. Annual report of the United States Geological Survey to the Secretary of the Interior . ILLINOIS PLATE XUV. 333 PLATE XLIV. Characteristic Fossils of the Austin Chalk. Fig. 1. Radiolites austinensis, 2, 2a, 2b, 2c. Terebratulina guadalupse 3,3a. Ammonites dentatocarinatus 4, 4a. Mortoniceras texanum 2 is enlarged 5 times; other figures are natural U. S. GEOLOGICAL SURVEY TWENTY-FIRST ANNUAL REPORT PART VII PL. XLIV. CHARACTERISTIC FOSSILS OF THE AUSTIN CHALK THE MERIOEN GRAVURE CO. UririrtRYOf THKUNIVERSITY of ILLINOIS HILL.] FOSSILS OF AUST
. Annual report of the United States Geological Survey to the Secretary of the Interior . ILLINOIS PLATE XUV. 333 PLATE XLIV. Characteristic Fossils of the Austin Chalk. Fig. 1. Radiolites austinensis, 2, 2a, 2b, 2c. Terebratulina guadalupse 3,3a. Ammonites dentatocarinatus 4, 4a. Mortoniceras texanum 2 is enlarged 5 times; other figures are natural U. S. GEOLOGICAL SURVEY TWENTY-FIRST ANNUAL REPORT PART VII PL. XLIV. CHARACTERISTIC FOSSILS OF THE AUSTIN CHALK THE MERIOEN GRAVURE CO. UririrtRYOf THKUNIVERSITY of ILLINOIS HILL.] FOSSILS OF AUSTIN CHALK. 335 projected in parallel benches. Even in the thick bed there is muchvariation in their resistance to weather, which is governed by thevarying percentage of siliceous material of which they are composed. From the base upward for about 200 feet the limestone is in itspurest state and occurs in massive beds from 2 or 3 to 10 feet thick,with borders of slightly argillaceous limestone. Mortoniceras texanum(see PI. XL1V, tigs. 1, 4a) and a large species of Inoceramus arefound. Typical exposures of this portion occur along the immediatebank of Brushy Creek from 2i to <6\ miles below Round Rock. The succeeding strata become slightly arenaceous, containing about10 per cent of silica, and occur in slightly thinner beds, which aremuch more fossiliferous than the more chalky layers below. Theseslightly arenaceous beds, the exact thickness of which is not kn
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