240 THEORIES REGARDING THE EARTH S INTERIOR. 



the radium is concentrated near the surface of the earth and 

 the heating' effects therefore would be confined to a zone near 

 the surface. 



These researches then lead us to the conclusion that the 

 earth consists of a self-heating crust resting upon a solid 

 nucleus. Is there any reason to believe that this central 

 nucleus is hot or cold ? Until very good evidence is adduced 

 to the contrary the earth centrosphere must be regarded as 

 having the temperature of outer space, that 15 — 273° C, plus 

 any heat that may have radiated out into it from the surface 

 layer. For consider how this earth's centre has been formed. 

 Meteorites large and small have come together and have been 

 consolidated by mutual gravitation; heat by impact of new 

 meteorites has been generated and the whole has swung round 

 for immeasurable time in an extremely cold medium. When 

 the earth was small, or even as large as the moon, the radia- 

 tion of heat into space would have been rapid enough to cool 

 down the whole to the temperature of outer space. Only 

 later, when water became abundant on the surface and the 

 outer rocks became disintegrated, changed by chemical pro- 

 cesses, enriched by the concentration of radio-active substances 

 through solution and deposition and distorted by earth folds 

 and quakes, could the internal generation of heat keep pace 

 with the radiation of heat into space and the crust of the earth 

 could thus become habitable. If we accept Prof. Chamberlin's 

 planetismal hypothesis, then it seems inevitable that we must 

 regard the earth's centre as a cold body like that of the moon. 

 In a small body, a cold nucleus surrounded by a hot surface 

 would gradually assume the temperature of the surface, but 

 in a vast body like the earth the penetration of the heat into 

 the interior is stopped by absorption in chemical and physical 

 changes near the base of the crust, and at most the tempera- 

 ture cannot be in excess of that existing in the surface laver i.e., 



1530° C. . ' . 



The production of molten lava m volcanoes is to be attri- 

 buted to local causes in the outer 10 or 12 miles of the crust; 

 where movement and consequent frictional heat is being de- 

 veloped ; as this heat is brought in lava streams and hot 

 springs to the surface and there dissipated into space, the melt- 

 ing of the rock under these circumstances does not add to ihe 

 general body heat of the earth. Or to look at the question 

 ill another way: since the heating effects in the earth's crust 

 in the end can be traced to the agency of water, so we can 

 state roughly that the limit of warmth in the earth is deter- 

 mined by the percolation of water; as granite has been formed 

 with the help of water we can define this limit of heat at that 

 depth to which the earth's substance has the elasticity of 

 normal rocky substances, namely: 30 miles. 



In other words, the temperature increment in the earth's 

 crust will increase as we go downwards till a limit is reached, 

 and beyond that, there will be a decrease. Is there good 

 evidence for this theoretical deduction in actual fact ? South 

 Africa is peculiarly well situated to answer this question. From 



