Experimetiis en Whinjione and Lava. 6^ 



a fingle large cone, the greateft part of which has funk during fome violent eruption, pro- 

 bably that which took place in the time of Pliny, leaving a fragment of its bafis, now called 

 the mountain of Somma. This fragment retains its original fhape; and on the fide front- 

 ing the towns of Somma and Otajano, the external conical furface, along which the an- 

 cient lavas had flowed, is flill entire. Fronting the centre of the cone, Somma breaks oft' 

 abruptly, and prefents a vertical cragg, fome hundred feet in height, which is concave in- 

 wards. From the gulf, produced by the ruin of the ancient mountain, though not exa£lly 

 from its centre, have arifen the exploGons which, by repeated accumulation, have formed 

 the prefent cone of Vefuvius. Next the fea, this cone has extended itfelf fo as completely 

 to cover all remains of the ancient one, forming a continued flope from the crater to the 

 foot of the mountain. On the oppofite fide it meets the bafe of the craggs of Somma, and 

 forms an angle, into which many fucceflive ftreams of lava have flowed, producing a nar- 

 row horizontal valley, in the form of a crefcent, called the ^trio del Cavallo. From this 

 valley the craggs of Somma prefent a complete view of the internal ftruiSture of the an- 

 cient mountain, correfponding, in moft things, to what might have been fuppofed. 



The various fubftances, depofited fuccefllvely on the external furface of the ancient 

 cone, being cut vertically in this cragg, their fucceflion is diftin£tly feen, the fe£lion of 

 each ftratum prefenting to the view part of a horizontal circle ; the whole confifts of alter- 

 nate layers of thin ftreams of lava, and very thick beds of loofe frothy rapilll, which laft 

 being thrown into the air in a foft ftate, had fallen in fhowers on tlie fides of the moun-. 

 tain. 



In various places the regularity of this arrangement is interrupted 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 diftin£lion, through the folid beds, and 

 through the loofe ones. It immediately occurred to us*, that thefe lavas mud have flowed 

 in fiffures of the ancient mountain ; and we accounted for them by fuppofing, that a melted 

 ftream, flowing along the external furface, had met in its courfe with one of thofc cre- 

 vices which are formed in all great eruptions, and had flowed into it fo as to return again, 

 into the heart of the mountain. This conjefture very nearly agrees with thofe advanced by 

 M. Dolomieu, and by M. Breiflack, who both mention thefe vertical lavas of Somma f . 



I have fince been induced to confider this phenomenon, which formerly feemed to pre- 

 fent only an amufing variety in the hiftory of volcanic eruptions, as of the utmoft confe- 

 quence in geology, by fupplying an intermediate link between the external and the fubter- 



• I faw this place in company witliDr. J. Home in 1785. 



+ M. Dolomieu conceives thefe lavas to have flowed over the lips of the crater, (IJles Ponces, p. 100.) ;, 

 M. Breiflack, that they had firft filled the open cavity of the crater, and from thence had flowed into ere- 

 vices formed in its fides, " che una lava aveiido riempita la cavita del cratere fi fofle infmuata per quefte 

 *' fcnditure^" (Topografia Fifica. della Campania, p. 115. J This laft mentioned work, published in 1798,, 

 contains many interefting and accurate defcriptions. Should tlie circumftances of the times permit, the 

 authoi: will have itiahis power to follow out, with every nUvautage, the hints I have fuggefted. 



raneous. 



