi tpeo localy trmed
opalline granite,, a mass of serpenltine (alterd Ooridobti) near
Oxford, a mass of inetadiort, and a fIsite dike t of C1
have also been mapped separately.+
. The a distribution of the gneise.ad hiat iA dvnr
dent primarily on their fmaor sural iatibitsbut i#
modified by gneous intusion. Te mor ai Of i Ildb
haw a geneenl for thtred ke iffhe'con of