Humboldt's Theory of the Volcano of Jorullo. 61 



tumescence of this semi-liquid substance alone above the fu- 

 marole of the lava ;* and the mobility of parts, occasioned by 

 this process, favouring the action of the concretionary forces, 

 probably gave rise to the agglomeration of the clay into the 

 foliated and concentric balls, of which the hornitos partly con- 

 sist. At Pont du Chateau, in Auvergne, is an example of 

 even a very coarse calcareovolcanic conglomerate having as- 

 sumed this precise variety of concretionary structure ; and I 

 suspect, from their being imbedded in the black clay, and 

 consisting of a fine-grained rock very different from the Dole- 

 ritic basalt of Jorullo, that the globular concretions of the 

 hornitos are not a true basalt, but merely hardened nodular 

 balls of volcanic ashes. They are, in fact, described by M. 

 de Humboldt as fragile and easily crumbled, and totally dif- 

 ferent from the syenitic lava of the current of Jorullo. 



It remains only to notice the supplementary facts produced 

 by M. de Humboldt in support of the explanation he adopts 

 of the appearances of Malpais, which I conceive tend much 

 more strongly to confirm the opinion of its being merely the 

 surface of a great bed of lava, which, up to the period of M. 

 de Humboldt's visit, retained much of its internal heat. 



These confirmatory facts are, 



1. The noise made by the steps of a horse upon the plain. 



2. The frequent formation of cracks or fissures across the 

 plain, and the occasional occurrence of partial subsidences. 



3. That two rivers, the Cuitimba and San Pedro, lose 

 themselves below the eastern extremity of the plain, and re- 

 appear as hot springs (at 52° cent.) at its western limit. 



1. With regard to the first mentioned circumstance, viz. 

 the sound produced when the surface of the plain is struck 

 by the hoofs of a horse, or, I presume, in any other mode of 

 percussion, it is evidently the same phenomena of reverbera- 

 tion, to which the mimo-phoneutical term rimbombo is so well 

 applied in Italy, and which, by a vulgar error, is often sup- 

 posed to indicate a great cavity below the spot so resounding 

 when struck. It is perfectly true that the roof of every large 



• " The Hornitos are hollow. When a mule steps upon them they 

 break in."— Humboldt- 



