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Between the frog and the horse-shoe lie our various camps, mostly in radiating hollows, open either to the east or west, but sheltered from cross fires by rough kopjes of porphyritic boulders that have turned brown on the surface by exposure to sunshine.
This phenomenon, analogous to that which I observed in the crater of Jorullo, deserves the more attention, as muriatic acid abounds in the greater part of volcanoes, and as M. Vauquelin has discovered it even in the porphyritic lavas of Sarcouy in Auvergne. I sketched on the spot a view of the interior edge of the crater, as it presented itself in the descent by the eastern break.
Fitton describes the islands, from the specimens which were submitted for his inspection, to be of primitive formation; and notices the following rocks: Compact felspar of a flesh-red hue, enclosing a few small crystals of reddish felspar and of quartz; Coane porphyritic conglomerate of a reddish hue; Serpentine; Slaty clay which forms the general character of the Percy Islands.
The Livistona palm and Cochlospermum gossypium grew on the ridges; the tea-tree, the stringy-bark, the leguminous Ironbark and Eugenia were useful timber. The whole country was most magnificently grassed. A Porphyritic sienite cropped out at the head of the first swamp, about a mile from our last camp. A low range was seen at the south-east end of the large swamp on which we encamped. Nov. 26.
Differently from what we have seen throughout Chile, the coast here is formed not by the granitic series, but by an escarpment of the porphyritic conglomerate formation, between two and three thousand feet in height.
On this view of there being a tendency in the old points of eruption to become the points of subsequent injection and disturbance, and consequently of denudation, it ceases to be surprising that the streams of lava in the porphyritic claystone conglomerate formation, and in other analogous cases, should most rarely be traceable to their actual sources.
Considering the number and thickness of the streams of porphyritic lava, and the great thickness of the beds of breccia-conglomerate, there can be little doubt that the sources of eruption must originally have been numerous: nevertheless, it is now most difficult even to conjecture the precise point of any one of the ancient submarine craters.
Extending from one degree north of Quito to Cuzco, and from Cuzco to Chili, it was quite as long as the two Pacific railroads, and its wild route among the mountains was far more difficult. The roadway, paved with well-cut, dark porphyritic stone, was twenty feet wide, and rested on deep foundations. This road was marvelous.
Knowing that the porphyritic conglomerate formation consists of alternate streams of submarine lavas and of the debris of anciently erupted rocks, and that the strata of the upper gypseous formation sometimes include submarine lavas, and are composed of tuffs, mudstones, and mineral substances, probably due to volcanic exhalations, the richness of these strata is highly remarkable when compared with the erupted beds, often of submarine origin, but NOT METAMORPHOSED, which compose the numerous islands in the Pacific, Indian, and Atlantic Oceans; for in these islands metals are entirely absent, and their nature even unknown to the aborigines.
Above this fossiliferous mass, there is a stratum of very fine-grained, pale brown mudstone, including numerous laminae of selenite. All the strata appear horizontal, but when followed by the eye for a long distance, they are seen to have a small easterly dip. On the surface we have the porphyritic gravel, and on it sand with recent shells.
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