THE K AT MAI REGION. ALASKA 605 



SUMMARY AND CONCLUSIONS 



A preliminary account has been presented of observations made 

 by the writer as geologist of the expedition sent in 1919 by the 

 National Geographic Society, in co-operation with the Geophysical 

 Laboratory, to the Katmai region. As a result of this work many 

 of the observations made by the director of previous expeditions 

 have been confirmed and supplemented. With regard to one or two 

 others, somewhat different interpretations are given in this article 

 from those of previous publications, but it is believed that on these 

 matters also all concerned are now in agreement. With respect to 

 still other phenomena, which had not been previously described, 

 evidence has been found that affords a basis for extending con- 

 siderably our ideas respecting the processes at work during the 

 eruption. 



It has been found that the volcanoes of this region, which form 

 a continuation of the Aleutian loop or festoon, are situated in an 

 area of sedimentary rocks remarkable for the absence of folding 

 or obvious faulting. The more recent lavas are basic andesites, 

 contrasting greatly in composition with the highly siliceous rhyoHte 

 of the last eruption. 



In the area of the Valley of Ten Thousand Smokes, it is believed 

 that the injection of a sill or closely similar body of magma into 

 the underlying strata at the beginning of the eruption caused 

 shattering of the rocks above it, and these openings permitted 

 the ascent of magma. The extrusion and inflation of this magma 

 gave rise to a great ash- or sand-flow, analogous in many respects 

 to the nuees ardentes of Pelee and La Soufriere, and led to the 

 formation of the parasitic cone of Novarupta. The fumaroles are 

 thought to be due to the continued evolution of volatile constitu- 

 ents from this body of magma. The development of the new vent 

 of Novarupta is ascribed to the enlargement of a channel along 

 one of the fissures. The later extrusion of the stiff lava forming 

 the dome of Novarupta is found to have been similar in many 

 respects to that of the "spine'' of Pelee. 



At Falling Mountain the most interesting features are those 

 resulting from fumarolic action. Evidence of a process of solution 

 and transfer of rock material in the gaseous medium was found 



