VULCANISM 389 



when felt by rocks already heated above what would be their melt- 

 ing points at lowered pressures, has been held to be a possible cause 

 of vulcanism. Such relief of pressure is assigned to faulting and 

 denudation. But many volcanoes are located in the bottom of 

 the ocean, where denudation does not take place, and faulting that 

 would give relief of pressure is not always related to vulcanism in 

 any clear way. Melting by crushing has been suggested, but in the 

 deeper parts, crushing involves increase of pressure, which opposes 

 melting. Depression to the zone of high temperature, under 

 accumulated sediments, is also assigned as a cause of melting, but 

 there is very little sedimentation in the ocean far from land where 

 many volcanoes are situated. 



If the earth grew up by slow accessions of matter, and if its 

 interior heat is due chiefly to the internal compression resulting 

 from growth, the distribution of internal temperature would be 

 as shown in Fig. 288, p. 362. With like conductivity, the flow of 

 heat from the deep interior to the middle zone of the earth would 

 be greater than the loss from this zone to the superficial shell. The 

 middle zone might thus rise in temperature. This zone is, under 

 this view, supposed to be composed of various kinds of matter, 

 mixed as they happened to fall in. If the temperature rises, the 

 fusion-points of some of these constituents will be reached sooner 

 than those of others. A fusion or solution of the more soluble 

 portions may thus take place while the rest of the rock remains 

 solid. To the liquid part the gases and volatile constituents in 

 the original material would obviously unite, as being also lique- 

 fiable parts. With a continued rise of temperature, the lique- 

 faction would extend itself until adjacent pockets or threads of 

 lava found means of uniting, and the lighter portions of the fluid 

 would be forced upwards and work their way toward the surface 

 by fusing and fluxing. 



As these portions rise, the pressure upon them becomes less 

 and less, and hence the temperature necessary for liquefaction 

 gradually falls, leaving them a constantly renewed margin of tem- 

 perature available for melting their way through the upper horizons. 

 Thus it is conceived that these fusible and fluxing selections from 

 the middle zone might thread their ways up to the zone of fracture, 



