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All around had disappeared in the abyss; the inferior cone of Mount Franklin, rent asunder by the explosion; the lava jaws of Shark Gulf, the plateau of Prospect Heights, Safety Islet, the granite rocks of Port Balloon, the basalts of Dakkar Grotto, the long Serpentine Peninsula, so distant nevertheless from the center of the eruption.
Crags, towers, and pinnacles of porphyry were mingled with huge convoluted masses of light brown trachyte, of tufa either pure white or white veined with crimson, of black and gray columnar basalts, of red, orange, green, and black scoria, with adornments of obsidian, amygdaloids, rosettes of quartz crystal and opalescent chalcedony.
The two principal members of the Plutonic family of rocks are Granite and Syenite, each of which, with their varieties, bear very much the same relation to each other as the trachytes bear to the basalts. Granite is a compound of feldspar, quartz, and mica, the feldspars being rich in silica, which forms from 60 to 70 per cent of the whole aggregate.
This information led me into an error. As the coasts of Portugal consist of basalts covering calcareous rocks containing shells, I imagined that a trappean formation, like that of the Vicentin in Lombardy, and of Harutsh in Africa, might have extended from the banks of the Tagus and Cape St.
Vincent as far as the Canary Islands; and that the basalts of the Peak might perhaps conceal a secondary calcareous stone. These conjectures exposed me to severe animadversions from M. G.A. de Luc, who is of opinion that every volcanic island is only an accumulation of lavas and scoriae. M. de Luc declares it is impossible that real lava should contain fragments of vegetable substances.
As the Peak rises amid a system of basalts and old lava, and as the whole part which is visible above the surface of the waters exhibits burnt substances, it has been supposed that this immense pyramid is the effect of a progressive accumulation of lavas; or that it contains in its centre a nucleus of primitive rocks. Both of these suppositions appear to me ill-founded.
The base of the volcanic series is seen at Carsaig and Gribon to rest on Cretaceous and Jurassic rocks, like those of Antrim; hence the Tertiary age is fully established by the evidence of superposition. This was further confirmed by the discovery by the Duke of Argyll, some years ago , of bands of flint-gravel and tuff, with dicotyledonous leaves amongst the basalts of Ardtun Head.
I have seen serpentines, basalts, and lavas which internally were dark, and which from their weight, I should suppose, must contain oxide of iron, superficially brown or red, and decomposing. Undoubtedly this was from the action of water impregnated with air upon their ferruginous elements. The Unknown. You are perfectly right.
The two principal families of trappean or volcanic rocks are the basalts and the trachytes, which differ chiefly from each other in the quantity of silica which they contain. The basaltic rocks are comparatively poor in silica, containing less than 50 per cent of that mineral, and none in a pure state or as free quartz, apart from the rest of the matrix.
The small part of the island of Graciosa which we traversed, resembles those promontories of lava seen near Naples, between Portici and Torre del Greco. The rocks are naked, with no marks of vegetation, and scarcely any of vegetable soil. A few crustaceous lichen-like variolariae, leprariae, and urceorariae, were scattered about upon the basalts.
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