PHYSIOGRAPHY OF EASTERN NEW GUINEA, ETC. 99 
gases, (b) sudden crustal warping of considerable extent, 
(c) the admission of large quantities of sea-water to the 
highly heated nucleus, or (d@) a combination of one or more 
of these conditions. Normally, the fumaroles act as safety 
valves in allowing release of the gases up to a certain point. 
Considering the explosive nature of the eruptions of 
this system, it is probable that the rift system has played 
an important part in the later phases of activity, especially 
as ihe waters of the harbour are so closely associated with 
it. It seems probable, judging from the deseription, that in 
the 1878 eruption the east-west vitt-line opened as the 
result of crustal movements, allowing the admission of vast 
quantities of sea-water to the intensely heated area below; 
the sudden pressure release, rock fusion, gas expansion in 
the subterranean passages, oxidation, and other thermo- 
chemical mechanism providing the forces necessary to bring 
about a severe and continuous erupiton. This is further 
borne out by the fact that the lava was thrown out entirely 
in the form of pumice and dust.° 
It is also quite possible that this process is continuous 
in a small way, for it is practically certain that water 
percolates slowly into the heated interior. In addition to 
the waters of the harbour, there is the large catchment of 
water resulting from an annual rainfall of upwards of 
100 inches. It is, then, quite probabie that steam pressure 
and even the explosicn of combinations of water vapour 
and other gases may be causes of some of the shocks, 
especially those accompanied by explesive rumblings. 
(5) Age and Future Activity—Modern investigation 
in voleanie areas has added very considerably to our know- 
ledge of their history and formation. It is now generally 
considered ‘‘that explesive activity is an old-age feature 
of veleanism.’’* The same applies to yellow ash and einder- 
cones which are associated with explosive activity. All the 
evidence goes to show that the craters of the Rabaul area 
represent the expiring stages of the activity of the system. 
The eraters are more or less in a solfaturie condition. There 
is now no evidence of anything but explosive action. The 
* See Prof. J. W. Judd: ‘‘Krakatoa,’’ 1884. 
*Cf. Jaggar, loc. cit., in a full discussion of the principles of 
volcanism. 
