REPORT OF MEETINGS FOR 1901 49 



the 8ame way as a blast of dynamite does in a quarry : it 

 loosens the masses around, and loosens most of all those which 

 overlie the spot where the explosion has taken effect. Thus 

 a further relief of pressure ensues ; more explosions follow ; 

 still more rock is softened ; additional quantities of water 

 find their way in ; the lateral pressure squeezes the softened 

 rock upwards, just as a painter squeezes oilcolours out of a 

 tube ; and so, step by stop, one earthquake follows another 

 with increasing rapidity and violence, until, eventually, an 

 explosion more violent than usual opens up a communication 

 with the surface, and a volcano starts into existence. 



When the volcanic stage is reached other factors come into 

 play. (1) The sea-water which finds its way down to the 

 volcanic focus is driven off at the surface in the form of steam ; 

 hence its dissolved constituents, such as the potassium and 

 sodium compounds, which are not so driven off, are steadily 

 concentrated, and thus play a part of ever-increasing importance 

 in the volcanic economy. The frequent expulsion of softened or 

 liquefied rock-material from the interior of the Earth's crust 

 sets up a lateral flow of other rock-material from the areas 

 around to fill up what would otherwise be a vacuity. This, in 

 its turn, favours an increase of terrestrial movement around, 

 and thereby generates more heat. Thus the action and reaction 

 go on, until, eventually, changed physical conditions lead to 

 the gradual transfer, step by step, of the centres of volcanic 

 action in one area to new centres in the areas adjoining. 



As regards what one maj' term minor features connected 

 with volcanic action only a few call for notice here ; if there 

 were space for more it would hardly be necessary for the 

 purpose in view to discuss them. Amongst these some refer- 

 ence must be made to the products of volcanic action. First 

 of all we may notice what is called tuff, which used to be 

 called " ashes." During explosive eruptions in a volcano the 

 imprisoned water, and the partly-dissociated gases 2 H2 + O2 

 of which water is composed, constantly tend to pass into 

 steam. The superheated water usually does so with explosive 

 violence, and generally gives rise to a succession of explosions 

 rather than to a single powerful one. The resulting sound 

 is rather like that of file-firing, as distinguished from volley- 

 firing, in musketry. The steam liberated at each exjilosioa 



