EOCENE HISTORY OF MEXICAN CULF COASTAL AREA 497 



gocene time. The lower beds, so far as now determined, are con- 

 fined to the region south of the Panuco, while the upper stretch 

 northward beyond the old Eocene barrier nearly to the Rio Grande. 



MIOCENE 



In Texas, as we have seen, such Oligocene deposits as may occur 

 are overlain directly by the Miocene (Oakville) and the sands of 

 one are almost indistinguishable from the sands of the other, both 

 being probably of fresh- or brackish- water deposition. 



In Mexico the yellow clays and sands of the Oligocene are also 

 overlain by similar yellow clays and sands of Miocene age and the 

 deposits themselves are of very similar character and thus far no 

 evidence of a decided break in sedimentation between them has 

 been found, the only change being in the fossils contained in them, 

 those of the Miocene being characteristically developed in the 

 beds around Tuxpam. 



TAMPICO EMBAYMENT . 



The importance of the Tamaulipas Range as a structural feature 

 is, therefore, clearly shown, and it is now apparent that at the 

 beginning of the Tertiary and through the Eocene the waters of the 

 Mexican Gulf covered only that portion of its present coast line 

 in Mexico which lies north of Tordo Bay about 225 miles south 

 of the mouth of the Rio Grande. 



It would also appear that the waters of the Gulf and those of 

 the embayment south of the Tamauhpas Range, although they 

 were evidently separated by a comparatively narrow strip of land, 

 did not have such a connection as would permit a comminghng 

 of the two faunas, since they have only four or five forms in common. 



The Topila well mentioned above is less than twenty miles 

 from the present shore line of the Gulf of Mexico and is much 

 nearer to its present waters than is any bed of similar age in the 

 region north of the Tamaulipas Range. There is no indication of 

 a barrier to prevent the extension of these deeply buried Alazan 

 beds with their Pacific fauna eastward or southeastward until 

 they actually underlie the present waters of the Gulf. It is clearly 

 evident, therefore, that during the Middle Eocene the Gulf waters 

 did not reach below Tordo Bay on its present coast line, but were 



