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him, we have found to occur indiscriminately throughout the entire thickness of the formation.

While engaged in the exploration of Upper Red River under Capt. R. B. Marcy, fossil shells, principally Gryphœa Pitcheri, and fragments of silicified monocotyledonous and dicotyledonous fossil wood, were, upon several occasions, observed in the upper part of the Many Clay formation; but as these appeared to be worn by attrition, and were also abundantly distributed through the overlying Quaternary drift, little attention was at that time paid to them. During the same journey we also found in the clay, considerably below the surface, a fossil coral, which Prof. Hitchcock refers to the genus Scyphia, and which lie regards as being closely allied to species that are characteristic of the Cretaceous rocks in other parts of the world.

Subsequently, while exploring near the sources of the Brazos and Big and Little Witchita Rivers, we again found the same shells in the Marly Clay formation, many feet below the top, associated with Ostrea and other forms that we have since detected in the Upper Cretaceous limestone farther southward.

The fossil wood mentioned by Mr. Marcon as occurring so abundantly along the Canadian, and which, we presume, is the same as we have obtained on Red River, only a few miles farther south, cannot be relied upon as evidence of the Triassic age of the Marly Clay formation, since fossil wood occurs abundantly in the Cretaceous rocks of Texas, as will be seen by referring to the Journal of this report, where it is mentioned that specimens were encountered at frequent intervals along our route from Indianola to El Paso. On the Frio and Nueces Rivers fragments of the trunks of trees, some of them several feet in length and weighing over five hundred pounds, were observed. After a careful comparison of some of these specimens with those obtained from the Brazos and Red River country, we are unable to detect any difference whatever. Fossil wood is also mentioned by Roemer and other explorers as occurring abundantly in the Cretaceous strata of Texas. The evidence derivable from fossil wood is therefore altogether in favor of the Cretaceous age of the Marly Clay formation of the Canadian and Red Rivers.

At Fort Washita the layers of the inferior part of the Cretaceous Group contain Ammonites, chiefly A. vespertinus, in a good state of preservation. These are confined principally to the clay, the included layers of sandstone being almost entirely destitute of organic remains.

The shell figured by Mr. Marcon as Ostrea Marshii, and regarded by him as characteristic of the American Jurassic, we have found in the Upper Limestone of Fort Washita, in the same beds with Gryphœa Pitcheri, Hemiaster elegans, Holaster simplex, and Ammonites vespertinus. This shell was described under the name of Ostrea subovata, by Dr. B. F. Shumard, in Capt. Marcy's Report of the Red River of Louisiana (p. 205, pl. 5, fig. 2), who regards it as being quite distinct from 0. Marshii, though it is doubtless a closely allied