96 
GEOLOGY 
sufficient evidence of the presence of Jurassic formations, and the Llano and other plateaux referred to 
that age are not Jurassic, but Cretaceous. 
The evidence brought forward to show the presence of the Jurassic, consists of one species of 
Gryphwa and one of Ostrwa. They were obtained from the upper strata of Pyramid Mount — one of the 
mounds separated from the Llano Estacado by erosion. The Gryphsea is said to have the greatest 
analogy with G. dilatata of the Oxford clay of England and France , and was provisionally called G. 
Tucumcarii. The Ostirea is reported to bear much resemblance to O. Marshii of the inferior Oolite of 
Europe’). In the text accompanying the map the species are announced as identical, one with G. di- 
latata, the other with 0. Marshii. Even if this identity bo admitted, it does not authorize the conclusion 
that the strata are beyond question Jurassic ; or if it did , the occurence of Jurassic at that one point 
on the Canadian, would not authorize us to conclude that the formation extends for more than a thous- 
and miles on both sides of the mountains. The genus Gryphwa in America is eminently characteristic 
of the Cretaceous formation, and species which very closely resemble G. Tucumcarii, if not in fact iden- 
tical with it, are very abundant in Alabama and New Jersey in the Cretaceous formation. Moreover, all 
the species are found with many variations according to the locality. The abundance and variety of 
the species of this genus render it unsafe to regard G. Tucumcarii, however much it may resemble G. 
dilatata, conclusive evidence of the presence of oolitic formations. Specimens of the Gryphwa are found 
in the government collection, but there are none of the Ostrwa. 
Some of the evidences of the Cretaceous age of the Llano may now be presented. If we follow 
the strata in which the Gryphwa was procured, westward, we find them extending across the mountain 
chain, through the passes, into tho valley of the Rio Grande, and here near the summit of the table- 
lands just south of Santa Fe, Mr. Marcou reports tho presence of Cretaceous fossils. Farther west, at 
Poblazon near the Puerco, Lieutenant Abert obtained several specimens of Jnoceramus '’) from horizontal 
strata. The topography at this point is the same as along the valley of the Canadian, the strata are at nearly 
the same elevation , and their mineral characters are similar. Numerous specimens of Inoceramus have also 
been obtained by Simpson'), Wislizenus and others along the valley of the Canadian river not far from 
Pyramid Mount, where the Gryphwa was procured. Farther east on tho False Washita and near the 
Canadian, the Cretaceous fossil Gryphwa Pilchcri occurs in abundance and near the great beds of gypsum. 
Leon Spring , in the southern part of the Llano, has afforded abundance of Cretaceous fossils, and this 
place is represented on the map as Jurassic. Cretaceous fossils wex'o also obtained by Capt. Pope from 
the bluffs of the Llano at tho Sulphur Springs of the Colorado and from the surface of the plateau near 
the Sand Hills ''). Tho Llano of Texas is well known and is undoubtedly tho continuation of tho Llano 
Estacado. The bluffs are fdled with Cretaceous fossils already described by Ferdinand Roemer. They 
are correctly represented as Cretaceous on the map. 
The map displays a most remarkable relation of position between the Cretaceous and tho « Jurassic » 
along the valley of tho Rio Grande between El Paso and the mouth of the Pecos. The river has cut 
its valley downwards through the horizontal formations of the Llano which form bluffs on each side. 
On the map wo find tho valley of the stream colored as Cretaceous, while tho higher strata of the Llano, 
are colored as Jurassic. Thus, according to this representation, the Jurassic strata overlie the Cretaceous. 
This conclusion is unavoidable unless we are ready to believe that tho Cretaceous strata were deposited 
since tho erosion of the valley of the Rio Grande. The same alternative is presented to us along the 
■) See Resume of a Geological reconnoissance, etc. Report of Lieut. A. W. Whipple, U. S. Top. Engrs., H. 
Doc. 129, ch,ip. vi. 
’’) Described and figured by Prof. Bailey — Report by Lieut. J. W. Abert, U. S. Top. Engrs. of an Examination 
of New Mexico. 
') Report and Map of the Route from Fort Smith, Arkansas, to Santa Fe, by Lieut. J. 11. Simpson, U. S. Top. 
Engrs., Washington, 1850. 
•') Report on the Geology of th# Route surveyed by Bvt. Capt. Pope, U. S. Top. Engrs., 4to. Washington, 1856. 
[Pacific R. R. Exp. and Surveys. ] 
