CAUSES OF VULCANISM 



243 



kinds of matter, mixed as they happened to fall in. If its tempera- 

 tun- rises, the fusion-points of some of its 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. The gases and volatile constituents in the original material 

 would obviously unite with the liquid part. With continued rise 

 of temperature, the liquefaction would extend itself until adjacent 

 pockets or threads of lava 

 united, and the lighter portions 

 of the fluid would be forced 

 upward and would work their 

 way toward the surface by fus- 

 ing and fluxing. 



As the lavas rise, the pres- 

 sure on them becomes less, and 

 hence the temperature neces- 

 sary for liquefaction gradually 

 falls, leaving them a constantly 

 renewed margin of temperature 

 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 way up to the zone 

 of fracture, and thence, taking 

 advantage of fissures and 

 cracks, reach the surface (Fig. 

 233). It is conceived that such 

 liquefaction and extrusion 

 would carry the excess of tem- 

 perature received by the lower 

 part of the outer zone toward 

 the surface, or even out to it. 

 The outward movement of the 

 lava would tend to lower the temperature below, forestalling gen- 

 eral liquefaction, and keeping the zone as a whole, solid. The 

 independence of volcanoes is assigned to the independence of the 

 liquid threads that work their way to the surface. Nothing like a 

 reservoir or molten lake enters into the conception. The prolonged 



Fig. 233. Ideal section of a portion of 

 the early earth, illustrating its assigned 

 modes of vulcanism. C, center; S, sur- 

 face; a-n', fragmental zone; a'-f, zone of 

 continuous rock below melting tempera- 

 ture at the surface; ff-c, interior portion 

 whose temperatures rise from the surface 

 melting-point at /-/ to a maximum at C; 

 V, V, threads or tongues of molten rock 

 rising from the interior to various levels, 

 many of these lodging within the frag- 

 nu-ntal /.one as tongues, batholiths, etc.; 

 PPP, explosion pits formed by volcanic 

 gases derived from tongues of lava below. 



