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{TRANSACTIONS Of SEOTION C. 721 
respects modern geology owes so deep a debt of gratitude, became utterly reckless 
in their demands for time, demands which even the requirements of their own 
science, if they had adequately realised them, did not warrant. The older geolo: 
pists had not attempted to express their vast periods in terms of years. The 
indefiniteness of their language fitly denoted the absence of any ascertainable 
limits to the successive ages with which they had to deal. And until some 
evidence should be discovered whereby these limits might be fixed and measured 
by human standards, no reproach could justly be brought against the geological 
terminology. It was far more philosophical to be content, in the meanwhile, with 
indeterminate expressions, than from data of the weakest or most speculative kind 
to 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.t 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 
‘ The Doctrine of Uniformity in Geology briefly refuted.’ * 
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 reformin 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 
present time is in direct opposition to the principles of natural philosophy.’* In 
pressing once more the original argument derived from the downward increase of 
terrestrial temperature, he now reinforced it by two further arguments, the one 
based on the retardation of the earth’s angular velocity by tidal friction, the other 
on the limitation of the age of the sun. 
These three lines of attack remain still those along which the assault from 
physics is delivered against the strongholds of geology. Lord Kelvin has repeatedly 
returned to the charge since 1868, his latest contribution to the controversy having 
been pronounced two years ago.t While his physical arguments remain the same, 
the limits of time which he deduces from them have been successively diminished. 
The original maximum of 400 millions of years has now been restricted by him to 
not much more than 20 millions, while Professor Tait grudgingly allows something 
less than 10 millions.° 
Soon after the appearance of Lord Kelyin’s indictment of modern geology in 
1868, the defence of the science was taken up by Huxley, who happened at the 
time to be President of the Geological Society of London. In his own inimitably 
brilliant way, half seriously, half playfully, this doughty combatant, with evident 
relish, tossed the physical arguments to and fro in the eyes of his geological 
brethren, as a barrister may flourish his brief before a sympathetic jury. He was 
willing to admit that ‘the rapidity of rotation of the earth may be diminishing, 
that the sun may be waxing dim, or that the earth itself may be cooling.’ But he 
went on to add his suspicion that ‘ most of us are Gallios, “who care for none of 
1 Trans. Roy. Soc. Edin. vol. xxiii. (1862). 
2 Proc. Roy. Soc. Edin. vol. v. p. 512 (Dec. 18, 1865). 
% Trans. Geol. Soc. Glasgow, vol. iii. (February 1868), pp. 1, 16. 
4 ‘The Age of the Earth,’ being the Annual Address to the Victoria Institute 
June 2,1897. Phil. Mag. January 1899, p. 66. 
5 Recent Advances in Physical Science, p. 174. 
1899. 3A 
