AGE OF THE GYPSUM FORMATION. 
25 
The observations on the nature of the strata of this portion of the survey are very few, and I 
cannot, therefore, present many remarks upon them. In the Diary of Mr. Byrne, under date 
of April 15th, bluffs of red sandstone at the crossing of the Colorado are mentioned. Again, on 
one of the tributaries of the Brazos the G-ypsum formation is mentioned, and its resemblance to 
that of Delaware creek is noted. April 19th, Captain Pope passed over a bluff or bank show¬ 
ing limestone and red sandstone in horizontal strata, with decomposed gypsum strewed thickly 
about. The water, too, was strongly impregnated with gypsum, or probably with- salts of mag¬ 
nesia. From these facts, and the peculiar topography and the vegetation, I conclude that, from 
the time of leaving the Llano, at the springs of the Colorado, east, to the vicinity of the Clear 
fork of the Brazos, the route was upon the red sandstone formation, containing the enormous 
deposits of gypsum, and underlying the lighter-colored sandstones and limestones of the Llano— 
the continuation, in fact, of the same deposits which were traversed along the Pecos and Dela¬ 
ware creek, already noted and described. The quantity of gypsum, however, which is met with 
in this part of the formation, does not at all compare with that found on the Pecos, but the 
characteristic peculiarities of the formation were preserved. 
Age of the Gypsum formation .—As in the first part of this report no observations upon the 
geological age of the Gypsum formation were presented, I now propose to briefly consider this 
subject, before passing to a description of the rocks around Fort Belknap, and from thence to 
Preston. 
Much light has already been thrown upon the geology of this region by the labors of Doctor 
G. G. Shumard, in connexion with the exploration made by Captain Marcy. 1 The Bed-sand¬ 
stone and Gypsum formation has also been described by President Hitchcock; 2 but he does not 
decide upon its age, both from the absence of fossils, and the want of sufficient observations 
upon its position relatively to other formations, the age of which is well determined. For 
the same reasons, I shall not attempt to establish its geological age. Its position relatively 
to the Carboniferous formation, which it overlies, indicates that it may belong to the upper 
portion of that series. It may also be of Permian or Triassic age, to which formations it bears 
great resemblance in its mineral characters. It may also be Cretaceous, and fossils of this 
period only, have been brought from that region. That it is of the age of the Trias is the opinion 
of Mr. Marcou, who traversed the formation along the Canadian river with Lieutenant Whip¬ 
ple; and in the preliminary report on Captain Pope’s collection, which he prepared, he distinctly 
states it to be Triassic, and overlaid by Jurassic strata. We have already seen that the evi¬ 
dence of the geological age. of the upper strata of the Llano favors the conclusion that they are 
Cretaceous; and there is no evidence or indication of their Jurassic age, nor is there in the 
collection any fossil from the underlying red clays and sandstones by which their age can be 
established. Dr. Shumard, in his geological section from Arkansas to Fort Belknap, has 
represented the Cretaceous strata at Fort Washita as resting directly upon the coal-measures; 
and Professor Tuomey,'the geologist of the State of Alabama, finds them in the same position, 
and without any intervening Triassic or Jurassic strata. The section which Dr. Shumard pre¬ 
sents does not show whether the Cretaceous strata correspond in dip, or are conformable with 
the coal-measures below; nor is it easy to decide, from the descriptions, whether the coal- 
measures are horizontal or have a slight dip. An outcrop of bluish-grey limestone, about one 
hundred miles southwest of Fort Smith, is said to dip at an angle of'30°. A ridge of granite 
succeeds, and then sandstone is found nearly to Fort Washita, where it is covered over by the 
Cretaceous strata. The Carboniferous sandstones at Fort Belknap are described as nearly hori¬ 
zontal. It would thus appear that the Cretaceous formation of that region is found to lie con¬ 
formably upon nearly horizontal Carboniferous strata. If this is true, may we not consider the 
red clays and sandstones of the Gypsum formation, upon which the Cretaceous of the Llano rests, 
as portions of the Carboniferous series? It is true, we are entirely without adequate data upon 
1 See Report of Captain R. B. Marcy, of an Exploration of the Red River in 1852—Appendix D, Geology, p. 179. 
2 Ibid, p. 164:. 
