Effects of Heal mod'ified hy Compression. 20i 



vals of calm. On many occasions, however, this spiracle 

 seems to have been entirely closed by the consolidation of 

 the lava, so as to suppress all emission. This happened to 

 Vesuvius during the middle ages. All appearance of fire had 

 ceased for five hundred years, and the crater was covered 

 with a forest of antient oaks, when the volcano opened with 

 fresh vigour in the sixteenth century. 



The eruptive force capable of overcoming such an ob- 

 stacle must be tremendous indeed, and seems in some cases 

 to have blown the volcano itself almost to pieces. It is im- 

 possible to see the mountain of Somma, which, in the form, 

 of a crescent, embraces Mount Vesuvius, without being 

 convinced that it is a fragment of a large volcano, nearly 

 concentric with the present inner cone, which, in some 

 great eruption, had been destroyed all but this fragment. 

 In our own times, an event of no small magnitude has taken 

 place on the same spot ; the inner cone of Vesuvius having 

 undergone so great a change during the eruption in 1794, 

 that it now bears no resemblance to what it was when I saw 

 it in 1785. 



The general or partial stagnation of the internal lavas at 

 the close of each eruption seems, then, to render it neces- 

 sary, that in every new discharge the lava should begin by 

 making a violent laceration. And this is probably the cause 

 of those tremendous earthquakes which precede all great 

 eruptions, and which cease as soon as the lava has found a 

 vent. It seems but reasonable to ascribe like effects to like 

 causes, and to believe that the earthquakes which frequently 

 desolate countries not externally volcanic, likewise indicate 

 the protrusion from below of matter in liquid fusion pene- 

 trating the mass of rock. 



The injection of a whinstone-dike into a frail mass of 

 shale and sandstone, must have produced the same effects 

 upon it that the lava has just been slated to produce on the 

 loose, beds of volcanic scoria. One stream of liquid whin, 

 bavj '1 uved into such an assemblage, must have given it 

 ional weight and strength ; so that a second stream, 

 ■he fiist, would be opposed by a mass the lace- 

 fa' ■' hu h wuulJ produce an earthquake if it were over- 

 come, 



