THE K AT MAI REGION, ALASKA 6oi 



evidence of fumarolic action that many of them bear is in accord 

 with such a situation. These might account for the rock that has 

 disappeared except that they are quantitatively insufiEicient. We 

 are left, then, to consider crater subsidence versus incorporation 

 in the new magma. Without trying to decide in this article 

 whether all of the material may be accounted for by incorporation, 

 the evidence that a large quantity has been taken up in this man- 

 ner has been considered. According to this evidence, numerous 

 specimens of andesite show attack. The processes involve either 

 an intimate penetration and consequent softening of the whole 

 mass, followed by dispersal of the phenocrysts; or the breaking up 

 of the fragments by attack along fissures, simultaneously with 

 solution of the groundmass around the periphery of the fragments 

 and eventual setting free of the phenocrysts. Finally, multitudes 

 of phenocrysts of the kind that the andesites carried are found to 

 appear in the later strata of the pumice, though the earlier strata 

 are practically free from them. Their instability with respect to 

 their surroundings is indicated by the active disintegration that 

 they are undergoing. The fact that quartz phenocrysts properly 

 belonging to the magma have no association with them is also 

 significant. These facts taken together seem to form strong 

 evidence identifying the phenocrysts of the pumice with those of 

 the former wall-rock, and the disappearance of large quantities of 

 wall-rock is thereby accounted for. 



Some of the materials in the ash-strata deserve further atten- 

 tion. The obsidians that have been described, when heated in the 

 laboratory, swell up to a frothy white pumice closely resembling 

 the pumice found in the field. When their powder is heated in a 

 closed tube it yields water, hydrogen sulphide, hydrochloric acid 

 or a chloride, and some gas having a fetid organic odor. It is 

 planned to investigate these gases with care. 



Although many phenocrysts of extraneous origin are found in 

 the pumice, the only phenocrysts that properly belong to the magma 

 are quartz and acid plagioclase. These had probably crystallized 

 out before the magma rose into the crater. The quartz, which is 

 easily recognized, is never associated with the groups of xenocrysts 

 mentioned, but, on the contrary, is found in the purest rhyolitic 



