16 Clarence King — Age of the Earth. 



That the application of the criterion of solidity here made 

 to Kelvin's method is open to the objection of being based on 

 the physical relations of an extremely superficial fraction of 

 radius is obviously true. Ignorance of the deeper, interior 

 distribution of specific materials and of their relations to the 

 degree of heat and the range of pressure to which they are sub- 

 jected, forbids the construction of a generalized line of melting 

 temperatures for the whole of radius. 



It might, therefore, be contended that a reversal of the 

 diabase conditions is possible, and the deeper materials 

 may possess the property of ice-fusion ; their melting temper- 

 atures suffering depression instead of elevation. The high 

 densities required in lower earth-depths have constantly 

 suggested the concentration there of heavy metals and the ex- 

 amples of meteorites has further influenced the idea of a metal- 

 lic nucleus chiefly of iron. And as iron at normal pressure 

 unmistakably exhibits ice-fusion, any great iron mass at the 

 center might be supposed to exist as a liquid in spite of the 

 enormous pressure there exerted. 



The distribution of materials and of " state " under this as- 

 sumption involves a metallic (iron) nucleus, liquid from ice-fus- 

 ion, overlaid by less dense couches which at some unassignable 

 depth pass into silicates of the diabase type, solid from com- 

 pression under the law shown for diabase, and solid to the sur- 

 face as required by tidal effective rigidity. 



Ice-fusion, however, is an exceptional phenomenon, nor have 

 we any but the most limited data as to its range as regards 

 temperature and pressure. Iron is conceded to contract in the 

 act of fusion, bnt cold iron is more dense than the substance 

 either just above or just below the fusion point. It is not 

 beyond the range of probability that excessive pressures might 

 bring about the same density in iron that cooling does, and 

 thus isothermally convert ice-fusion into the normal type and 

 produce a solid nucleus. However that may be, tidal effective 

 rigidity excludes fusion of either type for at least '2 of radius. 



Other methods have been used for obtaining a measure of 

 the earth's age or for some definite portion of geological time. 



Earth- Age from Tidal Retardation. 



Kelvin's comparison of the earth's present figure with that 

 of a thousand millions of years ago when the terrestrial day 

 wonld have been only half its present length is one of the 

 most interesting. The earth, if then plastic, would have 

 yielded to four times the present centrifugal force at the equator 

 and shown a correspondingly greater flattening at the poles 



