THE KATMAI REGION, ALASKA 597 



dark, dense material in small fragments. These, to the number 

 of hundreds, have been studied under a binocular magnifier. Their 

 indentification has not been difficult. Sediments excluded, they 

 consist usually of andesites containing small phenocrysts or groups 

 of phenocrysts of plagioclase, hornblende, pyroxene, and mag- 

 netite in a felsitic groundmass. In some, however, the pheno- 

 crysts are absent. Most are dense but some are vesicular. In 

 microscopic section the small feldspars of the groundmass are fre- 

 quently seen to be arranged in flow-lines. It is evident that some 

 of the specimens belong definitely to surface types of rocks and 

 all of them may well be such. The possibility that hypabyssal 

 rocks also may be present cannot be wholly excluded, but no 

 evidence of this origin for any of them has been recognized. In 

 composition they are medium to basic andesites, apparently no 

 different from the rocks that form the walls of the crater pit. 



Evidence is at hand regarding the absorption of these andesites 

 by the new magma, but before we proceed to consider this matter 

 it is necessary to digress for a moment. 



As previously indicated, it is supposed that the fragments of 

 andesite found in the ash-fall, or at least a large proportion of 

 them, represent wall-rock that collapsed and became immersed in 

 the pool of lava. One might expect, therefore, that similar pieces 

 would frequently be found as inclusions in the pumice. This does 

 not seem to be the case; on the contrary, their mode o^ occurrence 

 is nearly always as detached particles. This is a matter that 

 requires examination, and several features have been noted which 

 have a bearing upon the subject. It is found, first, that not only are 

 inclusions of andesites rare in the pumice but likewise inclusions 

 of shale and of all other dense material except the phenocrysts; 

 second, many of the fragments, although not now inclosed in 

 pumice, present evidences of previous immersion, such as films 

 of glass adhering to their surfaces, and corrosion effects; third, 

 although the pumice does not carry inclusions, the obsidians, 

 which must have been derived from the same lava-pool as the 

 pumice, carry great quantities of both shale and andesite. From 

 these facts it seems that the conclusion to be drawn is not that 

 the andesites were never immersed in the magma, but that, in 



