206 GEOLOGICAL TIME 



attempt to measure geological periods by a chronology 

 of years or centuries. 



In the year 1862 a wholly new light was thrown on 

 the question of the age of our globe and the duration 

 of geological time by the remarkable paper on the 

 Secular Cooling of the Earth communicated by Lord 

 Kelvin (then Sir William Thomson) to the Royal Society 

 of Edinburgh. 1 In this memoir he first developed his 

 now well-known argument from the observed rate of 

 increase of temperature downwards from the surface 

 of the land. He astonished geologists by announcing 

 to them that some definite limits to the age of our 

 planet might be ascertained, and by declaring his belief 

 that this age must be more than 20 millions, but less 

 than 400 millions of years. 



Nearly four years later he emphasised his dissent 

 from what he considered to be the current geological 

 opinions of the day by repeating the same argument 

 in a more pointedly antagonistic form in a paper of 

 only a few sentences, entitled, c The Doctrine of 

 Uniformity in Geology briefly refuted.' 2 



Again, after a further lapse of about two years, when, 

 as President of the Geological Society of Glasgow, it 

 became his duty to give an address, he returned to 

 the same topic and arraigned more boldly and explicitly 

 than ever the geology of the time. He then declared 

 that ' a great reform in geological speculation seems 

 now to have become necessary,' and he went so far 

 as to affirm that ' it is quite certain that a great mistake 

 has been made — that British popular geology at the 



1 Trans. Roy. Soc. Edin., vol. xxiii. (1862). 



2 Proc. Roy. Soc. Edin., vol. v. p. 512 (Dec. 18, 1865). 



