92 PROCEEDINGS OF THE AMERICAN ACADEMY. 



is to keep fluid the top part of the lava column during the volcano's 

 activity. The conception as a whole may therefore be called the gas- 

 fluxing hypothesis. For vents occupied by highly fluid lava this 

 hypothesis as just stated seems to suffice. For craters floored with 

 more viscous lava, the emanating gas issues under more or less high 

 pressure and may function as a melting blast, making more perfect the 

 analogy with an artificial blowpipe. 



Revival of Activity at the End of a Dormant Period. 



One of the leading problems in vulcauism relates to the periodicity 

 of central eruptions. This also seems to find explanation on the gas- 

 fluxing hypothesis. We have seen that the accumulation of gas 

 bubbles in the conduit must be a very slow process. So long as tho 

 vent is open, the escape of the gas from the magma is specially facili- 

 tated. That is true, not because the pressure on the main part of the 

 lava column is less than in times of dormancy, but because of the rapid 

 freeing of gas into the open air, with the consequent rapid production 

 of heavy, gas-freed lava which sinks and thus hastens the two-phase 

 convection. The tendency is, therefore, sooner or later to exhaust the 

 gas concentrated at the lower end of the conduit. With sufficient re- 

 moval of the heat-producing and heat-transferring agent, the forces of 

 cold temporarily win in the never-ceasing struggle and the lava solidi- 

 fies at the surface ; a plug of greater or less thickness is formed. The 

 crater may become temporarily so dead that even solfataric action 

 ceases and a forest may flourish within the crater, as has been the case 

 with Vesuvius. 



On account of the small horizontal dimensions of the average vent, 

 the consolidation of such a lava plug may be completed in a few years. 

 This new rock is characteristically tough ; when cooled, it is the 

 strongest rock in the average volcanic cone. In the text-books on 

 dynamical geology and in special vulcanological memoirs, the removal 

 of the plug is usually stated to be due to simple explosion of the gases 

 accumulating below it. Yet it is obvious that in the normal cone, 

 which is largely built of loose ash deposits of very low tensile strength, 

 the weakest place in the pile is on its flank and not at the main central 

 plug. By the orthodox view, therefore, the new crater, the main one 

 for the succeeding period of activity, should have a different location 

 from that of the earlier main crater. The fact is, that, in very 

 many cases, the main vent is located at the same place through 

 the many diff'erent periods of activity of the greater cones. The 

 beautiful symmetry of a Fujiyama or of a Mayon is the result. The 

 removal of the plug at the close of a dormant period is clearly not the 



