Vol. VIII] DUMBLE— GEOLOGY TAMPICO EMBAYMENT AREA 125 



"This division is much the most important in Chiapas 



It consists of Hmestones and dolomites which generahy occur in 

 quite thick beds and only occasionally as intercalated lenses. 

 Occasionally beds of limestone of brecciated structure are 

 found. In the lower part there sometimes occur beds of lime- 

 stone with chert concretions, but the upper part consists gen- 

 erally only of gray limestone with interbedded dolomite. It 

 may be said that these strata everywhere contain rudistes, 

 especially radiolites." 



He adds that he himself has never observed beds in this 

 vicinity which might with certainty be assigned to the Neo- 

 Cretaceous. 



NEO-CRETACEOUS 



The upper members of the Cretaceous section (Neo-Cre- 

 taceous series of Mexican authors) as determined by Cum- 

 mins from their occurrence in Northeastern Mexico^^ com- 

 prise a series of thin to medium-bedded limestones, with ino- 

 cerami and ammonites, called by him the San Juan lime- 

 stones, overlain conformably by a great thickness of dark 

 shales, without fossils, called the Papagallos. 



The San Juan Hills are made up of a series of thin to 

 heavy-bedded limestones interstratified with thin beds of yel- 

 lowish clay. Toward the base the limestones are shaly, dark 

 gray in color and weather gray to whitish. Toward the sum- 

 mit the limestones are of bluish shade, weathering white. The 

 uppermost beds are sandy and weather to a reddish or rusty 

 brown color. They carry numerous impressions of ammon- 

 ites, oysters and inocerami which are of forms referable to the 

 Taylor or Austin Chalk. 



The Papagallos consists of a series of very fine-grained 

 blue or black limy clay shales, leaching brown, yellow or 

 white. At their northern end, the type locality, and for some 

 distance south, they carry both selenite and barite and break 

 up into slaty particles. When broken down and fully weath- 

 ered, they form a black clay which when wet makes a very 

 stiff mud like the black waxy soils of central Texas. 



The Cretaceous age of the San Juan was fully proved by 

 its fossils and that the Papagallos shales, at the type locality, 

 were also of Cretaceous age was evidenced by the fact that 



'^Tertiary Deposits of Northeastern Mexico, pp. 170 to 174. 



