7 o EXPERIMENTS on WHINStONE and LAVA. 



tions of lire, may be expected to afford numberlefs opportuni- 

 ties of purfuing the comparifon between thefe two claffes *. 



A most interefling fcene for fuch a comparifon occurs like- 

 wife on Vefuvius. The hiftory of this volcano is fimpler than 

 that of iEtna, for it has been evidently formed, with all its ap- 

 pendages, by the continued action of external eruptions, which 

 have raifed it, at fome remote period, from the bottom of a fea, 

 occupying all the Campi Phlegr.xi, and warning the furround- 

 ing Appenines. The whole volcano feems once to have con- 

 lifted of a fingle large cone, the greateft part of which has funk 

 during fome violent eruption, probably that which took place 

 in the time of Pliny, leaving a fragment of its bans, now call- 

 ed the mountain of Somma. This fragment retains its original 

 fhape ; and on the fide fronting the towns of Somma and Ota- 

 jano, the external conical furface, along which the ancient lavas 

 had flowed, is ftill entire. Fronting the centre of the cone, 

 Somma breaks off abruptly, and prefents a vertical cragg, 

 fome hundred feet in height, which is concave inwards. 

 From the gulf, produced by the ruin of the ancient moun- 

 tain, though not exactly from its centre, have arifen the ex- 

 plofions which, by repeated accumulation, have formed the 

 prefent cone of Vefuvius. Next the fea, this cone has extend- 

 ed itfelf fo as completely to cover all remains of the ancient one, 

 forming a continued Hope 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 

 flreams of lava have flowed, producing a narrow horizontal val- 

 ley, in the form of a crefcent, called the Atrio del Cavallo. 

 From this valley the craggs of Somma prefent a complete view 

 of the internal ftructure of the ancient mountain, correfpond- 

 ing, in mofl things, to what might have been fuppofed. 



The various fubftances, depofited fucceflively on the external 

 furface of the ancient cone, being cut vertically in this cragg, 



their 



* M. Dolomieu has obferved this diftin&ion ; but fuppofes that the maffes which 

 we conceive to have flowed fubterraneoufly were erupted at the bottom of the fea. 



