THE LOWER RIO BRAVO. 47 



the sinuosities of the Gulf Coast, there cannot be any mistake as to the relation of the one to the 

 other. The various cretaceous terraces of the lower Rio Bravo basin, placed, as they are, one 

 above the other, appear in this position as so many antediluvial tide-marks of that vast sea ; on 

 the bottom of which secondary and also tertiary formations have been deiiosited, together with 

 all those fossil types of organic forms — each genus and species belonging to its respective 

 geological day. 



Like the tide-marks that are arranged in large concentric circles, and that have been brought 

 about by the oscillatory motions of the waters, the organic remains (leaves in the mighty book 

 of the history of creation) can tell each one a tale of the physiographic condition of the age 

 during which it rejoiced in the functions of life. 



Whilst the currents and other forces of the water, acting horizontally, placed the different 

 strata along the foot of the ancient coast of that secondary sea, a vertical force was needed to 

 bring the submarine strata above the level of the water. If there were no signs of such a 

 power, we could be content in believing that since the formation of the siibmarine strata the 

 salt water has receded. There is evidence, however, that does not admit any question of the 

 upheaval of the country. The volcanic dyke before spoken of is the strongest evidence in favor 

 of this conclusion ; these basaltic strata projecting through the cretaceous, and generally, also, 

 a part of the older secondary formation, have caused a rise in the whole of the cretaceous system. 

 Thus the peaks and branches of this dyke naturally became the caryatida of the great geological 

 edifice of this country, often bearing detached portions of the cretaceous strata far above their 

 general level. 



GENERAL VIEW OF ORGANIC REMAINS. 



With regard to the fossil remains of the regions of the lower Eio Bravo, which have come to 

 our knowledge, there are many among them common to the cretaceous formation of New Jersey. 

 With this locality they are also quite synchronous, according to Mr. Conrad, who has examined 

 them. The specimens of this character are mentioned in his enumeration of genera and species. 

 Many of these fossil remains, however, seem to belong solely to the cretaceous formation of 

 Texas, and are more particularly referable to the southern portion of the secondary strata, 

 which, on their part, are also nearly allied to analogous forms of the cretaceous formation of 

 southern Europe, especially that part lying about the Mediterranean. 



To go here more into the details of this supposed relationship between the southern and 

 northern cretaceous formations of both hemispheres is not the design of these pages. To such 

 of our readers, therefore, that may be desirous of examining this matter further,* we would refer 

 them to Dr. Roomer's work, already mentioned, which contains well-founded hints and proofs 

 on the subject. 



Besides the before mentioned interesting specimens, so important to the knowledge of the 

 general distribution of fossil forms, several new species have come to hand which seem to be 

 indigenous to the cretaceous formation of Western Texas. The enumeration by Mr. Conrad shows, 

 so far, six new sjjecies. Since then, however, another set of organic remains has been sent on for 

 examination ; and it seems to be not improbable that still other new forms may be met with in 

 this collection of ours, the second made on the lower Rio Bravo. It is probable that some of 

 the specimens of this last set will prove the prevalence of fossils of the tertiary system, which may 



" For a full exposition of this subject, see Dr. Hall's report, which concludes the Geology of the Boundary. 



