120 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Seh. 



I. C. White^° quotes the opinion of Dr. C. W. Hayes, sug- 

 gesting a Laramie age for the San Felipe and Valles beds. 



A number of the geologists who have worked in this area 

 and collected valuable data have been unable to publish it be- 

 cause of the character of their engagements. The writer 

 thankfully acknowledges the assistance through co-operation 

 and criticism of a number of these gentlemen. 



General Features 



At first appearance the geology of this area does not seem at 

 all complicated, but some misunderstanding and confusion have 

 arisen from the fact that through the entire area the predom- 

 inating material entering into and forming the floor of this 

 Tampico embayment is blue shale. At its northern end the 

 shale was proved to be Cretaceous by its position and as it was 

 unfossiliferous and little physical change was observed, this 

 interpretation was applied to cover all similar shales found 

 south of these. But, it transpires that in addition to these 

 Cretaceous blue shales there are also blue shales of Eocene and 

 Oligocene age and these predominate south of the Tamesi 

 River. 



The eastern face of the great plateau is composed of lime- 

 stones of Meso-Cretaceous age and the Rudistes limestones of 

 Micos canyon are found as far south as Chiapas. The dis- 

 turbed area at the foot and immediately in front of the main 

 mass shows the Meso-Cretaceous limestones folded, faulted, and 

 overlain by later beds which are also folded. From the San 

 Juan Hills in Coahuila to Aquismon in San Luis Potosi these 

 overlying beds appear to belong to the upper or Neo-Cre- 

 taceous. 



The beds found overlying the Meso-Cretaceous of the hill 

 country south of Aquismon have few of the characteristics of 

 the Neo-Cretaceous of the region northward and represent 

 such different conditions of sedimentation and fauna as to make 

 such a reference of them impossible. Fossils are scarce in 

 these beds but in the deposits overlying the Meso-Cretaceous in 

 Chiapas Bose found orbitoides and nummulites that were 

 clearly of Eocene age and similar forms occur south of Aquis- 

 mon. It is, therefore, proba ble that in the hill country be- 



" Bull. Geol. Soc. Am., Vol. 24, p. 253. 



