96 PROCEEDINGS OF THE AMERICAN ACADEMY. 



may be due to chemical reactions ; in part, it is due to the condensa- 

 tion of free gas constantly increasing in mass, within a closed chamber. 

 The increase in mass is assumed to be due to the exclusion of gas in the 

 crj'stallization at the walls of the abyssal injection, and perhaps to 

 other molecular transformations within the magma chamber. 



If the lava column is not kept supported, but withdraws for a time 

 from the plug, the compression-melting of the plug must await suf- 

 ficient accumulation of gas from beneath or the return of the fluid lava 

 (because of general strains in the earth's crust or for other reasons) into 

 the conduit. The mechanism is, however, the same as in the case just 

 discussed, and the base of the plug is gradually melted. 



The re-fused magma must become graduall}'' more and more charged 

 with gas. How much gas per unit weight of rock would be required 

 to fuse an average plug is obviously now impossible to declare, but the 

 maximum quantity of gas in solution may not need to be more than 

 two or three per cent of the total weight of the magma in the actual 

 conduit. The astounding explosive energy of newly awakened vol- 

 canoes, as shown in the vast heights to which fine ejecta are thrown 

 and by the excessive comminution of the respective plugs, seem to in- 

 dicate saturation of the magmas to an even higher degree. The " evis- 

 ceration " of some cones has possibly been due to the concentration of 

 juvenile gases beneath plugs not yet sufiiciently fluxed to permit of a 

 reopening of the former vents by more moderate explosions. In neither 

 case, however, is it probable that pure explosion could restore activity 

 to the dormant volcanoes. Here again, as in the continuance of ac- 

 tivity after the vent is opened, the problem is one of heat supply. 



Another cause for dormancy is to be found in the sudden emptying of 

 a lava-filled conduit by escape through a lateral fissure, forming satel- 

 litic intrusion, or distant surface flow, or both at once. This is a com- 

 mon event at both Kilauea and Mauna Loa. A multiple eff'ect is 

 produced. A large volume of specially concentrated juvenile gas is 

 taken out of the vent, just so far diminishing the motive power and 

 heat supply in that vent. As observed at Kilauea, the level of the 

 conduit lava may not be restored to its former height for months or 

 years. During that time the upper part of the conduit wall is cooling, 

 and, through decrepitation and initial weakness, large masses fall from 

 the wall and choke the vent. A resumption of activity at the surface 

 must be delayed by these processes. 



In the present argument we need not dwell on the fact that, if the 

 volcanic mechanism is nicely balanced, a minute effect, like tidal strain, 

 may pull the trigger and renew activity, for which the essential con- 

 ditions have been long preparing. However, it seems clear that cos- 



