Humboldt s Theory of the Volcano qfJorullo. 63 



stances in Auvergne. Wherever a bed of lava fills the bottom 

 of a valley, the river or torrent which drains the valley, dis- 

 appears below the upper extremity of the lava bed, and filter- 

 ing through the interstices of the scoriae which universally 

 form its substratum, reappears in copious springs at the lower 

 extremity or termination of the lava current. So long as the 

 lava retains a very exalted temperature in its interior, the wa- 

 ter percolating beneath it must be proportionately heated ; 

 and that this was the case with regard to the lava bed of Mal- 

 pais, at the time of M. de Humboldt's visit, was proved by the 

 numerous fumarolc on its surface. Hence it was to be ex- 

 pected that the rivers Cuitimba and San Pedro, which find 

 their way beneath it, should have had their temperature 

 raised before they issued again into the air at the opposite ex- 

 tremity of the superinduced lava bed. 



The noice heard on approaching the ear to any of the hil- 

 locks (hornitos) resembling that of a cascade, and which is, by 

 M. de Humboldt, attributed to the rivers flowing through the 

 hollow gulf below, is far more probably owing to the currents 

 of elastic vapour rushing through the fissures by which they 

 find a vent. A similar sound is produced by the rise of car- 

 bonic acid through the little cones of the mud volcanoes of 

 Maccaluba in Sicily ; and I have also observed a rushing 

 sound of the same nature to be produced by every powerful 

 fumarola of the lava currents of Vesuvius. 



M. de Humboldt mentions himself, that the heat of the 

 Hornitos decreases every year ; and I have the authority of 

 Mr. Bullock junior, who visited the spot a short time back, 

 for the fact, that they have now ceased almost entirely to emit 

 vapour, and that the hot springs are reduced to a very low 

 temperature, evidently from the congelation of the subjacent 

 bed of lava. This evidence is absolutely conclusive as to the 

 correctness of the opinions advocated here on the nature of 

 the plain of Malpais. 



I have given thus much space to the endeavour to reconcile 

 the phenomena presented by this plain to the ordinary and 

 well known modes of volcanic agency, because the opinion 

 expressed by M. de Humboldt of its surface having been 

 raised by intumescence in the manner of an enormous bladder 



