38 GULF TERTIARY OF TEXAS. 



RIO GRANDE SECTION. 



The beds on the Rio Grande, which stratigraphically seem to represent the 

 Timber Belt Beds to the northeast, come in direct contact with the Upper 

 Cretaceous glauconitic beds without any interposition of the Basal Clays. 

 Possibly a study of the paleontology may prove the existence of the equiva- 

 lent of these beds in that region, but their lithological representatives are not 

 there. Though considerable collections of fossils have been made in this 

 country, and are now in the hands of Professor Angelo Heilprin, they, un- 

 fortunately, have not, at the time this report must be published, been thor- 

 oughly studied, and therefore the position of the so-called "Laramie" on the 

 Rio Grande will have to be left for future discussion. It may be said, how- 

 ever, that Cretaceous fossils have been found at Eagle Pass, and from there 

 down the river to the Webb County line are found great quantities of Ammon- 

 ites, and other fauna of that epoch. In fact, it is not until we reach a point 

 three miles below the northwest corner of Webb County that true Tertiary 

 (or Laramie) forms are found. Supposing the Cretaceous and Tertiary part- 

 ing to cross the river at this point, we would do away with the iruch mooted 

 question of the westerly extension to Las Moras Creek, above Eagle Pass, as 

 drawn by Loughridge,* Conrad, and others, and the slight deflection to the 

 west could easily be accounted for by the supposition of an embayment on 

 the Rio Grande at the time of the deposition of these strata similar to that 

 which existed at the same time on the Mississippi. Roemerj- makes the line 

 of parting cross the Rio Grande at Presidio de Rio Grande, ten miles above 

 Laredo, while Schott| refers to all the country from the mouth of the Pecos 

 to the Gulf of Mexico as the " Cretaceous Basin of the Rio Bravo " (Rio 

 Grande). It seems probable now, so far as can be judged without a further 

 study of the fossils, that Roemer was nearer right than the others, and ■ that, 

 as has been pointed out by Hill, the line as drawn by Conrad was based on 

 certain Tertiary fossils which had been misplaced in the collection. 



The mineralogical constituents of the Upper Cretaceous and the Lower 



Tertiary (or Laramie) formations on the Rio Grande are very much alike, 



and nowhere can there be seen a break or an unconformity. It appears, so 



far as the lithological character of the rocks goes, to have been a continuous 



sedimentation from Eagle Pass to the sand beds, reaching down to below 



Roma, a distance of over 325 miles. The beds of course vary slightly in 

 » 



*" Report on the Cotton Production of the State of Texas," Tenth Census of the United 

 States, Vol. V., p. 6? 9. 



f" Contributions to the Geology of Texas," American Journal of Science and Arts, Second 

 Series, Vol. VI, p. 21, 1848. 



^Mexican Boundary Survey, Vol. II, Part II, Chapter II, p. 28. 



