60 Mr Scrope's Observations on 



hillocks (hornitos) with which the surface of the plain is thick- 

 ly studded. And here I must again have recourse to the re- 

 sults of the highly instructive eruption which took place from 

 Vesuvius in October 1822. 



All lava currents are well known both during their pro- 

 gress, and for a long time, often indeed many years after- 

 wards, to disengage torrents of aqueous and sulphureous va- 

 pour. If these are produced on any point in considerable 

 quantity, while the superficial lava is yet soft, their expansion 

 raises up a portion of this into a small dome or bubble, which 

 sometimes remains entire, at others it is broken through, leav- 

 ing the tattered fragments of lava that separate to give passage 

 to the vapour, in an upright position from their sudden con- 

 gelation.* This process is, in fact, one of the causes of the 

 numerous asperities that bristle the surface of most currents. 

 When, however, a deep coating of ashes has subsequently 

 fallen on this surface, its smaller roughness are effaced, and 

 the larger protuberances alone show themselves in the form of 

 small dome-shaped or sub-conical hillocks, which continue, 

 through various crevices, to give a passage to the vapours by 

 which they were at first thrown up. Many such hillocks, 

 rising five or six feet above the average level of the surface, 

 existed in the spring of 1823, on the Vesuvian lava currents 

 above-mentioned, and sent forth copious columns of vapour, 

 precisely of the same nature as that of the Hornitos of M. de 

 Humboldt; while other fissures, intersecting the intervening 

 surface of the small plain, formed by the lava on the Peda- 

 mentina, gave out similar vapours, presenting, on a small 

 scale, the identical phenomena for which Malpais has been so 

 long celebrated. -f- 



Where the quantity of ashes, covering the lava bed, and 

 mixed up into a paste with rain water, was great, as appears 

 to have been the case on the Malpais, it is probable that nu- 

 merous hillocks of this kind will have been formed by the in- 



* See Breislak's Institutions Geologiques, i. p. 251. 



•f That the eruptions of Jorullo threw up a prodigious quantity of vol- 

 canic ashes, is evident from the fact recorded by M. de Humboldt, that 

 the roofs of the houses at Queretaro, 144 miles distant in a straight line, 

 were thickly covered with them ! 



