202 Effects of Heat modified by Compression. 



come, or by which, if it resisted, the liquid matter would 

 be compelled to penetrate some weaker mass, perhaps at a 

 great distance from the first. The internal fire, being thus 

 compelled perpetually to change the scene of its action, its 

 influence might be carried to an indefinite extent ; so that 

 the intermittence in point of time, as well as the versatility 

 in point of place, already remarked as common to the Hut- 

 tonian and volcanic fires, are accounted for on our princi- 

 ples : and it thus appears that vvhinstone possesses all the 

 properties which we are led by theory to ascribe to an in- 

 ternal lava. 



This connection is curiously illustrated by an intermediate 

 case between the results of external and internal fire, dis- 

 played in an actual section of the antient part of Vesuvius, 

 which occurs in the mountain of Somma, mentioned above. 

 I formerly described this scene in my paper on whinstone 

 and lava ; and I must beg leave once more to press it upon 

 the notice of the public, as affording to future travellers a 

 most interesting field of geological inquiry. 



The section is seen in the bare vertical cliff several hun- 

 dred feet in height, which Somma presents to the view from 

 the little valley, in form of a crescent, which lies between 

 Somma and the interior cone of Vesuvius, called the Atria 

 del Cavallb. (Fig. 42. represents this scene, done from the 

 recollection of what I saw in 1785. ahc is the interior cone 

 of Vesuvius ; dfg the mountain of Somma; and cde the 

 Atrio del Cavallo). By means of this cliff (fd in fig. 42, 

 and which is represented separately in fig. 44), we see the 

 internal structure of the mountain, composed of thick beds 

 (kk) 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, showing 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 cliff 

 it mm and kk, fig. 45, 43, and 44). 



This assemblage of scoria and lava is traversed abruptly 



jHid vertically by streams of solid lava (.'//?, fig. 41), reaching 



8 from 



