54 STUDIES, SCIENTIFIC AND SOCIAL chap. 



that the crust floats on the fluid interior, and that it is so 

 thin and weak, comparatively speaking, that it cannot 

 resist a strain equal to its own weight, but must bend or 

 fracture so as to keep every part in approximate hydro- 

 static equilibrium. Volcanic action, especially continuous 

 and permanent volcanic action like that of Stromboli and 

 Kilauea, cannot, therefore, be explained by the mere 

 existence of a thin crust and a molten interior; but it is 

 well explained by the presence in the molten mass of vast 

 quantities of gases existing under enormous pressure, and 

 ready to escape with tremendous force whenever that 

 pressure is greatly diminished, and the molten material 

 that contains it lowered in temperature. 



Let us now endeavour to trace what will happen when 

 a fissure is opened gradually from below upwards till it 

 reaches the surface. Owing to hydrostatic pressure the 

 fluid will rise in the fissure, and in doing so will be subject 

 to some cooling and diminution of density, which, as we 

 have seen, will lead to a liberation of some of the con- 

 tained gas. The pressure of this gas will aid in extending 

 the fissure, and the liquid will continue to rise till it 

 reaches the level of hydrostatic equilibrium, which would 

 be somewhere about two miles below the surface. But 

 throughout the whole mass of the liquid in the fissure, 

 and for some depth below the under surface of the crust, 

 there would be a continual liberation of intensely heated 

 gases. These would no doubt carry with them in their 

 upward rush a portion of the liquid matter which had 

 risen from below, but they would also, owing to their 

 intensely heated condition, melt off some portions of the 

 rocky walls of the fissure, and thus give to the ejected 

 volcanic products a local character. We here see the 

 explanation of the supposed difficuliy of the individuality 

 of neighbouring volcanoes and the diversity of their 

 products, and also of the fact of an eruption of lava from 

 the crater of a lofty mountain while the liquid lava of one 

 close by, and thousands of feet lower, maintains its usual 

 level. Kilauea Ave may suppose to owe its permanently 

 molten lake to a siphon-like passage through which a 

 constant flow of heated gases is maintained, and which 



