84 AUTUMN VISIT TO LIPARI ISLES 



canic pipe, having been injected from it into the 

 surrounding country. From this we may fairly con- 

 clude that similar injections take place from time to 

 time beneath the base of existing and active cones, as 

 well as into the cones themselves. But a little con- 

 sideration will show that one effect of this will be in all 

 probability to reduce the pressure in the volcanic pipe, 

 and that rapidly. Thus in the case of the volcanic pipe 

 represented in the diagram (Fig. 18), we may suppose 

 the lava to sink from a to b as it fills the fissure d ; the 

 pressure will then be diminished -by the height of the 

 vertical column ab. On the other hand, we may suppose 

 the lava to well up from c to &, and then to escape into 



FIG. 18. Diagrammatic Section of a Volcano of the 

 Vesuvian type, a, b, c, the chimney ; d, a dyke 

 or sill. 



the fissure d. But this will also bring about a reduction 

 in the pressure, measured by the vertical height of the 

 column cb. Thus in either case a fall in pressure will 

 result, which may also be very rapid, even sudden. The 

 steam will consequently flash into a much greater volume, 

 and so blow the contents of the pipe out into the air. 

 Successive enlargements and injections of the fissure at 

 d will be accompanied by corresponding explosions of the 

 volcano. The liquid lava may be reduced to fine spray 

 by the violently escaping steam, and thus those finely- 

 divided particles spoken of as volcanic ash will be 

 produced. If this interpretation be correct, then every 

 spasmodic eruption of a volcano may be interpreted as 

 a fresh injection of some subterranean fissure, and, vice 



