236 



THEORIES REGARDING THE EARTH S INTERIOR. 



ture increment of only i° F. in every 200 feet, whereas the 

 latter may have as rapid an increment as 1° F. in every 28'i 

 feet (Anzin, near Valenciennes). In the British Isles we may 

 obtain in this confined area alone differences ranging from an 

 mcrement of 1° F. for every 34 feet to i ° F. for everv 130 

 feet. 



To explain the earth's temperature we have no reason to 

 postulate an internal molten sphere of rock, in fact, were there 

 any heat at all in the centre of the earth, life on the surface 

 would be impossible from the enormous additional heat re- 

 ceived by the crust by radiation. The source of this earth 

 temperature lies firstly in the chemical chanyes which go on 

 in the rocks themselves and secondly in the fact that all sili- 

 ceous rocks contain radium, and lastly in the movements in the 

 earth's crust, which produce frictional heat. 



To take the first case first. The earth's crust is a veritable 

 chemical laboratory, in which reactions are always going on. 

 The simplest process is the oxidisation of pyritic carbonaceous 

 shale rock, such as we find abundantly in the Lias of Europe 

 and above the Dwyka Conglomerate in South Africa. When 

 water containing" oxygen gains access to this rock through 

 cracks, there is immediately a reaction which sets up so much 

 heat that not infrequently steam and sulphurous vapours are 

 given off and escape at the surface as in solfataras near 

 volcanoes. In the Kimberley Mines, which pierce the carbon- 

 aceous shale of the Dwyka series near the surface, this action 

 has happened in many instances, and the reef has burnt for 

 years. It must be remembered, however, that the reaction 

 is reversible. An oxidised sediment deeply buried may be- 

 come pyritised and hence heat will be absorbed and locked 

 up so that pyritic shales will show abnormally high tempera- 

 ture increnients near the surface and abnormally low ones 

 below the range of surface waters. 



The following figures are given by Sir A. Geikie for the 

 Rose Bridge Colliery Shaft at Wigan, and will show at a glance 

 how dependent on the rock encountered is the temperature in- 

 crement. 



