Effects of Heat modified ly Compression. 203 



from top to bottom of the cliff. These last I conceive to 

 have flowed in rents of the antient mountain, which rents 

 had acted as pipes thxpugh which the lavas of the lateral 

 eruptions were conveyed to the open air. This scene pre- 

 sents to the view of an attentive observer a real specimen of 

 those internal streams which we have just been considering 

 in speculation, and they may exhibit circumstances decisive 

 of the opinions here advanced. For, if one of these streams 

 had formerly been connected with a lateral eruption dis- 

 charged at more than 600 feet above the Alrio del Cavallo, 

 it might possibly contain the carbonate of lime. But could 

 we suppose that depth to extend to 1 70S feet, the inter- 

 ference of air-bubbles, and the action of a stronger heat than 

 was merely required for the fusion of the carbonate, might 

 have been overcome. 



Perhaps the height of Vesuvius has never been great 

 enough for this purpose. But could we suppose ^-Etna to 

 be cleft in two, and its structure displayed, as that of Ve- 

 suvius has just been described, there can be no doubt that 

 internal streams of lava would be laid open, in which the 

 pressure must have far exceeded the force required to con- 

 strain the carbonic acid of limestone, since that mountain 

 occasionally delivers lavas from its summit, placed 10*954 

 feet above the level of the Mediterranean *, which washes 

 its base. I recollect having seen, in some parts of yEtna, 

 vast chasms and crags, formed by volcanic revolutions, in 

 which vertical streams of lava, similar to those of Somma, 

 were apparent. But my attention not having been turned 

 to that object till many years afterwards, I have only now 

 to recommend the investigation of this interesting point to 

 future travellers. 



What has been said of the heat conveyed by internal vol- 

 canic streams, applies equally to that deeper and more ge- 

 neral heat by which the lavas themselves are melted and pro- 

 pelled upwards. That they have been really so propelled, 

 from a great internal mass of matter, in liquid fusion, seems 

 to admit of no doubt, to whatever cause we ascribe the luat 



• Philosophical T. 



of 



