October 2isf, igo2.'\ PROCEEDINGS. iii 



under favourable circumstances deposits itself around the vent, 

 whose top level may easily be raised to the level of the intake 

 without in the least reducing the flow, even if the process illus- 

 trated by Mr. Yarrow is at work. In nature, the underground 

 temperature contours must adapt themselves to the surface 

 contours of the rising land around the vents, and the greatest 

 heat will then be applied to the up-flowing water in the vent 

 and not to the down-flowing water in the porous strata. 



Thus far the action in hot springs and in the miniature 

 volcanoes is identical, for wherever in a boiler the scale is 

 thickest, there will the underlying boiler plate with its fire on the 

 outside be the hottest, and, of course, the temperature of thick 

 scale will be hotter than that of the surrounding thin scale. 



Should the action of the hot spring continue for a consider- 

 able length of time, then its vent might rise to a considerable 

 height above the intake, but only if the downtake channels 

 descend to considerable depths. For obvious reasons this con- 

 dition is most generally to be found near ocean shores, and it 

 is here also that high volcanoes are most frequent. On first 

 thoughts it might seem impossible for volcanoes like Chimborazo 

 to have been built up in this manner, for the difference of 

 pressure represented by a height of 27,000 feet above sea level, 

 is greater than the difference of pressure due to down-flowing 

 cold water and up-rushing superheated water and steam. Sup- 

 pose, however, that this volcano had not grown ; suppose that 

 it had once been a mountain and that imprisoned superheated 

 water and steam had blown off its cap, then it is necessary to 

 explain why this water, which must have been led downwards 

 through some natural channel, should not flow back into the 

 ocean instead of overcoming an earth pressure represented by at 

 least twice the height of the mountain. It is possible to conceive 

 this action with low mountains, and it is notorious that the most 

 violent eruptions have occurred with them, but it is impossible 

 that high volcanoes should have been formed in this manner. 



One notable feature of high volcanoes is that they are always 

 active and their vents are always open ; under such conditions, 



