EOCENE HISTORY OF MEXICAN CULF COASTAL AREA 491 



which is one of its most characteristic forms. On the Nueces 

 and Rio Grande we find, besides this oyster, numerous other forms 

 which Hnk it positively with the Claiborne. This is especially 

 true of the exposures on the Rio Grande which have a width of 

 fully forty miles with many fossil beds. These include: 



Venericardia planicosta, Lam. 

 Anomia ephippioides, Gabb. 

 Leda opulenta, Con. « 

 Crassatella protexta, Con. 

 Tellina mooreana, Gabb. 

 Cytherea bastropensls, Har. 

 Conus sauridens, Con. 

 Voluiilithes petrosus, Con. 

 Levifusus trabeatoides, Har. 

 Lacinia alveata, Con. 

 Pseudoliva vetusta, Con. 

 Turriiella nasuta, Gabb. 



South of the Rio Grande it is found in a broad belt extending 

 southeastward by way of Mier, Aldamos, China, and Mendez to 

 the Conchos and beyond as far as the Chorreras, where it finally 

 passes under cover of the Oligocene. 



Between the Conchos and the Chorreras the Fayette appar- 

 ently forms a portion of Mt. Corcovada east of Burgos, being 

 elevated by a basaltic extrusion. 



The Frio clays which follow are found overlying the Fayette 

 sands from the Conchos River to the Guadalupe. On the Conchos 

 and on the Ramones Mountains lying north of it they show a con- 

 siderable thickness of yellow and darker clays which weather white 

 and are accompanied by considerable quantities of disseminated 

 gypsum and also beds of this material. Farther north the Frio 

 beds are not so thick and in many places only a few feet of the 

 formation are found between the sands of the Fayette and the 

 overlying Oakville sands of Miocene age. Such fossils as have 

 been found are apparently of Claiborne age and the clays are, 

 therefore, referred to that stage. 



The Middle Eocene of this Gulf area represents a succession of 

 alternating marine, lagunal, and swamp conditions. The marine 

 conditions were most constant in the Rio Grande region and farther 



