6o2 CLARENCE N. FENNER 



phases of the pumice. Its presence supphes information regarding 

 the upper Hmit of temperature within the magma chamber just 

 prior to extrusion. The transition point between quartz and 

 tridymite at atmospheric pressure is 870° C. Probably great 

 pressure will have a perceptible eiTect in shifting the inversion- 

 point — a thickness of 20,000 feet of rock strata might possibly 

 raise it 1 00°— but we can be fairly sure that a temperature of 

 less than 1000° prevailed. 



The distinctly stratified form of the ash-fall, with its indications 

 of a waning and renewal of activity many times repeated, harmo- 

 nizes with the other evidence presented that the melted rock accumu- 

 lated in a pool in the crater rather than that it was discharged as a 

 continuous stream as soon as some hypothetical obstruction, which 

 had previously restrained its escape from the depths, was removed. 



From the evidence that has been brought together certain 

 deductions may now be made. We see that the magma, as it 

 issued from the depths of the earth, did not at first show a tendency 

 to evolve its gases explosively; that is, did not have an extremely 

 high vapor-pressure; but that this was developed after a short 

 period of standing under the new conditions, and explosive erup- 

 tions ensued. From this we conclude that in the enormous change 

 of conditions consequent upon rapid extrusion internal equilibrium 

 did not keep pace with external changes, and that prior to extrusion 

 such internal combinations prevailed that the tendency of the 

 gases to escape was not extremely great. 



As an indication of the depth in the conduit at which explosions 

 occurred the relative amounts of the various sorts of foreign 

 material ejected with the pumice is of interest. Pieces of andesites 

 from the crater walls are very common; fragments of shale and 

 sandstone from the sedimentary platform are frequently found, 

 but the quantity is not so great as of the andesitic rocks; and 

 pieces of deep-seated granitoid rocks are almost lacking, though a 

 few specimens were found. This relative abundance seems to 

 show that the violently explosive action was exerted only at the 

 surface or at a moderate depth in the conduit. 



If the renewal of activity at this vent after a long period of 

 quiet were due to an accumulation of imprisoned forces until they 

 reached a magnitude where they were capable of blasting away 



