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Whilst the country to the north-east of the lakes is mostly composed of rocks, of great age, geologically, such as schists, quartzites, and old dolerytic rocks, with newer but still ancient trachytes, that to the south-west of them is formed principally of recent volcanic tufas and lavas, the irruption of which has not yet ceased.
At 10.20 A.M. we had passed out of the cultivated region to the Montijo, or Monte Verde, the laurel-region. The latter show the anatomy of the land tufas, lavas, conglomerates, trachytes, trachydolerites, and basalts of various kinds. Most of the rocks are highly magnetic, and are separated by thin layers of humus with carbonised plant-roots.
This last formation resembles the tufas of Pausilippo, and the strata of puzzolana, which I found in the valley of Quito, at the foot of the volcano of Pichincha. The amygdaloid has very long pores, like the superior strata of the lavas of Vesuvius, arising probably from the action of an elastic fluid forcing its way through the matter in fusion.
Volcanoes of earlier date than the sandstone and calcareous rocks have been for ages extinguished; those which are yet in activity are in general surrounded only with breccias and modern tufas; but nothing hinders us from admitting, that the archipelago of the Canaries may exhibit some real rocks of secondary formation, if we recollect that subterranean fires have been there rekindled in the midst of a system of basalts and very ancient lavas.
There are also numerous other kinds: for instance, in Campania, red and black tufas; in Umbria, Picenum, and Venetia, white tufa which can be cut with a toothed saw, like wood. All these soft kinds have the advantage that they can be easily worked as soon as they have been taken from the quarries.
Since then there have been several eruptions; and so late as 1857 it threw out volumes of smoke, and probably ashes. The whole country is volcanic. For scores of miles every rock is trachytic, and the earth decomposing tufas. The lake itself is like an immense crater with its perpendicular cliffs.
On the contrary, in addition to old granitic and other igneous rocks, all explorers find only either innumerable undulations of sandstones, schistose, and quartzose rocks, or such tufaceous and ferruginous deposits as would naturally occur in countries long occupied by lakes and exuberant jungles, separated from each other by sandy hills, scarcely any other calcareous rocks being found except tufas formed by the deposition of landsprings.
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