EOCENE HISTORY OF MEXICAN CULF COASTAL AREA 489 



overlapping them to contacts with the Escondido beds and the 

 Papagallos shales of the Cretaceous. 



South of the Rio Grande the Carrizo sands are well exposed 

 and form the crest and eastern slope of various small groups and 

 ranges of hills lying between the Rio Grande and Azulejo and of 

 the larger range south of that point, known as the Ceja Madre. 

 These hills have a total length of about ninety miles and reach 

 nearly to the Salado River. 



South of the Salado the Carrizo is not so prominent and was 

 observed only in a few localities. 



The Marine beds in places, as on the Rio Grande and the 

 Atascosa, are directly connected with the Carrizo sands by transi- 

 tional beds of lignitic shales, but at other places, and especially 

 in east Texas, no such connection can be found and beds of green- 

 sand or of brown sandstone are directly superposed upon the 

 Carrizo, although usually without apparent unconformity. Local 

 movements during Marine time are, however, registered in the 

 sediments in some parts of the area. 



The Marine beds have a thickness of 800-1,000 feet. The 

 eastern portion of the deposits seems to have been most highly 

 glauconitic, although the beds everywhere give evidence of the 

 presence of this mineral in greater or less quantity. 



In the eastern portion of the state the alteration of this glau- 

 conite has resulted in the formation of extensive deposits of excellent 

 iron ores, but in the western portion the resulting ferruginous 

 matter is widely disseminated and simply forms a cement for the 

 brown ferruginous sandstone which covers such a great area in 

 Southwestern Texas. 



On the Rio Grande the carbonaceous shales of the Carrizo find 

 their culmination in the San Pedro and San Tomas beds of cannel 

 coal between which are beds bearing characteristic Claiborne 

 fossils. 



The exposure of the Marine beds on the Rio Grande has a width 

 of more than fifty miles. They narrow rapidly to the south and 

 where they cross the Salado west of Guerrero the outcrop is not 

 more than five or six miles wide. 



