MODIFIED by COMPRESSION. 165 



It appears, at firft fight, that a lava having once pene- 

 trated the fide of a mountain, all fubfequent lavas mould conti- 

 nue, as water would infallibly do, to flow through the fame 

 aperture. But there is a material difference in the two cafes. 

 As foon as the lava has ceafed to flow, and the heat has begun 

 to abate, the crevice through which the lava had been pafling, 

 remains filled with a fubflance, which foon agglutinates in- 

 to a mafs, far harder and firmer than the mountain itfelf. This 

 mafs, lying in a crooked bed, and being firmly welded to the 

 fides of the crevice, mufl oppofe a moft powerful refiflance to 

 any ftream tending to purfue the fame courfe. The injury 

 done to the mountain by the formation of the rent, will thus 

 be much more than repaired ; and in a fubfequent eruption, 

 the lava mufl force its way through another part of the moun- 

 tain or through fome part of the adjoining country. The 

 action of heat from below, feems in moft cafes to have kept a 

 channel open through the axis of the mountain, as appears 

 by the fmoke and flame which is habitually difcharged at the 

 fummit during intervals of calm. On many occafions, how- 

 ever, this fpiracle feems to have been entirely clofed by the 

 confolidation of the lava, fo as to fupprefs all emiflion. This 

 happened to Vefuvius during the middle ages. All appearance 

 of fire had ceafed for five hundred years, and the crater was 

 covered with a foreft of ancient oaks, when the volcano open- 

 ed with frefh vigour in the fixteenth century. 



The eruptive force, capable of overcoming fuch an ob- 

 ftacle, mufl be tremendous indeed, and feems in fome cafes 

 to have blown the volcano itfelf almofl to pieces. It is im- 

 poffible to fee the Mountain of Somma, which, in the form of a 

 crefcent, embraces Mount Vefuvius, without being convinced 

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



with 



