206 Jagcjar — Yolcanologic Inmsiiqaiions at Kilauea. 



within the mountain at 700 feet of depression, if the phenomena 

 of streaming and foiintaining in the liquid lake were functions 

 independent of the form of the higher lava pit, and of contact 

 with the atmosphere ? The lava was sluggishly present in the 

 depths of the pit for it would occasionally begin puffing and 

 glowing, and its sulphur smoke was always rising. 



The answer to these questions would seem to imply that 

 lake magma convection at these low levels becomes very slug- 

 gish or stationary, owing to lack of stimulation with oxygen, 

 and that at the higher levels with increasing sinkhole suction 

 of free air, larger area of surface crusts, and assimilation of 

 air-filled talus blocks, the lava gradually acquires an increased 

 fluidity in proportion as its upper edifice becomes increasingly 

 a furnace. Meanwhile the bench magma, matrix for the 

 furnace of the liquid lake, remains always a sluggish, heavy 

 and stiff substance sucli as lay dormant in the depths during 

 the intervals of low level. 



Heating Mechanism in Bench Magma. 



The heating mechanism by oxidation of combustible gases 

 which has here been outlined, was first mentioned in relation 

 to the heating of the bench magma. The discussion so far has 

 been directed chiefly to the rapid engulfment of air in the 

 liquid lake. We have seen that an excess of air engulfed in 

 glazed crusts becomes built into the viscous bottom of the lake 

 and that heavy crusts and talus blocks, full of air in vesicles, 

 become buried and incandescent, by a gradual subsidence 

 under overflows, in the mass of the bench magma column. 



The maintenance of this inner heat in the bench magma 

 column is accomplished by the slow circulation of its lava, by con- 

 duction from the conduits and sinkholes which perforate it, by 

 the burial of incandescent lava flows within it, by the burial of 

 hot bottom layers of the lake upon it, and lastly by actual percola- 

 tion through it of the volcanic gases under pressure. It is 

 probably this last process which discovers buried air cells, and 

 the gases, uniting with the oxygen so encountered, set up a 

 distributed liberation of heat. This reduces viscosity and so 

 aids the slow circulation of the mass, which as before men- 

 tioned, proceeds by peripheral depression under weighting, and 

 central upflow under the lake. 



It is difficult to prove this, for the interior of the bench 

 magma is rarely under observation. This much, however, is 

 certain, that the rock of the islands and bench crags, when 

 revealed by collapse only a few weeks after it has solidified, is 

 intensely oxidized. The ferrous iron in the basalt and the 

 magnetite crystals have gone over to masses of earthy brown 

 limonite and red hematite. The lava when it solidified was on 

 the surface black, glistening and glassy, while within it was 

 gray-black and lithoidal. Such rapid decomposition through 



