CHAPTER IX 



VULCANISM 



Vtdcanism is the term applied to all movements of lava toward 

 the surface of the earth, and is made to include certain other phe- 

 nomena closely connected with these movements. In its rise, some 

 lava reaches the surface, giving rise to eruptive or volcanic phenom- 



I "}.'. 221. A dike two feet \vide, cutting through sandstone. Arran, coast of 

 Scotland. (H. M. Geol. Surv.) 



ena; and some intrudes itself into the outer formations of the earth 

 and congeals there. The first gives rise to volcanic rocks, and the 

 second to plulonic. 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. 



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