TABLE OF ROCKS OCCURRING IN

ARKANSAS

I. IGNEOUS ROCKS

NAME

Aplite

Fine, even grained igneous rocks)

Dike

Varieties include aplites, pegmatites, peridotites, etc., which see) Felsite

Fourchite

Granite

(For blue granite, see Pulaskite; for gray, see eleolite syneite; for brown granite, see eleolite syenite dike

rocks)

Hornstone

Kimberlite (Picrite porphyry) (Variety of peridotite)

See page 125

a.

Idiomorphic orthoclase crystals a. Saline County region distinctly microperthitic. Оссаsional small crystals of plagioclase. Quartz in small masses between the feldspars, also minor amounts of bluish green amphibole.

in paleozoic rocks

b. Aplitic Syenite

Intrusive

b. Dominantly orthclase feldspar with b. Saline County region
some plagioclase and minor amounts
of amphibole, crystals mainly allo-
triomorphic

Intrusive rocks with an outcrop long
in one direction and narrow in the
other. Composition variable

Term used for finely crystalline va-
rieties of acid igneous rocks almost
or wholly devoid of phenocrysts;
therefore macroscopically dense in
appearance

Pulaski, Saline, Montgomery, Pike, Conway, Garland, Hot Spring, Scott counties

Of questionable occurrence in Magnet Cove

Distinctly porphyritic, holo crystal- Fourche Mountain region
line, characterized by large augite
crystals imbedded in dull black
ground mass in which are small
grains of titanite and leucoxene

Term erroneously applies to syenite Pulaski,

rocks with granitic texture

Name sometimes applied to dark gray
to black fine grained microcrystal-
line rocks of igneous origin

Saline, Hot

Spring and Garland counties

Magnet Cove and Fourche
Mountain region

Dark colored, somewhat green, heavy Pike and Scott counties
porphyritic rock composed of crys-
tals and grains of olivine, yellow to
brownish yellow mica, yellow grains
of perovskite, some magnetite, in a
dence ground mass largely com
posed of lath-shaped crystals of
augite. Occasional diamonds are
found

NAME

IGNEOUS ROCKS-Continued

DESCRIPTION

LOCALITIES

Limestone-Crystalline (Suggestion is made that this rock is of igneous origin largely because of its association with the other igneous rocks of the Cove region)

Monchiquite

Ouachitite

Pegmatite

Peridotite

Phonolite

Porphyry

Pulaskite (Blue granite)

Pumice

Syenite

Composed of extremely large inter- Magnet Cove region
locking calcite rhombohedrons,
many of which exceed an inch in
cross section. A variety of min-
erals are associated with this lime-
stone, the most conspicuous of
which are perovskite, magnetite,
apatite, biotite, vesuivianite, and
monticellite

Porphyritic combination of olivine,
augite, and a devitrified or glassy
base with which may be associated
hornblende or (and) mica
Similar to monchiquite and Fourchite
except that the mineral olivine is
is lacking and the minerals biotite
and augite are abundant
Term applied to extremely coarse
grained dikes occurring in Arkan-
sas, associated with granitic intru-
sions

Fourche Mountain, Mag
net Cove, Potash Sul-
phur Springs and Saline
County region
Magnet Cove region

Fourche Mountain, Salins
County and Magnet
Cove region

Pike, Hot Spring, Garland. Saline and Scott counties

Granitoid rocks consisting of Olivine
and pyroxene with little or no feld-
spar; variety names depend on the
kind of pyroxene present or on its
absence in favor of related minerals
Rocks of porphyritic or feldsitic text- Magnet Cove region
ture consisting of orthoclase, nephe-
lite, and pyroxene with many other
accessory minerals such as eleolite,
sodalite, aegirite, biotite, melanite,
titanite

Textural term applied to any crystal-
line igneous rocks where large crys-
tals of any mineral are associated
with smaller crystals of the same
species

Pulaski

Saline, Garland, Hot
Spring and
counties

A hypidiomorphic granular or granitic Pulaski county
porphyritic rock composed of vary
ing quantities of the following min-
erals: Orthocla se, hornblende.
augite, biotite, eleolite, sodalite, ti-
tanite, apatite, etc.

Glassy cellular extrusive rock

Granitoid rocks consisting typically
of orthoclase with one or more fer-
romagnesian minerals, occasionally
with some plagisclase. All of the
so-called granites in Arkansas are
really syenites

One occurrence reported in Clay county Pulaski, Saline, Hot Spring and Garland counties

NAME

Tinguaite

Vein

Volcanic Ash

IGNEOUS ROCKS-Continued

DESCRIPTION

LOCALITIES

Spring and Saline counties

Rocks consisting of alkali-feldspar, Pulaski, Garland, Hot
nephelite, and abundant aegirite
which usually occur in dikes
Term applied to a rock similar in
shape to a dike, sometimes of pri-
mary igneous origin but perhaps
more frequently due to other than
igneous action

Composed of material blown from vol-
canoes, usually microcrystalline or
glassy with fragmental or brecciated
appearance

II. SEDIMENTARY ROCKS

localities west

Various
central and southwest-
ern Arkansas

Distributed along stream
valleys throughout the
State, sometimes on the
uplands between
streams in eastern and
southern Arkansas, es-
pecially conspic-
uous along the crest of
Crowley's Ridge
Found throughout eastern
and southern Arkansas
Widely distributed
throughout the Ozark
Mountain region
Ouachita Mountain re-
gion, less conspicuous
Occurrences in various
other parts of the State
Distributed along Crow-
ley's Ridge and adja-
cent to the Mississippi
River, also along the old
Mesozoic shore line
north of Little Red
River

Minerals of Commercial Importance

ANTIMONY

Composition. Antimony is a metallic element. It is usually found in its natural state as sulphide of antimony, or stibnite. It is also found as an impurity in other minerals, such as lead sulphide, silver, and bismuth.

Uses.-Metallic antimony is used principally in the manufacture of lead alloys, such as babbit and hard lead, solder, type metal, bullets, battery plates, Britannia metal, etc. It is also used in the manufacture of vulcanized rubber and rubber goods, enamel on meta! ware, chemicals, paints, and pigments.

Occurrence. The antimony deposits of Arkansas occur in a rather narrow belt of steeply folded Mississippian shales and sandstones (Stanley Shale) in northern Sevier County, southern Polk and northwestern Howard Counties (see accompanying map). The stibnite deposits' are associated with veins of quartz which penetrate the shales and sandstones and have somewhat northeast-southwest trends. The ore is associated with small amounts of copper, iron, zinc, and bismuth sulphides. These were probably deposited by the circulation of underground waters which dissolved the metals from deep-seated volcanic rocks and deposited them along veins a considerable distance above. The quantity of antimony ore in Arkansas has not been definitely estimated. A considerable amount of drilling would have to take place before an accurate estimate of the quantity of available ore could be made. The ore lies in lenses or pockets associated with quartzitic veins which are not continuous for any great distance. Some of the lenticular masses exceed 100 feet in vertical dimension and vary in width from three to forty feet, and vary in thickness from a feather-edge to two and one-half feet. The following are analyses of stibnite samples:

*Sample from Antimony Bluff Shaft

*U. S. Geological Survey Bulletin 340d, "Arkansas Antimony Deposits," by Frank L. Hess.

Mining is carried on by sinking shafts into the ore beds and drifting out from the main shaft. The shafts vary in depth from 20 feet to 220 feet.

Production. The production of antimony ore in Arkansas has gone on since about 1873, when antimony was discovered on the Otto property. Work was commenced that year in the Otto mine and has continued intermittently, depending largely on the market price of antimony. About 5,000 tons of ore have been removed since 1873.

There have been no State figures for the production of antimony since the severance tax law went into effect on April 23, 1923.

The available figures of the Arkansas production as given by the U. S. Geological Survey are as follows:

Following is a list of names of the mines which have numbers corresponding to those on the accompanying map, with approximate depth and probable tonnage, according to E. E. Vaughan of the Gilham Antimony Corporation.

Producers. Gilham Antimony Corporation, E. E. Vaughan. manager, Gilham, Arkansas.