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CLARENCE N. FENNER 



manner in which this new vent was developed in the floor of the 

 valley is important. There are undoubted evidences of explosive 

 action, but nothing that may not well be attributed to actions 

 going on after the vent had been opened. What we have to 

 account for here is the formation of a rather small, circular orifice, 

 through which a great amount of pumice was ejected and a small 

 amount of lava was extruded, situated in an area which is much 



Fig. 13. — Profile of lower part of Novarupta, and a portion of the inclosing 

 crater wall. Photograph by P. R. Hagelbarger, 1918. 



fissured but in which other vents of comparable size are lacking. 

 The formation of an orifice of this description is sometimes attrib- 

 uted to the assumed ability of a subterranean body of magma to 

 perforate by explosive action a great thickness of overlying strata 

 and form a cylindrical pipe or conduit (diatreme) for the escape of 

 lava. It is difiicu:lt, however, to form a conception of the manner 

 in which such action has been carried out without attributing to 

 the magma properties for which the evidence seems insufficient. 

 It would be necessary to assume not only that an enormous 



