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



to the hills north of Azulejo, where it turns and runs a 

 little east of south, crossing the Salado River near Rod- 

 riguez, the Sabinas near Piedras Pintas and the Salinas at 

 Ramones. It was not traced between the Salinas and the 

 Conchos rivers, but it was found on the latter stream near 

 Panalito and traced in a general way southeastward to the 

 Zarzizal. Actual contacts were found in a number of places 

 and the relations determined in others by such proximity of 

 the deposits of the two formations as renders the line here 

 given a fair approximation of the existing conditions. 



The upper beds of the Escondido formation are well ex- 

 posed on the Rio Grande between Las Isletas and the 

 Arroyo Caballero. They consist chiefly of greenish blue 

 shales with calcareous bands overlain by brownish ferru- 

 ginous sandy clays and sands. The calcareous bands of 

 the lower division carry Voliitonwrpha texana Con., Buc- 

 cinopsis parryi Con., and other gasteropods in large numbers. 

 The overlying sands carry Sphenodiscus pleurisepta Con., 

 and large Turritella. The following section was made a 

 few yards above the mouth of the Arroyo Caballero : 



Feet. 



Alluvial material 4 to 6 



Tertiary : 



Yellowish clay 6 to 8 



Hard bluish gray sandstone with Ostrea 



pulaskensiSj Turritella, etc 6 



Cretaceous : 



Bluish sand with Sphenodiscus pleuri- 

 septa - 6 



Black hard sandstone 2 



Laminated blue clay 6 



Other exposures showing similar contacts were seen in 

 the vicinity.^ No evidence was observed of erosion of the 

 Escondido beds before deposition of the Midway, but the 

 beds of the two formations dip at different angles and to 

 the southward the Midway is found in contact with what 

 are seemingly lower horizons of the Escondido and with the 



4 Note — For details of similar contacts on the Texas side, along the river bluffs and 

 in the uplands, see Stephenson, L. W., "The Cretaceous-Tertiary Contact," U. S. G. S. 

 Prof. Paper 90-1, 1915. 



