CHAPTER X. 



c. THE EXTRUSIVE PROCESSES. 



Outward movements. — In the preceding chapters movements toward 

 the center have been considered. The complementary processes of 

 outward movem,ent now invite attention. Without doubt these are 

 mainly but a resultant of the centripetal actions. For each pound 

 of material moved outwards an equivalent is quite surely moved inwards. 

 Notwithstanding this, the outward movements have a peculiar nature 

 of their own, and serve a function of radical importance in the economy 

 of the globe. Some minor phases have been incidentally considered, 

 such as the upward flow of springs and deep-seated waters, but here 

 the des(i'ehding and ascending factors are alike, and are closely and 

 otviously connected. 



VULCANISM. 



The great example of ascensive action is the movement of fluid rock 

 from the interior outwards. The term vulcanism will be used to embrace 

 not only volcanic phenomena in the narrower sense, but all outward 

 forcing of molten material, whether strictly extrusive or merely ascensive. 



The philosophy of this ascensive action, taken as a whole, is simple. 

 In the effort at concentration under the powerful action of the earth's 

 gravity, the material of high specific gravity is urged more strongly 

 toward the center, volume for volume, than that of less specific gravity, 

 and as gravity is perpetually active, it follows that whenever any move- 

 ment, molecular or molar, takes place which permits a readjustment 

 of the positions of the two kinds of matter, the heavier sinks toward 

 the center and the lighter rises, or at least tends to do so. So also 

 where there are stress- differences, the mobile matter tends to flow from 

 the regions of greater stress toward those of lesser stress. In so far as 

 any portion of the interior becomes liquid, it is free to move up or down 

 according to the balance of stress brought to bear upon it, and adapts 

 itself to any line of least resistance available to it. As a natural result, 

 therefore, the portion of the interior which becomes fluid most largely 



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