118 GEOLOGY AND PALEONTOLOGY. 



The same species of Inoceramus was brought from between the Big and Little Blue rivers, 

 (tributaries of the Kansas river,) by Captain Stansbury. A collection from several points in 

 Arkansas, made by Colonel Fremont in his late expedition, and sent to me for examination in 

 1854 contains also specimens of Inoceramus prohlematicus, associated with a few other fossils. 



The cretaceous fossils which occur in the vicinity of Corrilitas correspond in position, being 

 on the eastern slope of the Kocky Mountains in nearly the same meridian of longitude' 

 and between 31° and 32° of latitude. In each of these explorations the points mentioned 

 were the farthest west at which cretaceous rocks with fossils were obtained. 



The identity of fossils, the occurrence of the same species of Inoceramus in all these localities, 

 and its association at Poblazon with Hippurites, as in the collections of the Boundary Survey, 

 indicate very clearly the same geological horizon for the strata of all these localities from the 

 Kansas river to New Mexico. 



The dip of the strata in the localities is influenced by the igneous rocks in immediate prox- 

 imity, and is therefore variable, often inclining to the west ; while the general dip of the 

 formation is in the opposite direction.* 



To the west of the last named localities there occur various stratified, partially metamorphic 

 rocks, some of which may be of cretaceous age ; but the information possessed warrants no more 

 than a probable inference. One of the specimens is a somewhat coarse and rather loosely 

 aggregated calcareous gray sandstone, and another is a partially metamorphic silicious slate. 



The principal features developed by this collection show the existence of a broad belt of 

 cretaceous rocks, in almost uninterrupted continuity, along the Rio Grande, from below Laredo 

 to beyond San Vincente. On either side are igneous rocks occupying a greater or less extent ; 

 and beyond the junction of the Eio Pecos these igneous belts become of more frequent occurrence 

 and of greater extent. The older tertiary deposits occupy isolated basins in- the cretaceous 

 formation, and both are covered indiscriminately by the alluvium. 



In many places, these drift or alluvial deposits, consisting of waterworn materials, with saline 

 efflorescences, gypsum, &c., are spread out over large areas of the cretaceous formation which 

 forms the fundamental rock of the Llano Estacado or Staked Plain. 



The almost constant occurrence of the carboniferous limestone, with these igneous and meta- 

 morphic belts, along a great north and south extent, taken in connexion with our knowledge of 

 the existence of this formation on the west and northwest of the Mississippi valley and in 

 Arkansas, offers almost conclusive evidence that nearly or quite all the intermediate space is 

 occupied by the same strata underlying the cretaceous formation. 



We already know of a similar association of the carboniferous limestone, over a large extent 

 of country, in the vicinity of the Great Salt Lake, and at intervals farther to the south ; and 

 the facts, in connexion, afford a very probable inference that it occurs in similar associations 

 from the southern boundary of the United States, or latitude 28°, to above the 42d parallel. 



determination, and teeth of sharks. These fossils prove that the strata from which they were taken belong to the cretaceous 

 formation. The existence of vast beds of this formation on the east side of the Rocky Mountains, and extending from the upper 

 Missouri to Texas, is well known. The occurrence of the same formation on the western side of the primary axis of the Rocky 

 mountains, is quite interesting." 



" The dip of the rocks at Poblazon is to the west, or from the Rocky IMountains ; and this proves that these mountains have 

 been elevated since the deposition of the cretaceous beds. It is, therefore, probable that the cretaceous beds on both sides of 

 the Rocky Mountains were made by the same ocean." 



• The inferences in regard to dip, &c., are founded on observations and sections furnished by Dr. C. C. Parry. 



