198 Jaggar — Yolcanologlc Investigations at Kilauea. 



a porcellanoiis glaze which is stalactitic and impervious. It is 

 gas-tight, for the crusts are frequently ballooned upward by 

 gas. Doubtless the blocks become completely encased in such 

 a glaze when they sink. There can be no question, to the 

 writer's thinking, but that such up-ending blocks, spongy in 

 their vesicularity, exposed to sweeping w4nds, are filled with 

 air. Green was of the same opinion. The mechanism of their 

 solidification implies a cooling interstitially by air which 

 replaced the hot gases of the unsolidified state. The persist- 

 ence of those hot gases in tension in the vesicles is unthinkable, 

 particularly as the crusts, after reaching a certain thickness, 

 always acquire the impervious glaze beneath, and are openly 

 porous above. 



Simultaneous with the foundering of the crust area, a dome- 

 shaped mass of bubbles, or "fountain," bursts having quite 

 the appearance of an ebullition center in boiling milk; this 

 dome may end the explosion, or it may be continued with a 

 series of high flings of drawn slaggy openwork (tig. 14Z>), or 

 prolonged gushes of spray and flame, the noise being like surf 

 accompanied with puffing. Lava fountaining is not the expan- 

 sion of a single great bubble, but that of a swarm of bubbles, 

 a true eifervescence, and as it progresses the fluid heats and 

 loses viscosity markedly, flinging small droplets of glass such 

 as the normal lava never makes. While the fountain lasts, 

 engulfment and inward sncking to its center continues, and 

 the surface currents set towards it, increasing in speed at the 

 center, from some distance away. Foundering at the spot 

 does not always precede fountaining, and fountaining may 

 occur abortively without downsucking, raising convulsively the 

 surface without breaking it, or merely throwing back a flap of 

 crust without exploding through the under layer. 



Great balloons of crust are sometimes blown by quiet gases, 

 the bellying mass finally tearing at the end, when the gas 

 escapes and burns. For an instant a glowing cavern is seen 

 within with a bubbling liquid floor, and then the skin col- 

 lapses. The true fountain, however, doming and spurting, is 

 always followed by indraft of crusts. A continuous fountain 

 is always a place of rapid downsucking and engulfment. 



I have said that a fountain forms where currents conflict. 

 Such conflict may be of three kinds, surface uieeting of two 

 or more streams, deflection of a current against the shore, or 

 sub-surface meeting of currents following down opposed bot- 

 tom slopes with the convection. This last case reaches a 

 maximum when such submerged downstreaming meets from 

 various directions at a sinkhole where the united flowings 

 descend. A fountain over such a sinkhole reappears from 

 time to time in the same place, and if the streaming process 

 down the sinkhole is constant the fountain wdll be rhythmic 



