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. 



The general or partial ilagnation of the internal lavas at 

 the clofe of each eruption feems, then, to render it necefTary, 

 that in every new difcharge, the lava mould 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 eaufes, 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 male 

 and fandflone, muft have produced the fame effects upon it that 

 the lava has juft been Hated to produce on the loofe beds of 

 volcanic fcoria. One fiream 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 refilled, 

 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 fcene of its 

 a&ion, 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. 



