44 



the top of Katmai during the eruption is estimated at 11,000X10* 

 cubic yards (8400 X 10" cubic meters). 



Associated with the change in the character of the activity was 

 an equally great change in the composition of the magma concerned. 

 The old lavas are dark-colored basicandesites with a silica content 

 of about 60 per cent. 



But the new magma is a white, acid rhyolite with 75 per cent 

 of silica. 



This change in composition of the magma, wljile witliout any 

 particular bearing on the point at issue here, is of great significance 

 in interpreting other aspects of the eruption, for it enables us to 

 gain considerable insight into the processes operation before and 

 during the explosions. 



A second line of proof that the Incandescent Sand Flow could 

 not have been of the type supposed by Dr. Escher is that the slopes 

 of Katmai show no evidence of such a flow having passed over 

 them. As Dr. Escher rightly asserts, a lahar erodes in the upper 

 steep portion of its course. Erosion would have been particularly 

 marked if such a flow had passed down the slopes of Katmai, since 

 they were covered with ice, which would have melted away with 

 great rapidity before a hot lahar. Yet the slopes down which Dr. 

 Escher assumes the lahar to have coursed are still clothed by the 

 glaciers which originally covered them. To be sure, the heads of 

 these glaciers were blown away in the explosions of the summit of 

 the mountain and their toes were melted back by the flow of in- 

 candescent sand across them from Novarupta down the Valley. But 

 these accidents to the extremities only serve to emphasize the un- 

 disturbed condition of the middle slopes down which the hypothe- 

 tical lahar is supposed to have run. 



Instead of having flowed down the slopes of Katmai, the mass 

 clearlj^ moved transversely across the base of the volcano. The high 

 sand mark, i.e. the edge of the flow, slopes steadily from south to 

 north across the foot of Katmai. Its altitude at the south edge of 

 the glaciers is several hundred feet greater than at the north edge, 

 thus indicating that it flowed north along the foot of Katmai rather 

 than westward from its heights. 



A third circumstance which makes it impossible to assign the 

 origin of the flow to Katmai volcano is the distribution of its material. 

 A more detailed contour map then that published with Dr. Escher's 

 argument (see page 45) makes it clear that the greater part of any 

 fluid poured down the western slopes of Katmai would pass through 

 the East arm of the Valley of Ten Thousand Smokes between Knief 



