
<a 8 . ‘ 1 
Part L Sect. i§ 2] VOLCANIC FISSURES. 213 
series of six parallel fissures opened on the side of the mountain. One 
of these, with a breadth of two yards, ran for a distance of 12 miles, 
in a somewhat winding course, to within a mile of the top of the cone. 
Similar fissures, but on a smaller scale, have often been observed on 
Vesuvius; and they are recorded from many other volcanoes. 
Two obvious causes may be assigned for the fissuring of a volcanic 
cone :—(1) the enormous expansive force of the imprisoned vapours 
acting upon the walls of the funnel and convulsing the cone by 
successive explosions; and (2) the hydrostatic pressure of the lava- 
column in the funnel, which may be taken to be about 120 Ib. per 
square inch, or nearly 8 tons on the square foot, for each 100 feet of 
depth. Both of these causes may act simultaneously. 
Into the rents thus formed the molten lava naturally finds its way, 
or is forced, and it solidifies there like iron ina mould. The cliffs of 
many an old crater show how marvellously they have been injected 
by such veins or dykes of lava. Those of Somma, and the Val del Bove 
on Etna (Fig. 35), which have long been known, project now from the 
woes SE 












































































































Fie. 35.—View or Lava-pykES, VAL DEL Bove, Hrna (ABICH). 
softer tuffs like walls of masonry. The crater cliffs of Santorin also 
present an abundant series of dykes. Such wedges of solid rock 
driven into the cone must widen its dimensions, for the fissures are 
not due to shrinkage, although doubtless the loosely piled fragmentary 
materials in the course of their consolidation develop lines of joint. 
Sometimes the lava has evidently risen in a state of extreme 
fluidity and has at once filled the rents prepared for it, cooling 
rapidly on the outside as a true volcanic glass, but assuming a dis- 
