I 212 ACTION OF HEAT MODIFIED, &C. 



separately in Fig. 44.) > we see the internal structure of the 

 mountain, composed of thick beds (k k) of loose scoria, which 

 have fallen in showers ; between which thin but firm streams 

 (mm) of lava are interposed, which have flowed down the 

 outward conical sides of the mountain. (Fig. 43. is an ideal 

 section of Vesuvius and Somma, through the axis of the cones, 

 shewing- the manner in which the beds of scoria and of lava 

 lie upon each other; the extremities of which beds are seen 

 edgewise in the cUff at m m and k k, Fig. 42. 43. and 44.) 

 Explauation, This assemblage of scoria and lava is traversed abruptly and 

 vertically, by streams of solid lava ^w n, Fig. 44.) reaching from 

 top to bottom of the ciitf. These last 1 conceive to have flowed 

 in rents of the ancient mountain, which rents had acted as pipes 

 through which the lavas of the lateral eruptions were conveyed 

 to the open air. This scene presents 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 oi these streams had formerly been connected with a late-; 

 ral eruption, discharged at more than 600 feet above the Atria 

 del Cavallo, it might possjibly contain the garbonate of lime. 

 But could we suppose that depth to extend to 1708 feet, the 

 interference of air-bubbles, and the action of a stronger heat 

 than was merely required for the fusion of the carbonate, migh$ 

 have been overcome. 

 jLarger scale of Perhaps the height of Vesuvius has never been great enough 

 Mount^luL. f°^ ^^"^ purpose. But could we suppose .Etna to be cleft in 

 two, and its structure displayed, as that of Vesuvius 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 constrain the carbonic acid o^ 

 limestone ; since that mountain occasionally delivers lavas from 

 its summit, placed 10.954 feet above the level of the Mediter- 

 ranean,* which washes its base. I recollect having seen, in some 

 parts of ^tna, vast chasms and crags, formed by volcanic re- 

 volutions, 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. 



CTo be concluded in mir next. J 

 * riiil. Trans. 1777, p. 595. 



