GEOLOGICAL CHANGE, AND TLMK. ^'2') 



Ikhmi liijiher than it actually is; nor more than lour hundred millions of 

 years aiio, for then there would ha\'e been no sensible increase at all. Me 

 was inclined, Avhen lirst dealing with the subject, to believe that from 

 a review of all the evidence then axailable, some such period as one 

 hundred millious of years would eml)ra('e the whole geological history 

 of the globe. 



It is iu)t a pleasant experience to discover that a fortune whicli one 

 has unconcernedly believed to be ami)le has somehow taken to itself 

 wings and disap])eared. When the geologist was siuldeidy awakened 

 by the energetic warning of the physicist, who assured him that he had 

 enormously overdrawn his account with past time, it was but n.atural 

 under the circumstances that he shoidd think the accountant to be mis- 

 taken, who thus returned to him dishonored the large drafts he had 

 made on eternity. He saw how Avide were the limits of time deducible 

 from physical considerations, how vague the data from which they had 

 been calculated. And though he <-ould not help admitting that a limit 

 nnist be fixed beyond whicjh his chronology could not be extended, he 

 consoled himself Avith the rellection that after all a hundred millions of 

 years was a tolerably ample period of time, and might i)ossibly have 

 been quite sufflcieiit for the transaction of all the prolonged se(pience 

 of events recorded in the crust of the earth. He Avas therefore dis- 

 l)osed Ut accpiiesce in the limitation thus imposed upon geological his- 

 tory. 



But ])hysical in»iuiry continued to be pushed forward with regaid to 

 the early history and antiquity of the earth. Further consideration of 

 the influence of tidal friction in retarding the earth's rotation, and of 

 the sun's rate of cooling, led to sweeping reductions of the time allow- 

 able for the evolution of the ])lanet. The geologist found himself in 

 the plight of Lear when his bodyguard of 100 knights was cut down. 

 "What need you flve-and-twenty, ten or five'?" demands the inex- 

 orable physicist, as he remorselessly strikes slice after slice from his 

 aUowance of geological time. Lord Kelvin is willing, I believe, to grant 

 us some twenty Uiillions of years, but Professor Tait would have us 

 content with less than ten nullions. 



In scientilic as in other nnindane questions there may often be two 

 sides, and the truth nuiy ultiiimtely be found not to lie wholly with 

 either. I frankly confess that the demands of the early geologists for 

 an uidiiiuted series of ages were extravagant, and even, for their own 

 l»urposes, unnecessary, and that the physicist did good service in re- 

 ducing them. It may also be freel\ admitted that the latest conclu- 

 sions from physical considerations of the extent of geological time re- 

 quire that the interi)r(^ration given to the record of the rocks should 

 b(! rigorously i<;vised, with the view of ascertaining lunv far that inter- 

 ])retalion may be capable of modification or amendment. l>ut we nuist 

 also remeud)er that the geological record constitutes a voluminous body 

 of eviden(;e regarding the earth's history which can not be ignored, and 



