178 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



Fayette 

 Buff sandstone with greenish, sandy clay, hgnitic clays 

 with concretions and some lignite, opalized wood and chal- 

 cedony. Fossils are abundant in these beds along the Rio 

 Grande; the most characteristic being Ostrea alabamiensis 

 contracta, Cornulina armigera heilpriniana Har., Cerithium 

 plicifcrum Heilp. 



Frio 

 Gypseous clays with sands. Clays gray and green in 

 color, often weathering white and containing leaf impressions 

 and ferruginous and calcareous concretions. The fossils, 

 which are not numerous, are oysters of smaller size than 

 those of the Fayette, Corbula, etc. 



AREAL DISTRIBUTION OF THE TERTIARIES 



Midway and Wilcox 

 While there are numerous localities at which it is possible 

 to distinguish the various stages of the Eocene, by their 

 several lithologic and faunal characteristics, it will require 

 more detailed work to show the exact areal distribution of 

 the lower members. This is due to general similarity of ma- 

 terials, scarcity of distinguishing fossils at many places, the 

 successive overlapping of the upper members upon the lower, 

 and the widespread occurrence of the Reynosa, which covers 

 them over many square miles. This applies especially to the 

 Midway and Wilcox, and to the Carrizo Sands of the Clai- 

 borne. For this article, therefore, the area occupied by the 

 deposits of these stages will be treated as a unit, noting the 

 various occurrences of each where identified, but leaving the 

 area as a whole undifferentiated. The area has for its west- 

 ern boundary, along which we may find any or all of these 

 deposits, the Cretaceous-Tertiary contact just described. Its 

 eastern border, which is the line of contact of the Marine 

 substage with the Carrizo Sands, crosses the Rio Grande 

 just south of the mouth of Espado Creek and, running south- 

 eastward by Hidalgo, crosses the International Railroad near 

 Jarita, 16 miles west of Laredo, and the Salado at Los 

 Moros, 10 miles west of Guerrero. It then takes a southerly 



