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species. At the same locality we have found in the clay and overlying limestone examples of Gryphæa, which present no specific differences from those figured by Mr. Marcon as G. dilatata (G. Tucumcari ), which this author considers peculiar to the so-called American Jurassic. These difficulties can hardly be reconciled unless we refer the upper rocks of Fort Washita, with their rich array of well-marked cretaceous fossils, also to the Jurassic.

That the clays and sandstones of Fort Washita are really of the same geological age as those farther west, can, we think, hardly admit of a doubt. They are found reposing beneath the same limestone and rest immediately upon the Coal Measures, while their lithological characters are very similar, the only difference being in the color of the clay, which, at Fort Washita, is blue instead of red; but, as will be seen by referring to the Journal, this color also prevails to some extent towards the south and southwest.

Near San Antonio de Bexar well marked specimens of Exogyra costata and Inoceramus crispii, both of them highly characteristic fossils of the Cretaceous formation of Texas and various other portions of the United States, were obtained from near the bottom of the well, which was sunk in the many clay to the depth of a hundred and fifty feet. (Vide Journal.) Specimens of Exogyra costata were also abundantly met with in the same formation a few miles west of San Antonio.

With the exception of that at Fort Washita, the best locality for fossils of the Marly Clay occurs near the Rio Pecos, and is mentioned in the notes of May 17, 1855. (Vide Journal.) Here the Marly Clay is well exposed, subordinate to thick strata of limestone, and is crowded with Gryphœa, identical with those figured by Mr. Marcon as Gryphæa Pitcheri, and which is admitted by him to be a cretaceous species. The same shell is well known to occur at many localities in the upper limestone of the Cretaceous Group throughout Texas.

In looking over the journals of other explorers we find still farther confirmation of the cretaceous age of this formation. Specimens of Inoceramus, and other fossils considered characteristic of the Cretaceous formation, were obtained by Lieut. Abert from near the Raton Pass, latitude 70 degrees 41 minutes, longitude 104 degrees 07 minutes, and at Pablozon, on the Rio Puerco. From the published notes of that officer we judge that one, and probably both, of these localities occur in the Marly Clay formation. Well marked specimens of Inoceramus of the Cretaceous System have also been obtained by Wislizenus and Lieut. Simpson along the valley of the Canadian River, only a short distance from Pyramid Mount, where the Gryphœa Tucumcari and Ostrea Marshii, mentioned by Mr. Marcon, were obtained. On the False Washita, near the Canadian River, Gryphœa Pitcheri has been observed in great abundance in the vicinity of extensive gypsum deposits. Cretaceous fossils were also procured by yourself from the vicinity of the Sulphur Springs of the Colorado. These last, however, as well as those procured by you from the neighborhood of the Sand Hills