CHAPTER X 

 VULCANISM 



The term vulcariism is here used to include all movements of 

 lava toward the surface of the earth, as well as certain other phe- 

 nomena connected with these movements. The rise of lava assumes 

 two general phases. The one includes movements by which lava 

 reaches the surface, giving rise to eruptive or volcanic phenomena; 

 the other, movements by which lavas intrude themselves into the 

 outer formations of the earth and congeal underground. The 

 first gives rise to volcanic rocks, and the second to plutonic rocks. 

 The first are extrusive; the second, intrusive; the first constitute 

 eruptions; the second, irruptions. The fundamental nature of 

 the two phases of vulcanism is the same. 



1. INTRUSIONS 



Fluid rock forced into fissures and solidified there forms 

 dikes; forced into chimney-like passages, it forms pipes or plugs; 

 insinuated between beds, it forms sills; accumulated in consider- 

 able bodies which arch the strata up over them, it forms laccoliths 

 (Fig. 6) ; if the overlying beds are faulted up, bysmaliths, while in 

 greater aggregations underground, it constitutes batholiths. Dikes 

 (Fig. 289) and sills vary greatly in thickness, from inches to 

 hundreds of feet. Batholiths may be many miles across, but their 

 depths are not known. Laccoliths and bysmaliths may be looked 

 upon as small batholiths with special features. 



The heating of the adjacent rock by intrusions varies with the 

 mass and temperature of the lava. Thin dikes and sills produce 

 little effect, while greater masses metamorphose the adjacent rock 

 notably. The metamorphism results in part from (1) the heat, in 

 part from (2) the pressure incident to the intrusion, and in part 



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