MODIFIED ly COMPRESSION. 165 
Ir appears, at firft fight, that a lava having once pene- 
trated the fide of a mountain, all fubfequent lavas fhould 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 fubftance, 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, muft oppofe a moft powerful refiftance 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 muft 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 
fammit 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 emiffion. 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. 
Tue eruptive force, capable of overcoming fuch an ob- 
ftacle, muft be tremendous indeed, and feems in fome cafes 
to have blown the volcano itfelf almoft 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 
