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Travertine, usually called Lapis Tiburtinus, a straw-coloured volcanic limestone excavated in the plain below Tivoli, which has the useful property of hardening on exposure, was now used as the principal building-stone instead of the former lavas and tufas; and the Colosseum, entirely constructed of travertine, which was treated in the middle ages as a quarry, out of which were built many of the palaces and churches of Rome, attests to this day the beauty and durability of this material.
The road was a good one, having been made by the late President, Senor Fernando Guzman, who seems to have done what little lay in his power to develop the resources of the country. The soil was entirely composed of volcanic tufas, and was covered with fine grass; but there were no springs or brooks, all the moisture sinking into the porous ground.
Seen from Funchal, the profile of Garajao is that of an elephant's head, the mahaut sitting behind it in the shape of a red-brown boss, the expanded head of a double dyke seaming the tufas of the eastern face. We distinguish on the brow two 'dragons, puny descendants of the aboriginal monsters. Beyond Garajao the shore falls flat, and the upland soil is red as that of Devonshire.
The lower levels are everywhere dotted with white farmhouses and brown villages. The colours of the wall are the grey of basalt, the purple of volcanic conglomerates, and the bright reds and yellows of tufas.
A similar disengagement of vapours, joined to the elastic force of the gases, which penetrate strata softened and raised up, appears sometimes to have given great extent to the caverns found in trachytes or trappean porphyries. They are lined with sulphur, and differ by the enormous size of their openings from those observed in volcanic tufas* in Italy, at Teneriffe, and in the Andes.
Primitive formations are nowhere seen above ground; we find only what belongs unquestionably to volcanoes: feldspar-lava, dolerite, basalt, conglomerated scoriae, tufa, and pumice-stone. Among the limestone formations we must distinguish those which are essentially subordinate to volcanic tufas* from those which appear to be the work of madrepores and other zoophytes.
The basaltic crust, formed by ancient volcanoes, seems everywhere undermined; and the currents of lava, seen at Lancerota and Palma, remind us, by every geological affinity, of the eruption which took place in 1301 at the island of Ischia, amid the tufas of Epomeo.
Even the barren, arid, and windswept eastern slopes glowed bright with the volcanic muds locally called laterites, and the foliated beds of saibros and macapes, decomposed tufas oxidised red and yellow.
Most of the land, resulting from the decomposition of the tufas, is of extreme fertility; and, therefore, we find on the Pacific side of Nicaragua, indigo, coffee, sugar, cacao, and tobacco growing with the greatest luxuriance. Nicaragua is thus divided into three longitudinal zones. The most easterly is covered by a great unbroken forest; the principal products being india-rubber and mahogany.
Gillan calls somewhat vaguely, mountains of hard ferruginous clay, are nothing but an alluvium which we find at the foot of every volcano. Strata of clay accompany basalts, as tufas accompany modern lavas.
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