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WILLIAM H. HO BBS 



of the roll of sediments beneath a relatively competent member, 

 these inferior sediments support, as we have seen, a considerable 

 portion of the entire superincumbent load. After fusion this 

 support to the competent arch, which has hitherto been in excess of 

 half the load, is now replaced by the molten magma of high incom- 

 pressibility and perhaps also high viscosity. The compressive 

 stress is now exerted through the under limb of the anticline in such 

 a way as to squeeze or compress the macula. Under this action 

 the under limb may suffer greater or less extension and consequent 

 attenuation, but there must also be a tendency for the mass of 

 magma to find an outlet along the path of least resistance, and it 

 may in consequence fuse a course for itself upward toward the 

 earth's surface (Fig. 26). In the upper levels any fractures that 



Fig. 26. — Diagrams to illustrate the tendency to reduction in the volume of 

 progressively underturned anticlines and a consequent cause of the elevation of lava 

 toward the earth's surface. 



may exist are likely to be followed, and more especially fracture 

 intersections. Recent studies make clear that magmas have the 

 capacity of melting their own way by the process of overhead 

 stoping. If such magma arrives at the surface along essentially 

 vertical paths, its loci of emergence will constitute a series of arcs 

 parallel to, and generally behind, the folded mountain arcs beneath 

 which the maculae were developed. Thus we encounter in a con- 

 sideration of the mechanics of folded mountain arcs a possible 

 explanation of the position of volcanic arcs, and a possible solution 

 of the vexed problem of the cause of elevation of magma in volcanoes 

 of markedly Pacific type. 



Volcanic vents once secured at the surface, magma should be 

 exuded or ejected, and partial and further temporary relief be 

 secured below from the compressive stress. Broad relationships 



