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early Uniformitarians, the founders of modern geology. Their 
reaction against the older catastrophic school led them constantly to 
lay great stress on the extreme slowness of geological processes, and 
they thus came to assume unlimited time for the past changes to which 
the stratified rocks bear witness. To Hutton there was " no vestige 
of a beginning, no prospect of an end " : in other words he regarded 
geological time as infinite, and could no more contemplate reckoning 
it in centuries than numbering the sands on the shore. Later this 
position was reinforced from another quarter, as Darwin's doctrines 
gained acceptance ; for these were held to push back to an immeasur- 
ably remote epoch the beginning of life on the globe. Geologists 
and biologists alike saw no reason for limiting their prodigal drafts 
on the bank of time. 
From this comfortable attitude they were startled, as by a bomb- 
shell, just fifty years ago, when William Thomson, afterwards Lord 
Kelvin, published the first of his contributions from the mathematical 
side to this and cognate subjects. He pointed out that, apart from 
any changes on the surface of the globe, our planet as a whole must 
be undergoing a change of a secular, and so irrevocable, kind ; viz., a 
continual loss of energy in the form of heat, as proved by the observed 
temperature-gradient. Since the store of energy cannot be inex- 
haustible, we must deduce both a beginning and an end of the existing 
geological regime ; and Thomson endeavoured to set a limit to its past 
duration from a discussion of the rate of cooling of the globe. A 
parallel line of argument was based on the cooling of the sun. 
Now as regards the validity of the general criticism there can be, 
of course, no doubt. Huxley's halting defence of what was then the 
orthodox position was easily broken down, and a wholesome check 
was given to the extravagance of the geologists. AVhen we turn, 
however, from the destructive to the constructive part of Kelvin's 
argument, the case is different. The time to be allowed for the 
geological record was stated at first with considerable latitude, but was 
subsequently narrowed down, until, in 1899, Lord Kelvin concurred 
in Clarence King's conclusion that the globe was a molten mass about 
24 million years ago. It is rather remarkable that so many geologists 
were found willing to submit to this narrow limitation. Doubtless 
they were impressed by the prestige of Lord Kelvin's authority, and 
perhaps some of them w^ere influenced by a vague feeling that a result 
arrived at by strict mathematical reasoning is thereby entitled to 
credence. But, as has been so often pointed out, and so often for- 
