GEOLOGY OF THE LLANO.—CRETACEOUS FOSSILS. 
17 
thoroughly examined. According to Lieutenant Whipple and Mr. Jules Marcou, the geologist 
who accompanied him, the base of the Llano along the Canadian is composed of the red and 
blue clays or marls containing gypsum; above them are white and yellowish sandstones and 
limestones, also in horizontal strata. 
The geological age of these strata of red clays, sandstones, and gypsum, has not yet been 
satisfactorily determined; the formation appears to he singularly devoid of fossils, and, as yet, 
the only indications of its place in the geological series are its mineral characters and relative 
positions. The age of the overlying rocks of a lighter color is also obscure, hut there is much 
reason to regard them as Cretaceous and Tertiary. The only fossils which I find in the collec¬ 
tion from the Llano are Cretaceous, and serve to indicate the development of that formation at 
the Big springs of the Colorado, and a point on the Llano twenty miles east of the Sand hills. 
There are no fossils from the gypsum formation, nor any specimens which serve as a guide to 
its age. The fossils referred to are well preserved specimens of Gryphcea Pitcheri and Exogyra 
Texana; several specimens of limestone from the bluff at the Big springs of the Colorado, which 
contain a small Gryphcea like G. vomer; and a mass of shells much broken, but containing frag¬ 
ments of Gryphcea from a point twenty miles east of the Sand hills. Descriptions of these fossils 
will be found accompanying this report. They are all characteristic of the Cretaceous forma¬ 
tion ; Gryphcea Pitcheri being a well-known fossil of wide distribution in the Cretaceous forma¬ 
tions of the United States. The altitude of this locality of Cretaceous fossils at the Big springs, 
according to the profile from the Pecos to the Big springs, is 3,844 feet above the sea. 
The existence of the Cretaceous formation in the valley of the Bio Grande, between El Paso 
and Dona Ana, has been determined by Major Emory, by observations in connexion with the 
United States boundary survey. That part of the valley has an elevation of near 4,000 feet 
above the sea, and the west edge of the mesa opposite Dona Ana is 4,460 feet, which, according 
to the profile, is about the mean elevation of the Llano Estacado. The identification of hori¬ 
zontal Cretaceous formations at this point, at the same elevation with the formations of the 
Llano, also horizontal, is good evidence of their similarity in age. We have, therefore, three 
points at nearly the same elevation, yet separated by many miles, which are shown to be Creta¬ 
ceous by the fossil remains. Two of these localities are upon the Llano, and the other is on the 
Bio Grande river, and the elevation of each is that of the general surface of the Llano in that 
latitude. It would appear, therefore, that its upper strata are of the age of the Cretaceous. 
Further observations and collections of fossils from the plain are, however, very desirable to 
verify these indications and probabilities. 
The specimens of the rocks from the Llano Estacado have a general resemblance to each 
other, and show the nearly uniform nature of the geological formation. They consist, for the 
most part, of light-colored, whitish, or grey calcareous sandstones, with specimens of compact 
amorphous limestone, which resembles the specimens containing the Cretaceous fossils from the 
Big springs. Three or four specimens consist of sand and gravel united by a calcareous cement, 
and looking as if these materials had been exposed to the infiltration of water highly charged 
with carbonate of lime. (See descriptions and catalogue Nos. 23 and 24.) These have a very 
modern aspect, and much resemble the sandstones and conglomerates of the Tertiary age. Indeed, 
all the specimens of the sandstones of the Llano are so much like those of the Tertiary, that if 
it were not for the presence of the Cretaceous fossils, I would have been led to regard them as of 
that period. 
Mr. Jules Marcou, who passed over the Llano with Lieutenant Whipple, near the parallel of 
35°, regards the base of the Llano, or the formation of red clays, sandstone, and gypsum, as 
of the age of the Trias, and the upper strata—the light-colored sandstones and limestones—as 
Jurassic. In the preliminary observations he made upon this route, based upon Captain 
Pope’s notes and collections, and published in the preliminary 8vo. edition of his report, he 
assigns the same ages to the corresponding parts of the Llano on the line; and says that the 
rocks covering the variegated marls of the Trias, and forming the whole plain of the Llano 
