166 EFFECTS of HEAT 
with the ‘prefent inner cone, which, in fome great. eruption, 
had been deftroyed all but this fragment. In our own times, 
an event of no fmall magnitude has taken: place on the fame 
fpot ; the inner cone of Vefuvius having undergone fo great a 
change during the eruption in 1794, that it now bears no re- 
femblance to what it was when I faw it in 1785. 
Tue general or partial ftagnation of the internal lavas at 
the clofe of each eruption feems, then, to render it neceflary, 
that in every new difcharge, the lava fhould begin by. ma- 
king a violent laceration. And this is probably the caufe 
of thofe tremendous earthquakes which precede all great erup- 
tions, and which ceafe as foon as the lava has found a vent. It 
feems but reafonable to afcribe like effects to like caufes, and 
_to believe that the earthquakes which frequently defolate coun- 
tries not externally volcanic, likewife indicate the protrufion 
from below of matter in liquid fufion, penetrating the mafs of 
rock, 
- THE injection of a whinftone-dike into a frail mafs of hale 
and fandftone, muft have produced the fame effects upon it that 
the lava has juft been ftated to produce on the loofe beds of 
volcanic fcoria. One ftream of liquid whin, having flowed into 
fuch an aflemblage, muft have given it great additional weight 
and ftrength : fo that a fecond ftream coming like the firft, would 
be oppofed by a mafs, the laceration of which would produce 
an earthquake, if it;were overcome ; or by which, if it refifted, 
the liquid matter would be compelled to penetrate fome weaker 
mafs, perhaps at a great diftance from the firft... The internal 
fire being thus compelled perpetually to change the {cene_ of its 
action; its influence might be carried to an indefinite extent: 
So that the intermittance in point of time, as well as the verfa- 
tility in point of place, already remarked as common to the 
Huttonian and Volcanic fires, are accounted for on our princi- 
ples. 
