596 



INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



5, Above this there are several hundred feet of very massive light limestone, which is clearly 

 the equivalent of the Edwards limestone. 



6. The last and highest of the series is a compact limestone only some half a hundred feet 

 thick, which is the equivalent of the Buda. It is separated from the massive beds below by 

 some 20 feet or more easily weathering ledges which are frequently filled with organic fragments 

 and which correspond to the Del Rio clay. 



At Terlingua both the Buda and the Del Rio zones are much heavier than to the east of the 

 Chisos. These formations clearly thin out or entirely disappear in this direction. 



H 13. TEXAS, NEAR EL PASO, AND CHIHUAHTrA, MEXICO. 



Near El Paso, Tex., occur strata of Cretaceous age, which represent the Fred- 

 ericksburg and Washita groups of the Comanche series. They consist of about 700 

 feet of limestone, shale, and sandstone of which Stanton and Vaughan^*^ gave a 

 detailed section. This section is quoted below, with a correction in the relations of 

 divisions 8, 9, and 10. As observed in the field and as described in the published 

 section these beds were inverted. 



The following is a description of the section beginning with the highest bed : 



Columnar section of Cretaceous near initial monument of Mexican boundary survey, on west side of 



tJie Rio Grande from El Paso, Tex. 



Feet. 



8. Hard limestone with the same species of Exogyra in the top 15 



9 . Clay shales, with bands of limestone nodules, containing great numbers of a large Exogyra 40 

 10. Sandstone, white, yellow, or brownish, with shale beds. . : 200± 



7. Sandstone, white or brown 30+ 



6. Upper part of the bed, clay shales alternating with brown sandy flags. Brown sandy 

 calcareous layers at base, containing Gryphxa washitaensis Hill and Nodosaria texana 

 in great abundance. The G. washitaensis is confined to base; 30 feet above the base, 

 Ostrea subovata; Ostrea qvudriplicata Shum, very abundant between 30 and 40 feet; 

 40 feet above base zone of Kingena wacoensis 60 



5. Flaggy argillaceous limestone, with shale partings. Layer of sandy flags at base. In 

 the sandy flags, Cyprimeria. In the limestone bands Schlcmbachia leoneums, Gryphxa 

 was/jitaensis, Protocardia, Trigonia emoryi. The zone oiG. washitaensis ieskt the top.. 100 



4. Clay shales, with indurated calcareous bands. No fossils were found .' 125 



3. Ledges of hard limestone. Fossils not numerous; at the base there is a considerable 

 number of an undetermined species of Gryphsea, Gryphxa tucumcarii, Neithea, and 

 Schlcenbachia 25 



2. Alternations of clay and soft argillaceous limestone ledges. Fossils: Exogyra texana, 

 Gryphxa tucumcarii, Gryphxa fornieulata, Schlcenbachia peruviana, two other species of 

 Schlcenbachia, and a large Neithea 24 



1. Argillaceous limestone, in thick ledges, weathering into nodular limestone, the nodules 

 surrounded by clay. Fossils: Protocardia texana, Tylostoma pedemalis, Trochus, 

 Turritella, Exogyra texana, an anchuroid genus, two species of echinoids, Pleuromya 

 Pn I. knowltoni, Requienia, etc.; 11 feet from the top Ostrea subovata (?) 79 





698 ± 



Taking the fauna of the section as a whole, its essential identity with that of the noted 

 Tucumcari region in New Mexico and of other localities on the western and northern borders of 

 the Lower Cretaceous area is at once apparent. 



The section studied by Stanton and Vaughan at El Paso occurs in the Cerro 

 de Muleros, which extends into Mexico and has been investigated by Bose,^^ who 

 presents a different interpretation of the sequence. He says : 



We have elsewhere stated that we differ on several points from the ideas of the authors 

 mentioned. The erz'ors of Stanton and Vaughan resulted from the fact that they did not clearly 



