The Geology of Texas - Vol. 1 - pages 439-455

Nomenclature.—The name Austin limestone was first used by B. F. Shumard (1463, pp. 583, 585) in 1860. for the limestone typically exposed at Austin, which was located by Shumard correctly above the thinned Eagle Ford (fish bed), but incorrectly below the Comanche Peak formation. Shumard says that the State House and several public buildings in Austin were made of the stone. The "Pinto" limestone of Dumble is a synonym.

⁵¹Literature.—North-Centrol Texas: Alexander, 31b; Hill, 803; Shuler, 1454; Stephenson, 1530; Taff, 1575; Bullard, 177; Lahee, 969. South-central Texas: Adkins, 11, 16; Hill, 803, 808; Vaughan, 1686; Sellards, 1402; Liddle, 992; Taff, 1574. Trans-Pecos Texas: Udden, 1625, 1626. 
Paleontology: Bohm, 126; Heine, 702; Heinz, 703, 703a; Kniker, 947; Lasswitz, 976; Prather, 1256, 1257; Schlöter, 1372; Stanton, 1518; Stephenson, 1540; Adkins, 13; Roemer, 1331; Reeside, 1295a. Igneous rocks: Lonsdale, 1009; Ross, 1353.

 Stratigraphic position and contacts.—East of Sherman a little below the base of the typical Austin there is a fish-bed conglomerate with associated sands and clays (thicker to the east) and pebbles of phosphate, chert and quartz. Stephenson regards this as evidence of an unconformity, and places the "fish bed conglomerate" at the base of the Austin chalk. Stephenson has traced Taff's Fish bed conglomerate from the Red River region southward to Hays County. It contains fossil material reworked from the underlying Eagle Ford, including several kinds of oyster shells and the teeth of several kinds of fish. In Travis and Hays counties small borings extend from the base of the Austin downward into the Eagle Ford clay to a depth of as much as 18 inches and these are filled with glauconitic chalk exactly like the basal chalk layer of the overlying Austin (1539, p. 1328). In McLennan County the Austin-Taylor contact is unconformable, and is marked by a thin but persistent phosphatic conglomerate. At a locality near Bynum, Hill County, the base of the Taylor cuts across bedding planes of the upper Austin chalk. The phosphatic conglomerate at the base of the Taylor has been traced across Hill, McLennan and Ellis counties to the Dallas County line, where it apparently dies out. According to Stephenson, a layer containing numerous specimens of Inoceramus undulato-plicatus Roemer lies at the top of the exposed Austin chalk at Waco but 250 feet below the top at Dallas and well down in the Austin chalk at a place 6 miles northeast of Austin, showing that the unconformity at Waco represents the removal of at least 250 feet of upper Austin chalk (1539, p. 1330). East of eastern Smith County, Austin rests on Woodbine, and east of Upshur-central Gregg counties where the Woodbine disappears, on Comanche rocks.

 Facies.—The type Austin is an alternation of white chalky limestone and limy marl strata with some layers of shelly marl especially near the top. East of Grayson County on the outcrop, clay (Bonham) and sand (Blossom) appear. In the East Texas oil fields east of Smith County the Austin is sandy, and in southwestern Arkansas its partial equivalent, the Tokio, contains much sand interbedded with clay. Likewise in the northern Louisiana oil fields it contains sand. Two other intergrading facies occur in west Texas. From Val Verde-Terrell counties westward to beyond Terlingua, the Austin consists of thin limestone flags, chalkier to the east, more crystalline to the west. Near Terlingua these are thin and are interbedded with much marly material, so as to weather down to flats. To the northwest the formation becomes more marly until at Chispa Summit it consists of marl, with thin subordinate amounts of marly and platy limestone flags. This facies is here called the Colquitt formation.

 Near Pilot Knob, Travis County, the Austin locally occurs in a reef facies, a rather pure white shelly limestone, in part coquina, containing interbedded serpentine at places, with echinoids, caprinids, oysters and various other mollusca.

 Areal Geology.—Eastwards from the eastern line of Grayson County (roughly east of the axis of the Preston anticline) through Fannin, Lamar and Red River 1 counties, the Austin is represented by four equivalents, in ascending order as follows: (1) "Fish bed conglomerate" and associated sands and clays; (2) Ector chalk; (3) Bonham clay; (4) Blossom sand. In the Black Prairie region the Austin is represented by the Austin chalk proper, overlain by the Burditt marl. In Trans-Pecos Texas it is represented by unnamed flags generally similar to the underlying Boquillas but less vividly colored, and farther northwest by an unnamed clay facies. In the Sabinas coal basin (northern Coahuila) a local marginal facies of coarse grits, sands and conglomerates appears.

 Topography and vegetation.—In the humid region, where the Austin is composed mostly of limestone ("chalk"), and typically forms prairies, the central or inner portion of the black land belt, two general topographic types prevail: the canyon topography occurs along streams cutting across the Black Prairie, and the interstream areas are prairie land. Indurated limestone layers produce steep bluffs along streams, but the softer strata weather to more rounded forms, and the canyons are not intrenched. The uplands are widely but shallowly dissected by small headwaters, especially on cuesta slopes. The upland type of Austin is most typically shown on top the White Rock escarpment, as in Dallas and Ellis counties, where through-going drainage is eliminated and only small headwaters remain. Here a deep fertile soil is formed, and the topography is flattened. Near large streams, much soil is removed and bare patches of limestone rock may mark hill slopes. The most characteristic soil of the Austin is the "Houston" clay, typically a dark-brown or grayish-brown clay superficially, grading at a foot or more depth into grayish-yellow clay containing friable limy material, which deeper grades into the chalky marl or chalky limestone of the Austin. Where derived from the chalky limestone, the subsoil is a greenish-yellow, chalky, friable clay, grading into soft, white chalky material at depths of less than 3 feet. The dried soil may be crumbly, and in color light or dark ashy gray. The prairie white rock land originally supported a growth of mesquite grass, with occasional post oak, elm and hackberry. The bare Austin chalk, especially on escarpments, is covered with a thicket growth of small trees and shrubs, "cedar" (== juniper), oaks, elm, hackberry, box elder, red bud, honey locust, sumac, mesquite, smilax, prickly pear and bear grass. The main oaks are Quercus brevilocata (shin oak), and Quercus texana.

 In the semi-arid region the Austin group is largely of a soft facies. At most places it consists of thin interbedded marls and flags and forms flats. In the Chispa Summit district, it is largely marls, and forms miniature bad lands.

 From Fannin County eastwards on the outcrop, the Austin consists of the four units described below. Southwards from Fannin County, these lose their identity, and the group consists of typical Austin limestone overlain by Burditt chalk marl.

 This member was named by Stephenson in 1919 (1530, p. 149). It is the basalmost member having the chalk facies, in this section. From Dallas northwards the Austin chalk has thickened considerably, and in the Sherman-Leonard section in southwestern Fannin County its top includes chalk of Taylor age perhaps continuous eastward with the Gober chalk. East of the axis of the Preston anticline the Austin portion of the chalk changes facies, becoming the Bonham clay, except that its base retains the chalk facies and extends from a point south of Ector northeastwards to near Ivanhoe, Fannin County. This is the Ector chalk member, 50 feet or less thick near Ector. It is underlain by the shaly clay, thin sands and fish bed conglomerate previously mentioned. The shaly clay grades into the chalk through a transition zone only about one foot thick. The easternmost known locality of the Ector (1530, p. 150) is on the old Bonham-Ravenna road about 1 ½ miles southeast of Ravenna. The member contains Gryphaea aucella Roemer and "Radiolites" austinensis Roemer.

 Southwest of Bonham the thick central portion of the Austin chalk changes in a northeast direction into the Bonham clay. It was named by Stephenson (1534, p. 8) in 1927, the type locality being at small exposures a short distance north of Bonham, Fannin County, Texas. It is a greenish-gray waxy clay, weathering yellowish green, about 400 feet thick. A little above the middle there is a stratum of calcareous and strongly glauconitic clay with fragments of Inoceramus large species, Ostrea congesta Conrad and Ostrea plumosa Morton, which Stephenson considers the westward extension of the Blossom sand. To a certain extent the transition from Austin chalk to Bonham clay has been observed in Grayson County east of Sherman. In the area between Luella, Bells and Whitewright, the basal half of the Austin is largely impure argillaceous chalk and chalky clay, and as far west as Luella, highly argillaceous, slightly bituminous shaly chalk or chalky clay. The Bonham clay is mapped eastward, across Fannin, Lamar and Red River counties, to Red River north of Clarksville, between Pecan Bayou and Silver City. In southern McCurtain County, Oklahoma, and in southwestern Arkansas the same member is called the Tokio. The Bonham clay has been traced as a narrow strip through southern Grayson and northern Collin counties as far south as McKinney (Alexander and Smith, 31b).

 This sand, first called by Veatch (1691, p. 25) "sub-Clarksville" sand from wells near Clarksville, was named Blossom by Gordon (609, p. 19), who correlated it with the sandy upper portion of the Eagle Ford at Sherman. The type locality is at Blossom, eastern Lamar County.

 The upper part of the Bonham clay about halfway between Ector and Randolph, southwestern Fannin County, is calcareous and chalky and eastwards is distinctly glauconitic; this develops into brown, sandy, ferruginous glauconitic beds interlaminated with thin clay beds, outcropping as a sandy belt several miles wide. The Blossom outcrop passes eastward across central Fannin, Lamar and Red River counties, through Paris, Blossom, Detroit and Bagwell, and ending at the edge of Red River valley near the mouth of Pecan Bayou.

 Stephenson states that the fossils from the Blossom indicate the equivalence of the Blossom with the upper part, perhaps the upper half, of the type Austin near Austin. At least 11 species are common to the type Austin, and the following indicate synchroneity with the upper part of the type Austin: Inoceramus aff. /. deformis Meek, Ostrea congesta Conrad, Ostrea aff. 0. diluviana Linne, Gryphaea aucella Roemer, Exogyra ponderosa Roemer, Liopistha elegantula (Roemer)?, and Baculites asper Morton. No Eagle Ford species were found. The cephalopoda listed are: Nautilus  sp., "Hamites" sp., Baculites asper Morton, Placenticeras sp., and Prionotropis?.

BLACK PRAIRIE REGION
TYPICAL AUSTIN CHALK

  From eastern Grayson County southwards, the Austin is in general typical. It forms a wide, prominent and important outcrop, the substratum of a part of the fertile black land belt (Black Prairie) devoted largely to cotton raising. On it are located many important towns: Sherman, McKinney, Dallas, Waxahachie, Midlothian, Waco, Temple, Austin and San Antonio. The total chalk in eastern Grayson County is probably as much as 1000 feet thick; near Dallas 700 feet; at Corsicana 425 feet; 480 feet in the Powell field and 440 feet in the Mexia-Richland area (969, p. 331); near Groesbeck, southern Limestone County, 350 feet; in Bell County 550-604 feet; in Milam County about 500 feet; in Williamson County, 325-342 feet; in Travis County, about 420 feet (275-400 in wells) ; in Medina County, about 350 feet; in eastern Uvalde County, 350 feet; western Uvalde County, about 350 feet [F. M. Getzendaner, personal communication].

 Although the literature contains records of 350 to 400 feet of Austin in Bexar County, many geologists now consider that these thicknesses include some rocks of Taylor age. The Austin chalk is stated to be 110 feet thick in eastern Bexar County [L. W. MacNaughton, personal communication], and the 443 feet of Austin recorded (888) in southwestern Bexar County is stated to include the Anacacho. Likewise it is unknown how much, if any, of the upper part of the type Austin chalk in Travis County should be separated from the true Austin. These questions require extensive zonal work and a paleontological redefinition of the type sections.

 At the type locality the lower two-thirds of Austin consists of irregular strata of variable thickness, from thin-bedded to massive, and with often indefinite limits, generally alternately harder and softer. They are composed of a gray-white chalky limestone in the harder layers, and a dark blue or blackish marly limestone or limy marl weathering dead white or light gray, and in texture unevenly flaky or laminated. A few of the limestones are indurated, some are shelly. Some contain much debris of oysters, inocerami and other shells. At certain levels considerable glauconite, dispersed as small specks, occurs; the formation contains imbedded balls, cylinders and irregular botryoidal masses of pyrite with radiating internal structure; and locally veins, seams and joint cracks filled with calcite. In youthful stream cuts vertical cliffs of alternately projecting and receding strata occur; on hillsides and upland prairies a rounded topography prevails. On many patches of upland, headwater erosion is sufficiently rapid to strip the outcrop of soil. 

Generally harder and softer ledges are not topographically well expressed. The upper part of the formation has considerable calcareous marl, in beds up to 30 feet thick, and some very shelly marl (with Exogyra), generally in beds 5 feet or less thick. Such a marl in northern Travis County seems to be characterized by the presence of Exogyra tigrina Stephenson and "Ostrea" centerensis Stephenson, and contains other species listed below. A notable feature of this level is the presence of a wide range and variety of Exogyra, probably referable to several species.

 The Austin consists of beds of impure chalky limestone, containing 85 per Cent or more of calcium carbonate, interstratified with beds of softer marl. It is usually of an earthy texture, free from grit, and on fresh exposure softer, so that it can be cut with a hand saw, but on exposure more indurated. In thin slices the material shows calcite crystals, particles of amorphous calcite, finely crystalline calcareous material, foraminiferan shells and fragments, fragments of the prismatic layer of Inoceramus often in great abundance, debris of pelecypods, gastropods, echinoids, and other organic fragments. The material has the typical crystalline structure of limestone. Some slices show abundant glauconite specks; some show a sparse to medium amount of "spherical bodies" (see page 365) ; and some show a finely crystalline texture almost devoid of organic material. Typical analyses show calcium carbonate 82 per cent; silica and insoluble silicates 11 per cent; ferric oxide and alumina 3 per cent; magnesia 1 per cent.

 The water-filled subterranean chalky limestone is usually of a blackish-blue to bluish-gray color, as in most cores. The air-dried material is generally glaring white and of a matte texture. The dried marls may be more blackish or bluish. They weather mostly into abrupt slopes capped by harder ledges. Some ledges become indurated and crystalline; others, less crystalline, weather into irregular small conchoidal flakes with an earthy fracture. The harder strata have an irregular, ragged conchoidal fracture. Locally in the more massive layers, there occurs a large conchoidal flaking, superficially resembling exfoliation. On prolonged disintegration, the Austin weathers into a black residual soil, characteristic of the Black Lands belt of east-central Texas. Locally as near Pilot Knob, the Austin is metamorphosed, and occurs as a porous, redeposited and recrystallized limestone in medium beds, soft enough on fresh exposure to be sawed, nearly pure, and producing an excellent building stone. The German Lutheran Church just north of the Capitol at Austin is built of this stone. Formerly the ordinary Austin was somewhat used as a building stone, but its softness, marly partings and iron stain make it less desirable than other stones available in central Texas.

 The outcrop covers the southeastern one-fourth or more of Grayson County. Marly lime is more prominent in the lower half of the formation, and medium bedded to thick massive chalky limestone in the upper half. In Dallas County the base of the chalk capping soft upper Eagle Ford shales, clays and flags, forms the prominent west-facing White Rock escarpment which proceeds southwards to the Brazos. It is well exposed in road cuts and cement plants on the Fort Worth road about 5 miles west of Dallas.

 In the quarry of the Texas Portland Cement Company 3 miles west of Dallas, the base of the Austin chalk, just above the Eagle Ford contact, is marked by a layer of phosphatic pebbles (1530, p. 148, and PL XXVII-A). The basal part, 150 feet of the formation, consists of heavy-bedded, massive chalk layers separated by thin shaly layers, the most resistant beds being contained in the basalmost 50 feet (1454, p. 19). The basal part contains an abundance of nodular, spherical or cylindrical masses of pyrite.

 The middle part, about 250 feet thick, has fewer massive layers, and is characterized by thick, and often indurated shaly layers which show a fine lamination. This part does not show in stream cuts as marked expression of projecting and receding layers as does the basal part. The uppermost part contains more shaly limestone and less chalk. The colors are predominantly blue and yellow. Some sandy strata occur. At the Austin-Taylor boundary, there is a sharp transition from massive flaggy chalk (containing "large ammonites,' most of them Parapuzosia) to gray shale, some of it calcareous.

 In the Waco region the basal chalk is well exposed in Cameron Park. Here it consists of medium to thick massively bedded strata with some alternating receding ledges. By undercutting of Bosque River large blocks fall down the slopes and disintegrate. Flaking and exfoliation are extensive. In the cuts of Brazos River across the Austin chalk outcrop, considerable small scale faulting, with the development of V- and A-shaped grabens and horsts, is present. Such local faulting occurs in White Rock Creek near the Harrington well.

 The base of the chalk, and locally the hard Eagle Ford flags, form the crest of the west-facing Bosque escarpment across McLennan  County. The Bosque is deflected parallel to this scarp and follows it northward to the point where the Brazos cuts through the scarp. Along the scarp, there is considerable faulting in disconnected lines. The medial part of the chalk consists of medium bedded chalky limestone softer marly layers, weathering white. The Austin-Taylor contact occurs in and near the Baylor University Campus. Here the topmost chalk stratum is a massive chalky limestone containing Inoceramus undulato-plicatus and other species, and is followed with a sharp lithological break by blue-black Taylor shale. Stephenson considers that about the upper 250 feet of chalk is here absent by subsequent erosion. The top of the chalk occurs in several creeks just east of the Waco-Austin highway. In these creeks Dr. Pace has demonstrated a persistent line of faulting, possibly the eastern limit of the Balcones fault zone in this area.

 Southward from Waco, the typical hard Austin chalk is overlain by a chalk marl of variable thickness, generally referred to the Taylor because of its foraminifera. In or near the base of this marl at many localities, large ammonites of the genera Parapachydiscus and Parapuzosia occur, and their zone of abundance marks approximately the top of the Austin chalk. Unfortunately their exact zonation has not yet been published. They occur in several localities southwest of Robinson and Rosenthal, and east of Temple, west of Holland in the branches of Darr's Creek, north of Austin in Big and Little Walnut creeks, on the Rio Grande at Tequesquite Creek between Del Rio and Eagle Pass, and near San Carlos, Coahuila. Some of these "cart-wheel" species reach large size; two of them have been described from Tequesquite Creek by Scott and Moore. The eastern border of the chalk is exposed near Garland, east of a line between Waco and Eddy, in Deer Creek, in northern Bell county near Little Flock Church east of Temple, in various creeks west of Holland, in Brushy Creek south of Hutto, and in various creeks in Travis County.

 In San Antonio the excavations in Brackenridge Park represent the Austin chalk at levels between 100 and 150 feet above the base. The rock is evenly bedded in strata of from six inches to several feet in thickness. It is light gray, tinged with yellow. Oxidized pyrite nodules are present. Near the base of the quarry, a layer rich in Gryphaea shells occurs. In this county the upper 200 feet of the Austin is a soft bluish calcareous clay or mud. In these beds Stephenson found Scaphites sp., Placenticeras sp., and a large Baculites.

 The Austin in Medina County (992, pp. 46-48) consists of  alternations of soft chalky limestones and marls, argillaceous shales, or clays. Near the top the formation is mostly chalky limestone. The basal half of the Austin as far west as Hondo River contains ledges which are highly glauconitic. The upper part especially has vertical cylindrical concretions of pyrite with radiating structure. The basal 75 feet is quite highly indurated and lithologically somewhat resembles the Buda, but lacks the calcite veins characteristic of that formation. At a level about 275 above the base there is a layer of Gryphaea aucella shells, about 4 to 5 feet thick, which persists across Medina and Bexar counties, and is possibly the same as the prominent Gryphaea layer above the middle of the formation at Austin.

 In the Uvalde area the Austin consists of soft, white and yellow chalky limestone and limy marl alternating. Two large exposures are on the east side of Nueces River opposite Soldiers Camp Spring, where the upper 150 feet is exposed; and a basal 200 feet in the high bluff on the west side of the Nueces between the Southern Pacific Railroad and the West Nueces. Gryphaea aucella Roemer, Inoceramus aff. digitatus Sowerby, and Mortoniceras texanum (Roemer) are recorded from the Austin in this area.

BURDITT MARL

 Nomenclature.—Hill included this chalk marl with the Austin chalk, and stated that the top is transitional to the Taylor. Taff (1574, p. 353) segregated the upper marly lime zone of the Austin chalk, and considered it lithologically transitional to the Taylor marl. This chalk marl is here called Burditt, from Burditt School, Travis County, and the type locality is along Little Walnut Creek downstream from the Austin-Cameron road.

 Stratigraphy.—Stephenson states that the Taylor in McLennan County unconformably overlies the Austin, and that its base contains a phosphatic pebble zone. There appears to be no marked

 break in Travis County, although a prominent layer of phosphatic nodules and fossils occurs in the Burditt near the type locality.

 Areal geology.—In south-central McLennan County above the solid Austin chalk, there are chalk marls some of which may be of the same age as the Burditt in Travis County. In Bell County the solid chalk is overlain by a chalk marl which contains many thickshelled Exogyra, some Ostrea centerensis, Parapuzosia, and several other fossils. This chalk marl outcrops in Deer Creek, the headwaters of Little Elm Creek, in Cottonwood Creek, and in the branches of Darr's Creek west of Holland. Only about 35 feet has been recorded from The Pure Oil Company Hill core test west of Rogers, but it seems to be thicker on the outcrop. At many places in this zone in McLennan, Bell, and Travis counties large Parapuzosia occur, and seem to be distinctive of the level.

 Taff records the zone from the high bluffs on San Gabriel River, between 1 and 2 miles below Jonah. Lithologically it is transitional to the Austin chalk. At least 20 feet of marly, chalky lime occurs, interbedded with marl, and with some thin seams of indurated sandy marl. Taff records that this marl contains "large Exogyra ponderosa, an oyster resembling 0. subovata, and a small narrow-beaked oyster." A sandy flagstone layer above the marly lime is regarded by TafF as the top of the Austin.

 At the type locality on Little Walnut Creek about 5 mfiles northeast of Austin, the Burditt marl is about 40 feet thick. It is a lightgray, somewhat shelly, calcareous clay overlying the hard chalk. Several species or varieties of Exogyra (laeviuscula?, ponderosa, tigrina, and costate, subcostate, imbricate, spinose, and subcancellate kinds), cart-wheel ammonites (mostly Parapuzosia), Mortoniceras, Glyptoxoceras, Nautilus (Eutrephoceras) and "Ostrea" centerensis occur here.

 At the crossing of the new Del Rio-Eagle Pass road over Tequesquite Creek, Exogyra tigrina and various large ammonites including Parapuzosia similar to those in the Burditt marl occur in the limestone. At localities between San Carlos and El Moral, Coahuila, exceptionally large ammonites occur at this level.

 A soft, thin member at the top of the Austin chalk in Medina and Bexar counties is likely equivalent to the Burditt.

 This unit is distinguishable lithologically from McLennan County probably to Medina County. Paleontologically it is approximately the zone of large cart-wheel ammonites (Parapuzosia).

 Paleontology.—This unit is the zone of abundance of Parapuzosia sp. and the oysters Exogyra tigrina Stephenson, Exogyra n. sp. (called the "Chalk Ponderosa"), and "Ostrea" centerensis Stephenson. In addition it contains fossils of undetermined range: Nautilus sp., Mortoniceras sp., a pachydiscid, Neancycloceras?, Baeulites, Exogyra ponderosa, E. laeviuscula?, and several other mollusca. The foraminiferal fauna includes a large proportion of species common to Austin and Taylor, and apparently a few Austin markers, according to Mrs. Helen Jeanne Plummer.

 West of the Pecos, the Austin outcrops in four general areas. In Terrell County there is a considerable area of Austin chalk in thin to medium bedded white chalky limestone ledges along the Shumla-Dryden highway. These beds, perhaps 200 feet remaining, contain typical Inoceramus and other fossils.

 In Brewster County, in the Chisos and Terlingua quadrangles the Austin is irregularly exposed, and forms the lower part of Udden's Terlingua formation. The Austin locally may be considered bounded below by a widespread caprock of siliceous limestone in medium or thin-bedded flags, containing Crioceras n. sp. (with short spines), a belemnite, one or two species of Scaphites, and some unidentified discoidal ammonites. It outcrops on the hill north of No. 16 head shaft of the Chisos Mining Company at Terlingua, at the south end of Grace Canyon, at many places between Terlingua and Hen Egg Mountain, on the north and west slopes of Mariscal Mountain, and elsewhere. The Austin extends up to and grades into typical Taylor clay. In lithology it is entirely different from type Austin, but somewhat similar to Val Verde or Boquillas flags. It consists of thin to medium bedded, laminated, slightly shaly limestone flags, blackish-blue to gray interiorly, weathering to whitish-gray in the more calcareous layers, blackish-blue in the more shaly and carbonaceous layers, brittle, jointed vertically, and breaking into diamond-shaped blocks of various sizes or in more indurated flags weathering to large, hard, ringing slabs. Near the top there is an alternation of softer shaly and harder limy strata, which on domes weather to circular benches with low infacing cuesta faces and long outdipping back slopes. The formation is less resistant, and forms less prominent hills, than the underlying Boquillas flags (Eagle Ford); it is more resistant than the Taylor, which weathers to clay flats and slides, bad lands and outlier topography. Among the fossils are: Mortoniceras several species, Baculites, Inoceramus subquadratus Schliiter, Inoceramus several species, Durania cf. austinensis (Roemer), and others. 

 A notable area of Austin is in the southeastern part of the Chisos quadrangle along the Rio Grande between San Vicente and the mouth of Tornillo Creek, where numerous Inoceramus cf. digitatus Sowerby occur. In the Tierra Vieja, Van Horn and Eagle Mountains, Austin formation outcrops. Between Chispa Summit and the abandoned San Carlos coal mine, it is in almost entirely a clay facies, with small intercalated bodies of thin limestone flags. This formation, about 1200 feet thick, bounded below by the Chispa Summit (Eagle Ford) and above by Taylor clays, is here called the Colquitt formation. The type locality is on the Colquitt ranch below Chispa Summit, western Jeff Davis County.

 In northern Brewster and western Pecos counties, Austin chalky limestone occurs on the east slope of the Davis Mountains beneath the Taylor clay.

 Paleontology.—But little progress has been made toward a zonation of the Austin chalk and its equivalents. The following are some fossils from the formation with their probable ranges. The earliest Mortoniceras52a occur in the Austin; the genus is represented by a complicated, as yet unpublished, flowering of species as in the Washita genus Pervinquieria. M, minutum Lasswitz is common; M. quattuornodosum Lasswitz and M. texanum (Roemer) are rare. The rudistids Durania and Sauvagesia are common in Austin and Taylor. Inoceramus subquadratus Schliiter (often misnamed /. crippsi) is perhaps the commonest Chalk Inoceramus. A rarer tall form is /. subquadratus var. austinensis Heinz (703a). Practically typical /. digitatus Sowerby occurs in the Big Bend. /. undulato-plicatus Roemer characterizes the upper part of the Austin

^52aThe ammonite genus Mortoniceras as applied to Austin chalk and higher species has been renamed Texanites by Spath (1511b), but for the convenience of readers the older name is retained in this discussion.

 chalk, but its total range is unknown. At many levels both basal and upper, forms like Haploscapha grandis Conrad and H. niobrarensis Logan occur. Other rather characteristic species are Pecten bensoni Kniker, Spondylus guadalupae Roemer, Liopistha elegantula (Roemer), Gryphaea aucella Roemer, Hemiaster texanus (Roemer), Exogyra laeviuscula Roemer, Exogyra n. sp. Bose (so-called "Chalk Ponderosa," practically confined to the Austin chalk marl at the top of the  formation), Exogyra tigrina Stephenson, "Ostrea" centerensis Stephenson, and others. In a lowermost portion in Travis County, no Mortoniceras have yet been reported; to this portion likely belong: Phlycticrioceras n. sp., Parapuzosia aff. stobaei, and Coilopoceras austinense Adkins. This portion contains large Inoceramus (up to 2 feet in diameter).

 A middle portion of the chalk contains Mortoniceras minutum Lasswdtz, and several species similar to it, M. planatum Lasswitz, M. quattuornodosum Lasswitz, M. texanum (Lasswitz), M. aff. emschere, and other species, Parapachydiscus (small sp.), Parapuzosia aff. corbarica, Barroisiceras dentatocarinatum (Roemer), pelecypods and echinoids, and a new ammonite genus closest related to Mortoniceras.

 Near the top of the chalk there are found: Parapuzosia (large species), Parapachydiscus (large species), "Hamites" Baculites, and other ammonites. The overlying chalk marl with Exogyra n. sp. (Chalk Ponderosa), Exogyra tigrina and "Ostrea" centerensis, contains Baculites, "Hamites" and Parapachydiscus (small sp.).

 The following species are on record from western Texas: (a) probably from the basal Austin equivalents, Coniacian = Spath's Gauthiericeratan age, Peroniceras aff. czörnigi (Reeside, from Seco Creek and Tequesquite Creek); P. aff. czörnigi, with very prominent alations (Arick; Capote Ranch), P. aff. westphalicum (Reeside; Uvalde County), Gauthiericeras aff. margae (Terlingua area), (b) From upper levels, Lower Santonian = Mortoniceratan, Parapuzosia americana Scott and Moore, P. bosei Scott and Moore (Tequesquite Creek). Canadoceras flaccidicostum (Roemer) and Turrilites wysogorskii Lasswitz also probably are Austin chalk species. Some of these ammonites were described by Reeside (1295a).

 Of the rudistids, Sauvagesia aff. degolyeri Stanton, S. acutocoslata Adkins, S. aff. belti Stephenson, and Durania austinensis (Roemer) occur in the chalk. Fish and mososaur remains have been found in the Austin chalk.

The following are among the most distinctive Austin ostracoda:

Foraminifera do not in general afford an exact distinction of Austin from either Eagle Ford or basal Taylor; however, the following foraminifera are found in the Austin chalk in central Texas:

⁵⁸ in this list, prepared by Helen Jeanne Plummer, the species marked (*) are thought to be most characteristic of the Austin.