Judging from the height, the general appearance, and the white colour of the patches visible on the hill sides, the uppermost plain, both on the north and western side of the Strait of Magellan, and along the eastern coast of Tierra del Fuego as far south as near Port St.Polycarp, probably belongs to the great Patagonian tertiary formation, These higher table-ranges are fringed by low, irregular, extensive plains, belonging to the boulder formation (Described in the "Geological Transactions" volume 6 page 415.), and composed of coarse unstratified masses, sometimes associated (as north of C.Virgin's) with fine, laminated, muddy sandstones.The cliffs in Sebastian Bay are 200 feet in height, and are composed of fine sandstones, often in curvilinear layers, including hard concretions of calcareous sandstone, and layers of gravel.In these beds there are fragments of wood, legs of crabs, barnacles encrusted with corallines still partially retaining their colour, imperfect fragments of a Pholas distinct from any known species, and of a Venus, approaching very closely to, but slightly different in form from, the V.lenticularis, a species living on the coast of Chile.Leaves of trees are numerous between the laminae of the muddy sandstone; they belong, as I am informed by Dr.J.D.Hooker, to three species of deciduous beech, different from the two species which compose the great proportion of trees in this forest-clad land.("Botany of the Antarctic Voyage" page 212.) From these facts it is difficult to conjecture, whether we here see the basal part of the great Patagonian formation, or some later deposit.
SUMMARY ON THE PATAGONIAN TERTIARY FORMATION.
Four out of the seven fossil shells, from St.Fe in Entre Rios, were found by M.d'Orbigny in the sandstone of the Rio Negro, and by me at San Josef.
Three out of the six from San Josef are identical with those from Port Desire and S.Julian, which two places have together fifteen species, out of which three are common to both.Santa Cruz has seventeen species, out of which five are common to Port Desire and S.Julian.Considering the difference in latitude between these several places, and the small number of species altogether collected, namely thirty-six, I conceive the above proportional number of species in common, is sufficient to show that the lower fossiliferous mass belongs nearly, I do not say absolutely, to the same epoch.What this epoch may be, compared with the European tertiary stages, M.d'Orbigny will not pretend to determine.The thirty-six species (including those collected by myself and by M.d'Orbigny) are all extinct, or at least unknown; but it should be borne in mind, that the present coast consists of shingle, and that no one, I believe, has dredged here for shells; hence it is not improbable that some of the species may hereafter be found living.Some few of the species are closely related with existing ones; this is especially the case, according to M.d'Orbigny and Mr.
Sowerby, with the Fusus Patagonicus; and, according to Mr.Sowerby, with the Pyrula, the Venus meridionalis, the Crepidula gregaria, and the Turritella ambulacrum, and T.Patagonica.At least three of the genera, namely, Cucullaea, Crassatella, and (as determined by Mr.Sowerby)Struthiolaria, are not found in this quarter of the world; and Trigonocelia is extinct.The evidence taken altogether indicates that this great tertiary formation is of considerable antiquity; but when treating of the Chilean beds, I shall have to refer again to this subject.
The white pumiceous mudstone, with its abundant gypsum, belongs to the same general epoch with the underlying fossiliferous mass, as may be inferred from the shells included in the intercalated layers at Nuevo Gulf, S.
Julian, and S.Cruz.Out of the twenty-seven marine microscopic structures found by Professor Ehrenberg in the specimens from S.Julian and Port Desire, ten are common to these two places: the three found at Nuevo Gulf are distinct.I have minutely described this deposit, from its remarkable characters and its wide extension.From Coy Inlet to Port Desire, a distance of 230 miles, it is certainly continuous; and I have reason to believe that it likewise extends to the Rio Chupat, Nuevo Gulf, and San Josef, a distance of 570 miles: we have, also, seen that a single layer occurs at the Rio Negro.At Port S.Julian it is from eight to nine hundred feet in thickness; and at S.Cruz it extends, with a slightly altered character, up to the Cordillera.From its microscopic structure, and from its analogy with other formations in volcanic districts, it must be considered as originally of volcanic origin: it may have been formed by the long-continued attrition of vast quantities of pumice, or judging from the manner in which the mass becomes, in ascending the valley of S.Cruz, divided into variously coloured layers, from the long-continued eruption of clouds of fine ashes.In either case, we must conclude, that the southern volcanic orifices of the Cordillera, now in a dormant state, were at about this period over a wide space, and for a great length of time, in action.
We have evidence of this fact, in the latitude of the Rio Negro, in the sandstone-conglomerate with pumice, and demonstrative proof of it, at S.
Cruz, in the vast deluges of basaltic lava: at this same tertiary period, also, there is distinct evidence of volcanic action in Western Banda Oriental.
The Patagonian tertiary formation extends continuously, judging from fossils alone, from S.Cruz to near the Rio Colorado, a distance of above six hundred miles, and reappears over a wide area in Entre Rios and Banda Oriental, making a total distance of 1,100 miles; but this formation undoubtedly extends (though no fossils were collected) far south of the S.