MEXICAN BOUNDARY LINE. 139 



REMARKS UPON THE TERTIARY FORMATION. 



In addition to the Eocene marine tertiary fossils of the Boundary collections, there are 

 numerous specimens of regularly stratified sandstone, conglomerate, clay, etc., which are refer- 

 rible to the tertiary period. The relations of the beds of these different series and their great 

 thickness indicate that they belong to a system of deposits having a wide extent in that part of 

 the country from which the specimens were derived. The mineral aspect of the specimens, the 

 association of clays, sandstones, and conglomerates, show a close similarity with the tertiary 

 formation of the Mauvaises Terres of Nebraska. Further explorations, collections, and com- 

 parisons are certainly necessary before this identity can be fully established ; but the numerous 

 intermediate explorations, from the Missouri river to the Mexican boundary line, indicate the 

 occurrence of similar formations along the entire distance. The collections from the tertiary 

 basin east of the Cordilleras likewise present specimens of lithological aspect and conditions so 

 similar to those of the Eio Grande on the east, that there would appear to be some relation 

 between the formations at these distant points. 



The tertiary materip,ls in the collections do not appear to me sufficient to furnish more than 

 grounds for a probable inference regarding the identity of the formations, and in the almost 

 entire absence of fossils from the western basin, it might be unwise to express any positive 

 opinion regarding the equivalency of these beds. 



Without possessing the means of a detailed section of the successive beds along the line of the 

 boundary survey, we are able, from the specimens of rocks and fossils in the collection, to present 

 the order of succession and general features of the formations and their relative position. 



The following section will serve to show the order of succession and relative age of the forma- 

 tions crossed by the Boundary survey : 



TERTIARY FORMATION. 



Tertiary formations on the west coast, probably of Miocene age. 



Tertiary formations east of the Cordilleras, consisting of beds of sandstone, sand,* conglome- 

 rates, etc., with subordinate beds of sandstone. Formations in the valley of the Rio Grande, 

 consisting of sandstone and conglomerate, with their calcareous beds, resembling in many 

 respects the tertiary formations of the Mauvaises Terres in Nebraska. 



Calcareous beds, with marine fossils of the Eocene tertiary, apparently underlying uncon- 

 formably the preceding strata. 



CRETACEOUS FORMATION. 



Argillaceous beds, containing Exogyra costata, etc. 



Calcareous beds of a buff color, and similar beds of a lead color, with beds of white limestone, 

 containing Gryphosa Fitcheri, Cardium multistriaium, Toxaster, Holectypus, Cyphosoma, Pyrina, 

 Ammonites Texanus, Hippurites, Caprina, Nerinea, etc. 



Sandstone of various colors, white, brown, red, etc., with beds of clay, etc. 



CARBONIFEROUS FORMATION. 

 Upper carboniferous limestone, containing Spirifer cameratus, S. lineatus, Terebratulasubtilita, 

 and other fossils of the age of the coal measures. Perhaps, also, some other members of the 

 coal measures, represented in the altered sandstones and slates of the collection. 



* The drifting gand» of the southwest, like those of the north, appear to be denved from Ihc sandttonet of the tertiary period 



