THAT VOLCANOES ARE " BURNING " MOUNTAINS 191 



tents of the globe, which is covered with a comparatively 

 thin crust (from thirty to fifty miles thick) that has cooled 

 off during past ages and is now in a condition in which 

 organic beings can live upon its surface. A volcano is 

 simply a hole in this crust through which the melted matter 

 of the interior and the steam produced by the infiltration 

 of water are ejected. Several causes may contribute to 

 the ejection of this volcanic material, amongst the prin- 

 cipal being the following: 



1. The access of sea water through one or more fissures, 

 thus producing enormous pressure, a pressure so great 

 that dust and cinders have been projected to a height of 

 10,000 feet. That sea water is the cause of at least some 

 eruptions is rendered probable by the large proportion of 

 chlorides present in the ejected matter. 



2. The pressure of deposits at the bottom of the ocean, 

 these deposits consisting of material washed down from 

 mountain ranges and other regions through which large 

 rivers flow. For while the average pressure over the entire 

 globe would not be disturbed by this action, it is very 

 evident that large local deposits over a limited area might 

 easily cause the comparatively slight disturbance which 

 would be necessary to produce volcanic phenomena. These 

 phenomena, when compared with the vast amount of ma- 

 terial carried out to sea by some of our large rivers, are 

 small. Of the amount of this material few people have 

 any conception. The greatest works of man in moving 

 rocks and earth are insignificant when compared with it. 

 The weight of this material might easily cause a local sink- 

 age of the crust quite sufficient to set a volcano in action 

 or to open up a new vent at some distant point along the 

 line of least resistance. 



