350 THE AGE OF THE BAUTH. 



geologically recent sni)erficial inequalities is siifticient tor a^Uiisting the 

 tiguve 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 applica- 

 tion of subterranean energy. 



It therefore appears that no time measure can be deduced from the 

 sui)posed tixing of the present ellipticity at some past date. 



ASTKONOMICAL MEASURE OF EARTH TIME. 



Oroll's hyi)othesis from which it was proposed to fix dates by secular 

 variations of eccentricity and to correlate the climatic eiiects of those 

 variations with geological operations, and thus measure certain inter- 

 vals of geological time, retiuired so much (juestionable physical geogra- 

 phy and left so many physical doubts that few have been found to accept 

 the excessively complex chain of effects lying between eccentricity data 

 and geological facts. The objections of Prof. Newcomb, noticed rather 

 tlian answered, left Croll's doctrine where it was permissible to believe 

 that there was something in it, but not necessarily that definite sequence 

 of climates Avhich is the core of the idea. 



The gap in Croll's scheme seems to have been successfully stopped 

 by Sir llobert Ball, whose interesting proof of the seasonal inequality 

 of the thernuil element in climate due to position of the equinoxes and 

 its intensification in periods of high eccentricity offers a new hope for 

 the accurate dating ol at least very modern geological climates. From 

 this point of view late geological history requires re-examination, and 

 if it should appear that a sequence of climates has existed closely par- 

 allelling the thermal variations which the astronomical values seem to 

 afford, an extremely probable case will have been made out. And t his 

 case would be practically substantiated if the hypothesis of H. Blytt 

 should yield the confirmation for which he hopes. Blytt * proposes 

 and has already attempted to correlate the secular attractional changes 

 due to varying eccentricity and precession with the observed succes- 

 sive shifting of beach lines. 



So far as he has proceeded it is of interest to note that his time esti- 

 mates are more in harmony with the physical than the stratigraphical 

 figures. 



Periodic changes in the figure of the hydrosphere relatively to the 

 solid earth, due to alterations of attraction, might be predicted with 

 some confidence if it were clear that the lithosphere would under the 

 slow stresses involved continue to exercise a degree of rigid resistance 

 conq^arable with that it opposes to the tidal stress, but there is no proof 

 that it would. 



Since we find the solid earth undergoing slow deformations to-day 



*The probable cause of the displacement of beach lines. H. Blytt— 1889, Chris- 

 tiania Videnskabs Foidhaudlinger No. 1— Additional note 1889— second additional 

 note 1889. {Smithaonian liepori lor 1889, p. 353.) 



