Vol. V] DUMBLE— TERTIARY DEPOSITS IN NORTHEASTERN MEXICO 159 



CRETACEOUS FLOOR 



The deposits of Cretaceous age which appear in this 

 area in connection with the Tertiary are : The Escondido 

 Beds, the Papagallos Shales and the San Juan Limestones. 



Escondido Beds 



These beds as described from the Rio Grande section^ 

 comprise the materials lying between the top of the Coal 

 Series (that division of the Taylor marls which contains 

 the coal deposits of Eagle Pass, Fuente, Sabinas, Esperanza, 

 etc.), and the basal Tertiary and consist of alterations of 

 clays and sands, more or less glauconitic, with an abundant 

 and characteristic fauna, which is as yet only partially de- 

 scribed. Sphenodiscus pleurisepta Con., and Ostrea cortex 

 Con., are probably the most abundant species in it, although it 

 also carries a large gasteropod fauna, including Buccinopsis 

 parryi Con., Volutoinorpha texana Con. 



On the Rio Grande the rocks of this formation extend 

 from Eagle Pass to the mouth of Caballero Creek, where 

 we found its contact with the Midway, or basal Eocene. 

 This contact was traced southward as far as the Rancho 

 del Pescado, a few miles southwest of the Laguna de la 

 Leche, where we found the last exposure of the clays with 

 Ostrea cortex. It is probable that the brown clays and shales 

 occurring on the Salado River and tributaries north of 

 Rodriguez, may belong to the Escondido, but we found no 

 fossils that would enable us to place them there with cer- 

 tainty, nor were we able to recognize them farther south. 



The Escondido is the latest Cretaceous known in the 

 Texas area. In its lower part it carries Exogyra costata, but 

 this is absent from its upper beds. Stephenson says of it:^ 



"In this connection it should be stated that the Rancocas 

 and Manasquan formations of New Jersey, which carry only 

 a meager fauna, are thought to be somewhat younger than 

 the Exogyra costata zone, and the upper part of the Escon- 

 dido formation of southwestern Texas may be a little younger 



\ 1 Dumble, E. T., Notes of the Geology of the Valley of the Middle Rio Grande, 



Bull. Geol. Soc. of A., Vol. 3, p. 227, 1892. 



2 Stephenson, L. W., "Cretaceous-Eocene Contact," U. S. G. S. Prof. Paper 90-T, 

 p. 157, 1915. 



