The Bank of Time - Four Revolutions in the Earth Sciences - page 14

dition of the globe at present, it cannot be more than from ten to twenty million
years since it was at a temperature in which life on it would have been impos-
sible.” 9 He then calculated the amount of time that had elapsed since the beginning
oftheCambrian period,estimating itat95million years.Addingonhisestimate of
the time before the Cambrian, Reade concluded “that the earth's age geologically
speaking must be, as inferred in the Presidential Address, somewhere between 100
million to 600 million years.” 10 This was of course almost exactly the range that
Kelvin had originally allowed for the age of the Sun.
The address to which Reade referred had been given to the British Association
in 1892 by its new president, the geologist Sir Archibald Geikie (1835-1924).
Some twenty-five years had elapsed since Geikie had first staked out a position on
the age of the Earth. In 1867, newly appointed as head of the Geological Survey
for Scotland, he had said that if scientists were to calculate geologic ages, it would
be “by the labours of the astronomer rather than that of the geologist.” 11 Kelvin
was not an astronomer per se, yet he was estimating the age of a star and a planet,
so no doubt Geikie had Kelvin's calculations in mind.
In an 1868 paper titled “On Modern Denudation,” Geikie removed any doubt
on where he stood. 12 (In time, geologists came to use “erosion” instead of “de-
nudation.”) Some writers treat “the great geological domain” as if it demanded
“no previous scientific knowledge,” he wrote. 13 Rejecting Lyell's strict uniform-
itarianism, Geikie said that geologists had no warrant to conclude that geological
agencies “have always acted in precisely the same proportion and at exactly the
same rate.” The circle of geologists' experience was too narrow for them to as-
sume that the present rate is the only possible one or that “uniformity of causation”
has been established as true, Geikie wrote. He did not abandon all of uniformitari-
anism, only the substantive variety in which everything remains constant. Geikie's
position was that “pure catastrophism will certainly lead us into error: mere uni-
formitarianism will not bring us the whole truth” (187).
The evidence from denudation “best attest[s] the enormous duration of geolo-
gical periods,” Geikie wrote (188). But geologists based their calculations on the
assumption that the past rate of denudation has been the same as the present rate.
Geikie thought instead that denudation is a “far more gigantic and rapid process”
than geologists had been apt to believe. If the hourglass had run much faster in
the past, then “our demands for enormous periods . . . are unnecessary. The whole
chain of reasoning . . . seems to break down when it is tested by the facts of mod-
ern denudation.” Geikie cast his lot with Kelvin: “The unlimited ages demanded
by geologists cannot be granted,” Geikie said. “We have been drawing recklessly
upon a bank in which it appears there are no further funds at our disposal. It is