Ilalemaumau Lava Lake. 



341 



Now, it is obvious that the crust, ordinarily, cannot sink as 

 a horizontal sheet, but whenever it is enabled to turn and sink 

 vertically it will do so, and this vertically descending material 

 should be understood to include with the crust itself, whether 

 flexible and bending or brittle and broken, the immediately 

 underlying sheets of gas free, partially cooled and more or less 

 viscous lava which may all be comprised in the general term 

 " heavy surface layers." 



Let us now suppose these surface materials moving normally 



Fig. 2. 



Fig. 2. Showing striation of lake surface by division of flow. General 

 flow is from farther end towards the observer. View looking- West. 



eastward, when a lava fountain bursts into play. A great 

 dome of glowing liquid is thrown up, boiling furiously, while 

 a cloud of bluish vapors, rising above it, indicates the escape 

 of gases, the flash of whose flame is invisible by clay. The 

 fountain lava, cooled and condensed, then subsides into the 

 lake — whose surface level is already depressed locally by the 

 absence of displacement of the great gas bubbles just vanished 

 — and, by its momentum and greater density, continues to sink 

 while the surface layers, surging inward from all sides, turn 

 clowm wards and follow, often for a considerable time. 



But the general trend of the surface current is toward the 



