347 I'^K Oligocene of the Southern United States 37 



by Mayer to the beds from the geographical name of this sedlion 

 in which the upper Oligocene is so typically developed. 



In general the white and the grey limestones of Agen are of 

 freshwater origin, while the sandstone of Bazas is marine. But 

 to the westward, the limestones merge into marine beds. 



White limestone of Agen. The white limestone of Agen, in 

 the vicinity of Bazas, although a thin layer, is rich in freshwater 

 fossils such as Planorbis solidus, P. declivis, Limncsa pachygaster, 

 L. urceolata and Helix. lyayers of clay and sand with a brackish 

 fauna occur near Balizac, where Lutraria sanna, Cyretia Brongn- 

 iarti, Cytherea undata, Circe Deshayesi, Cerithiiwi corrugatiun, C. 

 girondicum and C. plicatiun are also found. 



Near Bordeaux the Aquitanian outcrops along a creek which 

 flows through Saucats, I^ariey and other hamlets. Tournouer, in 

 1862, described the lowest bed as a blue marl with Neritina pi£la, 

 Cerithium calculosum, C. plicatum. C. pseudothiarella and Lucina 

 dejitata. But at the base is a freshwater layer with Helix oxy sto- 

 ma and Planorbis cornu. This indicates that the whole bed, 

 though largel}^ marine, represents the white limestone of Agen. 



The blue marls, characfterized by Neritina piFla, are best ex- 

 posed in a spring in a vineyard near La Salle (commune of La 

 Brede). The owner obligingly dipped out a quantity of the fine, 

 dark blue marl, which was found to be filled with this beautifully 

 decorated species. 



The sandstone of Bazas. The ferruginous sandstone of 

 Bazas^ is quarried in many localities as a building stone. 



The fauna of this horizon is not varied. The most important 

 species zxo. Sciitella Bonali, Psamniobia aquitanica, Ostrea produfla, 

 Ma^ra triangula , Area cardiiformis, Turritella Desniaresti, T. vas- 

 ate?isis, Proto Basteroti, Monodonta Araonis and Ce?'itlmun plica- 

 turn. These fossils usually are found only as internal casts, but, 

 in a few localities, as at Balizac, the shells are well preserved. 



The grey limestone of Agen. The grey limestone of Agen, 

 near Brazas, is a thin deposit charadlerized by Planorbis and 



* Degrange-Touzin, Notes G^ol. sur le Bazadais, C. R. des Seances de la 

 Soc. Linn, de Bordeaux, vol. XLII, 1888, pp. 1-49. 



