"The small conical mounds called 'hornitos' or little ovens may resemble those five or six small hillocks which existed in 1823 on the Vesuvian lava, and sent forth columns of vapor, having been produced by the disengagement of elastic fluids heaving up small dome-shaped masses of lava. The fissures mentioned by Humboldt as of frequent occurrence, are such as might naturally accompany the consolidation of a thick bed of lava, contracting as it congeals; and the appearance of rivers is the usual result of the occupation of the lower part of the valley or plain by lava, of which there are many beautiful examples in the old lava currents of Auvergne. The heat of the 'hornitos' is stated to have diminished from the first; and Mr. Bullock, who visited the spot many years after Humboldt, found the temperature of the hot spring very low,--a fact which seems clearly to indicate the gradual congelation of a subjacent bed of lava which, from its immense thickness, may have been enabled to retain its heat for half a century. The reader may be reminded, that when we thus suppose the lava near the volcano to have been, together with the ejected ashes, more than 500 feet in depth, we merely assign a thickness which the current of Skaptar Jokul attained in some places in 1783. "Another argument adduced in the support of the theory of inflation from below, was, the hollow sound made by the steps of a horse upon the plain; which, however, proves nothing more than that the materials of which the convex mass is composed are light and porous. The sound called "_rimbombo_" by the Italians, is very commonly returned by _made ground_ when sharply struck, and has been observed not only on the sides of Vesuvius and of other volcanic cones where a cavity is below, but also in plains, such as the Campagna di Roma, composed in a great measure of tuff and other porous and volcanic rocks. The reverberation, however, may be assisted by grottoes and caverns, for these may be as numerous in the lavas of Jorullo as in many of those of Etna; but their existence would lend no countenance to the hypothesis of a great arched cavity, four square miles in extent, and in the centre 550 feet high.[64] "Mr. Burkhart, a German director of mines, who examined Jorullo in 1827, ascertained that there had been no eruption there since Humboldt's visit in 1803. He, went to the bottom of the crater, and observed a slight evolution of sulphurous acid vapors, but the "hornitos" had ceased entirely to give forth steam. During the twenty-four years intervening between his visit and that of Humboldt, vegetation had made great progress on the flanks of the new hills, and the rich soil of the surrounding country was once more covered with luxuriant crops of sugar cane and indigo, and there was an abundant growth of natural underwood on all the uncultivated tracts."[65] * * * * *