Lavis — On the Structure of Rocks. 131 



in power — a fact thoroughly borne out by experience. Under the 

 same conditions, in a lateral opening of a very high volcano, such 

 as Etna, the amount of lava will be greater as the chimney is 

 higher above the outlet, since it will hold more. But beyond this, 

 the amount of output will be more than that contained in the 

 chimney above the level of exit, but also indirectly as the pressure 

 of that amount is removed. When the lava pours out laterally 

 from the chimney, its superincumbent weight, being removed, will 

 allow expansion of the elastic matter below the level of exit, so 

 that as this rises to establish a balance, lava will continue to pour 

 out from the lateral outlet until total equilibrium is obtained. In 

 this way the amount of lava spread over the surface will be much 

 more than that contained in the chimney above the level of the 

 exit. This fact I have been able to verify on various occasions in 

 the recent small eruptions of Vesuvius, which are in many ways 

 more instructive than great ones on account of permitting near 

 approach to be made. This, I believe, will eventually be proved 

 to be the true mechanism of lateral eruptions. As examples, we 

 may compare the bulk of the lava products from the lateral craterets 

 of Vesuvius and Etna. 



Conditions which determine the Extinction of a Volcano. — We have 

 already seen how a dyke that has not manifested itself at the sur- 

 face may solidify, and so divert igneous action to other localities. 

 In the case of explosive eruption, the expansion that takes place in 

 the magma increases its volume, in the form of a pumiceous froth, 

 to such an extent that it occupies many times its original volume. 1 

 A large portion of this spongy magma escapes, leaving the fissure 

 still filled to such depths as to the point where expansion would be 

 prevented, choked, as it were, by this vesicular paste, which may 

 even have solidified by the loss of heat in volatilization, and so 

 may effectually have plugged up the exit. Some such process 

 would really seem to occur in eruptions like those of Krakatoa. 

 We have an illustration of this in opening a bottle of champagne 

 well up, in which case more than half the liquid contents may 

 escape in the form of froth. In the expanding magma there 

 would be no distinct line of demarkation between the pumiceous, 



1 The volume of steam at 100° C. is 1696 times that of water at the same tempera- 

 ture. 



L2 



