115 CALIFORNIA ACADEMY OF SCIENCES [Peoc. 4th Sei. 



roads are only trails. Good exposures of the rock materials 

 are, therefore, scarce except along drainage channels and cer- 

 tain hillsides, making it difficult to trace the continuations of 

 any of the formations over any considerable area. If we add 

 to this the fact that fossiliferous horizons are comparatively 

 few and frequently discontinuous, the difficulty of accurate 

 correlation of the beds of separated areas will be readily appar- 

 ent. It is for this reason, doubtless, that some confusion has 

 arisen. 



Publications 



The publications bearing directly on the geology of this 

 area are not very numerous. 



Among the earlier papers relating to the eastern coast of 

 Mexico, those by Deshayes, Heilprin and Sapper give only the 

 results of their observations on the Pliocene of Yucatan. 



The first definite statement regarding the geology of this 

 particular district is that of Bose in his itinerary of the trip 

 from San Luis Potosi to Tampico, published in the Guide 

 Book for the excursions of the International Congress of 

 Geologists in 1906. 



Bose regards that part of the massive limestones with rudis- 

 tes near Tamasopa and Micos, and which is last seen between 

 El Abra and Taninul, as Meso-Cretaceous and equivalent to the 

 Cenomanian or Vraconian. These include the limestones now 

 called Tamasopa. The shales and marls with limestone bands 

 which overlie these and are well exposed between Valles and 

 El Abra he classes as Neo-Cretaceous, although no fossils 

 were found in them. The yellow to gray argillaceous shales in 

 the plain east of Taninul he says probably belong to the Ter- 

 tiary, although he found no fossils, and states that they re- 

 semble the Pliocene of Tuxpam and Papantla. 



This was followed and added to by Villarello in his Report 

 on the Oil Regions of Mexico^, which gives clear and satisfac- 

 tory descriptions of the various geological formations of the 

 region, although later discoveries may necessitate a different 

 reference as to the age of some of the deposits there described. 



Villarello refers the massive grayish limestones along the 

 front of the Sierra Madre Oriental to the Meso-Cretaceous. 



"Bull. 26. Inst. Geol. Mex. 1908. 



