6^8' GEOLOGY. \ 



(3) Immediately under an anticlinal arch there may doubtless be some 

 relief of pressure within the limits of strength of the arch, which is not 

 great (p. 582). The pressure under the arch as a whole is greater than 

 before it was bowed up by lateral thrust, and in depth this excess becomes 

 distributed so as to obliterate the local relief under the center of the 

 arch, and so adds the effects of folding to the average pressure of the 

 crust. Besides, as a matter of fact, volcanoes do not appear to be 

 especially associated with mountain folds where arching reaches its 

 best expression. 



(4) The same general considerations bear on the assignment of lique- 

 faction to relief of pressure in connection with the more general deforma- 

 tion of the earth's body. Besides, while relief of pressure might account 

 for liquefaction, it leaves the extrusion without an obvious cause ; indeed, 

 it would seem, to furnish a condition opposed to extrusion, and if pres- 

 sure were subsequently added to force the liquid out, it would tend to 

 restore the solid condition. 



Hypothesis 5. Lavas assigned to melting by crushing. — Mallet ^ and 

 others have attributed melting to the crushing of rock. Crushing, in 

 the ordinary sense of the term, can only take place in the zone of frac- 

 ture, and that is apparently too shallow to meet the requirements of 

 the case. Below this zone, the pressure on all sides is too great to per- 

 mit any separation of fragments, and a solid mass can only change its 

 form by what is called '^ solid flowage." The rock under these condi- 

 tions may be compressed, and this compression must give rise to heat, 

 but at the same time the melting-point is raised, according to all experi- 

 ments. It seems improbable that melting can be produced in this way. 

 If great pressure could be brought to bear upon a tract of rock so as to 

 heat it by compression, and if then the pressure were relaxed before 

 the heat generated could be distributed by conduction, and if re-expan- 

 sion did not follow, possibly melting could be effected, but this makes 

 the process complicated and apparently inapplicable. It is scarcely 

 possible that such a sequence of events can have affected all the tracts 

 that are now volcanic, much less all those that have been such through- 

 out geologic history. As noted in the preceding case, relaxation w^ould 

 seem to be unfavorable to expulsion. Besides, volcanoes do not seem 

 to be confined to tracts that show signs of great crumpling and crushing, 

 as the Alps, the Appalachians, and the closely folded ranges generally. 



1 Phil. Trans., 1873. 



