The Interior of the Earth 



ably lower than this mean density. Thus the 

 density of marble is 2.7, of lava 2.4, of granite 

 2.6; hence to compensate for this the deep 

 layers must be heavier. Calculations by Roche 

 and experiments by Airy make it probable that 

 this internal density is about 7. This is approxi- 

 mately the density of the common metals, zinc, 

 iron, and copper. The available data lead to the 

 conclusion that the still denser metals silver, 

 mercury, gold, platinum must be confined to 

 the more central regions of the globe, so that 

 disturbances and displacements capable of 

 bringing them to the surface must have been 

 relatively much rarer. This would explain 

 why the most precious metals are also the 

 heaviest. 



In any case one fact is certain : that for a con- 

 siderable depth, greatly in excess of that of the 

 layer of crust altered by external influences, the 

 density of terrestrial materials increases with the 

 depth. Such a result would be inexplicable unless 

 at some period the fluidity of these materials 

 had enabled them to grade according to density. 

 We are thus certain that the earth has been 

 liquid; it is a globe in process of cooling, and 



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