33 
seen with the naked eye. The plutonic magma because 
of its size has a high heat content and the immediate 
temperature difference between the marginal part of tie’ 
magma and the adjacent intruded rock is not great, so 
consequently cooling is relatively slow, with the result 
that crystals grow to a larger size. 
Granite is the most popularly known plutonic rock 
Much of the material spoken of as granite is really 
Granodiorite or one of its near relatives. Rocks of this 
group, which differ slightly chemically and mineralogic 
ally from the true granites, are frequently found in the 
cores of the old fold mountains. The plutonics of the 
d ’Aguilar Range in S.E. Queensland belong here. The 
huge, elongate, domed masses formed by the granites 
and granodiorites are known as batholiths. Granites and 
granod iorites belong to the acid group chemically. The 
intermediate group consists of the Donates, and the basic 
representatives of the plutonics are the Gabbros, which 
are not very common. Again, there is a marked dark- 
ening of colour towards tin* basic end oi the sequence. 
Ore minerals are usually associated with intrusive 
rocks. They appeal- to be deposited in adjacent country 
rocks from vapours and hot solutions derived from the 
cooling magma. 
Beside the plutonic group there are other intrusive 
rocks which usually occur in smaller tabular or lens- 
shaped bodies. These minor intrusives have a texture 
intermediate between those of the plutonic and volcanic 
groups, and this is indicative of intermediate conditions 
of cooling. Frequently the crystals arc ol two sizes. The 
minor intrusives apparently represent both highly pene- 
trative tongues from plutonic masses and feeders or 
branches from feeders of volcanic rocks. 
It is significant that of all the igneous rock granite 
(the term being broadly used) and basalt are of most 
common occurrence. Seismological investigations show 
that in the continental areas of the earth there exists a 
sub-crust of material comparable in its physical proper- 
ties to granite, and this is above a layer resembling 
basalt. Under the oceans the granitic layer thins or is 
lacking altogether. Herein may lie the key to the com- 
plex problem of the origins of the igneous rocks. 
