30 
NOTES ON ROCKS, No. 1. 
By JACK T. WOODS 
The rosks of the earth’s crust enter into man's 
environment in many ways other than as the source of 
the minerals, the coal, and the oil, so important in his 
present civilisation. Prom the rocks are derived the 
soils, which sustain the natural vegetation and the culti- 
vated domestic plants. The topography of an area is 
dependent to a certain degree on the rock types present 
in that area, and this in turn is a factor in the control 
of the local climate. The rocks together with tin 1 waters 
and the atmosphere above them, and the radiated energy 
from the sun, form the inorganic background to the 
organic world, 
A rock may be defined as a coherent, naturally 
occurring mineral aggregate, homogeneous within certain 
limits. A mineral is a naturally occurring substance of 
fixed chemical composition, possessing a definite internal 
atomic structure. 
It has been estimated that the crustal part of the 
earth solidified about 2,000 million years ago. Forces 
of both terrestrial and cosmic origin have constantly 
upset the equilibrium of the crust, and the resultant 
geological processes, operating over this vast period of 
time, sometimes infinitely slowly, sometimes with explos- 
ive rapidity, have left their record in the rocks. Every 
rock mass has been produced by a series of physical and 
chemical processes which have operated towards the at- 
tainment of local equilibrium in space and time. 
Igneous Rories 
There is a causal relationship between many, if not 
all, geological processes and the difficult task of finding 
Die perfect scheme of classification, based on the origins 
of (lie rocks, has not been accomplished. However, it is 
possible to divide rocks into three major categories — 
Igneous. Sedimentary and Metamorphic. The term 
igneous is used for those rocks which are believed to 
have crystallised from melts or magmas which have had 
their origin at depth in the crust. 
Volcanic Rocks 
The Volcanic rocks belong to the igneous group, 
and many of the phenomena associated with their em- 
