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



THE AREA 



Lyinj^ to the east of that portion of the main body of the 

 mountainous highlands or Cordilleras of Mexico known as 

 the Sierra Madre Oriental, which, beginning at the Sierra 

 Carmen on the Rio Grande border, extends southeastwardly 

 by way of Monterey and Tamasopa, there is a broad valley, 

 interrupted in many parts by hills. This valley in turn is 

 bordered on the east by disconnected ranges and groups of 

 hills, which, as a whole, are roughly parallel to the main 

 range and to the course of the Rio Grande. Among these 

 groups and ranges may be named the San Antonio, San 

 Juan, Vallecillo, Picachos, Papagallos, San Carlos and 

 Tamaulipas. Prof. Cummins has proposed that these be 

 known collectively as the Tamaulipas Range, which is seem- 

 ingly warranted by the common origin of the groups. 



This range consists of deposits of shales and limestones of 

 late Cretaceous age, more or less altered and disturbed by 

 igneous activity and by folding, and as the trend of the 

 coast in this region is a little west of south, the southeast 

 course of the Tamaulipas Range brings it rapidly nearer the 

 Gulf until, in the region around Tordo Bay, fifty miles north 

 of Tampico, the hills of this range are within ten miles of 

 the coast and scattered peaks and ridges of later eruptives 

 occur within four miles or less of the Gulf shore. 



In the triangle thus formed by the Rio Grande, theTamauli- 

 pas Range and the coast line we find the occurrence of Ter- 

 tiary deposits which are the direct continuation of those of the 

 Texas area, but the Tertiary beds, which, along the Rio 

 Grande, form the surface rocks for a distance of 150 miles, 

 narrow rapidly toward the south, the lower beds disappearing 

 in turn by reason of successive overlaps of the later, until, at 

 the southern end, on the Zarzizal River, just north of Tordo 

 Bay, the entire exposure shows no Tertiary beds below the 

 Oligocene, which has here a width of a very few miles. 



The Tamaulipas Range thus marks the extreme western 

 and southern limits of these beds and, so far as our investi- 

 gations go, this area contains the last appearance in Mexico 

 of the Eocene beds as known in Texas, since the beds of this 

 age which are found south of the Tamaulipas Range have a 

 fauna more nearly related to those " of the deposits of the 

 Pacific Coast. 



