EOCENE HISTORY OF MEXICAN CULF COASTAL AREA 487 



is seen no more and the Lignitic is itself in contact with the shales 

 of the Cretaceous. 



The most southerly exposure of the Lignitic was seen on the 

 Conchos River a mile west of Angeles, where it is uncovered for 

 a short distance by the erosion of that stream. 



At the close of the Lignitic its sediments emerged and were 

 strongly eroded prior to the incursion of the Claiborne sea. 



In Lower Eocene time, therefore, the area north of the Tamaul- 

 ipas Range was submerged and marine waters prevailed as far 

 south as the latitude of Monterey. At the close of this period' 

 the marine waters receded and after a period of erosion were 

 followed by the submergence of the deposits under fresh or brack- 

 ish waters. The close of this second or brackish-water period of 

 sedimentation was followed by its emergence and erosion. 



In the region west and south of the Tamaulipas Range no 

 deposits have been found which can certainly be referred to the 

 Lower Eocene as represented north of that range by the Midway 

 and Wilcox. 



Beginning near the foot of the high plains of the interior of 

 Mexico there is a series of blue and gray shales which have a wide 

 development, forming a large part of the surface rock of the valley 

 between the Cordilleras and the Tamaulipas Range and continu- 

 ing southward beyond the Tuxpam River as far as examinations 

 were carried. In the northern part of this area these shales have 

 been traced to a direct connection with those found in the San 

 Juan and Papagallos mountains, which, by their relations to the 

 basal Tertiary and the fossils found in limestones immediately 

 underlying them, were proved to be Upper Cretaceous. 



There is undoubtedly a great thickness of these shales, but no 

 fossils have ever been found in them. There is, therefore, a possi- 

 bility that in their upper portion some representative of early 

 Eocene time may be present, but, if so, it has not yet been differ- 

 entiated. 



So far as yet recorded no fossils characteristic of the Midway 

 or Lignitic have been found between Tordo Bay and Yucatan, 

 although fossils of both periods are known to exist in Venezuela 

 and elsewhere to the south. 



