>From specimens and notes given me by Lieutenant Stokes, it appears that the lower bed consists of soft muddy sandstone, like that of S.Josef, with many imperfect shells, including the Pecten Paranensis, d'Orbigny, casts of a Turritella and Scutella.On this there are two strata of the pale brown mudstone, also like that of S.Josef, separated by a darker-coloured, more argillaceous variety, including the Ostrea Patagonica.Professor Ehrenberg has examined this mudstone for me: he finds in it three already known microscopic organisms, enveloped in a fine-grained pumiceous tuff, which Ishall have immediately to describe in detail.Specimens brought to me from the uppermost bed, north of the Rio Chupat, consist of this same substance, but of a whiter colour.
Tertiary strata, such as here described, appear to extend along the whole coast between Rio Chupat and Port Desire, except where interrupted by the underlying claystone porphyry, and by some metamorphic rocks; these hard rocks, I may add, are found at intervals over a space of about five degrees of latitude, from Point Union to a point between Port S.Julian and S.
Cruz, and will be described in the ensuing chapter.Many gigantic specimens of the Ostraea Patagonica were collected in the Gulf of St.George.
PORT DESIRE.
A good section of the lowest fossiliferous mass, about forty feet in thickness, resting on claystone porphyry, is exhibited a few miles south of the harbour.The shells sufficiently perfect to be recognised consist of:--1.Ostrea Patagonica, d'Orbigny, (also at St.Fe, and whole coast of Patagonia).
2.Pecten Paranensis, d'Orbigny, "Voyage, Pal." (also at St.Fe, S.Josef, S.Julian).
3.Pecten centralis, G.B.Sowerby (also at S.Julian and S.Cruz).
4.Cucullaea alta, G.B.Sowerby (also at S.Cruz).
5.Nucula ornata, G.B.Sowerby.
6.Turritella Patagonica, G.B.Sowerby.
The fossiliferous strata, when not denuded, are conformably covered by a considerable thickness of the fine-grained pumiceous mudstone, divided into two masses: the lower half is very fine-grained, slightly unctuous, and so compact as to break with a semi-conchoidal fracture, though yielding to the nail; it includes laminae of selenite: the upper half precisely resembles the one layer at the Rio Negro, and with the exception of being whiter, the upper beds at San Josef and Nuevo Gulf.In neither mass is there any trace to the naked eye of organic forms.Taking the entire deposit, it is generally quite white, or yellowish, or feebly tinted with green; it is either almost friable under the finger, or as hard as chalk; it is of easy fusibility, of little specific gravity, is not harsh to the touch, adheres to the tongue, and when breathed on exhales a strong aluminous odour; it sometimes contains a very little calcareous matter, and traces (besides the included laminae) of gypsum.Under the microscope, according to Professor Ehrenberg, it consists of minute, triturated, cellular, glassy fragments of pumice, with some broken crystals.("Monatsberichten de konig.Akad.zu Berlin" vom April 1845.) In the minute glassy fragments, Professor Ehrenberg recognises organic structures, which have been affected by volcanic heat: in the specimens from this place, and from Port S.Julian, he finds sixteen Polygastrica and twelve Phytolitharia.Of these organisms, seven are new forms, the others being previously known: all are of marine, and chiefly of oceanic, origin.This deposit to the naked eye resembles the crust which often appears on weathered surfaces of feldspathic rocks; it likewise resembles those beds of earthy feldspathic matter, sometimes interstratified with porphyritic rocks, as is the case in this very district with the underlying purple claystone porphyry.From examining specimens under a common microscope, and comparing them with other specimens undoubtedly of volcanic origin, I had come to the same conclusion with Professor Ehrenberg, namely, that this great deposit, in its first origin, is of volcanic nature.
PORT S.JULIAN.
(FIGURE 17.SECTION OF THE STRATA EXHIBITED IN THE CLIFFS OF THE NINETYFEET PLAIN AT PORT S.JULIAN.
(Section through beds from top to bottom: A, B, C, D, E, F.))On the south side of the harbour, Figure 17 gives the nature of the beds seen in the cliffs of the ninety feet plain.Beginning at the top:--1st, the earthy mass (AA), including the remains of the Macrauchenia, with recent shells on the surface.
Second, the porphyritic shingle (B), which in its lower part is interstratified (owing, I believe, to redisposition during denudation) with the white pumiceous mudstone.
Third, this white mudstone, about twenty feet in thickness, and divided into two varieties (C and D), both closely resembling the lower, fine-grained, more unctuous and compact kind at Port Desire; and, as at that place, including much selenite.
Fourth, a fossiliferous mass, divided into three main beds, of which the uppermost is thin, and consists of ferruginous sandstone, with many shells of the great oyster and Pecten Paranensis; the middle bed (E) is a yellowish earthy sandstone abounding with Scutellae; and the lowest bed (F)is an indurated, greenish, sandy clay, including large concretions of calcareous sandstone, many shells of the great oyster, and in parts almost made up of fragments of Balanidae.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:--1.Ostrea Patagonica, d'Orbigny, "Voyage, Pal." (also at St.Fe, and whole coast of Patagonia).
2.Pecten Paranensis, d'Orbigny, "Voyage, Pal." (St.Fe, S.Josef, Port Desire).
3.Pecten centralis, G.B.Sowerby (also at Port Desire and S.Cruz).
4.Pecten geminatus, G.B.Sowerby.
5.Terebratula Patagonica, G.B.Sowerby (also S.Josef).
6.Struthiolaria ornata, G.B.Sowerby (also S.Cruz).
