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" Pant I. Sect. i.§ 2] LAVA-STREAMS. 291 
the lava makes its appearance at the surface will largely depend 
upon the structure of the ground. Two causes have been assigned 
on a foregoing page (p. 213) for the fissuring of a volcanic cone. 
As the molten mass rises within the chimney of the volcano, con- 
tinued explosions of vapour take place from its upper surface. The 
‘violence of these may be inferred from the vast clouds of steam, 




















































































































































































































































Fic. 40.—Virw oF Housres sURROUNDED AND PARTLY DEMOLISHED BY THE 
Lava oF VEsuvius, 1872. 
ashes, and stones hurled to so great a height into the air, and from 
the concussions of the ground which may be felt at distances of more 
than 100 miles from the volcano. It need not be a matter of sur- 
prise, therefore, that the sides of a great vent exposed to shocks of 
such intensity should at last give way, and that large divergent 
fissures should be opened down the cone. Again, the hydrostatic 
pressure of the column of lava must, at a depth of 1000 feet below 
the top of the column, exert a pressure of between 70 and 80 tons on 
each square foot of the surrounding walls. We may well believe 
that such a force, acting upon the walls of a funnel already shattered 
by a succession of terrific explosions, will be apt to prove too great 
for their resistance. When this happens, the lava pours forth from 
the outside of the cone. On a much fissured cone lava may issue 
freely from many points, so that a volcano so affected has been 
graphically described as “ sweating fire.” 
In a lofty volcano lava occasionally rises to the lip of the crater 
and. flows out there; but more frequently it escapes from some fissure 
or orifice in a weak part of the cone. In minor volcanoes, on the 
other hand, where the explosions are less violent, and where the 
