《地球的年齡》

Holmes opens on the long quarrel over the Earth's age — from Archbishop Ussher's reading of 4004 B.C., through the geologists who demanded vast spans of time to lay down the strata (Hutton, Lyell), to the physicists who tried to put a hard number on it. He frames the book as an attempt to settle that quarrel with measurement rather than assertion.

He reviews the methods then in play and finds each wanting: the cooling of a once-molten Earth (Kelvin), the slow salting of the oceans by rivers (Joly's sodium clock), and the rate at which sediment piles up. Each depends on assumptions — a starting condition, a steady rate — that cannot be trusted over hundreds of millions of years.

Kelvin had allowed geological time only some 20–40 million years, assuming the Earth simply loses its primordial heat. But radioactivity — unknown when Kelvin calculated — continually generates heat inside the Earth. Holmes argues this single fact dissolves Kelvin's limit and reopens the question of deep time.

The book's core: uranium and thorium decay to lead and helium at rates fixed by physics, so the amount of lead locked beside the uranium in a mineral measures how long it has existed. Using the lead ratios he had published in 1911, Holmes assigns about 370 million years to a Devonian rock, with Carboniferous and older Palaeozoic figures behind it — the first numerical geological timescale.

Carried back to the oldest minerals he could find, the method points to ages near 1,600 million years — far beyond anything most of his contemporaries were willing to accept, and a floor, not a ceiling, on the age of the Earth itself.