THE AGE OF THE EARTH. 349 



Other methods have been used for obtaining ;i measure of the earth's 

 age, or for some deiiiiite portion of geological time. 



EARTH AGE FROM TIDAL RETARDATION. 



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

 a thousand millions of years ago, when the terrestrial day would 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 cen- 

 trifugal force at the equator and shown a correspondingly greater flat 

 tening at the poles and bulging at the eiiuator, 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 mod- 

 ified 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 deformations from very sloicli/ applied 

 stresses. So strongly has this belief taken root that many American 

 geologists accept "isostasy" and consider it to be an expression of a 

 fluid equilibrium for the earth. 



From abundant geological observation plasticity must be admitted 

 for slow deformations enormously in excess of the small change of fig- 

 ure which the stress of tidal attraction would produce but for elastic 

 resistance. 



Although rigidity prevents a sudden tidal deformation of 5 feet, it 

 does not prevent a slow radial deformation of 5 miles of the surface 

 matter. How, then, can it be supposed to resist the slow change of 

 stress due to tidal retardation of rotation! The excess of the etpia- 

 torial 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 Cfpials present values, the earth's figure could never 

 have been a survival from some assumed earlier epoch when centrifu- 

 gal 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 



