168 
GEOLOGICAL REPORT—THIRTY-FIFTH PARALLEL. 
characteristic of the neocomien of Europe. Further, at Fort Washita, the neocomien is covered 
by the green-sand, containing very fine Hemicister, large Ammonites, &c. 
This neocomien has been almost wholly destroyed and carried away by denudations, for it is 
only found on the summits of the hills, resembling the remains of ancient buildings ; it occupies 
actually only a width of three or four miles. Probably at the time of the deposite it covered 
more space ; hut, as at Fort Washita, where it has been very little denuded, it is only twenty- 
five or thirty miles wide. This shows it to have been hut a narrow bend in the immense basin 
of the prairies. 
The second point where the expedition passed from the strata of the Trias to a more recent 
formation, is at the place where we crossed the Llano Estacado. The base of the Llano is 
formed wholly of the upper strata of the Keuper , which reaches half way up the height of the 
plateau. These strata, which are of a red color, are suddenly replaced by white sandstone, con¬ 
taining numerous calcareous concretions, then by a compact, white limestone, sometimes oolitic, 
that forms the summit of the Llano. These beds are superposed in concordant stratification 
upon those of the Keuper. 
The Llano Estacado consists of two table-lands of different elevations. We crossed the lower 
one but forty miles further west. Near Fossil creek and Tucumcari mount there is a second 
steppe, one hundred and fifty feet higher than the first, also forming a vast mesa, which extends 
to the Pecos. This second mesa is entirely formed of blue clay at the base, then yellowish 
sandstone ; and, finally, the summit is again a very compact, white silicious limestone. 
In the whole, this formation of the Llano Estacado does not exceed four hundred feet in 
thickness. 
This formation is not limited to the Llano, but it forms the summits of all the plateaux that 
are seen to the north, in the direction of the Canadian river, and between the Canadian and 
the Raton mountains, as well as the majority of the mesas, which extend from the Rio Pecos to 
the foot of the Sierra de Sandia. Our survey has also met with it on the other side of the Rio 
Puerco, forming with volcanic lava almost the whole road between Covero and the Sierra Madre, 
and finally between Inscription Rock and the Pueblo of Zuhi, where it again forms mesas, which 
extend in the direction of Fort Defiance and the Canon de Chelly. It will be seen that those of 
the Llano Estacado occupy, geographically, a large place in the geology of the Rocky mount¬ 
ains ; but as regards their relative age, they are still more important, for they fill a void here¬ 
tofore left in the series of stratified rocks of North America ; these rocks belong to the Jurassic 
or Oolitic epoch. Fossils are very rare in the sandstone and limestone; but the beds of blue 
clay which are found in the middle of this formation contain in abundance a Gryphcea which has 
the greatest analogy with the Grxyphcecft dilatata of the Oxford clay of England and France, and 
which I call, provisionally, Grypliceci Tucumcari , and a very large Ostrea having much resem¬ 
blance to the Ostrea Manshift of the inferior oolite of Europe. I found also trigonia and a species 
of Carclinia. This American Jurassic presents, at least thus far, one point of considerable 
difference from the Jurassic of Europe and Asia, where such large quantities of Cephcdopods are 
found, such as Ammonites and Belemnites; while here the Ammonites are only found in the green¬ 
sand, and the Belemnites in the marly chalk ; and even there these fossils are never so abundant 
as in the corresponding strata of Europe. 
1 Mr. Marcou has since decided that this fossil is identical with G. dilatata of the Oxford clay. I append here a copy of 
his recent description of this shell, and of Ostrea Marshii. —W. P. B. 
Gryphcea dilatata, Sow. (pi.-, figs. 1, 2 et 3.)—Cette coquille, dont nous faisons reprdsenter l’e'tat adulte et l'e'tat jeune, 
tie laissera aueun doute sur son identite avec cclle de l oxford-clay de l'Europe occidentale. La figure 3 repre'sente une 
forme plus allongee, que nous distinguons provisoirement a titre de variety, en lui assignant le nom de la montagne de 
Tucumcari {Gryphiasa Tucumcarii) pres de laquelle nous l’avons trouvee et qui est ce'lebre dans les plaines de l’Ouest. Les 
caracteres essentiels de la charniere sont d’ailleurs ceux du type de l’espece. 
2 Ostrea Marshii, Sow. (pi. --, fig. 4.)—La seule valve de cette coquille que nous ayons rencontre'e avec l’espece prdcd- 
dente, nous parait e'galement ne pas diffe'rer de celle a laquelle nous la rapportons ; et est identique aux specimens d' Ostrea 
Marshii, trouve'es en Augleterre et en France. 
