EXPERIMENTS on WHINS70NE and LAVA. 71 



their fuccefllon is diftindlly feen, the fedion of each ftratum 

 prefenting to the view part of a horizontal circle ; the whole 

 confifts of alternate layers of thin flreams of lava, and very thick 

 beds of loofe frothy rapilli, which lafl: being thrown into the 

 air in a foft flate, had fallen in fhowers on the fides of the 

 mountain. 



In various places the regvilarity of this arrangement is in- 

 terrupted by certain vertical lavas, from two feet to ten or 

 twelve in thicknefs, which crofs the ftrata juft defcribed in an 

 irregular manner, and pafs upward, without diftindlion, through 

 the folid beds, and through the loofe ones. It immediately 

 occurred to \is *, that thefe lavas mull have flowed in fiflures 

 of the ancient inountain ; and we accounted for them by fup- 

 pofing, that a melted ftream, flowing along the external furface,. 

 had met in its courfe with one of thofe crevices which are 

 formed in all great erviptions, and had flowed into it fo as to 

 return again into the heart of the mountain. This conjedlure 

 very nearly agrees with thofe advanced by M. Dolomieu, and 

 by M. Breislack, who both mention thefe vertical lavas of 

 Sommia f. 



I HAVE fince been induced to confider this phenomenon, 

 which formerly feemed to prefent only an amufing variety in 

 the hiftory of volcanic eruptions, as of the utmofl; confequence 

 in geology, by fupplying an intermediate link between the 

 external and the fubterraneous produ<5lions of heat. I now 

 think, that, though we judged rightly in believing thofe lavas 



to 



* I SAW this place in company with Dr J. Home in 1785. 



f M. Dolomieu conceives thefe lavas to have flowed over the lips of the crater, 

 (IJIes Ponces, p. loo.J ; M. Breislack, that they had firft filled the open cavity 

 of the crater, and from thence had flowed into crevices formed in its fides, " che 

 " una lava avendo riempita la cavita del cratere fi fofTe infiniiata per quelle fendi- 

 " \.-Axt" (Topografia Fijica delta Campania, p. \\^.). This lafl: mentioned work, 

 publidied in 1798, contains many interefling and accurate defcriptions. Should the 

 circumfliinces of the times permit, the author will have it in his power to follow out,, 

 with every advantage, the hints I have fuggefled.. 



