This State lies chiefly on the western slope of the Cordillera, and is cut up by hills and genial vallies. The highest point within its limits is the Peak of Tancitaro, which, in all probability, is an extinct volcano. East of this, and south of the village of Ario, the Volcano of Jorullo burst forth on the night of the 29th of September, 1759.
The great region to which this mountain belongs has been already described in our account of the geological structure of Mexico. The plain of Malpais forms part of an elevated platform, between 2,000 and 3,000 feet above the level of the sea, and is bounded by hills composed of basalt, trachyte, and volcanic tuff, clearly indicating that the country had previously, though probably at a remote period, been the theatre of igneous action. From the era of the discovery of the New World to the middle of the last century, the district had remained undisturbed, and the space, now the site of the volcano, which is thirty leagues distant from the nearest sea, was occupied by fertile plains of sugar cane and indigo, and watered by the two brooks, Cuitimba and San Pedro. In the month of June, 1759, hollow sounds of an alarming nature were heard, and earthquakes succeeded each other for two months, until, in September, flames issued from the ground, and fragments of burning rocks were thrown to prodigious heights.
"Six volcanic cones, composed of scoriæ and fragmentary lava, were formed on the line of a chasm which ran in a direction from N. N. E. to S. S. W. The least of these cones was 300 feet in height, and Jorullo, the central volcano, was elevated 1,600 feet above the level of the plain. It sent forth great streams of basaltic lava, containing included fragments of rocks, and its ejections did not cease till the month of February, 1760.
"Humboldt visited the country more than forty years after this occurrence, and was informed by the Indians, that when they returned, long after the catastrophe, to the plain, they found the ground uninhabitable from the excessive heat. When he himself visited the place, there appeared around the base of the cones, and spreading from them, as from a centre, over an extent of four square miles, a mass of matter of a convex form, about 550 feet high at its junction with the cones, and gradually sloping from them in all directions towards the plain. This mass was still in a heated state, the temperature in the fissures being on the decrease from year to year, but in 1780 it was still sufficient to light a cigar at the depth of a few inches. On this slightly convex protuberance, the slope of which must form an angle of about 6° with the horizon, were 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 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.