6 
The Queensland Naturalist 
October, 1&31 
Firstly, volcanic activity ; secondly, deep-seated intrusions ; 
thirdly, minor intrusions into the outer portion of the 
earth's crust. Sometimes there is a repetition of the vol- 
canic or extrusive phase. 
Volcanoes may be divided into two main types: (1) 
those associated with fissure cracks, and (2) those asso- 
ciated with particular centres of eruption. The first type 
is much the more efficient and is usually much quicker in 
its mode of action than the central type. As a result of 
the association with the Mediterranean regions of the Cen- 
tral type of eruption, we have had handed down from our 
ancestors very much about the Central types of eruption, 
and very little about the fissure type. The central types 
of volcanoes are much more spectacular and generally have 
a good deal of explosive phenomena associated with their 
action, but as far as pouring out lava on the surface is 
concerned, they are of comparatively little importance. 
Enormous quantities of volcanic rocks in Eastern Austra- 
lia and the Deccan in India, and many other places, have 
been poured out from the fissure types of eruption. Fissure 
eruptions are associated with fissure cracks or vertical 
movements of the earth 's crust, whereas central types of 
eruption are associated with regions of folding movement. 
In the Pacific regions we have normally developed 
that type of central eruption which is very much akin to 
the fissure eruption as far as the efficiency of outpouring 
of lava is concerned ; the Hawaiian volcanoes are the type 
examples of this. 
The volcanoes of the Pacific region have been grouped 
into two sets (a) Circumpacific volcanoes which have as- 
sociated with them lavas of an andesitic type; and (b) 
Intrapacific volcanoes which are characterised by basaltic 
lavas. 
The volcanoes of the Andes, Central America, Alaska, 
Japan, Philippine Is., New Guinea. New Zealand, and An- 
tarctica, come within the first category, whereas those of 
the Hawaiian Is., Society Is., Samoa, and other groups in 
Polynesia, Melanesia, and Micronesia, come within the 
second. It is of interest to note that the Intrapacific vol- 
canoes wb ( ich occur in distinct groups throughout the 
Pacific generally, are developed along a line which has a 
north-westerly and south-easterly trend. 
From a study of volcanic action throughout the world 
it has been established that volcanoes of the fissure type 
have lavas richer in alkalies and poorer in lime and mag- 
nesia, than the normal type of volcanic rocks. The vol- 
canoes associated with folding movements have the nor- 
mal type of lava. These two types are knowrn respectively 
as the alkaline and calcic types of volcanic rocks. Owing 
