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. River of S. Cruz; here 280 feet above sea.)
In ascending the valley of the S. Cruz, the upper strata of the coast- cliffs are prolonged, with nearly the same characters, for fifty miles: at about this point, they begin in the most gradual and scarcely perceptible manner, to be banded with white lines; and after ascending ten miles farther, we meet with distinct thin layers of whitish, greenish, and yellowish fine-grained, fusible sediments. At eighty miles from the coast, in a cliff thus composed, there were a few layers of ferruginous sandstone, and of an argillaceous sandstone with concretions of marl like those in the Pampas. (At this spot, for a space of three-quarters of a mile along the north side of the river, and for a width of half a mile, there has been a great slip, which has formed hills between sixty and seventy feet in height, and has tilted the strata into highly inclined and even vertical positions. The strata generally dipped at an angle of 45 degrees towards the cliff from which they had slided. I have observed in slips, both on a small and large scale, that this inward dip is very general. Is it due to the hydrostatic pressure of water percolating with difficulty through the strata acting with greater force at the base of the mass than against the upper part?) At one hundred miles from the coast, that is at a central point between the Atlantic and the Cordillera, we have the section in Figure 18.
The upper half of the sedimentary mass, under the basaltic lava, consists of innumerable zones of perfectly white bright green, yellowish and brownish, fine-grained, sometimes incoherent, sedimentary matter.