82 AUTUMN VISIT TO LIP ART ISLES 



point of intersection, where one of the radial cracks 

 crosses the peripheral crack. It is through these fissures 

 that molten matter finds its way to the surface. 



The eruption of Stromboli is obviously due to the 

 escape of confined steam. We may picture to ourselves 

 a long pipe descending through the axis of the volcano 

 to a great but unknown distance below the ground, and 

 filled with molten lava supplied by some subterranean 

 reservoir. The lava is charged with steam, and this 

 throughout its mass. But the liquid lava exerts a hydro- 

 static pressure upon the steam, equal to about one pound 

 per square inch for every vertical foot, so that no deeper 

 down than the bottom of the cone i.e., at a depth of 



FIG. 17. Diagrammatic Section through a Volcano of the 

 Stromboli type. The black dots represent ascend- 

 ing bubbles of steam, enlarging as they go. 



2,000 feet the pressure is equal to 2,000 pounds, or 170 

 atmospheres, per square inch. 



The lava is in a state of slow circulation in the pipe, 

 the hotter lava rising up the axis, bringing its steam with 

 it, the cooler descending by crawling down the sides 

 (Fig. 17). 



Under the pressure of the lava at the base of the cone 

 the steam is highly compressed, and perhaps exists in a 

 state of solution ; but as it rises the pressure corre- 

 spondingly diminishes, little bubbles begin to appear 

 and grow larger as they ascend, and these gather together 

 beneath the tough, thick film of cooler lava, w r hich covers 

 like a scum the upper surface of the molten column. 

 The steam does not immediately escape owing to the 

 tenacity of this tough film, but when a sufficient quantity 



