thousands of flattish conical mounds, from six to nine feet high, which as well as large fissures traversing the plain, acted as fumeroles, giving out clouds of sulphuric acid and hot aqueous vapor. The two small rivers before mentioned disappeared during the eruption, losing themselves below the eastern extremity of the plain, and reappearing as hot springs at its western limit. Humboldt attributed the convexity of the plain to inflation below; supposing the ground, for four square miles in extent, to have risen in the shape of a bladder to the elevation of 550 feet above the plain in the highest part. But this theory is by no means borne out by the facts described; and it is the more necessary to scrutinize closely the proofs relied on, because the opinion of Humboldt appears to have been received as if founded upon direct observation, and has been made the ground work of other bold and extraordinary theories. Mr. Scrope has suggested that the phenomena may be accounted for far more naturally by supposing that lava flowed simultaneously from the different orifices, and principally from Jorullo, united with a sort of pool or lake. As it poured forth on a surface previously flat, it would, if its liquidity was not very great, remain thickest and deepest near its source, and diminish in bulk from thence towards the limits of the space which it covered. Fresh supplies were probably emitted successively during the course of an eruption which lasted a year; and some of these, resting on those first emitted, might only spread to a small distance from the foot of the cone, where they would necessarily accumulate to a great height.
"The showers, also, of loose and pulverulent matter from the six craters, and principally from Jorullo, would be composed of heavier and more bulky particles near the cones, and would raise the ground at their base, where, mixing with rain, they might have given rise to the stratum of black clay which is described as covering the lava.
"The small conical mounds called 'hornitos' or little ovens may resemble those live 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,
