MEXICAN BOUNDARY LINE. 105 



other sources, and comparison of other collections from rocks of the same age, and particularly 

 those of the cretaceous period from western localities and from New Jersey. These results are 

 given in the chapter upon the relative position of the cretaceous fossils of the Boundary Survey 

 with other known cretaceous formations in the United States. 



The relations of the carboniferous limestone of the Rocky Mountain range, I have endeavored 

 to make more clear by a few pages upon the carboniferous rocks, and a section of the principal 

 members belonging to that period, as known in the Mississippi valley. 



The specimens from the Tertiary formation of the southwest, although indicating a general 

 similarity to the formations of Nebraska, are nevertheless insufficient to give the means of care- 

 ful comparison and reliable conclusions. 



Our knowledge of the geological formations of the west is now so rapidly progressing, and 

 the materials accumulating in such abundance, that whatever may be presented to-day as 

 new and in advance of previous knowledge, will to-morrow be regarded only as a historical 

 record of our progress. The facts here presented, and the conclusions deduced from these and 

 other collections which I have heretofore examined, may serve as a contribution towards a more 

 perfect elucidation of the geology of this great central region, which has been traversed by the 

 Boundary Survey commission. 



I have the honor to remain, very respectfully, your obedient servant, 



JAMES HALL. 



14 M 



