Vol. V] DUMBLE— TERTIARY DEPOSITS IN NORTHEASTERN MEXICO JgQ 



Oligocene 



Overlying the beds we have here referred to the Eocene, 

 we find a series of yellow sands, clays and calcareous beds 

 which carry an Oligocene fauna. We have called these 

 the San Fernando from the fine exposures of the beds in the 

 vicinity of the town of that name on the Conchos River. As 

 will appear, these beds all belong to the Upper Oligocene 

 and up to this time no beds of the Lower Oligocene, like those 

 of the Buenavista River region with Orhitoides papyracea, 

 etc., have been recognized in the area north of the Tamaulipas 

 Range. From our present knowledge it would seem that 

 while the Lower Eocene deposits show a gradual overlapping 

 southward until the Conchos is reached, the Oligocene, on the 

 contrary, shows an overlapping northward to the same region, 

 so that along the Conchos the uppermost beds of the Oligocene 

 are in contact with the members of the Eocene there exposed. 



In the region of San Jose de las Rusias,'' which occupies 

 the extreme southern portion of this area, we have numerous 

 exposures of the Upper Oligocene. It apparently immedi- 

 ately overlies the Cretaceous and is penetrated by eruptive 

 rocks which are connected with or extend eastward from the 

 Tamaulipas Range. In places these eruptives are of a porphy- 

 ritic texture but at others they are basalts. They occur as 

 masses, ridges or isolated peaks throughout the region and 

 the Oligocene beds in immediate contact with them are more 

 or less metamorphosed and show at times considerable dips. 

 These eruptive rocks, in places, extend to within two or three 

 miles of the Gulf coast. 



The lower beds of the Oligocene in this region are yellow 

 clays, which are altered in places and appear as hardened 

 shales, and clayey limestones carrying Cristellaria, Num- 

 mulites, corals and molluscan forms followed by yellow sands 

 and clays with an extensive fauna. The beds have a general 

 southeast dip. At San Rafael on the Zarzizal River at the 

 extreme southern end of the district, the contact of the erup- 

 tives with the yellow clays is well shown. 



Northward of San Rafael toward the ranch of San Jose 

 de las Rusias, the principal exposures are of eruptive rocks, 



7 An excellent description of this region will be found in "Boletin del Institute 

 Geologico de Mexico," No. 26, Juan D. Villarello. 



