24 EXPLORING EXPEDITION FROM SANTA FE 



s] token. This encroachment of the sea took place from east to west, and the invading 

 flood is now represented by the Gulf of Mexico. This is indicated by the facts that 

 the Lower Cretaceous sandstones thin out and disappear toward the south and east, 

 where nearly the entire Cretaceous series is represented by marine and organic sedi- 

 ments ; while, as we go toward the west and northwest, on the contrary, the sandstones 

 increase in thickness, the superincumbent strata become less and less calcareous, until 

 ultimately, as we approach the shores of the continent not submerged by the Creta- 

 ceous ocean, the mechanical sediments greatly preponderate over the organic, and beds 

 of sandstone and shale of enormous thickness, the direct ilcltrix of the land, represent 

 all the subdivisions of the chalk formation. 



I have spoken in my former report of the parallelism between the Lower Creta- 

 ceous strata of New Mexico and those of Nebraska, as described by Meek and Hay- 

 den, and those of New Jersey, by Professor Cook. How accurately the Lower Cre- 

 taceous rocks of the district now under consideration represent those of the regions 

 just mentioned will be seen in the following section : 



Section of Lower Cretaceous rocks at the crossing of Pawnee Fork. 



No. 1. Soft, coarse sandstone, dark reddish-brown, yellow, or nearly white; lower 

 part finer, and containing impressions of dicotyledonous leaves (Salir, occ.) to summit 

 of cliff, about 50 feet. 



2. Light dove-colored clay, with lignite and broken leaves, 10 feet. 



3. Yellow sandy clay with vegetable impressions to bed of stream ; about 15 feet 

 exposed. 



I had not time fully to explore this locality, but the fossils are quite numerous and 

 very accessible. The species of Salix referred to is apparently identical with S. J/yr/vV, 

 obtained at Smoky Hill by Meek and Hayden. The dove-colored clay which under- 

 lies the sandstone is scarcely distinguishable from that which holds a similar position 

 in New Jersey, and which, like this, is filled with vegetable matter. 



By reference to the section of the rocks of Kansas given by Messrs. Meek and 

 Hayden, in the Proceedings of the Philadelphia Academy, January, 185!), it will be 

 noticed that the "red, brown, and yellowish coarse-grained sandstone, containing 

 leaves of dicotyledonous trees, forming the summits of the Smoky Hills," is underlaid 

 by " whitish, very fine-grained, argillaceous sandstone, with bluish, purple, and ash- 

 colored clays." Though containing at that locality no vegetable impressions, it is 

 evident that these latter beds are the equivalents of those exposed beneath the sand- 

 stone on Pawnee Fork. 



PAWNEE FORK TO GROSSING OF CIMARRON. 



TERTIARY STRATA. 



After leaving Pawnee Fork the Santa Fe road crosses the margin of an immense 

 Tertiary basin, to which I first called attention in my report on the geology of the 

 Colorado country, and designated by the name of the " Tertiary basin of the 

 Arkansas." 



