MEXICAN BOUNDAET LINE, 



135 



From the facts before us it appears that Nos. 4 and 5 of the upper division, which are so 

 largely developed on the upper Missouri, become gradually attenuated towards the southwest, 

 and lose in a great measure their distinctive fossils. 



Although the specimens in the boundary collections do not clearly indicate the occurrence in 

 that region of the sandstone No. 1 of the Nebraska section, it is nevertheless quite certain, 

 from other collections in my possession, that»the same rock occurs on the Arkansas river, 

 possessing the same characters as in Nebraska. 



Dr. Shumard, in his examinations in Arkansas, speaks of a sandstone which clearly holds 

 the place of No. 1 of our section. On page 181 of report, before cited, he says : ''' Passing this 

 range, the sandstone again reappears, and constitutes the prevailing rock to within a short 

 distance of Fort Washita, where it disapjjears, and is succeeded by strata of the cretaceous 

 period." Although not recognized by Dr. Shumard as a member of the cretaceous formation, 

 still it holds, in regard to the beds in Arkansas, equivalent to Nos. 2 and 3 of Nebraska, the 

 same relative position as the sandstone No. 1 on the Missouri river.* 



The observations made in the course of the boundary survey, and in all the other surveys in 

 the southwest, show the occurrence of various colored sandstones and clays below the fossili- 

 ferous beds identified, as above, with Nos. 2 and 3 of the Nebraska section. Indeed, we have 

 evidence, from numerous observations, of the occurrence of a similar sandstone at so many 

 points from Missouri to New Mexico as to render it certain that the formation is continuous over 

 this wide extent of country. 



The observations of Mr. Marcou, on the line traversed by one of the Pacific railroad surveys, 

 induced him to regard these sandstone as of older date than the cretaceous formation. In a 

 section of Pyramid Mountain given by Mr. Marcou, (Bulleten Soc. Geol. de France, tome 12, 

 p. 878,) he recognizes a series of sandstones and clays beneath limestones which are of unques- 

 tionable cretaceous age. 



Corresponding to Nnhraska, 

 seclion Sand 3. 



Comiponding to Nebraska, see. 1. 



While, siliceous limestone. 



Sandy limestone of a deep yellow color. 

 Blue clay, 30 feet. 



White sandstone, 25 feet. 



Hard, yellow sandstone, 80 feet. 

 White sanditone, 6 feet. 



900 feet. 



" a. Bed of variegated marls in contact with the Jurassic formation." 



"6. Alternations of calcareo-argillaceoiis marls of variegated colors — red, green, and white." 



"c. Bed of GryphoM dilatala and of Oslrea Marshii."f 



* I believe that Dr. B. F. Shumard regards this sandstone, in part, or altogether, as of carboniferous age ; but it is 

 difficult to understand these relative positions, since in numerous localities, from the Missouri river to Te.xas, tlie upper 

 carbon iferou.s liuiestone is the highest determined carboniferous rook, and underlies the sandstone No. 1 of the Nebraslea 

 section. 



t These explanations of the section quoted above are those given by Mr. Marcou. The designations at the right hand 

 are those given by him in the text accompanying the section. 



