THE CHOCTAW AND GRAYSON TERRANES OF THE 



ARIETINA. 



RY F. W. CRAGIN. 



The " ram's horn oyster," Exogyra arietina, F. Roemer, 

 is the characteristic fossil of a column of sediments, the 

 so-called Exogyra arietina marl, that in Hays, Travis and 

 Williamson counties, Texas, consists mostly of calcareo- 

 argillaceous, and more or less ferruginous marl, and attains 

 a thickness of sixty to eighty feet, occupying the interval be- 

 tween the top of the Washita limestone of Shumard and the 

 base of the Shoal creek limestone of Hill. This column was 

 recognized by Dr. Slnimard as an important member of the 

 Cretaceous section of Texas, and was named by him, the 

 Exogyra arietina marl; and this name, either as it originally 

 stood or under such slight variations of form as " the Exogyra 

 arietina clay," "the Arietina marl," etc., has been used for it 

 by most later writers. The same formation outcrops, with 

 more or less interruption by faulting, mantling, etc., and with 

 more or less variation in thickness and lithological character 

 southwestward to the region of El Paso. Its detail in this 

 direction being less known than that of its northward exten- 

 sion, will not here be discussed. From Austin to the Red 

 river valley in Cooke and Grayson counties, the Arietina be- 

 comes, as Taff has shown, gradually reduced in thickness and 

 decidedly more calcareous. For this calcareous northern 

 phase of the Arietina which, in the Red river valley, occupies 

 the entire interval between the summit of the Pawpaw clays 

 of Hill and the base of the Dakota sandstone, Hill has recently 

 proposed the name, Main-street limestone.* 



The Main-street limestone, however, consists of two mem- 

 bers. Its dual character has been independently determined 

 in the field by the present writer. But the members that 

 compose it were first recognized as terranes by Taff in his 

 second Report on the Cretaceous Area North of the Colorado 



* Bulletin of the Geological Society of America, V., 330. 



