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At this junction of the clay and the gypsum the majority of French geologists have always drawn the line between the Middle and Lower Tertiary, or between the Miocene and Eocene formations, regarding the Fontainebleau sands and the Ostrea cyathula clay as the base of the Miocene, and the gypsum, with its mammalia, as the top of the Eocene group.
Out of these three beds, I procured the following twelve species, of which the two first were exceedingly numerous in individuals, as were the Terebratulae and Turritellae in certain layers: Ostrea Patagonica, d'Orbigny, "Voyage, Pal." Pecten Paranensis, d'Orbigny, "Voyage, Pal." Pecten geminatus, G.B. Sowerby. 5. Fusus Patagonicus, G.B. Sowerby. 8. Fusus Noachinus, G.B. Sowerby. 9.
Hence M. d'Orbigny, not having himself examined this country, has concluded that there are here two distinct formations; but the Spirifer and T. aenigma were certainly included in the same bed with the Pecten and Ostrea, whence I extracted them; and the geologist M. Domeyko sent home the two Terebratulae with the other-named shells, from the same locality, without specifying that they came from different beds.
The shells included in these beds have been named for me by M. d'Orbigny: they consist of: Ostrea Patagonica, d'Orbigny, "Voyage" Part. Pal. 2. Ostrea Alvarezii, d'Orbigny, "Voyage" Part. Pal. 3. Pecten Paranensis, d'Orbigny, "Voyage" Part. Pal. 4. Pecten Darwinianus, d'Orbigny, "Voyage" Part. Pal. 5. Venus Munsterii, d'Orbigny, "Voyage" Pal. 6. Arca Bonplandiana, d'Orbigny, "Voyage" Pal. 7.
Hence, though both sand and mud originated simultaneously, the one near the land, the other far from it, the sands in every locality where a shore became submerged might constitute the underlying deposit. Ostrea columba. Syn. Gryphaea columba. Ostrea carinata. Chalk marl and chloritic sand. Terebrirostra lyra, Sowerby. Pecten 5-costatus. White chalk and chloritic sand.
It proved to be a pearl-oyster, as Max had supposed, and on being opened was found to contain eleven seed-pearls. Eiulo presented the shell and its contents to Johnny, who seemed to value the former, quite as much as the latter, and presently ran off in search of Arthur, to inquire whether it should properly be classed with the "genus ostrea," or the "genus mytilus."
Such are the Littorina scabra, on the trunks and branches of mangroves among islets in Trinity Bay; a Phasianella inhabiting the trunks and branches of Rhizophora at the Percy Isles; a Littorina on the leaves of Aigaeceras fragrans at Port Curtis, Auricula angulata, and rugulata on the trunks of mangroves at Port Essington, and Monodonta viridis on their roots at Night Island; a new and very beautiful Ostrea was found on the roots of mangroves among Low Islets in Trinity Bay.
Pecten Dufreynoyi, d'Orbigny, "Voyage, Part Pal." This species, which occurs here in vast numbers, according to M. D'Orbigny, resembles certain cretaceous forms. Ostrea hemispherica, d'Orbigny, "Voyage" etc. Also resembles, according to the same author, cretaceous forms. Terebratula aenigma, d'Orbigny, "Voyage" etc. Is allied, according to M. d'Orbigny, to T. concinna from the Forest Marble.
At the bottom of these sands a green clay occurs, containing a small oyster, Ostrea cyathula, Lam., which, although of slight thickness, is spread over a wide area. This clay rests immediately on the Paris gypsum, or that series of beds of gypsum and gypseous marl from which Cuvier first obtained several species of Palaeotherium and other extinct mammalia.
On the other hand, taking into account the evidence derived from the cretaceous character of these three shells, and of the Hippurites, Gryphaea orientalis, and Ostrea, from Coquimbo, we are driven back to the provisional name already used of cretaceo-oolitic. From geological evidence, I believe this formation to be the equivalent of the Neocomian beds of the Cordillera of Central Chile.
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