Dec, 1918] Are the Ten Thousand Smokes Real Volcanoes? 115 



neighbor, as may be judged from (a) the larger size of the 

 cinders thrown out; (b) the greater depth of the deposits 

 immediately around the vent, (see cut below) ; (c) their lesser 

 distribution, for they cannot be identified beyond a few miles 

 from the volcano. At the close of this explosive period a con- 

 spicuous crater ring was thrown up around the vent. This was 

 followed by the gradual extrusion of the lava plug. 



Concerning the size, condition, and geological relations of 

 the other portions of the mass of subterranean magma, we are 

 of course, left entirely to speculation. There are the best of 



Photograph by R. F. Griggs 

 A FAULT SCARP ON BROKEN MOUNTAIN. 



The man gives the scale. This is the same fissure as that shown in the distance 

 on page 98. The visible face of the scarp is composed of stratified ash. mostly 

 from Novarupta. The sequence of the ash strata shows that Novarupta 

 burst forth before the explosion of Katmai, but this faulting occurred after 

 the eruption. 



reasons for supposing that it approaches closely to the surface 

 over the whole area of the Valley, and over Katmai Pass as 

 far as Observation Mountain as well, for over all of this area 

 there is clear evidence of fumarole action. The sections through 

 the tuff described above, which gave us our first intimation 

 that the steam might reach the surface through the sandstone, 

 were almost at the very foot of the Valley, 13 miles from 

 Novarupta. Some of the largest and hottest of all the volcanoes, 

 e. g., that pictured on page 100, occur far down toward the foot 



