Clarence King — Age of the Earth. IT 



and bulging at the equator and " therefore " (as Tait expresses 

 it) " as its rate of rotation is undoubtedly becoming slower and 

 slower it cannot have been many millions of years back when 

 it became solid, else it would have solidified very much flatter 

 than we find it." This implies that because a computed earlier 

 and greater value of ellipticity does not now exist it could never 

 have existed, in other words, that terrestrial rigidity has been 

 and is of such value that a form taken in the remote past by 

 the solid earth would not be modified by the tidal retardation 

 of rotation and its attendant change of centrifugal force. 



There is in modern geology a growing body of evidence 

 which is believed to prove the very general plasticity of the 

 lithosphere, by which it may experience important deforma- 

 tions from very slowly applied stresses. So strongly has this 

 belief taken root that many American geologists accept " isos- 

 tasy " and consider it to be an expression of a fluid equilibrium 

 for the earth. 



From abundant geological observation plasticity must be ad- 

 mitted for slow deformations enormously in excess of the small 

 change of figure which the stress of tidal attraction would pro- 

 duce but for elastic resistance. 



Although rigidity prevents a sudden tidal deformation of 

 five feet it does not prevent a slow radial deformation of 

 five miles of the surface matter. How then can it be sup- 

 posed to resist the slow change of stress due to tidal retarda- 

 tion of rotation ? The excess of the equatorial over the polar 

 axis is now roughly 25 miles while the radial range of surface 

 inequalities of the lithosphere is about 12 miles, of which a large 

 part dates from this side of the beginning of Tertiary time. 

 If past plasticity equals present values, the earth's figure 

 could never have been a survival from some assumed earlier 

 epoch when centrifugal force was greater, but must always 

 have been a function of the slowly diminishing rate of rotation. 



If the conclusions of the earlier portion of this paper are 

 true they go further and exclude the idea of a formerly fluid 

 earth and any epoch of solidification. With any admissible 

 assumption of initial excess nearly the whole earth must have 

 been solid from the date of the first collocation of its matter. 



To whatever radial depth plasticity may descend, what is 

 enough for geologically recent superficial inequalities is suffi- 

 cient for adjusting the figure of the earth to existing forces of 

 rotation. 



The same coast lines which remain stationary under tidal 

 stress are slowly rising and falling in a hundred places under 

 the slow application of subterranean energy. 



Am. Jour. Sci.— Third Series, Vol. XLV, No. 265.— January, 1893. 

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