6oo CLARENCE N. FENNER 



penetration by the magma reached a fairly advanced stage before 

 the final inflation occurred, these dark masses as well as the light 

 glass assumed the porous condition; both must have been charged 

 with vapors. 



Although, under some circumstances, the bands or schlieren 

 that have arisen from these pasty masses remain sharply distinct 

 for several inches and perhaps much more, it is not unusual, on 

 the other hand, for bands to disappear within a short distance. 

 The facility with which the banding has become obliterated in 

 these cases shows that its sharp definition is not a property that 

 persists in spite of turbulent movements. 



In the pumice of the early strata of the ash-fall, phenocrysts are 

 almost lacking; in later strata they become exceedingly abundant. 

 Their appearance at this later stage is ascribed to the setting free of 

 phenocrysts from the andesitic wall-rock in the manner described. 

 Their character in the two environments has been carefully com- 

 pared. Those in the andesites have been studied in microscopic 

 sections, and also under a binocular magnifier in such specimens 

 as show surface corrosion, which has left them in partial relief. 

 The phenocrysts of the pumice have, many of them, been set free 

 by explosions and occur loose in the ashy strata. These are easily 

 studied with the binocular magnifier. Others, still inclosed within 

 a matrix of pumice or obsidian, have been examined in thin sec- 

 tions. In the andesites the phenocrysts occur both as isolated 

 crystals within the felsitic groundmass and as aggregates of the 

 kind that Judd has called glomeroporphyritic groups, and have 

 certain characteristics in regard to size, form, and grouping. The 

 component minerals are pyroxene, hornblende, plagioclase, and 

 magnetite. In the pumice we find the same minerals, isolated or 

 in the same sort of groups as before, apparently duplicating in all 

 respects the phenocrysts of the andesites. 



Let us review briefly the evidence that has been presented on 

 this matter. An immense amount of material has disappeared 

 from the top of the mountain and from the crater walls, and must 

 be accounted for. In the pumiceous strata fragments of andesites 

 are found that have the characteristics of surface-flow rocks, and 

 correspond to what is known of the rocks in the crater walls. The 



