496 E. T. DUMBLE 



These Oligocene deposits are found resting upon the Frio clays 

 along the Conchos River and the eastern front of the Pomeranes 

 Range, and they extend northward to within fifty miles of the Rio 

 Grande, beyond which point we find no trace of them, possibly 

 because of the extensive development of the Reynosa surficial 

 deposits in that region. 



South of Abasola the Oligocene overlaps the Frio and is in 

 direct contact with the Papagallos shales on the west and extends 

 eastward to the Gulf shore. 



The corals found in some of these beds have been studied by 

 Dr. Vaughan, who says that they indicate an Upper Oligocene 

 horizon, about equivalent to the Chattahoochee of Georgia. This 

 is borne out by such of the molluscan remains as have been exam- 

 ined, and so far our collections from this area have not yielded any 

 forrns characteristic of the Lower Oligocene. Among the corals 

 determined are: Favites (?) polygonales Duncan, Goniastrea 

 antiquaensis Duncan, Aero par a ? sp., Orhicella cellulosa Duncan, 

 Orhicella n. sp., Geneopora sp. very similar to, or identical with, 

 an Antiguan species. 



South of the Tamaulipas Range the yellow clays and sands of 

 the Oligocene cover a broad area. Between the Buena Vista 

 River and the Tamiahua Lagoon many exposures are found and 

 the beds are highly fossiliferous. Cristalleria and Nummulites 

 abound and OrbUoides papyracea Bou. is found from the Buena 

 Vista to the Tancochin at Cerro del Oro. Dr. Dall states that this 

 form occurs in the Lower Oligocene beds in Alabama, indicating 

 a Lower Oligocene age for this portion of the deposits. This 

 Oligocene belt has a known width of twenty miles east of the Buena 

 Vista River and has been observed twenty miles west of that 

 stream resting upon the shales of the Cretaceous. 



In addition to the fossils named there are many others, includ- 

 ing several echinoderms, pectens, and other bivalves and numerous 

 gasteropods. 



While a detailed examination of these beds has not been made, 

 enough has been done to prove that there is in this part of Mexico 

 very considerable thickness of these Oligocene deposits and that 

 they represent portions, if not all, of both Lower and Upper Oli- 



