386 Marcou's Geological Map of the United States. 



much broader coloring representing the trias. Yet there is no 

 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 Gryphcea and one of Ostrcea. 

 They were obtained from the upper strata of Pyramid Mount — 

 one of the mounds separated from the Llano Estacado by ero- 

 sion. The Gryphaea 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 Ostrasa is reported to 

 bear much resemblance to 0. Marsha of the inferior Oolite of 

 Europe.* In the text accompanying the map the species are 

 announced as identical, one with G. dilatata, the other with 0. 

 Marshii. Even if this identity be admitted, it does not author- 

 ize the conclusion that the strata are beyond question Jurassic ; 

 or if it did, the occurrence of Jurassic at that one point on the 

 Canadian, would not authorize us to conclude that the formation 

 extends for. more than a thousand miles on both sides of the 

 mountains. The genus Grypheea in America is eminently char- 

 acteristic of the Cretaceous formation, and species which very 

 closely resemble G. Tucumcarii, if not in fact identical with it, are 

 very abundant in Alabama and New Jersey in the Cretaceous 

 formation. Moreover, all the species are found with many varia- 

 tions according to the locality. The abundance and variety of 

 the species of this genus render it unsafe to regard G. Tucum- 

 carii, however much it may resemble G. dilatata, conclusive 

 evidence of the presence of oolitic formations. Specimens of the 

 Gryphasa are found in the government collection, but there are 

 none of the Ostrsea. 



Some of the evidences of the Cretaceous age of the Llano 

 may now be presented. If we follow the strata in which the 

 Gryphcea was procured, westward, we find them extending across 

 the mountain chain, through the passes, into the valley of the 

 Rio Grande, and here near the summit of the table-lands just 

 south of Santa Fe, Mr. Marcou reports the presence of Cretace- 

 ous fossils. Farther west, at Poblazon near the Puerco, Lieuten- 

 ant Abert obtained several specimens of Inoceramus\ from hori- 

 zontal strata. The topography at this point is the same as along 

 the valley of the Canadian, the strata are at nearly the same ele- 

 vation, and their mineral characters are similar. Numerous 

 specimens of Tnoceramus have also been obtained by Simpson,^: 



* See Resume of a Geological reconnoissance, &c. Report of Lieut. A. W. Whip- 

 ple, U. S. Top. Engrs., H. Doc. 129, ehap. vi. 



f Described and figured by Prof. Bailey— Report by Lieut. J. W. Abert, IT. S. 

 Top. Engrs. of an Examination of New Mexico. 



% Report aad Map of the Route from Fort Smith, Arkansas, to Santa Fe, by 

 Lieut. J. H. Simpson, U. S. Top. Engrs., Washington, 1850. 



i 



