Rev. Maxwell H, Close — Extent of Geological Time. 453 



interests of geology, that the unit of gravitation may not be constant 

 throughout time, nor through the enormous length of interstellar 

 space traversed by the solar system mass since it first began to fall 

 together. We therefore cannot assert that we know what potential 

 energy was possessed by that mass at that time, and what amount 

 of heat for the sun to radiate at that time could be produced thereby. 

 If there is too much boldness in these speculations, it is not the 

 geologists who are accountable for that. Let us hope that the 

 physicists may not withdraw them when they find that they may be 

 turned to the advantage of geology. 



We now come to the argument from the figure of the earth, 

 taken in connexion with the circumstance that her rate of rotation 

 must be diminishing — diminishing in consequence of the excess of 

 the retardation caused by the tides over the acceleration caused 

 by her contraction in cooling. The argument is this — when the 

 earth consolidated she became as rigid as steel. Then, since the 

 earth's figure is now so very nearly that due to her present 

 rotation rate, she must have consolidated when her rotation was 

 but little higher than now — that is to say, a comparatively short 

 while ago. One reply which I have heard made by Professor 

 Haughton is this — that when the earth had first solidified, her high 

 temperature would cause rapid cooling ; this would produce rapid 

 contraction ; this would cause acceleration of her rotation, which at 

 first would overpower the tidal retardation, but after the lapse of 

 a long time would sink to an equality therewith. Then the tidal 

 retardation, even though remaining constant, would obtain the pre- 

 eminence, and after the lapse of another long time bring down the 

 rotation rate again to what it was at first solidification. So that, for 

 all that appears to the contrary, the time that has elapsed since first 

 solidification took place, consists of two long periods, and may be as 

 great as geologists need wish for. This answer accepts, at least for 

 the sake of argument, the steel rigidity of the globe, and is, I 

 conceive, sufficient. But I would beg leave to point out another. 



Sir William Thomson has himself very seriously shaken the 

 foundation of the doctrine of the steel rigidity of our globe. This 

 doctrine had formerly two grand supports, and, as far as I can find 

 out, only two. Of these. Sir William has completely shattered one, 

 that which rested on the amount of precession and nutation ; and 

 he has greatly discredited the other, for the present at least, that 

 which rested on the magnitude of the ocean tides. He still con- 

 siders that the evidence of the tides, as far as it goes, points to the 

 high rigidity of the earth, but he shows that the semi-diurnal and 

 diurnal tides are to be laid aside for one reason, and the semi-annual 

 and annual for another. The only tides that he looks to for the 

 determination of the amount of the earth's rigidity are the lunar 

 fortnightly and monthly tides. But now observe, he says, that the 

 Tide Committee of the Association "have not hitherto succeeded 

 in obtaining any trustworthy indications of these tides," but the 

 indications, such as they are, " seem to show possibly no sensible 

 yielding, or, perhaps, more probably some degree of yielding of the 



