32 
plug-like structures probably produced by the piling— 
up of very viscous lava above the vent. 
Large quantities of gases, including much steam, 
are associated with the molten lava which issues from 
t lie vent of a volcano. If the escape of tiie gases is pre- 
vented the ultimate eruption is likely to be explosive, 
and when this occurs much fragmental material is thrown 
into the air. The fragments rauge from large blocks 
down to particles of dust size. 
Accumulations of the finer material may be con- 
solidated to form the rock known as Tuff. Sometimes 
the gases do not completely escape and bubbles are 
trapped in the cooling lava, which assumes a vesicular 
structure. If the molten lava encounters water or in 
other eases of extreme gas escape, it may he whipped 
up into a froth, and the resultant rock. Pumice, is very 
light and buoyant. 
There are no active volcanoes in Australia at the 
present time, although it is thought that some of the 
volcanoes of Victoria may have become extinct only in 
Recent times, within the last 2.5,000 years. Other vol- 
canoes in Northern Queensland are probably not much 
older. 
Geysers, hot springs and fumaroles belong to the 
dying phases of volcanic activity. Most earthquakes 
have their centres in those areas experiencing some de- 
gree of activity, both sets of phenomena indicating con- 
ditions of structural instability in the crust. 
Plutonic Rode* 
Many igneous rocks are intrusive. At the time of 
(heir formation they do not break through To the sur- 
face, and are only revealed hv erosion of the covering 
rocks. Some of these rocks arc formed at considerable 
depths and occur as very large structures. These are 
usually referred to as the Plutonic rocks and they con- 
stitute the hulk of I lie intrusive group. The textures of 
the plutonic rocks, the structures that they form, and 
their relationship to the contiguous country rocks sup- 
port the hypothesis that their parentage is analogous to 
that of the volcanies, in that they crystallise from mag- 
mas: a hypothesis which for obvious reasons lacks direct 
verification. 
The grain size of the plutonic rocks is much coarser 
than that of the volcanies and the crystals can be readily 
