594 CLARENCE N. FENNER 



a rock of the composition of that in which they are found. More- 

 over, pieces of banded and variegated pumice are common, such 

 as those previously described as occurring in the Valley of Ten 

 Thousand Smokes (see page 583), and are attributed to solution of 

 basic rock in the new siliceous magma just prior to ejection. The 

 evidence on these matters will be discussed more fully a little 

 farther on. It appears to show that the new magma, when it 

 rose into the crater, possessed a sufficient degree of superheat to 

 cause it to attack corrosively the basic rock of the crater walls, 

 and, within a brief period, to effect sufi&cient solution to permit 

 the dispersal throughout its own mass of the basic phenocrysts 

 derived from great quantities of foreign rock-material. The heat 

 requirements seem to demand that in addition to the original store 

 large accessions should have been received, possibly from rising 

 gases.'' 



■ In any case a new synthetic magma is believed to have been 

 formed in large quantities. The rapidity of destruction of the 

 walls would be attributed to a combination of the shattering effect 

 of explosions and the corrosive action of magma lying in a pool in 

 the crater. The disappearance of much of the rock of the walls 

 may thus be accounted for. Whether all of it may be accounted 

 for by this process and by the ejection of fragments of undissolved 

 rock is not certain. Further evidence will be obtained from 

 analyses ^which Dr. Allen has undertaken) of selected material 

 representative of the new magma, little affected by digestion of 

 basic rock; and of other material, representative of the average 

 result attained by the digestion of foreign matter. 



The alternative hypothesis, that of crater subsidence, is one in 

 regard to which little or no direct evidence has been observed. 

 Apparently rock-slides of considerable importance have occurred 

 at several places in the crater, due to the failure of the vertical 



I Daly's discussion of the effect of rising gases {Igneous Rocks and Their Origin, 

 p. 267) is rather misleading. It is true that a bubble of gas, expanding and doing 

 work, loses energy approximately equivalent to the work done, but if the work be 

 expended in producing viscous flow in the surrounding magna, the energy lost by the 

 gas is taken up by the magma, and the system as a whole neither gains nor loses. We 

 may therefore disregard expansion and look upon gas rising from below into a cooler 

 region as a source of heat. 



