On the Planetismal Hypothesis the centre of the globe is cold, and these deductions from the temperature increment must therefore be wrong. In the first place the crust of the earth contains within itself sources of heat which render it self-heating. These are, (a) chemical changes produced in the rocks by circulating water, (b) movements which cause segments to grind against each other and yield frictional heat, and (c) all rocks on the surface of the earth contain radium which spontaneously gives out heat.
(a) Chemical changes are accompanied either by the giving out or by the absorption of heat. In rocks containing pyrites or iron sulphide the chemical action which reduces the mineral to an oxide is accompanied by considerable heat, so much so that when the rock containing the pyrites contains coaly matter the heat evolved is sufficient to ignite the mass. Such a case has happened many times round the Kimberley mines, where a black carbonaceous rock is exposed near the surface. The mining operations have opened up large quantities of the black shale, which has thus been brought into contact with water from rain; decomposition sets in, and consequently the shale catches fire and has in some instances continued to burn for many years. The Black Lias of Yorkshire and Dorsetshire catches fire in a similar way and from the same cause. The extraordinary variations in the Rose Bridge Colliery shaft above-mentioned are due to a similar cause; where chemical action is considerable the temperature increment is rapid, where it is less the increment is small. Pyrites occurs universally in rocks, and its oxidation serves as a type of chemical action which gives out heat. In the lower depths of the earth's crust the
