THE AGE OF THE EARTH. 351 



which are relatively perinaueut, while its efteotive elastic resistance to 

 tidal stress is sufticieut to iierniit a water tide, it ai)i>ears tliat either 

 the purely telluric stresses are greater thau the moon's attraction, or 

 that there is for the time rate of application of equal stress a transi- 

 tional value above which the elastic resistance of the earth-solid is 

 enough to conserve figure, aiul below which plastic defornuitioniseasy — 

 a relation of properties such as Kelvin suggests for the a'ther. Under 

 the former alternative, deformations due to purely telluric forces might 

 by upheaval or subsidence at any time mask or counteract astronomical 

 beach shifting. In the latter case, to make use of the astronomical data 

 for displacement of beaches, it is required to ascertain the time rate of 

 terrestrial plasticity accurately enough to know that relatively to the 

 duration of eccentricity and i)recessioii cycles and their correlative 

 attractional variations, the reaction of the lithosphere would differ 

 enough from that of the hydrosphere to allow of the beach shifting 

 sought. 



Beyond the most modern geological dates the grander earth deforma- 

 tions have carried ancient beach lines out of all recognizable radial 

 relations with each other and the several oceans of which they mark 

 the shores, or else, as is frecjuently the case with rising continents, they 

 have been wholly effaced by erosion. Evidently the Croll Blytt time 

 measure, interesting as it niayi)rove to be for recent dates, isat j)resent 

 inapplicable to any general determination of the earth's age. 



EARTH-AGE MEASURED BY SUN-AGE. 



Since the incrustment of the earth would be almost immediately fol- 

 lowed by a climate controlled wholly by the sun's heat, re-distribution 

 of the crust by water necessitates a sun heat received upon the earth's 

 surface sufficient at least to maintain the temperature above that of 

 permanent freezing. 



Newcomb* remarks: 



" If we reflect that a diminution of the solar heat by less than one- 

 fourth its amount would i)r(»bably mean an earth so cold that all the 

 water on its surface would freeze, while an increase of nuich more than 

 one-half would probably boil all the water away, it nuist be admitted 

 that the balance of cause which would result in tlie sun radiating heat 

 just fast enough to preserve the earth in its present state has probably 

 not existed more than 10,000,000 years." 



All we know of the earlier strata indicates a water mechanism for the 

 denudation, comminution, and dei)osition of rock. Exactly the division 

 of this work between tidal and river forces we may never know, but 

 all evidences confirm the conviction that life was continuous from its 

 earliest, or at least an early, appearance, and hence clinmte must have 

 been continuously suitable for the circulation of continental waters. 

 The range of temperature for the time since the beginning of the Huro- 

 nian must have been well within Newcomb's limits. So that unless the 



* Popular Astronomy, p. 511. 



