THE KATMAI REGION, ALASKA 589 



amount of energy is set free with great suddenness in a narrowly 

 confined space, but that in some manner this force is given a 

 definitely directed tendency upward. The inherent difficulties in 

 this conception are so great that we naturally look for a simpler 

 explanation. 



We might suppose, as a second possibihty, that the underlying 

 magma possessed great expansive powers because of dissolved 

 gases which were struggling to escape, and was thus enabled to effect 

 an upheaval of overlying material, but the natural result of this 

 would be the upturning of huge blocks over a rather wide area, 

 and the escape of pumiceous material from many widely open 

 fissures, accompanied by the ejection of portions of the fractured 

 blocks in large masses. No such evidence is visible around Nova- 

 rupta. The ejection of pumice seems to have been confined 

 mostly to a small opening, and there is no hint in the surface 

 contours of the ash that a widespread chaotic upheaval of strata 

 occurred. Moreover, the largest pieces of ejected sediments found 

 were about the size of one's fist. We turn, therefore, to a third 

 hypothesis, which is really the simplest of all: that fissuring was 

 first produced, either because of regional strain or because of 

 hydrostatic pressure due to the injection of a sill; and that the 

 magma rose along such fissures in much the fashion that any 

 liquid might do, except that a certain amount of solution was eft'ected 

 and that near the surface a sudden conversion into pumice resulted 

 in the violent abrasion of the walls. Under this conception Nova- 

 rupta would simply represent a channel along one of the fissures, 

 where chance conditions made escape specially favorable and which 

 therefore tended to enlarge the conduit rapidly and establish more 

 direct connection with the body of magma below. It will appear 

 farther on in this article, as various topics are discussed, that none 

 of the phenomena of the Katmai eruption seem to indicate that 

 these magmas exerted great explosive or expansive powers at 

 depths within the earth, and Novarupta conforms to this idea. 

 Undoubtedly explosions of a violent character occurred here after 

 the magma had reached the surface, but no evidence was found of 

 such explosions prior to its ascent. 



