This paper presents a zonation of the Austin chalk and the Burditt marl, divisions of the Austin group in the Jonah quadrangle, Williamson County, Texas. The Austin chalk consists of the Inoceramus subquadratus Schlüter faunizone, Gryphaea wratheri Stephenson faunizone, Inoceramus undulatoplicatus Roemer faunizone, Hemiaster texanus Roemer faunizone, Exogyra laeviuscula Roemer faunizone, and the Exogyra tigrina Stephenson epibole. The Burditt marl contains the Ostrea centerensis Stephenson faunizone. The Austin-Taylor contact has been traced from the southeastern to the northeastern corner of the area. The relations of the Taylor marl and some of the Terrace gravels are discussed. The outcrop, lithology, and paleontology of the Eagle Ford shale, which underlies the Austin group, are also included. / text