GEOLOGICAL AGE OF THE LLANO ESTACADO. 81 
is no good evidence of the uplift of the strata by the mass of the mountains. So, also, at and 
near Fort Defiance there are great dykes of trap, and, according to Mr. Campbell, the strata are 
much uplifted and disturbed. Mr. Marcou also records a prevailing dip of ten or fifteen degrees 
towards the north, in the red strata, along the Colorado Chiquito, to which the upper and white 
sandstone strata cuite conform. 
The fact that the strata are nearly or quite continuous from one side of the Anahuachian chain 
to the other, through the passes, and that they are nearly horizontal around the base of the 
moüntains, shows that the uplift took place before the cretaceous period, and that the mountains 
- were nearly or quite submerged beneath a cretaceous sea. "The higher peaks and ranges may 
have risen above the waters, and formed a chain of islands, trending nearly north and. south. 
We are thus led to refer the uplift of the chain, at least in the vicinity of the route, to the 
Secondary period, or between the close of the Gebot and the first Hoc fena of the Cre- 
taceous and Gypsum formation.! 
lieported Jurassic age of the Llano.—The formations of the Llano and idodi west of 
the mountains now described as Cretaceous, are not considered so by Mr. Marcou. In his 
Resumé, and later publications, he states that they are Jurassic, and they have been so colored 
by him on maps recently issued. The fossils which are mentioned in the Resumé in support of 
the Jurassic age of the strata, are as follows: ‘A Gryphea which has the greatest analogy 
with the Gryphea dilatata of the Oxford clay of England and France, and which I call, pro- 
visionally, G. Tucumcarii, and a very large Ostrea, having much resemblance to the Ostrea 
Marshii of the inferior oolite of Europe. I found, also, trigonia, and a species of cardinia.''? 
In the later publications by Mr. Marcou the G. Tucumcarii is stated to be identical with €. 
dilatata, and the Ostrea with O. Marshii. These fossils have also been figured by Mr. Marcou. 
Specimens of the Gryphæa are in the collection, but the other fossils do not appear. The only 
evidence, therefore, of the Jurassic formation, according to Mr. Marcou, which is before me in 
the collection: is dus single species of firi. which may or may not be identical with G. 
dilatata. It certainly has very considerable resemblance to that fossil, judging from figures ; 
but, even if specifically identical, it cannot be regarded as establishing the presence of Jurassic 
strata where it was found. The value of the evidence which the fossil presents, if, indeed, it can 
be regarded as furnishing any, is much lessened by the fact that the cretaceous formations of the 
Southwest are characterized by the number and variety of the species of Gryphea which they 
contain. Species are found with many variations at different localities, and specimens which 
very clearly resemble the G. T'ucumcarii are abundant in the undoubted cretaceous deposites of 
New Jersey and Alabama. It is such a variable fossil, and the same species appears under such 
different aspects, according to age and locality, that it certainly is not safe to regard the doubt- 
ful species, G. T'ucumcarii, as evidence of the Jurassic age of the stratum from which it was 
taken. It should rather be regarded as showing its cretaceous age. After a careful considera- 
tion of this subject, I am compelled to differ from Mr. Marcou in respect to the presence of the 
- Jurassic formation. Its presence has not been satisfactorily shown by the fossils, and all other 
- Observations lead to the conclusion that the strata of the Llano are Cretaceous. 
TERTIARY FORMATIONS. 
The geological collection furnishes little or no evidence of the presence of Tertiary formations 
along the line of the survey. There are no fossils which can be referred with certainty to this 
_ period, nor are there many rock specimens which have the aspect of such recent deposites. We 
may, however, except some of the specimens coming from the Llano Estacado, some of them 
much resembling Tertiary sandstones and marls. As, however, we cannot determine the age 
1 See, also, a paper, by the author, read before the American Association for the Advancement of Science, August, 1856 ; 
Albany meeting. 
? Resumé. Report of Lieutenant A. W. Whipple, 8vo., Chap. VI, p. 44. See, also, Appendix. 
15 es 
