File:Annual report of the United States Geological Survey to the Secretary of the Interior (1899) (14576848638).jpg

Identifier: annualreportofun2171geol (find matches)
Title: Annual report of the United States Geological Survey to the Secretary of the Interior
Year: 1880 (1880s)
Authors: Geological Survey (U.S.)
Subjects: Geological Survey (U.S.) Geology Water-supply Forest reserves Mines and mineral resources
Publisher: Washington : G.P.O.
Contributing Library: University of Illinois Urbana-Champaign
Digitizing Sponsor: University of Illinois Urbana-Champaign

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clay into soil. Laminations alsoshow in the clay on freshly weathered surfaces. These are especiallyapparent in the upper portion of the clay, where the flaggy layersoccur. The chief and most readily distinguishable feature of the clay,wherein it may always be easily identified, is the presence in enor-mous quantities of the unique and easily recognizable fossil known asExogyra arietina Roemer. This little oyster occurs by the millionsin certain portions of the clay, either as consolidated sheets of agglom-erate or as separate individuals, which often weather out so abundantlyas to cover the entire surface of the exposure. (See Pis. XYII andXXXVIII.) Attached to these shells, especially to the umbonalregion, are small cubes of iron pyrites, the decomposition of whichhas been described. Paleontology and fossil horizons.—This clay is abruptly limitedabove and below by limestone formations. The limestone upon which U. S. GEOLOGICAL SURVEY TWENTY-FIRST ANNUAL REPORT PART VII PL. XXXVIII
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EXOGYRA ARIETINA AGGLOMERATE NEAR ROUND ROCK, TEXAS. LLINOIS HILL.) FOSSILS OF THE DEL RIO FORMATION. 285 it rests is usually a massive stratum largely made up of the fossilKiiKjrna iracoensis. This limestone is stratigraphically the top of theGeorgetown limestone, but the fossil zone which it represents is thebase of the Main Street limestone of the Red River section. The Kingena limestone horizon is compact, while the clay is friablein structure and disintegrates readily when exposed. The fossils ofthe limestone extend for several feet up into the clay, mingling freelywith Exogyra arietina. The characteristic Exogyra arietina occurswherever the bed is found and throughout the clay, but its zone ofgreatest prevalence is near the center of the bed. Through a rangeof nearly 15 feet from the center of the bed upward this beautifulfossil is found in countless numbers in the clay and forms masses ofreck. Above the culminating zone of Exogyra arietina the clays aresomewhat barren of fossil

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