7.Fusus Patagonicus, G.B.Sowerby.
8.Fusus Noachinus, G.B.Sowerby.
9.Scalaria rugulosa, G.B.Sowerby.
10.Turritella ambulacrum, G.B.Sowerby (also S.Cruz).
11.Pyrula, cast of, like P.ventricosa of Sowerby, Tank Cat.
12.Balanus varians, G.B.Sowerby.
13.Scutella, differing from the species from Nuevo Gulf.
At the head of the inner harbour of Port S.Julian, the fossiliferous mass is not displayed, and the sea-cliffs from the water's edge to a height of between one and two hundred feet are formed of the white pumiceous mudstone, which here includes innumerable, far-extended, sometimes horizontal, sometimes inclined or vertical laminae of transparent gypsum, often about an inch in thickness.Further inland, with the exception of the superficial gravel, the whole thickness of the truncated hills, which represent a formerly continuous plain 950 feet in height, appears to be formed of this white mudstone: here and there, however, at various heights, thin earthy layers, containing the great oyster, Pecten Paranensis and Turritella ambulacrum, are interstratified; thus showing that the whole mass belongs to the same epoch.I nowhere found even a fragment of a shell actually in the white deposit, and only a single cast of a Turritella.Out of the eighteen microscopic organisms discovered by Ehrenberg in the specimens from this place, ten are common to the same deposit at Port Desire.I may add that specimens of this white mudstone, with the same identical characters were brought me from two points,--one twenty miles north of S.Julian, where a wide gravel-capped plain, 350 feet in height, is thus composed; and the other forty miles south of S.Julian, where, on the old charts, the cliffs are marked as "Chalk Hills."SANTA CRUZ.
The gravel-capped cliffs at the mouth of the river are 355 feet in height:
the lower part, to a thickness of fifty or sixty feet, consists of a more or less hardened, darkish, muddy, or argillaceous sandstone (like the lowest bed of Port Desire), containing very many shells, some silicified and some converted into yellow calcareous spar.The great oyster is here numerous in layers; the Trigonocelia and Turritella are also very numerous:
it is remarkable that the Pecten Paranensis, so common in all other parts of the coast, is here absent: the shells consist of:--1.Ostrea Patagonica, d'Orbigny; "Voyage Pal." (also at St.Fe and whole coast of Patagonia).
2.Pecten centralis, G.B.Sowerby (also P.Desire and S.Julian).
3.Venus meridionalis of G.B.Sowerby.
4.Crassatella Lyellii, G.B.Sowerby.
5.Cardium puelchum, G.B.Sowerby.
6.Cardita Patagonica, G.B.Sowerby.
7.Mactra rugata, G.B.Sowerby.
8.Mactra Darwinii, G.B.Sowerby.
9.Cucullaea alta, G.B.Sowerby (also P.Desire).
10.Trigonocelia insolita, G.B.Sowerby.
11.Nucula (?) glabra, G.B.Sowerby.
12.Crepidula gregaria, G.B.Sowerby.
13.Voluta alta, G.B.Sowerby.
14.Trochus collaris, G.B.Sowerby.
15.Natica solida (?), G.B.Sowerby 16.Struthiolaria ornata, G.B.Sowerby (also P.Desire).
17.Turritella ambulacrum, G.B.Sowerby (also P.S.Julian).
Imperfect fragments of the genera Byssoarca, Artemis, and Fusus.
The upper part of the cliff is generally divided into three great strata, differing slightly in composition, but essentially resembling the pumiceous mudstone of the places farther north; the deposit, however, here is more arenaceous, of greater specific gravity, and not so white: it is interlaced with numerous thin veins, partially or quite filled with transverse fibres of gypsum; these fibres were too short to reach across the vein, have their extremities curved or bent: in the same veins with the gypsum, and likewise in separate veins as well as in little nests, there is much powdery sulphate of magnesia (as ascertained by Mr.Reeks) in an uncompressed form:
I believe that this salt has not heretofore been found in veins.Of the three beds, the central one is the most compact, and more like ordinary sandstone: it includes numerous flattened spherical concretions, often united like a necklace, composed of hard calcareous sandstone, containing a few shells: some of these concretions were four feet in diameter, and in a horizontal line nine feet apart, showing that the calcareous matter must have been drawn to the centres of attraction, from a distance of four feet and a half on both sides.In the upper and lower finer-grained strata, there were other concretions of a grey colour, containing calcareous matter, and so fine-grained and compact, as almost to resemble porcelain-rock: I have seen exactly similar concretions in a volcanic tufaceous bed in Chiloe.Although in this upper fine-grained strata, organic remains were very rare, yet I noticed a few of the great oyster; and in one included soft ferruginous layer, there were some specimens of the Cucullaea alta (found at Port Desire in the lower fossiliferous mass) and of the Mactra rugata, which latter shell has been partially converted into gypsum.
(FIGURE 18.SECTION OF THE PLAINS OF PATAGONIA, ON THE BANKS OF THE S.
CRUZ.
(Section through strata (from top to bottom)):
Surface of plain with erratic boulders; 1,146 feet above the sea.
a.Gravel and boulders, 212 feet thick.
b.Basaltic lava, 322 feet thick.
c, d and e.Sedimentary layers, bed of small pebbles and talus respectively, total 592 feet thick.