98 PROCEEDINGS OF THE ROYAL SOCIETY OF QUEENSLAND. 
definite vent. It is quite certain that there is a deep- 
seated nucleus of great heat. The floor itself is hot, and 
immediately under the surface of the mud too hot to touch. 
The surrounding fumeroles, the very hot rocks, and the 
fact that well-waters around Rabaul, two miles away, are 
inconyeniently hot below 20 ft., all provide evidence of a 
centre of great heat. There is no direct evidence at the 
present time of a lava column of anv kind. 
The yellow ash, scoria, and pumice of the 1878 eruption 
prove the presence of lava during that period. However, 
the continuous tremers and swarms, and the occasional 
ereater tremors (coming from the direction of the craters), 
indicate the presence of stress movements there. If the 
observations made at the Halemaumau Seismometric Obser- 
ratory, Mt. Kilauea, Hawaii,’ provide any analogy, it is 
possible to assume the presence at least of a deep-seated 
lava nucleus. At Halemaumau these tremors and swarms 
have been definitely associated with recorded movements 
of the lava column. It is, however, possible that the 
tremors may be caused by other conditions present in the 
erateral area. 
(4) Causes of Activity—AIll the evidence seems to 
indicate that the causes of continued activity are to be 
found in the combination of tensional stresses and strains 
and the rift system. The action of the strains and stresses 
on the rift system result in the maintenance of the heat of 
the central nucleus, mainly by pressure release. This heat 
is further developed by thermo-chemical reactions. 
Strains and stresses are set up by (a) the fact that the 
whole area is unstable and seems to be in a continual state 
of readjustment, (b) periodic high tides, and (¢) Iluni- 
solar earth warping. All these factors operating in a rifted 
area will of necessity combine to set up crustal movements 
with accompanying earth tremors and shocks, and pressure 
release in the central heated nucleus. This seems to be the 
explanation of the present activity of the Rabaul volcanic 
system. The occasional outbursts of explosive energy may 
be due either to (a) the confinement of outflowing heat and 
*T. Jaggar: Seismometric Investigation of the Hawaiian Lava 
Column, 1920; Bulletin of the Seismological Society of America, vol. 
10, No. 4. 
