"natural oil"

means any liquid or solid hydrocarbon or combustible gas existing in a natural condition in the earth's crust, but does not include coal or bituminous shales or other stratified deposits from which oil can be obtained by destructive distillation, or gas arising from marsh or other surface deposits;

[Definition inserted by section 4(d) of the Income Tax Act, 1963 (Act No. 72 of 1963)]