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Show 378 CONVEXITY OF TilE PLAIN OJ? MALPAIS. remain thickest and deepest near their source, and diminish in bulk from thence towards the limits of the space which they 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 J orullo, 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 ( ca1led" hornitos,, or ovens) may resemble those five or six small hillocks which existed in 18~3, on the Vesuvian lava, and sent forth columns of vapour, 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 disappearance of rivers is the usual result of the occupation of the lower part of a valley or plain by lava, of which there are many beautiful examples in the old lava-currents of Auvergne. The heat of the "hor~ nitos" 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. Another argument adduced in 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 g-round when struck sharply, and has been observed not only on .the sides of Vesuvius and other volcanic cones where there IS a cavity below, but in plains such as the Campagna di Roma, composed in great measure of tuff and porous volcanic rocks. The reverberation, however, may, perhaps, be assisted by DWIGHT W. TAYLOR ERUPTION OF JORULLO, A.D. 1819. 379 grottos and caverns ' :£or th ese may be . of J orullo, as in many of th f as numerous m the lavas would lend no countenanc t os;h o l Etna; but their existence cavity, or bubble four sq e o 'el l~pothesis of a great arched ' uare m1 es m e t t d . five hundred and fifty feet hiO'h * x en 'an m the centre Jprullo happened in 1819 o . A. subsequent eruption of , accompamed b 1 but unfortunately no E uropean travellers Yh an e·a rt 1qu.a.k e; the spot,. and the only facts hitherto k ave smce VISited at the City of Guanaxuato h. 1 . n;.wn are that ashes fell dred and forty EnO'lish m'tl' wf Ic 1 Jls Istant about one hun- . . o es rom orullo i h . . as to he six inches deep in th t ' n sue quantities cathedral of Guadalaxara was teh s reetsd, and the tower of the rown own t. * See Scrope on Volcanos p 267 t For this information I am indebted to c' ap. t am. v'e tch, F.R.S. |
