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Deep Time

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Powell, James Lawrence. Four Revolutions in the Earth Sciences : From Heresy to Truth, Columbia University Press, 2014. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/vt/detail.action?docID=1830699. Created from vt on 2018-04-15 21:23:41.

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Gone Right Through It

In September 1846, the faculty of the University of Glasgow convened

to examine an applicant for its chair in natural philosophy, the previ-

ous holder, appointed in 1803, having passed on after a lengthy illness.

At age twenty-two, the candidate, William Thomson (1824–1907), was

easily mistaken for a student himself. In spite of his youth, the Cam-

bridge graduate had already accomplished more than enough to justify

his candidacy, but there was one additional hurdle. Before an appoint-

ment could become offi cial, the applicant had to write and deliver, in

Latin, an essay assigned by the faculty. Thomson’s topic was to be De

caloris distributione per terrae corpus : “The distribution of heat within the

Earth.” The title echoed that of Joseph Fourier’s famous book on heat

fl ow. Either Thomson himself or his father, James, a long-time member

of the Glasgow faculty and his son’s strongest booster, had proposed

the topic. 1 Both knew that no one was better qualifi ed to address the

question of the Earth’s heat than William, who at age sixteen had al-

ready mastered Fourier’s diffi cult mathematics.

William Thomson had learned of Fourier’s The Analytical Theory of

Heat in 1839 from the lectures of Professor John Nichol, who told the

teenager that “perhaps” he could understand this work of “transcen-

dent merit.” According to William’s later recollection, “I took Fourier

out of the University Library; and in a fortnight I had mastered it—

gone right through it.” 2 The following summer James Thomson took

his children on a trip to Germany. Before leaving, William had picked

up a book by Phillip Kelland, professor of mathematics at Edinburgh,

titled Theory of Heat . Fourier, Kelland alleged, had made an error. Indig-

nant at this slander of his hero and already a budding academic, Wil-

liam penned an article titled “On Fourier’s Expansions of Functions in

• • • •

The Abyss of Time

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4 • D E E P T I M E

Trigonometrical Series” and submitted it to the Cambridge Mathemati-

cal Journal , which published it. 3 The author was listed not as William

Thomson but, at the suggestion of his father, as the pseudonymous

P. Q. R. James Thomson evidently thought it inappropriate for his six-

teen-year-old son to rebuke in print a distinguished colleague. 4

As further evidence that young Thomson’s essay topic had not been

chosen at random, in 1842, at age eighteen, the precocious youngster

had published a memoir titled “On the Linear Motion of Heat.” 5 Using

Fourier’s approach, he solved the differential equation that describes

how to determine the temperature in a solid body at any time in the

future. For the rest of his long life, William Thomson would return to

this early paper and its implications, which “contain[ed] the germs

of many of his subsequent ideas.” 6 At the end of the paper Thomson

speculated on the effect were he to assign negative values to time. The

equations then gave impossible results, convincing him “that there

must have been an origin to the natural order of the cosmos. There

must have been a beginning.” 7

In looking back at his body of work from the vantage point of 1882,

Thomson wrote that this youthful essay “gave a very decisive limita-

tion to the possible age of the earth as a habitation for living creatures,

and proved the untenability of the enormous claims for TIME which,

uncurbed by physical science, geologists and biologists had begun to

make and to regard as unchallengeable” (186).

The implications of Fourier’s mathematics, fi rst encountered by

young William Thomson at age sixteen, would occupy him inter-

mittently but without surcease for sixty-eight years, until his death

in 1907. By then he was known as Lord Kelvin, the world’s most ac-

claimed and accomplished scientist.

High Priest of Uniformitarianism

Just before Fourier began his mathematical advances, geology began

to emerge as a true science. The pivotal insight came from the Scots-

man James Hutton (1726–1797). Trained as a physician and chemical

manufacturer, Hutton inherited several farms from his father, allowing

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T H E A B Y S S O F T I M E • 5

him the opportunity to roam the land and pursue his interest in geol-

ogy. Not content merely to observe, Hutton set out to explain.

Hutton believed that God had created the Earth for man. Since sec-

tions of the Earth are visibly eroding, some process must restore it; else

our planet would eventually become uninhabitable, surely not God’s

intent. Hutton came to believe that eroded sediments are deposited in

the sea and subsequently hardened, heated, uplifted, and returned to

the continents, where they erode to start the process again. He viewed

earth history as a series of endless cycles of decay and rejuvenation,

with, in his most famous phrase, “no vestige of a beginning,—no pros-

pect of an end.” 8

This view contrasted mightily with the rival theory of geology,

which saw earth history as ruled by catastrophe: earthquakes, volcanic

eruptions, and the like. Catastrophism fi t well with the short chronol-

ogy of Archbishop James Ussher and his followers, which allowed only

a few thousand years for all of geologic time. Hutton’s cycles required

vastly longer periods. As his devoted biographer and interpreter, John

Playfair, wrote, they required an “abyss of time.” 9

Hutton earned his position as the “Father of Geology” for a state-

ment that would become the guiding principle of geologic thought

and practice:

Not only are no powers to be employed that are not natural to the

globe, no action to be admitted of except those of which we know

the principle, and no extraordinary events to be alledged in order

to explain a common appearance . . . we are not to make nature act

in violation to that order which we actually observe . . . chaos and

confusion are not to be introduced into the order of nature, because

certain things appear to our partial views as being in some disorder.

Nor are we to proceed in feigning causes, when those seem insuf-

fi cient which occur in our experience. 10

Charles Lyell (1797–1875) extended Hutton’s theory in a book titled

Principles of Geology . The fi rst edition appeared in 1830 and the last,

published posthumously, in 1875. The book made Lyell the most in-

fl uential geological writer in history. Trained fi rst as a lawyer, Lyell’s

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6 • D E E P T I M E

Principles was a “passionate brief for a single, well-formed argument,

hammered home relentlessly.” 11

Like Hutton, Lyell believed that God created the Earth for humans.

But once he set the Earth going, never again did he intervene in its

workings. Natural laws are invariant. Moreover, not only are the pro-

cesses that we observe today the only ones that have ever operated,

but they also have always operated at the same rate. According to Ly-

ell, “If in any part of the globe the energy of a cause appears to have

decreased, it is always probable that the diminution of intensity in its

action is merely local, and that its force is unimpaired, when the whole

globe is considered.” 12 Lyell disdained catastrophism, writing in 1881

that he needed no “help from a comet.” 13

According to Lyell, while change is constant on Earth, it does not

lead anywhere. Our planet has always looked about as it does now, its

history revealing no evidence of progress. Even extinction does not

represent permanent change: “The huge iguanodon might reappear in

the woods, and the ichthyosaur in the sea, while the pterodactyl might

fl it again through umbrageous groves of tree-ferns.” 14

Had the Earth been on trial with Lyell as prosecuting attorney, the

defense might have pointed out that his thesis divides into two parts. 15

First, natural law and earthly processes do not vary. This we may call

the constancy of law and process. Second, neither the rate at which

those processes operate nor the overall condition of the Earth vary.

This we may call the constancy of rate and state. One could accept the

fi rst constancy without having to accept the second. In what Stephen

Jay Gould has called “the greatest trick of rhetoric . . . in the entire

history of science,” Lyell gave both arguments the same name: “unifor-

mity.” 16 William Whewell, who reviewed the second edition of Lyell’s

book, lumped the two meanings together under the unwieldy name

“uniformitarianism,” which stuck. Whewell posed the essential ques-

tion of geology: “Have the changes which lead us from one geological

state to another been, on a long average, uniform in their intensity, or

have they consisted of epochs of paroxysmal and catastrophic action,

interposed between periods of comparative tranquillity?” He sagely

predicted that the question “will probably for some time divide the

geological world into two sects, which may perhaps be designated as

the Uniformitarians and the Catastrophists.” 17

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T H E A B Y S S O F T I M E • 7

The defense would go on to point out that the constancy of law

and process, which Gould has called methodological uniformitarian-

ism , describes not how the Earth works but how geologists ought to

work. They reject supernatural explanations and employ common-

place, simple processes before appealing to rare, complicated ones. Of

course, this is not the only way that science works; it is nothing more

than common sense. William of Ockham expressed it well in the four-

teenth century: “One should not assume the existence of more things

than are logically necessary.” All scientists reason from effects back

to causes. Nothing is special about methodological uniformity; it says

merely that geology is a science.

Lyell believed so strongly in the uniformity of rate and state, which

Gould called substantive uniformitarianism , that he wrote,

But should we ever establish by unequivocal proofs, that certain

agents have, at particular periods of past time, been more potent

instruments of change over the entire surface of the Earth than they

now are, it will be more consistent with philosophical caution to

presume, that after an interval of quiescence they will recover their

pristine vigor, than to regard them as worn out. 18

But by the late nineteenth century, geologists had already found un-

equivocal evidence that substantive uniformity is false: earth history

is marked by change. Glaciers have advanced over the continents and

retreated; seas have drowned the continents and withdrawn; mountain

ranges have risen and worn away; parts of the Earth now cold were

once warm and vice versa.

The coup de grace to an unchanging Earth was the progression

shown by the fossil record, leading from the simplest life forms in

Precambrian rocks to modern Homo sapiens . Lyell accepted evolution

only in the 1866 edition of his Principles and then, Gould believes,

only because “it permitted him to preserve all other meanings of

uniformity.” 19

One type of uniformitarianism amounts to the statement that geol-

ogy is a science; the second, which requires the adoption and mainte-

nance of an a priori position regardless of the evidence, amounts to the

statement that geology is not a science.

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8 • D E E P T I M E

In a 1905 book titled The Founders of Geology , Sir Archibald Geikie,

from whom we will hear more, wrote that Lyell became “the great high

priest of Uniformitarianism—a creed which grew to be almost univer-

sal in England during his life, but which never made much way in the

rest of Europe, and which in its extreme form is probably now held by

few geologists in any country.” 20

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You Can’t Win, and Eventually You Lose

In 1892, the queen made William Thomson a peer of the realm. He

chose the name “Kelvin,” for a small stream that wends its way near

the University of Glasgow. 1 Some thirty years before, Kelvin, as we will

henceforth refer to him, had come to despise Lyell’s theory, but not

for any of the reasons already given. Indeed, Kelvin operated from too

lofty a perch to become embroiled in geology’s internecine squabbles.

His objection came from a higher plane: physics.

Lyell’s philosophy envisioned an eternal, unchanging Earth. To sup-

ply the energy necessary to keep the planet running, Lyell appealed to

chemical reactions in the Earth’s interior. They produce heat, he said,

which in turn generates electrical currents, which break up the com-

pounds produced in the reactions and start the process over again. But

Kelvin knew that such a scheme was impossible, for it would violate

the fundamental laws of nature.

The fi rst law of thermodynamics holds that in any process, energy

in the form of heat and work is conserved. As science students once

liked to joke, the fi rst law states: “you can’t win”—you cannot get out

more energy than you put in. The second law, jointly discovered by

Kelvin, can be stated in several different ways. The simplest may be to

say that heat fl ows spontaneously from hotter to colder places, never

the opposite. “Although mechanical energy is indestructible ,” Kelvin

said, “there is a universal tendency to its dissipation, which produces

gradual augmentation and diffusion of heat, cessation of motion, and

exhaustion of potential energy through the material universe.” 2 The

message of the second law is “eventually, you lose.”

In describing geologic time as infi nite and the Earth as unchanging,

Lyell claimed that the Earth is a perpetual-motion machine, one that

• • • •

A Great Mistake Has Been Made

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10 • D E E P T I M E

can not only win the energy battle but go on doing so forever. But the

fi rst and second laws prove that such a machine is impossible. Lyell’s

theory “violates the principles of natural philosophy in exactly the

same manner, and to the same degree,” Kelvin wrote, “as to believe

that a clock constructed with a self-winding movement may fulfi ll the

expectations of its ingenious inventor by going for ever.” 3

Kelvin carried the battle to the geologists, charging in an 1868 ad-

dress 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. There cannot be uniformity. The

Earth is fi lled with evidences that it has not been going on for ever in

the present state, and that there is a progress of events towards a state

infi nitely different from the present.” 4

Kelvin set out to refute Lyell by showing that the Earth was born at

some fi nite time in the past and will not survive beyond some fi nite

time in the future. The Earth is neither eternal nor unchanging. As the

second law dictates, like everything else our planet is running down.

Kelvin knew that Fourier’s mathematics and the second law showed

that when a body starts out with different internal temperatures at

different places, heat fl ows from hot to cold regions and eventually

removes the differences. In his 1846 candidate’s lecture, he had shown

that if one knew how fast the Earth is losing heat and how well rocks

transmit heat, one could work the equations back to the point when

the process began and thereby estimate the age of the Earth. 5

But before he took up that question, Kelvin began to puzzle over

the age of the Sun. It also had to obey natural law and therefore had

to have an energy source. Kelvin fi rst attempted to explain the Sun’s

heat by appealing to the idea that meteorites continually fall into the

Sun and give up their gravitational energy as heat. But calculations

showed that the process was inadequate to explain the Sun’s abundant

heat and light.

That theory having failed, the only apparent alternative was that

the Sun’s energy is left over from its birth. The second law then re-

quires that the Sun be running down. If the Sun’s energy is waning, it,

too, must have been hotter in the past and will one day be cooler, so

cool that it will no longer warm and illuminate the Earth.

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A G R E A T M I S T A K E H A S B E E N M A D E • 11

In an 1862 article in the popular Macmillan’s Magazine , Kelvin de-

scribed the results of his calculations of the age of the Sun:

It seems, therefore, on the whole most probable that the sun has

not illuminated the earth for 100,000,000 years, and almost certain

that he has not done so for 500,000,000 years. As for the future, we

may say, with equal certainty, that inhabitants of the earth can not

continue to enjoy the light and heat essential to their life for many

million years longer unless sources now unknown to us are prepared in

the great storehouse of creation . 6

To Kelvin, whether the Sun was 100 million years or 500 million years

old mattered little. The Earth could be no older. By limiting the age of

the Sun, Kelvin had refuted Lyell and uniformitarianism.

Now Kelvin turned to the Earth, using Fourier’s mathematics to cal-

culate its maximum age. Fourier had attempted the same calculation

and gotten 200 million years, such a seemingly absurd fi gure that he

did not even bother to write it down. 7 Kelvin estimated the Earth’s

initial temperature at 7,000°F, the measured melting point of igneous

rocks. A handful of observations suggested that temperature increased

with depth by about 1°F for every fi fty feet. Having these facts and

Fourier’s mathematics at his command, it was a simple matter for Kel-

vin, if not for many others, to calculate that the Earth consolidated

98 million years ago. But given the lack of precision in his starting

numbers, he broadened his estimate to say that the Earth’s formation

“cannot have taken place less than 20,000,000 years ago, nor more

than 400,000,000 million.” 8 His most probable estimate of the age of

the Sun, 100 million years, lay well within this range, seemingly giving

two independent calculations of the age of the Earth and the solar sys-

tem. This encouraged him to declare that he had refuted the “Doctrine

of Uniformity.” 9

Kelvin was no doubt vexed to fi nd that geologists took no notice

of his calculations. The latest edition of Lyell’s Principles continued to

claim that the Earth is a perpetual-motion machine. A heavier bar-

rage was needed, and, in an 1868 lecture to the Geological Society of

Glasgow titled “On Geological Time,” Kelvin loosed it. 10

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12 • D E E P T I M E

“A great reform in geological speculation seems now to have be-

come necessary,” he began. Oddly, or perhaps sagaciously, instead of

attacking the geologists of his day, Kelvin directed his fi re at the long-

dead John Playfair, who sixty-six years earlier had written “Illustrations

of the Huttonian Theory of the Earth.” 11 Hutton was a poor writer,

and his treatise contained long passages from the French, so that most

geologists had to learn of his views from Playfair. “The statement that

the phenomena presented by the earth’s crust contain no evidence of

a beginning, and no indication of progress towards an end,” Kelvin

wrote, “is founded upon what is very clearly a complete misinterpreta-

tion of the physical laws under which all are agreed that these actions

take place.” Much of Kelvin’s paper was taken up with a third method

of gauging the Earth’s age, using the slowing of the Earth caused by

tidal friction with the Moon, which provided another corroboration of

Kelvin’s view that the Earth could not have existed indefi nitely.

“Odious Spectre”

This was too much for geologists and biologists, both of whom needed

more time than Kelvin would grant. Indeed, in a letter to Alfred Russell

Wallace, the co-discoverer of natural selection, Charles Darwin wrote

that in order to explain the missing links that mark the fossil record he

would like to take advantage of the ample eons before the Silurian pe-

riod, but “then comes Sir W. Thomson like an odious spectre.” 12 To ex-

orcise the “spectre” came “Darwin’s Bulldog”: Thomas Henry Huxley.

In an 1860 debate over Darwin’s theory, so legend has it, Huxley

had defeated a famous public speaker, Bishop Samuel Wilberforce,

known as “Soapy Sam” for his unctuous delivery. Now evolution was

under attack not from a cleric armed with the Bible but from Kelvin,

the leading British scientist, armed with impeccable and, to Huxley,

impenetrable mathematics. Kelvin’s 1868 assault had been directed at

geology, and Huxley happened to be president of the Geological Soci-

ety of London at the time. In his 1869 presidential address, he used the

bully pulpit to rebut Kelvin.

Huxley found himself in the same seemingly inferior position in

which many geologists over the next hundred years were to fi nd them-

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A G R E A T M I S T A K E H A S B E E N M A D E • 13

selves: unable to counter an apparently superior quantitative argu-

ment from a physicist. Huxley well knew that he was unable to use

mathematics to refute Kelvin. But, he said, this left him no worse off

than “attorney[s] general,” who must “nevertheless contrive to gain

their causes, mainly by force of mother-wit and common-sense, aided

by some training in other intellectual exercises.” 13 Huxley pounced

on Kelvin’s selection of the long-dead Hutton and Playfair as his tar-

gets, pointing out that geologists had long since modifi ed the overly

rigid uniformitarianism of the founding fathers. “To my mind there

appears to be no sort of necessary theoretical antagonism between

Catastrophism and Uniformitarianism. On the contrary, it is very con-

ceivable that catastrophes may be part and parcel of uniformity,” Hux-

ley argued. 14

In Huxley’s most telling point, he elegantly summed up what today

we often put more crudely. “Mathematics may be compared to a mill

of exquisite workmanship, which grinds you stuff of any degree of

fi neness,” he said. “Nevertheless, what you get out depends upon what

you put in” (50).

Winding up, Huxley pointed out that Kelvin’s results also depended

on assumptions and suppositions. Asked Huxley, “Is the earth nothing

but a cooling mass and has its cooling been uniform? An affi rmative

answer to both these questions seems to be necessary to the validity

of the calculations on which Sir W. Thomson lays so much stress”

(52–53).

But Kelvin would not let Huxley off so lightly. In a response only

two weeks later, he began: “The very root of the evil to which I object

is that so many geologists are contented to regard the general princi-

ples of natural philosophy, and their application to terrestrial physics,

as matters quite foreign to their ordinary pursuits.” Kelvin added, “A

clever counsel may, by force of mother-wit and common sense, readily

carry a jury with him to either side, [but] I do not think that the high

court of educated scientifi c opinion will ever be satisfi ed by pleadings

conducted on such precedents.” 15

Huxley’s address was the last time for several decades that any scien-

tist would challenge Kelvin. He had shifted the ground of debate about

the Earth. No longer could Lyell’s limitless time be countenanced, nor

his unchanging Earth, nor reasoning by “mother-wit and common

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14 • D E E P T I M E

sense.” Whatever the fate of Kelvin’s argument about the age of the

Earth, geologists heard his message that their science could not remain

descriptive while all other sciences became increasingly quantitative.

Kelvin pushed geology toward the twentieth century, though once it

got there, it would reject his assumptions and his results.

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Those Confounded Millions of Years

Even using their own methods to calculate the age of the Earth, geolo-

gists could not escape Kelvin’s infl uence. Each calculation from geol-

ogy depended on its own assumptions, and each gave at least a slightly

different answer. How could an individual calculator tell whether his

result was anywhere close to right? In the second half of the nineteenth

century there was only one way, and that was to compare one’s result

with that of the lone external authority: Lord Kelvin. The temptation

proved irresistible to the geological calculators, who “produced an amaz-

ing variety of methods and an even greater homogeneity of results.” 1

One of the fi rst was John Phillips (1800–1874). “Nothing can be

simpler in aspect than the problem of the age of the stratifi ed crust of

the globe on the Uniformitarian hypothesis,” he wrote in 1860. “We

have only to fi nd out the rate of accumulation of sediment in the

sea—the thickness of deposits produced in a year, or century, or some

long historic period—and apply this measure or rate to the ancient

deposits.” 2 He reported that “the Ganges River delivers 1/111th of an

inch of sediment in a year.” Geologists had estimated the maximum

thickness of the sediments in the Ganges Basin at 72,000 feet. Divide

accumulation by rate, and you have the “calculated antiquity of the

base of the stratifi ed rocks = 95,904,000 years.” 3

We might describe Phillips’s method by analogy to an hourglass. If

you know the rate at which sand passes through the constriction, and

you know the amount of sand in the bottom of the hourglass, divide

amount by rate and you have the time elapsed since the hourglass was

turned over. Or, instead of using the amount of sand in the bottom,

analogous to sedimentation, you could use the amount remaining in

the top, analogous to erosion.

• • • •

The Bank of Time

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16 • D E E P T I M E

Phillips was well aware that his answer relied on several assump-

tions: that the rate of sediment accumulation was accurately known,

that it had not changed over geologic time, and that erosion had not

removed some of the accumulated sediments. Even making different

assumptions, however, Phillips said that his result “cannot be reduced

to so little as 38,000,000 years” (136). He also noted that the Ganges car-

ries more sediment than other great rivers, so that his 96-million-year

estimate for the age of the Earth’s crust “may be much too short” (126).

Another who attempted the hourglass method was none other than

Charles Darwin (1809–1882), who ever after regretted the attempt.

Darwin was a disciple of Lyell, whose monumental Principles of Geology

he had taken with him aboard the Beagle . Later Darwin wrote: “He who

can read Sir Charles Lyell’s grand work on the Principles of Geology,

which the future historian will recognize as having produced a revolu-

tion in natural science, and yet does not admit how vast have been the

past periods of time, may at once close this volume.” 4 Lyell repeatedly

referred to geologic time as “indefi nite,” thus allowing plenty for natu-

ral selection and evolution.

In the fi rst edition of Origin of Species , Darwin fell prey to one of

those impulses that writers give into in a weak moment but as soon

as they witness their idea in print wish they had resisted. Darwin off-

handedly threw in a calculation of how long it had taken erosion to

excavate a valley in southern England called the Weald. He estimated

the rate of erosion at “one inch per century,” a rotund number that has

the feel of an educated guess—or possibly just a guess. Estimating the

amount of rock that erosion had removed to leave the Weald as we fi nd

it today, and dividing by the rate, Darwin calculated that “the denuda-

tion of the Weald must have required 306,662,400 years; or say three

hundred million years.” 5 Almost immediately, critics pounced on what

they saw as an absurdly high result. Darwin began rapidly to backpedal

from what he came to call “those confounded millions of years.” 6

No Further Funds at Our Disposal

T. Mellard Reade (1832–1909), a British architect, engineer, and ama-

teur geologist, was another hourglass calculator. Phillips and others

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T H E B A N K O F T I M E • 17

had used the amount of sediment carried by the world’s great rivers,

but Reade thought he had a better method: the amount of “soluble

constituents”—dissolved salts like carbonates, sulfates, and chlorides—

in river water. 7 Using the rate of delivery of salt to the ocean and the

amount of salt in the ocean, Reade had a new hourglass.

Reade improved on some of Phillip’s assumptions but made at least

as many himself. After “laborious” calculations, his answer for the age

of the sedimentary rocks came out at 526 million years, which he re-

garded as a minimum. 8 Paraphrasing Hutton, Reade concluded, “We

may speculate on a beginning, but we can fi nd no trace of it by geo-

logical methods” (30).

Although Reade had said that “it defi es calculation to reach a maxi-

mum beyond which we can say the age of the earth does not extend,”

in 1893, he defi ed himself by making just such a calculation (27).

“Physicists say that from the thermal condition 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 impossible.” 9 He

then calculated the amount of time that had elapsed since the begin-

ning of the Cambrian period, estimating it at 95 million years. Adding

on his estimate 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-fi ve years had elapsed since Geikie

had fi rst 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 “ero-

sion” instead of “denudation.”) Some writers treat “the great geologi-

cal domain” as if it demanded “no previous scientifi c knowledge,” he

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18 • D E E P T I M E

wrote. 13 Rejecting Lyell’s strict uniformitarianism, Geikie said that ge-

ologists 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

assume that the present rate is the only possible one or that “unifor-

mity of causation” has been established as true, Geikie wrote. He did

not abandon all of uniformitarianism, only the substantive variety in

which everything remains constant. Geikie’s position was that “pure

catastrophism will certainly lead us into error: mere uniformitarianism

will not bring us the whole truth” (187).

The evidence from denudation “best attest[s] the enormous dura-

tion of geological 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 modern denudation.” Geikie cast his lot with Kel-

vin: “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 well, therefore,

to fi nd that our demands are really unnecessary; that even the facts

of our own science do not require these exorbitant drafts upon the

past” (189).

Little Short of a Swindle

Samuel Haughton (1821–1897), a professor of geology at Trinity Col-

lege in Dublin, disdained the “Geological Calculus” because he be-

lieved “that the time during which organic life has existed on earth

is practically infi nite.” To prove his point, he calculated when the

temperature of the Arctic had been at 122°F, “at which degree albu-

min [a form of protein found in egg white] coagulates.” Life could not

have existed at higher temperatures than that, Haughton wrote. Using

the measured cooling rate of basalt and other data, he calculated that

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T H E B A N K O F T I M E • 19

from the time the oceans formed up to the beginning of the Tertiary

period, 14 2,298,000,000 years had elapsed, a number that he said is

“practically infi nite . . . so great as to be inconceivable by beings of our

limited intelligence.” 15 To attempt to measure absolute ages was fruit-

less, Haughton concluded: the “stony tables that contain the history

of the earth . . . can never give up to even our most diligent research

all the secrets they once contained.” 16

Twelve years later in a paper in Nature , Haughton revisited the

“duration of geological time.” 17 Whereas in his earlier paper he had

concluded that even two billion years was too little, now he found

that “on comparing the rates of cooling of the earth with the maxi-

mum measured thicknesses of the several strata, we fi nd a remarkable

proportion between them.” 18 In other words, his method now gave

the same result as Kelvin’s. To reach that conclusion, however, Haugh-

ton had to engage in some mathematical sleight of hand. After an

assumption-rich analysis of past climate, he came to his new calcula-

tion. The estimated rate of erosion for large rivers ranged from one foot

in 729 years to one foot in 6,846 years, which Haughton averaged and

rounded down to one foot in 3,000 years. He raised this number to ac-

count for the greater area of the seafl oor compared to the land surface,

coming up with one foot in 8,616 years, or a rate of 0.000116 feet per

year. Haughton estimated the total thickness of sedimentary strata up

to the beginning of the Tertiary to be 33.5 miles, a much larger fi gure

than anyone else had assumed. To follow his method to its logical

conclusion, Haughton would now have divided 177,200 feet (about

33.5 miles) by 0.000116 feet per year to get 1.526 billion years for pre-

Tertiary time. But instead Haughton did this: “If we admit (which I am

by no means willing to do) that the manufacture of strata in geological

times proceeded at ten times this rate, or at the rate of one foot for every

861.6 years, we have for the whole duration of geological time, down

to the miocene tertiary [ sic ] epoch . . . 152,675,000 years.”

Estimating the time from the Miocene to the present at one-third

that amount, Haughton concluded that the whole duration of geologi-

cal time requires “a minimum of two hundred millions of years” (268).

Thus in order to bring his calculus into the range that Kelvin allowed,

after saying he was by no means willing to divide by ten to do so,

Haughton then divided by ten. As Kelvin’s biographer Burchfi eld put

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20 • D E E P T I M E

it, this juggling was “little short of a swindle.” 19 Haughton got away

with it, as geologists cited his calculation as further evidence that the

Earth is no more than a few hundred million years old.

American geologists soon began to make their own calculations.

One of the most thorough came from Charles D. Walcott (1850–1927),

the discoverer of the “wonderful life” of the Cambrian Burgess shale,

as Stephen Jay Gould titled his book on the subject. In 1894 Walcott

became director of the U.S. Geological Survey and in 1906 succeeded

Samuel Langley as secretary of the Smithsonian Institution. Walcott

decided to calculate the time required for the deposition of a particular

sequence of Paleozoic rocks he knew well and extrapolate from there to

the age of the Earth. 20 He distinguished between rocks like sandstone

that are deposited mechanically and limestones that are formed chem-

ically or biologically. After a number of assumptions and corrections,

Walcott estimated the duration of Paleozoic time at 17.5 million years.

He used estimates made by others of the length of the other eras—

really little more than educated guesses—to come up with a total for

geologic time of 55 million years. 21 Geologic time, Walcott concluded,

“is of great but not of indefi nite duration. I believe that it can be mea-

sured by tens of millions, but not by single millions or hundreds of

millions of years.” 22

Dropping with Harmonic Regularity

Geologists were not the only ones to fall sway to Kelvin. George Dar-

win (1845–1912), second son of Charles, like Kelvin had been “second

wrangler” at Cambridge: runner-up in the annual university mathe-

matics contest. George Darwin would go on to become a professor of

Astronomy at Cambridge, president of the Royal Astronomical Society,

and president of the British Association. His interests were quite differ-

ent than those of his father and much closer to Kelvin’s.

Kelvin and others had been interested in the “fi gure” of the Earth—

its departure from a perfect sphere—and of the effect of tides in the

solid Earth in slowing the planet’s rotation. In 1877, George Darwin

wrote a paper titled “On the Infl uence of Geological Changes in the

Earth’s Axis of Rotation,” just the sort of work that Kelvin admired. 23

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T H E B A N K O F T I M E • 21

After Kelvin wrote to George, father Charles expressed delight at the

budding association between his son and his long-time nemesis.

George Darwin made two calculations that bore on the age of the

Earth. The fi rst concerned the length of the day. As the Earth and the

Moon have been braking each other, working backward there must

have been a time when the length of a day, determined by the speed

of the Earth’s rotation, and the length of a month, determined by the

Moon’s orbit, were equal. At that time, Darwin calculated, the Moon

would have been within 6,000 miles of the Earth. This must have been

close to the time the Moon and the Earth separated: “something over

54 million years” ago. 24 In a second calculation, Darwin estimated that

in something less than 57 million years, the pull of the Moon would

have produced the current tilt of the Earth’s axis. He could not resist

adding, “It is particularly important to notice that all the changes might

have taken place in 57 million years; and this is far within the time

which physicists admit that the Earth and moon may have existed.” 25

Later George Darwin cautioned, “The actual period, of course, must

have been much greater,” saying that his calculation “is only a wild

speculation, incapable of verifi cation.” 26 But scientists ignored his cau-

tions and regarded his estimate of 54 to 57 million years for the age of

the Earth as yet another confi rmation, one by a rigorous and indepen-

dent method, of Kelvin’s result.

As scientists were coming up with one variation after another on the

hourglass theme, citing one another as authorities, Kelvin continued

to move the target. Each time he redid the calculation, using improved

data for heat fl ow and conductivity, the age of the Earth dropped

with “almost harmonic regularity.” From the 400 million years of his

1862 calculation, the permissible lifespan of our planet descended to

100 million in 1868, to 50 million in 1876, to a possible low of 20 mil-

lion in 1881, fi nally settling at 24 million years in 1897. 27

The 24-million-year estimate had come from the laboratory of an

American geologist named Clarence King (1842–1901), the founding

director of the U.S. Geological Survey and one of the oddest fi gures in

the history of American science. 28 Kelvin had assumed that the molten

Earth had initially been at a temperature of 7,000°F. King started there,

but he had more current information about the melting point of rocks

and the distribution of temperature within the Earth. Using those data,

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22 • D E E P T I M E

King found “no warrant for extending the earth’s age beyond 24 mil-

lions of years.” 29 He ended by saying that “the concordance of results

between the ages of the sun and earth throws the burden of proof

upon those who hold to a vaguely vast age, derived from sedimentary

geology.” 30 But for Archibald Geikie, the “astronomers” had fi nally

gone too far.

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A Fortune Taken Wings

As we saw, in 1868 Archibald Geikie had endorsed Kelvin’s 100-

million-year timescale. Over the decades, he watched as the fund of

time available to geology shrank until the science approached tempo-

ral bankruptcy. In his 1892 presidential address to the British Associa-

tion, Geikie defected. His erudite, eloquent, even romantic defense of

the science of geology is one that every student of the subject and any-

one interested in the history of science would benefi t from reading. 1

With hindsight, we can see how it bridged the descriptive methods of

the nineteenth century and the quantitative ones that were to arrive

in the twentieth.

Geikie acknowledged the successes that uniformitarianism had

brought to the understanding of the Earth, yet, he said, “We must

admit that the doctrine has been pushed to an extreme perhaps not

contemplated by its original founders.” 2 Pointing to the ice ages, he

said, “We recognise the catastrophe, while at the same time we see in

its progress the operation of those same natural processes which we

know to be integral parts of the machinery whereby the surface of the

earth is continually transformed” (17).

Hutton had been able to fi nd no vestige of a beginning, but accord-

ing to Geikie, Lord Kelvin had shown that there must have been one.

Kelvin, using the heat fl owing from the Earth, had estimated, as Geikie

put it, that “the surface of the globe could not have consolidated less

than twenty millions of years ago, for the rate of increase of tempera-

ture inwards would in that case have been higher than it actually is;

nor more than 400 millions of years ago, for then there would have

been no sensible increase at all” (18).

• • • •

Account Overdrawn

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24 • D E E P T I M E

Geikie wrote that Kelvin, when fi rst dealing with the subject, was in-

clined to believe that “100 millions of years would embrace the whole

geological history of the globe.” For the uniformitarian geologists who

had accepted a limitless Earth, “It was not a pleasant experience to

discover that a fortune which one has unconcernedly believed to be

ample has somehow taken to itself wings and disappeared” (18–19).

When the physicist assured the geologist

that he had enormously overdrawn his account with past time, it

was but natural under the circumstances that he should think the

accountant to be mistaken, who thus returned to him dishonoured

the large drafts he had made on eternity.

The geologist found himself in the plight of Lear when his body-

guard of one hundred knights was cut down. “What need you fi ve-

and-twenty, ten or fi ve?” demands the inexorable physicist, as he

remorselessly strikes slice after slice from his allowance of geologi-

cal time.

(19)

Kelvin had given geology a letter of credit in the amount of 20 mil-

lion years but would extend no more. His friend, fellow Scot, and

coauthor Peter Tait, who might well have been nicknamed “Kelvin’s

Bulldog,” reduced the allowance still further. In a lengthy 1869 re-

view of the Kelvin-Huxley debate, Tait derided geology as little more

than “beetle-hunting or crab-catching,” its practitioners incapable of

appreciating mathematics. Tait proved himself an even stricter book-

keeper than Kelvin, allowing geologists only “ten or fi fteen millions of

years . . . and with better experimental data, this period may be still

farther reduced.” 3

Physicists who could reject the fi ndings of geology and reduce the

age of the Earth from 100 to 50 to 24 to 10 million years might just

decide to keep on going. Speaking for his beloved profession, Geikie

cried Enough! “There must be some fl aw in the physical argument,”

he wrote. “Some assumption has been made, or some consideration

has been left out of sight, which will eventually be seen to vitiate the

conclusions.” 4

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A C C O U N T O V E R D R A W N • 25

Twenty Million in My Mouth

A new challenge to Kelvin came from Geikie’s colleague at the Geo-

logical Survey of Scotland, James Croll (1821–1890). Having taught

himself physics and chemistry, Croll took a job as the caretaker of a

Glasgow museum in order to have access to its books. He must have

read and understood them, for he began to correspond with Kelvin

about his (Croll’s) idea that variations in the Earth’s orbit might have

caused the ice ages. Croll turned out to be right: orbital variations were

the cause, though, understandably, he got the details wrong.

In an 1877 article, like Huxley, Croll pointed out that Kelvin’s result

depended on unprovable assumptions: “The utmost that any physicist

is warranted in affi rming,” said Croll, “is simply that it is impossible for

him to conceive of any other source [of the Sun’s energy]. His inability,

however, to conceive of another source cannot be accepted as a proof

that there is no other source.” 5

Another criticism of Kelvin’s approach came from close to home and

in mathematical form. John Perry (1850–1920) had been Kelvin’s stu-

dent and his assistant at the University of Glasgow. As would be re-

quired for anyone aspiring to assist Kelvin, Perry was himself an excel-

lent mathematician. Underlying all of Kelvin’s work was the fi rst law

of thermodynamics: energy is conserved. As England et al. point out,

in his work on the age of the Earth and the Sun, Kelvin made three

assumptions: the Earth is rigid as steel, is homogeneous in its physi-

cal properties, and has no undiscovered source of heat. 6 Kelvin clearly

spelled out each assumption, for example writing in his 1862 article

in Macmillan’s that his calculations for the age of the Sun were reliable

“unless sources now unknown to us are prepared in the great storehouse

of creation.” 7

Perry made a simple change to Kelvin’s assumption of a homoge-

neous Earth: he imagined that a thin, solid crust overlies a fl uid inte-

rior that conducts heat by convection. As the interior cooled, it would

transfer heat to the surface. According to Perry, that meant that Kelvin’s

fi gure of 100 million years is too low. If the interior conducted heat ten

times better than the surface, for example, Kelvin’s result would have

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26 • D E E P T I M E

to be multiplied by 56, Perry calculated, giving 5 billion years as the

age of the Earth.

Kelvin found Perry’s theorem “clearly right.” Though he thought

his original range for the Earth of “20 to 40 millions probably wide

enough,” it was possible that he “should have put the superior limit a

good deal higher, perhaps 4000 instead of 400.” 8 Kelvin could afford to

be generous with the possible maximum age of the Earth, since he still

had the limit of the Sun’s age, which he now said was restricted to a

score or very few score of millions of years. The Earth could be no older

than the Sun. Kelvin stuck by his estimate of 100 million years: “That

is all Geikie wants; but I should be exceedingly frightened to meet him

now with only 20 million in my mouth.” 9

In 1897, Kelvin had his last word on the subject of his lifelong ob-

session. He had not changed his mind. He began his address to the

Victoria Institute by noting that “thirty or forty years ago” geologists

had given “very little thought” to the age of the Earth. This attitude,

Kelvin said, “would have left geology much in the same position as if it

were impossible to ascertain whether the Battle of Hastings took place

800 years ago, or 800 thousand years ago, or 800 million years ago.” 10

Such a lax attitude was no longer acceptable, Kelvin said, for the analy-

sis of the heat escaping from the Earth “suffi ces to sweep away the

whole system of geological and biological speculation demanding an

‘inconceivably’ great vista of past time, or even a few thousand mil-

lion years, for the history of life on the earth.” 11 This analysis “utterly

refuted the Doctrine of Uniformity as taught by Hutton, Lyell, and

their followers” (20).

Kelvin cited his fi rst estimate of 20 million to 400 million years from

his 1862 paper, “On the Secular Cooling of the Earth.” Now, thirty-fi ve

years later, he said that new knowledge of the thermal properties of

rocks allowed a “closer estimate.” Ignoring other new developments,

such as Geikie’s defection and Perry’s logical refutation of his assump-

tions, Kelvin tightened the screws: “We now have good reason for

judging that it was more than 20 and less than 40 million years ago;

and probably much nearer 20 than 40” (20). But he could go even fur-

ther, endorsing King’s estimate of 24 million years (21).

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F I G U R E . Lord Kelvin in old age. Source : Gi of A. Rex Rivolo, courtesy AIP Emilio Segre Visual Archives.

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28 • D E E P T I M E

Certain Truth

Just as Kelvin had fi nally gone too far for Geikie, now he had gone

too far for the American geologist Thomas Chamberlin. Two months

after Science reprinted the text of Kelvin’s 1897 address, Chamber-

lin took it upon himself to respond. He began by praising Kelvin,

whose “contributions have been the most potent agency of the last

three decades in restraining reckless drafts on the bank of time.” Then

he buried Kelvin’s conclusions: “It must be recognized that any one

line of reasoning, however logically and rigorously followed, is quite

sure to lead astray if it starts from limited and uncertain premises.”

Moreover, “Lord Kelvin’s address is permeated with an air of retro-

spective triumph and a tone of prophetic assurance.” 12 Such dog-

matism offended Chamberlin’s professed ideal of multiple working

hypotheses. As did Kelvin’s choice of words, the most offensive of

which Chamberlin singled out: “strict limitations . . . sure assump-

tion . . . certain truth . . . no other possible alternative.” As Chamber-

lin saw it, “the most essential factor in his reasonings rests ultimately

upon an assumption ” that, as Kelvin put it, “the material of our pres-

ent solid earth all round its surface was at one time a white-hot liq-

uid.” 13 Chamberlin “begged leave to challenge the certitude of this

assumption.”

Consistent with his philosophy of the proper methods of science,

Chamberlin did not insist that he was right and that Kelvin was wrong.

Rather, in the most prescient part of his response, Chamberlin focused

on what scientists did not know:

Is present knowledge relative to the behavior of matter under such

extraordinary conditions as obtain in the interior of the sun suf-

fi ciently exhaustive to warrant the assertion that no unrecognized

sources of heat reside there? What the internal constitution of the

atoms may be is yet an open question. It is not improbable that

they are complex organizations and the seats of enormous energies.

Certainly no careful chemist would affi rm either that the atoms are

really elementary or that there may not be locked up in them ener-

gies of the fi rst order of magnitude. 14

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A C C O U N T O V E R D R A W N • 29

Chamberlin had accused Kelvin of using a “tone of prophetic assur-

ance,” but it was he, Chamberlin, who in this paragraph proved the

true prophet.

In 1924, George P. Merrill, the head curator of geology at the U.S.

National Museum, reviewed the various estimates of geological time

that scientists from Lyell to King had made. 15 Merrill divided them

into three groups depending on whether the estimate was supposed to

represent the time since life began on Earth, since the ocean came into

existence, or since the Earth was molten. The estimates ranged from 3

to 240 million years and averaged 83 million. None even approached

Kelvin’s original upper limit of 400 million, a generous allowance that

itself had lasted only a decade or so before the accountants redid the

books.

A Clock Made of Salt

One of the authors whom Merrill cited was John Joly, a professor of

geology and mineralogy at the University of Dublin. Joly was one of

the most prolifi c, inventive, and diversely curious scientists of his era,

writing 269 scientifi c articles and several books. 16 His interests ranged

over the age of the oceans and the Earth, paleontology, rainfall gauges,

the diet of sea birds, tectonics, continental drift, the use of radium to

treat cancer, and the canals of Mars. Like Kelvin, Joly owned a yacht

and loved to sail.

In 1715, the English astronomer Edmund Halley, friend of Newton,

had proposed an hourglass method to the Royal Society. If the amount of

salt in landlocked lakes like the Caspian Sea were measured at that time

and “after some centuries,” he said, then “we may by the rule of propor-

tion, make an estimate of the whole time wherein the water would ac-

quire its present degree of saltness.” In other words, the amount of salt

in the oceans could provide another kind of hourglass. Halley accepted

that “mankind has dwelt about 6,000 years,” but Scripture nowhere

revealed “how long the Earth had existed before this last Creation.”

Halley wished that the ancient Greek and Latin scholars had

thought to measure and record the amount of salt in the sea, but

since they had not, he recommended that the Royal Society begin to

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30 • D E E P T I M E

keep track of the “present degree of saltness in the Ocean . . . that they

may stand upon the record for the benefi t of future ages.” The great

Halley recognized a problem that we will come back to: the ocean

might have contained original salt, which would “contract the age

of the world.” In other words, if the oceans had contained salt at

their birth, as though sand was already present in the bottom of the

hourglass, the salt clock would give an age that was too old, but still

that age would be the maximum possible. Thus the method would

“refute the ancient notion of the eternity of all things; though perhaps

by it the world may be found much older than many have hitherto

imagined.” 17

By Joly’s day, scientists had accumulated enough information to al-

low them to perform Halley’s calculation. In 1899 Joly, unaware of

either Halley’s suggestion or of T. Mellard Reade’s similar work, did the

arithmetic, using sodium instead of sodium chloride to “avoid the ob-

scure question of its ionisation.” 18 Joly’s calculation had the simplicity

of the hourglass: divide the total amount of sodium in the ocean, esti-

mated at 1.5627 x 10 16 tons, by the rate at which rivers deliver sodium

to the ocean, estimated at 1.527 X 10 8 tons per year, and derive the age

of the ocean: 99 million years.

According to Patrick Wyse Jackson, the paper “fi red the imagina-

tion of both scientifi c and general audiences, and for perhaps a decade

this ‘sodium method’ held sway amongst geochronologists.” 19 The salt

clock would turn out to be the last grain of sand through the hourglass

of the geological calculators.

Joly continued to write and lecture until his death in 1933, but he

never completely forsook the method that had brought him a moment

of fame. By that time, scientists had plumbed the atom and shown

beyond doubt that the Earth is at least ten times older than Joly’s up-

per limit.

It seems paradoxical, even tragic, that though Joly participated in

the discovery and use of the new methods of measuring geologic time

based on radioactivity, even founding an institute to explore the use

of radium in medicine, he could never desert the hourglass of salt for

the rigor of the atom.

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A C C O U N T O V E R D R A W N • 31

Joly’s last publication was “History of the Irish Radium Institute” in

1931. 20 In that year, the U.S. National Research Council published a

report titled The Age of the Earth . The author of the chapter on radio-

activity and geologic time was a prolifi c Briton named Arthur Holmes

(1890–1965), whom we will meet repeatedly and for whom a good case

can be made as the greatest geologist of the twentieth century.

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Serendipity

The decade of the 1890s was a time of great progress in science. No year

in that decade was more eventful than 1895. In Sweden, Svante Arrhe-

nius was calculating the effect of atmospheric CO 2 on global tempera-

ture, while in Britain John Perry was exposing the fallacy of Kelvin’s

assumptions. The Scottish chemist William Ramsay discovered helium,

previously known only from the Sun’s spectrum, in an earthly mineral.

Helium would not only help explain what had eluded Kelvin—the true

source of the Sun’s energy—but help refute his claims about the age of

the Earth. In the town hall at Sceaux, France, a young Polish chemist

named Marie Sklodowska married her sweetheart, Pierre Curie.

That same year, at the University of Wurzburg in Germany, Wilhelm

Röntgen (1845–1923) was investigating the properties of cathode rays.

These mysterious beams, discovered in 1876, appeared when scien-

tists applied a voltage across the electrodes in an evacuated tube. As

sometimes happens in science, Röntgen made a serendipitous discov-

ery. After enclosing the cathode-ray tube in a black box to exclude all

light, then turning off the lights in the laboratory, Röntgen switched

on the current in the vacuum tube. To his surprise, a spectral shimmer

appeared on his benchtop a few meters away. He lit a match and saw

that the glow had come from a paper plate that he had coated with a

barium compound. When he switched off the current, the glow disap-

peared. He recognized that the cathode-ray tube must give off invis-

ible rays that had somehow excited the barium. Röntgen named them

X-rays, X for unknown. He found that when he placed a solid object

between the cathode-ray tube and a photographic plate, a ghostly im-

age of the object appeared on the plate. Röntgen used X-rays to make

a famous image of his wife’s hand, her bones and ring clearly visible.

• • • •

Strange Rays

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S T R A N G E R A Y S • 33

Some substances, like Röntgen’s barium compound, glow or “fl uo-

resce” when irradiated with X-rays. Others, including some uranium

minerals, “phosphoresce”: they continue to glow for a while even after

the irradiation is turned off. In 1896, the French physicist Henri Bec-

querel (1852–1908), suspecting that sunlight might also trigger phos-

phorescence, wrapped a photographic plate in black paper so that no

light could reach it, placed a specimen of a phosphorescent uranium

mineral called pitchblende on top of the wrapped plate, and sat the

whole thing in the sunlight. Sure enough, when he developed the

plate, he found that it had captured the image of the uranium crystal.

But when the Sun failed to shine for a few days, Becquerel temporar-

ily suspended the experiment, storing the crystal atop a photographic

plate inside a drawer where no light could reach it. When he later

developed the plate, expecting to fi nd at most a faint impression from

the residual phosphorescence of the uranium mineral, he found in-

stead an image of the crystal that was as sharp as it had been after the

mineral had sat in the sunlight. Becquerel realized that phosphores-

cence does not depend on sunlight but instead comes from within the

crystal itself. Like cathode-ray tubes, some minerals also emit strange

rays. Unlike the tubes, the minerals do so spontaneously.

Becquerel suggested that his student Marie Curie (1867–1934) take

up the study of the new rays, thus launching one of the most pro-

ductive careers in the history of science. For her Ph.D. thesis topic,

Marie Curie set out to discover whether other substances also emit the

uranium rays, a process that she and Becquerel had named radioac-

tivity. Instead of a photographic plate to detect radiation, she used a

sensitive instrument called an electrometer, invented by her husband

Pierre and his older brother. Her work soon disproved a scientifi c belief

that went back to the Greeks: that the atom is the smallest particle of

matter.

Marie Curie (and independently a German scientist) soon found

that the element thorium is also radioactive. She then discovered that

pitchblende emitted radioactivity at four times the rate of pure ura-

nium. She deduced that pitchblende must contain another element

or elements more radioactive than uranium. She and Pierre set out to

identify those elements and soon found two: one they named one po-

lonium, after her native Poland; the other they named radium.

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34 • D E E P T I M E

F I G U R E . Marie Curie ( – ). Nobel Portrait, c. . Source : Photograph by Gene- ralstabens Litogra ska Anstalt, courtesy AIP Emilio Segre Visual Archives.

In 1903, Marie Curie, her husband, and Becquerel each received the

Nobel Prize in Physics for these discoveries. In 1911, she won the No-

bel Prize in Chemistry, making her the only person to win the prize in

two different sciences.

Having no idea of the dangers of radiation, Marie Curie conducted

her work without the safety measures that we take for granted today.

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S T R A N G E R A Y S • 35

This was a time when workers would lick the tip of a paint brush, dip it

into a pot containing radium, and use it to coat the hands on a watch

so that they would glow in the dark. Then they would repeat the pro-

cess. Just as Kelvin had carried around a specimen of a radioactive min-

eral, so too did Madame Curie carry test tubes of radioactive material,

storing them in her desk drawer. In 1934, she died from leukemia, one

year before her daughter Irene Joliot-Curie and Irene’s husband Fredric

Joliot themselves won the Nobel Prize in Chemistry.

Half Lives

Physicists and chemists leapt on the exciting new discoveries and be-

gan feverishly to explore the strange new rays. The most productive

was a young New Zealander named Ernest Rutherford (1871–1937). 1

If any career personifi es the benefi ts to society of providing educa-

tional scholarships to deserving students, it is Rutherford’s. How else

would this young man have made the journey from a small family

farm to the Nobel Prize? His fi rst scholarship award allowed Ruth-

erford to attend Nelson College, close to the family farm on the

South Island. Having excelled there, he next received a scholarship

to Canterbury College of the University of New Zealand. Excelling

once again, in 1895 he won a scholarship to Cambridge University,

where his intellect and gifts as an experimentalist again made him

stand out. Rutherford went from Cambridge to the chair of physics

at McGill University in Montreal. In 1918 he became director of the

famous Cavendish Laboratory at Cambridge, where he had done his

post graduate work.

Ernest Rutherford ranks not only as one of the greatest experimen-

talists in the history of science but as one of the most inspiring men-

tors. Not only did he win the Nobel Prize in Chemistry in 1908 for his

work on radioactivity, under his direction four scientists at the Cav-

endish Laboratory themselves became Nobelists. Some of his students

would go on to lead the discovery of plate tectonics in the 1960s. Just

as the emissions from radioactive atoms induced radioactivity in other

nearby atoms, so proximity to Rutherford inspired his students and

colleagues to reach beyond themselves.

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36 • D E E P T I M E

While a student at the Cavendish, Rutherford and his thesis profes-

sor, J. J. Thomson (no relation to Kelvin), discovered that when X-rays

pass through a gas, they engender a host of charged atoms or, as Mi-

chael Faraday had named them, ions. Rutherford wondered whether

Becquerel’s new uranium rays would also ionize air. He found that they

did but also discovered that the uranium rays were not X-rays. Bec-

querel had already found that when the uranium rays pass through

a magnetic fi eld, some veer to one side, indicating that they have a

negative charge, while others swerve to the opposite side, showing that

they have a positive charge. Rutherford named the negative rays beta

and the positive rays alpha, the names by which we know them today.

(The French chemist Paul Villard discovered gamma rays in 1900 while

studying radium.) The beta rays turned out to be the electrons that

J. J. Thomson had discovered in 1897.

One of Rutherford’s McGill colleagues found that air currents could

waft the radioactivity emitted by thorium about the laboratory: the

thorium rays were a gas. Rutherford drew the gaseous rays, which they

called “Thorium Emanation,” into a tube and, using an electrometer

like the one that Marie Curie had employed, found that no matter how

much of the thorium gas was present at the start, every 54.5 seconds its

radioactive emissions would decline by one-half. In the next 54.5 sec-

onds, the activity would decline by half again, and so on. Other radio-

active substances also lost half their activity in a given amount of time,

but the amount differed for each. Thus was born the concept of half-life:

the amount of time it takes for half of any starting number of radio active

atoms to decay. Plotted on a graph, radioactive decay is exponential.

(“Thorium emanation” turned out to be an isotope of radon: Rn-220.)

Another component of thorium, which they called Thorium X, de-

cayed with a half-life of 3.6 days, but the radioactivity built up again

at the same rate. (“Thorium X” turned out to be Ra-224.) Rutherford

and his research partner Frederick Soddy (1857–1956) deduced the pro-

cess: “The normal or constant radioactivity possessed by thorium is an

equilibrium value, where the rate of increase of radioactivity due to the

production of fresh active material is balanced by the rate of decay of

radioactivity of that already formed.” 2

Soddy went on to discover that elements are composed of varieties

with the same chemical properties but different atomic weights, which

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S T R A N G E R A Y S • 37

he named isotopes. For this discovery Soddy won the 1921 Nobel Prize

in Chemistry. In his remarks at the ceremony, Soddy gave a succinct

description of isotopes: “Put colloquially, their atoms have identical

outsides but different insides.” 3

Rutherford and Soddy had found a strange new world: substances

decay so as to lose exactly half their original activity in a fi xed amount

of time, but in so doing they transmute themselves into other sub-

stances, which also die away but with a different half-life. Atoms, far

from being eternal, immutable, and the smallest subdivision of mat-

ter, may spontaneously split into pieces and vanish while atoms of an

entirely different element arise in their place. In a sense, the ancient

alchemists were right: one element can be transformed into another.

But although lead cannot be changed into gold, as the ancients had

hoped, lead did turn out to be the most important element in discover-

ing the age of the Earth.

In a series of classic papers, Rutherford and Soddy explained that

radioactive decay obeys the law of probability. 4 If we could observe

a particular atom, say of radon-220, we could not predict when that

atom would decay. It might happen within the fi rst millisecond of

observation, or take hours, days, weeks, or years. We can speak only of

the probability that the atom will decay. But if the number of atoms

observed is large enough, the laws of probability dictate that in one

half-life, exactly one-half that number will decay.

A law of probability also governs the outcome of tossing a coin,

with its 50-50 chance of landing on heads. Toss an honest coin only a

few times, and you might get several heads in a row, or several tails, or

any combination: the exact outcome is unpredictable. But as you toss

the coin over and over, the percentage of heads approaches and fi nally

reaches 50 percent to whatever number of signifi cant fi gures you have

the patience to achieve. Toss the coin 10 23 times, for example, and the

number of heads will be 50 percent to a mind-numbing number of sig-

nifi cant fi gures. Ten raised to the twenty-third power is approximately

the number of molecules in only a single gram-mole of any chemical

element (Avogadro’s Number: 6.02 x 10 23 ). Even minute traces of a

radioactive element contain so many atoms that they obey the law of

radioactive decay with complete fealty.

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How Old Is the Earth Supposed to Be?

In 1903, Rutherford carried out an experiment that his biographer

said demonstrated “remarkable ingenuity—even for Rutherford.” 1 He

separated the alpha and beta particles and measured the velocity of

the alphas, fi nding that they travel at the fantastic speed of 24,000

kilometers per second, or about 54 million miles per hour. Ruther-

ford observed that the alpha particles have about twice the mass of a

hydrogen atom, from which he could deduce that they were almost

certainly helium. Since the energy of a moving object is one-half its

mass times its velocity squared, Rutherford could calculate the energy

of the alphas, which he found to be far greater than that of the beta

and gamma rays combined. 2

Pierre Curie (1859–1906) had discovered that one gram of radium

gives off about 100 calories of heat per hour—enough to raise the tem-

perature of a gram of water from the freezing to the boiling point.

Rutherford had discovered why. As the alpha particles fl y outward,

they collide with other atoms. Since energy must be conserved, the

collision transforms the enormous kinetic energy of the alphas into

heat.

In a 1905 lecture at Yale, Rutherford said that “the weight of the

evidence points to the conclusion that the alpha particle from radium

is an atom of helium.” 3 Then, “if the rate of production of helium

from known weights of the different radio-elements were known . . .

it should thus be possible to . . . determine the age of the mineral”

(187–188). He recognized that some of the gaseous, unreactive helium

would have escaped from minerals, but even so the method would

allow “a minimum limit for the age of the mineral” (188). In 1907,

Rutherford and Thomas Royds, a graduate of Manchester University,

• • • •

An Hourglass of Great Precision

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A N H O U R G L A S S O F G R E A T P R E C I S I O N • 39

where Rutherford had moved by this time, proved that the alpha par-

ticles were indeed nuclei of helium atoms.

By measuring the amount of uranium and helium in a mineral and

by knowing the half-life of uranium, Rutherford had the amount of

parent atom, the amount of daughter atom, and the rate at which par-

ent changed to daughter: he had an hourglass. He applied the method

to a sample of a uranium mineral called fergusonite, fi nding that it

was at least 600 million years old (189). Another uranium mineral gave

about the same age. Rutherford said, “When the data required for these

calculations are known with more defi nitiveness [this] will prove one

of the most reliable methods of determining the age of the various geo-

logical formations” (190). The fi rst attempts to measure geologic ages

by radioactivity gave results many times greater than either Kelvin’s

100-million-year upper limit or the geological hourglasses.

Rutherford knew that helium was not the end product of uranium

decay but only an intermediate step in the decay chain. Rutherford cal-

culated that the fi nal product of uranium decay would have an atomic

weight of 206.5, close to the measured atomic weight of lead, at 206.9.

The Yale chemist Bertram Boltwood had noted that minerals high in

uranium also contain lead, which is chemically quite unlike uranium.

These facts suggested that lead is the fi nal product of uranium decay,

prompting Rutherford to say that the “percentage of lead in radio-

active minerals should be a far more accurate method of deducing the

age of the mineral.” Why? Because unlike gaseous helium, “the lead

formed in a compact mineral has no possibility of escape” (192). But

he thought that it would take “many years to prove or disprove experi-

mentally that lead is the fi nal product of radium” (192–193). On this

occasion, Rutherford turned out to be wrong.

His talks at Yale were not the fi rst time Rutherford had reported an

age much larger than Kelvin’s accountancy would allow. As his biog-

rapher reports, a year or so earlier, Rutherford was walking the McGill

campus, carrying in his pocket a specimen of pitchblende, the same

uranium mineral that Becquerel and Marie Curie had used in their ex-

periments. Meeting a colleague, Rutherford asked, “Adams, how old is

the earth supposed to be?” The answer came back at Kelvin’s then still

prevalent fi gure of 100 million years. “I know,” said Rutherford quietly,

“that this piece of pitchblende is 700 million years old.” 4

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F I G U R E . Ernest Rutherford ( – ). Oswald Birley’s portrait. Source : Royal So- ciety, courtesy AIP Emilio Segre Visual Archives.

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A N H O U R G L A S S O F G R E A T P R E C I S I O N • 41

A Prophetic Utterance

If one moment can symbolize the transition from Victorian to

twentieth-century science, when geology began to cast off the fetters

of unfounded assumptions, uncritical deference to authority, and in-

ability to measure and quantify, the 1904 meeting of the Royal Society

of London makes a good candidate. Slated to give the prestigious Ba-

ker ian lecture was Ernest Rutherford. His predecessors in the honor

had included George Darwin, Davy, Faraday, Lyell, Maxwell, Tyndall,

and many other notables.

The audience of nearly eight hundred included the cream of British

science, assembled to hear Rutherford report the results of his experi-

ments on radioactivity. Few in the audience could have fully under-

stood the brave new world of transmuting atoms that Rutherford de-

scribed. To some, his science may have seemed closer to alchemy. Lord

Kelvin was in the audience, as Rutherford was said often to recount:

I came into the room, which was half dark, and presently spotted

Lord Kelvin in the audience and realized that I was in trouble at the

last part of my speech dealing with the age of the earth, where my

views confl icted with his. To my relief, Kelvin fell fast asleep, but as

I came to the important point, I saw the old bird sit up, open an eye

and cock a baleful glance at me! Then a sudden inspiration came, and

I said Lord Kelvin had limited the age of the earth, provided no new

source was discovered. That prophetic utterance refers to what we are

considering tonight, radium! Behold, the old boy beamed upon me. 5

The discovery of radioactivity both falsifi ed Kelvin’s calculations

for the age of the Sun and provided the means of making a correct

calculation of the age of the Earth. “The discovery of the radio-active

elements, which in their disintegration liberate enormous amounts of

energy, thus increases the possible limit of the duration of life on this

planet,” Rutherford said, “and allows the time claimed by the geologist

and biologist for the process of evolution.” 6

Others, including George Darwin and John Joly, had noted before

Rutherford’s lecture that if radium were present in the Sun, “the supply

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42 • D E E P T I M E

of solar heat must no longer be regarded as affording a major limit

both to solar age and geological time,” as Joly put it. 7 This possibility

confi rmed what Chamberlin, Geikie, and Perry had all recognized: one

or more of Kelvin’s assumptions could as well be false as true.

The discovery of radioactivity reversed the downward trend in es-

timates of the Earth’s age, prompting Joly to write that “the hundred

million years which the doctrine of uniformity requires may, in fact,

yet be gladly accepted by the physicist.” 8 Uniformitarianism did not

require one hundred million years—that was Joly’s salt clock. But the

physicists soon showed that they would not be glad to accept 100 mil-

lion years, nor even 1,000 million.

In his cautions about “operations . . . which are impossible under

the laws to which the known operations going on at present in the ma-

terial world are subject” and “sources now unknown to us . . . prepared

in the great storehouse of creation,” Kelvin had seemed to anticipate

radioactivity. But after its discovery, he never acknowledged that the

existence of radioactive heat required him to change his assumptions.

At various times, he accepted radioactivity, denied it, and, fi nally, ig-

nored it.

According to Rutherford’s biographer, Kelvin agreed to a bet that

he would soon accept the new radioactivity. At the subsequent meet-

ing of the British Association in 1904, Kelvin paid off. 9 He may have

honored the bet, but his published statements took the opposite tack.

In a series of letters to the London Times in 1906, asserting that he had

spent “more hours in reading the fi rst and second editions of Ruther-

ford’s Radio-activity” than almost any other person, Kelvin denied that

radium could emit heat in perpetuity or account for the Earth’s heat.

As for the heat of the Sun, that was attributable not to radioactivity

but, as he had long said, to the release of gravitational energy. Kelvin

claimed that radium is not a chemical element but a molecular com-

pound composed of lead and fi ve helium atoms. 10

In one of the last papers of his life, Kelvin wrote that the heat emit-

ted by radium is not caused by alpha particles but by electrons, which

somehow become “loaded” with energy. This process, he said, can “go

on forever, without violating the law of conservation of energy, and

without any monstrous or infi nite store of potential energy in the loaded

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A N H O U R G L A S S O F G R E A T P R E C I S I O N • 43

Radium atom.” 11 Having begun his attack on geology in order to refute

Lyell’s claim that the Earth is a perpetual-motion machine, Kelvin now

made a rather similar claim himself.

Lord Kelvin died in 1907, the same year the statement above ap-

peared, four months after attending yet another meeting of the Brit-

ish Association, where according to Rutherford he had taken part in

a lively discussion on the “Constitution of the Atom.” 12 Though in

the end Kelvin turned out to be wrong about the age of the Earth and

the solar system, his insistence on quantitative methods and that the

Earth’s age measured not in the few thousand years the Bible allowed

but at least in a score or more million years helped propel geology into

a new century.

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Lead Is the Final Product of Uranium

To Kelvin and Rutherford, the age of the Earth and its constituent rocks

and minerals were of secondary interest. Kelvin was intent on cor-

recting the great mistake of British popular geology—belief in Lyellian

uniformitarianism—and the age of the Earth and the Sun provided the

means. He had enough other interests practical and theoretical to oc-

cupy several ordinary careers: thermodynamics; laying a cable under

the Atlantic Ocean; writing a textbook with Tait; and inventions that

included an improved compass, a machine to sound the depth of the

ocean, and one to predict the tides.

Rutherford wanted to use radioactivity to explore the atom. Once

he and his colleagues had revealed the basic principles of radioactiv-

ity and provided a few examples of its utility, he moved on to what

some might call his greatest scientifi c accomplishment: the Rutherford

model of the atom, with its central nucleus of protons and neutrons

surrounded by shells of electrons. Niels Bohr, his distinguished Dan-

ish collaborator, said that “Rutherford’s achievements are so great that

they provide the background of almost every word that is spoken at a

gathering of physicists.” Sir James Jeans called Rutherford “The New-

ton of atomic physics.” 1 In 1914 he was knighted as Baron Rutherford

of Nelson, for his hometown on the South Island. His list of honors

fi lls two pages. 2

One of the fi rst to stand on Rutherford’s broad shoulders was Robert

Strutt (1842–1919), the fourth Baron Rayleigh and a professor of phys-

ics at Imperial College in London. Like Rutherford, Strutt had studied

physics under J. J. Thomson at the Cavendish Laboratory. He subse-

quently wrote the biography of this “other Thomson,” who in 1906

won the Nobel Prize for his discovery of the electron.

• • • •

Geochronology

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G E O C H R O N O L O G Y • 45

Strutt was elected into the Royal Society in 1905 at age thirty. His

citation included thirteen scientifi c papers, each written before he took

up the topic that by itself would have ensured his place in scientifi c

history: pioneering the uranium-helium method of calculating geo-

logic ages.

From 1908 to 1910, Strutt measured the amounts of uranium and

helium in phosphatic bones, which can have fi fty times as much

uranium as does the average rock. Though he found ages as great as

141 million years, they failed to correlate with the known geologi-

cal ages of the specimens. Strutt concluded that “fossilized bones and

other materials do not always contain as much helium as would be

expected from their radioactivity and geological age.” 3 Apparently the

bones had leaked helium, as Rutherford had suspected. Strutt searched

for a common uranium-bearing mineral that would better retain he-

lium, soon settling on zircon, a refractory mineral that remains the

primary one used today for age measurements. Right away Strutt

found that the uranium-helium ratio in zircons “stands in very close

relation to the geological age of the specimen.” 4 The zircons gave ages

of several hundred million years and the oldest, from Precambrian

rocks in Ontario, dated to 715 million years. But the ease with which

helium can escape crystal lattices, even those of zircon, meant that

uranium-helium dating would always be suspect. Other methods soon

supplanted it.

While Strutt was experimenting with the uranium-helium method,

the Yale chemist Boltwood was following up on the possibility that,

as he and Rutherford suspected, lead is the end product of uranium

decay. For forty-three specimens of uranium minerals, Boltwood found

that the older the mineral, the more lead it contained. Using a method

suggested by Rutherford, which required an estimate of the decay rate

of the intermediate product radium, Boltwood calculated ages ranging

from 410 million years to the then amazing 2.2 billion. He recognized

that these were only “rough” calculations and that the import of his

study was that the ratio of lead to uranium “is greatest in minerals

from the locality which, on the basis of geological data, is the oldest.”

This he considered “proof that lead is the fi nal disintegration product

of uranium.” 5 Boltwood was more interested in radioactive decay and

its end products than in the practical use of the process.

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46 • D E E P T I M E

But even these fi rst, rough estimates of geologic ages using radio-

activity gave results well beyond Kelvin’s limit. And because at least

some of the minerals must have lost daughter atoms, the true ages

were even greater.

Biologists and geologists now had plenty of time; indeed, some had

more than they wanted. Kelvin’s mathematical mill had ground down

geologists and forced them to accede to an age of the Earth of no more

than 100 million years and possibly to as few as 20—even 10. Now a

new group of physicists, armed not with Fourier’s mathematics but

with startling new discoveries that seemed to border on alchemy, were

demanding that the geologists reverse themselves, throw out their

hourglasses, and capitulate to ages of at least several hundred million

years, and still these were only minimums. Estimates of geological time

had now swung back in Lyell’s direction, possibly measuring in the

thousands of millions, with the upper limit unknown. Geologists were

not going to be whipsawed in this fashion. None resisted more than

John Joly in defense of his salt clock.

Assumptions

In a lengthy 1911 article titled “The Age of the Earth,” Joly correctly

stressed that the hourglasses of the geologists were not the only ones

to require assumptions: so did the hourglass of radioactivity. He iden-

tifi ed three suppositions on which the new methods depended. 6 First

was that at the time the mineral being investigated had formed, it

had contained neither helium nor lead. If a mineral had contained

original daughter atoms, it would appear older than its true age. This

possibility suited Joly since the ages inferred from radioactive decay

were older than those from his salt clock. Second, the methods based

on radioactivity assumed that the specimen had neither gained nor

lost daughter atoms, which in either case would have rendered the

calculated ages meaningless. Again, because salt is removed from the

ocean and deposited in beds, the salt clock suffered from the same

problem. The third assumption was that the rate of decay of the par-

ent element—its half-life—remains constant. Joly took pains to avoid

suggesting that changes in heat and pressure might have altered the

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G E O C H R O N O L O G Y • 47

rate of decay, no doubt aware that both Madame Curie and Rutherford

had tried but failed to induce such changes. Instead, Joly wondered

whether variations in the rate of decay might have arisen “intrinsi-

cally, ultimately due possibly to conditions of origin.” The decay of

uranium, he wrote, “150,000,000 years ago may have been many times

what it is now.” 7 Given the number of surprising fi ndings in chemistry

and physics that had followed Röntgen’s discovery and how little was

known about the atom at the time, who could have said that Joly’s

skepticism was unjustifi ed?

His fundamental point was that if the ages of several hundred mil-

lion years, even billions, as measured from radioactivity are correct,

the hourglasses of sedimentation and salt would have to be off by a

factor of ten or more. Since he and the other hourglass calculators did

not see room for an error of that size in their estimates of sediment

accumulation, the mistake would have to lie in their assumed rate of

sedimentation. But it seemed absurd that the estimated rates could be

wrong by a factor of ten.

Having spent a decade refi ning his fi gures and adopting improve-

ments suggested by others, Joly found it impossible that the ages cal-

culated from geological uniformity and his salt clock could be so far

off. Even though he trained as a physicist, not a geologist, and even

though he understood radioactivity well enough to found the Irish Ra-

dium Institute in 1914, Joly evidently saw no good reason why geology

should once again submit to physics.

Even before Joly had voiced his doubts, George F. Becker (1847–

1919) of the U.S. Geological Survey anticipated them. In a 1908 paper

Becker redid the calculations of Kelvin and King, arriving at an age of

60 million years. Even “with better data,” he avowed, “this age will

not be changed by more than perhaps 5 million years.” Granting the

physicists no quarter, in another paper that same year Becker wrote

that although “geologists would assuredly rejoice in the discovery of a

valid method” of measuring rock ages, “one condition of acceptance

would clearly be that it should give periods of the same order of magni-

tude as is indicated by purely geological data.” 8 In other words, physics

would have to submit to the hourglasses of geology.

By the teens, in contrast to Becker and Joly, other prominent ge-

ologists, including Thomas Chamberlin, had begun to reconcile ages

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48 • D E E P T I M E

calculated from geologic evidence with the ages calculated from radio-

activity. One of the most infl uential was Joseph Barrell (1869–1919),

a structural geologist at Yale. In a 143-page paper in 1917, Barrell ex-

plained why the geological hourglasses had given false results. 9 First,

the rates of erosion and sedimentation throughout geological time had

varied greatly. Second, since today’s rate of erosion is believed to be

higher than the long-term average, geologic processes appear to have

run faster in the past than they actually had. Third, the rate of ero-

sion and the rate of sedimentation have not been equal, as the hour-

glass calculators had assumed. Fourth and fatally, gaps representing

unknown amounts of time riddle the sedimentary record. (A fi fth rea-

son is that nineteenth-century geologists had no way of knowing that

Precambrian time accounts for nearly 90 percent of all geologic time.)

For such reasons, Barrell wrote, “Geologic time is certainly much

longer—perhaps ten or fi fteen times longer—than the estimates based

on a strictly uniformitarian interpretation” (749). An earlier generation

of geologists, “feeling the need of meeting the demands of the physi-

cists,” he wrote, had with diffi culty compressed their estimates into

the range allowed by Kelvin and his followers. Now with the advent of

radioactivity, a new crop of “physicists destroy[ed] the conclusions pre-

viously built by physicists” and “granted upwards of 1,500,000,000”

years (749). “Many geologists,” Barrell wrote, “adjusted to the previous

limitations, shook their heads in sorrow and indignation at the new

promulgations of this dictatorial hierarchy of exact scientists” (749).

He said that the evidence from radioactivity showing that as much

as 500 million years might have elapsed since the Cambrian period be-

gan was “not in confl ict” with the geologic evidence. 10 Adding the vast

and unknown expanse of Precambrian time, it seemed probable to Bar-

rell that “the oldest known rocks are as much as 1,400,000,000 years

of age” (881).

A Precocious Young Man

In 1908 the Imperial College of London appointed Strutt its professor

of physics. He soon spotted in his classroom a student with a special

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G E O C H R O N O L O G Y • 49

aptitude for the new subject of radioactivity. Strutt invited the young

man, Arthur Holmes, to join his research. 11 He must have felt his con-

fi dence justifi ed when only a year later, at age twenty-one, Holmes

published his fi rst scientifi c article, “The Association of Lead with Ura-

nium in Rock-Minerals, and Its Application to the Measurement of

Geological Time.” 12 Holmes would go on to become an instructor at

Imperial College and professor of geology fi rst at Durham and then at

Edinburgh, and he would write many more articles and a book that be-

came a classic. No one would do more to validate the ages determined

using radioactivity than Holmes. He would become the pivotal fi gure

not only in establishing the age of the Earth, but as we will see in part

3, in keeping the theory of continental drift on life support after nearly

the entire community of geologists had pronounced it dead and good

riddance.

Holmes began his fi rst scientifi c article by rephrasing the three as-

sumptions necessary for radioactivity to give accurate geologic ages,

the same three that Joly noted. Let us state them as:

1. No original daughter atoms

2. No gain or loss of parent or daughter atoms: a “closed system”

3. Constant decay rate

Had a mineral contained original lead, the fi rst problem, its measured

age would appear to be older than its true age. The way to avoid or

minimize this problem, Holmes said, was to select specimens that in-

corporate much more uranium than lead when they crystallize, so that

the amount of original lead becomes negligible in comparison with

the lead produced by radioactive decay (radiogenic lead). One such

mineral is zircon.

To avoid the second problem, the scientist should select “fresh,

stable, primary rock-minerals.” Since it is inconceivable that different

minerals would lose parent and daughter atoms in the same propor-

tion, “if the analyses [of those different minerals] give consistent re-

sults one can only assume that any alteration has been inappreciable.”

Conversely, if a suite of minerals does not give consistent ages, then,

whatever the reason, that suite cannot be used for age dating.

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50 • D E E P T I M E

Madame Curie and Rutherford had been unable to change the rate

of radioactive decay, but no one could say it was impossible. The most

that could be said, as Holmes put it, was that “experimental evidence

consistently agrees in suggesting that [radioactive] processes are quite

independent of the temperatures and pressures which igneous rocks

can have sustained without becoming metamorphosed.” 13

After evaluating Boltwood’s uranium-lead ages and his own mea-

surements, which included an age of 370 million years for an igneous

rock from Norway, Holmes concluded that “wherever the geological

evidence is clear, it is in agreement with that derived from lead as an

index of age. Where it is obscure . . . the evidence does not, at least,

contradict the ages put forward.” 14

In 1913 Holmes published the fi rst of several books, this one titled

The Age of the Earth . After carefully reviewing the debates of the pre-

ceding fi fty years, he consigned Kelvin’s calculations to the dustbin of

history:

With these discoveries the long controversy was fi nally buried, and

Kelvin’s treatment of the problem was proved to have been falla-

cious. The discovery of radium did not only destroy the validity of

the older thermal arguments; but also, it led directly to the elabora-

tion of a new and more refi ned method . . . every radioactive mineral

can be regarded as a chronometer registering its own age with ex-

quisite accuracy. Indeed, if our interpretation is correct, some of the

oldest Archean rocks must date back 1600 million years. 15

Holmes then turned to Joly’s salt clock, noting that it incorporated

two fundamental and unprovable assumptions: “That all the sodium

liberated from igneous rocks is contained in the ocean, and that all

the sodium carried annually to the ocean has been liberated from such

rocks for the fi rst time.” 16 Holmes wrote that for Joly’s estimates to tally

up, igneous and sedimentary rocks “would be obliged to lose nearly

twice as much sodium as they actually contain!” (72). He concluded

that deductions using the sodium method “must be regarded as being

purely provisional” (75). In 1926, Holmes would go further, noting that

“many geologists have rejected [the sodium method] as worthless.” 17

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G E O C H R O N O L O G Y • 51

The fi nal chapter of the 1913 book set out the issue:

Of the various methods which have been devised to solve the prob-

lem of the earth’s age, only two, the geological and the radioactive,

have successfully withstood the force of destructive criticism. From

the mists of controversy which for half a century have hung over the

subject, the two hour-glass methods alone emerge, and the fi nal issue

must be fought out between them. 18

Holmes regretted that

many geologists feel it impossible to accept what they consider the

excessive periods of time which seem to be inferred. That there exists

a serious discrepancy obviously points to a fl aw in the underlying

assumptions of one or the other or both of the methods. The funda-

mental assumptions on which the arguments are based cannot both

be right. One of them must be rejected.

Uniformity . . . is involved equally in both calculations. If we

favour the uniformity of geological processes—a well-worn doctrine

which has done good service—then we must reject uniformity of

radioactive disintegration. 19

After reviewing the claims of Joly and others, Holmes came to his

conclusion: no independent evidence exists to show that the rates of

radioactive decay have varied. Therefore, “the discordance between

the time-estimates drawn from the rates of geological and radioactive

changes cannot be held to constitute a suffi cient reason for rejecting

current opinions unless it is conclusively demonstrated that the geo-

logical estimates are beyond question.” 20 He then went on to show, as

Barrell would reiterate a few years later, that the geological estimates

were not beyond question. Indeed, since “the modern hour-glass [of

erosion] is running at two-and-a-half to four times its average rate,”

those estimates from geological uniformity are too low by the same

factor. “In the geological evidence,” Holmes concluded, “there is noth-

ing impossibly at variance with the dictates of the radioactive miner-

als” (176).

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52 • D E E P T I M E

Billions, Not Millions

Soon after the First World War, the British Association and the Ameri-

can Philosophical Society held conferences to consider the age of the

Earth. By this time, scientists had measured many ages in the hundreds

of millions of years and some in the billions. Both conferences con-

cluded that the Earth is from 1.3 to 1.5 billion years old. In the sec-

ond edition of his book, published in 1927, Holmes parted company

with the geological hourglass methods. They are “incapable of provid-

ing exact results because the assumption of uniform rates [of erosion]

throughout the past cannot be granted,” he wrote. “If the present rates

are fi ve times greater than the average, then geological time must be of

the order of 1,500 million years.” 21

In 1926, the U.S. National Research Council, an arm of the National

Academy of Sciences, convened a large committee to prepare a report

titled Physics of the Earth . The purpose was to “give the reader, presum-

ably a scientist but not a specialist in the subject, an idea of its pres-

ent status together with a forward-looking summary of its outstand-

ing problems.” The subcommittee on the age of the Earth included a

physicist, an astronomer, and four geologists, one of whom was Arthur

Holmes. It produced a 1931 book of 487 pages, over two-thirds of them

written by Holmes in a section—really a book in its own right—titled

“Radioactivity and Geologic Time.” 22 The most striking thing to one

reading this section today is what a wealth of knowledge scientists had

produced in the twenty-fi ve years since Rutherford had pulled that

specimen of pitchblende from his pocket.

In one section of his chapter, Holmes calculated the age of the Earth

assuming that all the lead in average igneous rocks had come from

radioactive decay; in other words, that none was original. This com-

putation gave 3,000 million years as the age of the Earth. Since rocks

almost certainly contained at least traces of original lead, Holmes re-

garded this as the maximum possible age of the Earth. Based on his

review of the most suitable samples for age dating, he concluded that

“no more defi nite statement can therefore be made at present than

that the age of the earth exceeds 1460 million years, is probably not

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G E O C H R O N O L O G Y • 53

less than 1600 million years, and is probably much less than 3000 mil-

lion years.” 23

By 1931 a new generation of scientists, most of them chemists and

physicists with no allegiance to geology or uniformitarianism, had un-

covered the secrets of the atom. Armed with new understanding and

new instruments, they were ready to provide a precise answer to the

question that had puzzled scholars for centuries: how old is the Earth?

The answer would turn out to be greater than even Arthur Holmes had

conceived. Never again would a scientist espouse an age of the Earth

of 100 million years, nor even 1,000 million. The question now was

not whether the Earth is billions of years old but exactly how many

billions.

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Plumbing the Atom

Although Rutherford left behind his early interest in measuring min-

eral ages, his subsequent research, along with that of the other pio-

neers, led to a series of surprising discoveries that were crucial to es-

tablishing the true age of the Earth. These fertile early years were an

era of “little science”—research conducted on a laboratory bench at

minor expense with apparatus that today seems not much advanced

beyond string and sealing wax. But with only string and sealing wax,

Rutherford was a virtuoso.

Rutherford’s collaborator, Frederick Soddy, found that whereas the

lead in uranium minerals has an atomic weight of just over 206, the

lead in thorium minerals is heavier, weighing close to 208. Soddy de-

duced that lead and other elements exist in varieties that have the same

chemical properties but different atomic weights. These he named iso-

topes, from the Greek for equal place.

The explanation is that lead in nature is a mixture of isotopes of dif-

ferent atomic weights. Ores rich in uranium have more of the lighter

lead isotopes; those rich in thorium have more of the heavier ones.

By the mid-1930s, scientists had worked out the essential facts of the

decay of uranium and thorium to lead (the half-lives that follow are

the modern measured ones): U-235 decays to Pb-207 with a half-life of

713 million years; U-238 decays to Pb-206 with a half-life of 4,468 mil-

lion years; Th-232 decays to Pb-208 with a half-life of 14,050 million

years. Lead has one additional naturally occurring isotope, Pb-204,

that is neither radioactive nor radiogenic.

As the pioneers recognized, uranium and thorium do not decay di-

rectly to lead but through a long chain of in-between elements, such

as radium and radon. At fi rst it would appear that these complex decay

• • • •

Duck Soup

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D U C K S O U P • 55

chains, with their many intermediate products, some of them gases

that could easily escape, would invalidate the lead methods of age de-

termination. But in part because the half-life of each decay event in

the chain is so short, the process works as though uranium decayed

directly to lead.

Having three clocks running at once, two from uranium decay and

one from thorium decay, lead is unique among the elements used for

age dating. From the mathematics of the decay process, it turns out

that if one measures the Pb-206/Pb-204 ratio and the Pb-207/Pb-204

ratio in some specimen today and one knows or can reasonably as-

sume what those ratios were when the specimen originated, one can

calculate its age from that information alone. Neither the amount of

original lead nor the Pb-208 from thorium comes into the calculation.

Thus if one knew the lead-isotope ratios of modern lead minerals and

of the primordial Earth, one could calculate the age of the Earth.

There was just one problem: how could scientists discover the lead-

isotope composition of the primordial Earth? They couldn’t, but they

could do the next best thing: search for lead minerals that have the

lowest ratios of Pb-206/Pb-204 and Pb-207/Pb-204: in other words, that

have the most primordial, least radiogenic lead. Those oldest and most

primitive types of lead, when compared with the youngest, would al-

low scientists to calculate a minimum age for the Earth.

The pioneer in lead mass spectrometry, the technique used to mea-

sure isotope ratios, was Alfred Nier of the University of Minnesota. In

1941, Nier and his colleagues published analyses of galena, a lead sul-

fi de mineral, of different ages. Galena contains no detectable uranium

or thorium and thus should preserve its original lead-isotope ratios.

Nier found that a galena from Ivigtut, Greenland, had the least ra-

diogenic lead. In 1942, the Russian academician E. K. Gerling used

Nier’s data to calculate the time required for lead with the ratios of the

Ivigtut galena to evolve to the lead in one of Nier’s youngest galenas.

The calculation gave 3,950 million years. This led Gerling to write that

the age of the Earth “is not under 3,000–4,000 million years.” 1

After the war, two other scientists made similar calculations using

slightly different approaches. One was Arthur Holmes, who calculated

that “the most probable age of the earth is about 3,350 million years.” 2

The other was a German physicist named Friedrich G. Houtermans

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56 • D E E P T I M E

(1903–1966). 3 Because of wartime secrecy, neither was aware of Ger-

ling’s work.

One of the most remarkable characters in the history of science,

Houtermans could have stepped from the pages of an Eric Ambler spy

novel. He emigrated from Germany to England in 1933, then a year

later, out of allegiance to the expressed ideals of the Soviet Union,

moved to Kharkov. As Stalin’s reign of terror reached its peak, in 1937

the NKVD arrested and tortured Houtermans. To allow his wife and

children to escape, unaware they had already slipped out of Moscow to

Riga, Houtermans confessed to spying for Nazi Germany. After Hitler

and Stalin signed their infamous pact in August 1939, the NKVD extra-

dited Houtermans back to Germany, handing him over to the Gestapo

at the border. This put Houtermans in the unenviable position of being

able to compare the torture techniques of two of the most monstrous

organizations of the twentieth century: the NKVD and the Gestapo. He

found the NKVD to be “the more serious.” 4

In science if not in politics, Houtermans was ahead of his time. In

1929, he and a colleague made the fi rst calculations of thermonuclear

reactions in stars. As Houtermans remembered: “That evening, after we

had fi nished our essay, I went for a walk with a pretty girl. As soon as it

grew dark the stars came out, one after another, in all their splendour.

‘Don’t they shine beautifully?’ cried my companion. But I simply stuck

out my chest and said proudly: ‘I’ve known since yesterday why it is

that they shine’ ” (30).

A few years later, after scientists had discovered the neutron, Hou-

termans pointed out the possibility of a self-sustaining nuclear chain

reaction, the basis for the later atomic bomb. In August 1941, in a re-

port titled “On the Question of Unleashing Chain Nuclear Reactions,”

he identifi ed plutonium as a more likely element than uranium for in-

ducing a chain reaction, the same conclusion that Manhattan Project

scientists would later reach. In December 1942, the Chicago team of

researchers, led by Enrico Fermi, was working to achieve a controlled

nuclear chain reaction when they received a cable from Switzerland.

It said only, “Hurry up. We are on the track” (35). The wire had come

from Houtermans through an intermediary. The Germans never got

the bomb.

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D U C K S O U P • 57

In 1946 and 1947, Houtermans used Nier’s reported lead-isotope

ratios to make his own calculations. Using a diagram he called an

“isochrone,” he obtained an age of 2,900 million years. 5 Houtermans

thought this was either the age of the elements themselves or possibly

the age of the Earth’s crust.

Science, Science, Science

The calculations of Gerling, Holmes, and Houtermans were bound to

err because they depended on unknowable assumptions about the his-

tory of lead in the Earth. Even terrestrial leads with the lowest isotope

ratios are not likely to be truly primordial. Where on our living planet,

with its complex geological history, could scientists fi nd unaltered lead

that still retains its primordial isotopic abundances? In 1947 Houter-

mans proposed that the place to search for primordial lead was not

on the Earth but in rocks that fall from the sky: in meteorites that

had been isolated in the frigid depths of space since the beginning of

geologic time. 6

Scientists knew that one class of meteorites, the “irons,” which look

as though they could have come from a blast furnace, contain minerals

that have so little parent uranium that even in billions of years they

would not have generated measureable radiogenic lead. Thus these me-

teorites would retain their original lead-isotope ratios. Harrison Brown

of the University of Chicago, mentor to the next person we are to

meet, in the same year came to the same realization as Houtermans. 7

In different countries, minds were converging: to measure the age of

the Earth, use the primordial lead in meteorites. But in the aftermath

of a devastating world war, only the United States had the scientists,

methods, instruments, and funds to do the experiments.

Brown had worked during the war on the Manhattan Project, one of

whose major tasks was to separate the two isotopes of uranium, which

was necessary since the rarer U-235 is the only one of the pair that can

engender a neutron-based chain reaction. In order to monitor how well

the separation had succeeded, scientists analyzed the abundance of the

uranium isotopes using a mass spectrometer. When Brown returned to

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58 • D E E P T I M E

F I G U R E . Clair Cameron Pa erson ( – ). Source : Courtesy of the Archives, Cali- fornia Ins tute of Technology.

the University of Chicago after the war, he knew that scientists could

also use a “mass spec” to measure the isotopic composition of lead in

meteorites.

Brown soon moved to the California Institute of Technology, where

he joined forces with just the right person to undertake the measure-

ments: a graduate student named Claire Cameron Patterson. 8 Patter-

son had graduated from one of the nation’s fi ne liberal arts colleges,

Grinnell, earned a master’s degree at the University of Iowa, and when

the war began, joined the Manhattan Project, where he learned mass

spectrometry.

Brown assured Patterson that the use of meteorites would be “duck

soup” and make him famous as the person who fi nally measured the

age of the Earth. 9 Only one of these assurances would come true. Pat-

terson’s research took seven years and required the building of an en-

tire new laboratory and the invention of novel techniques.

Brown had based his optimism as to how long the work would take

on chemical analyses that had shown that iron meteorites contain

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D U C K S O U P • 59

enough lead to make it easy to separate and analyze. But Patterson

soon discovered that industrial lead is so pervasive in the environ-

ment, and the effects of its contamination so diffi cult to remove, that

almost all previously reported lead abundances in meteorites refl ected

the amount of contaminant lead rather than the amount inherent in

the meteorites themselves. The discovery of the extent of environmen-

tal lead pollution would eventually change Patterson’s career path, to

our everlasting benefi t.

The modern fi gure for the age of the Earth derives from Claire Pat-

terson’s classic 1956 paper “Age of Meteorites and the Earth.” 10 There

he reported the lead-isotope ratios from three stone meteorites, which

contain appreciable uranium and therefore have evolved a radiogenic

lead component, and two uranium-free iron meteorites, one of them

the so-named Canyon Diablo meteorite from Meteor Crater, Arizona,

which we will meet in part 3. Having no uranium, these two had pre-

served their primordial lead ratios. For the group, Patterson calculated

an age of 4.55 ± 0.07 billion years. This, he argued, was the age of the

meteorites, the Earth, and the solar system.

Patterson’s life and career serve as a model of the transition that many

scientists have found themselves making. At fi rst devoting himself to

purely scientifi c questions, Patterson’s attention inevitably turned to

the implications of environmental lead for human health. Rather than

play this up, Patterson preferred the role of the no- nonsense scientist

whose goals, as he put it, were nothing but “science, science, science.” 11

Patterson’s environmental awareness began with his need to

identify the source of the ubiquitous environmental lead. He soon

discovered high concentrations of lead in the surface waters of the

ocean and in otherwise pristine snow. Lead in snow could only have

come from the atmosphere, and it could only have gotten there from

leaded gasoline. To prove this point, Patterson participated in one

of the most arduous experiments in modern science. He measured

the amount of lead in hard-won ice cores from Greenland, fi nding

that the amount of lead in the atmosphere had risen slowly from the

beginning of the Industrial Revolution until the 1920s, when gaso-

line manufacturers began to add lead to their product. At that point,

the level of lead in the atmosphere shot up. It continued to climb

until by the 1970s it had reached two hundred times the natural

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60 • D E E P T I M E

level. Humans were polluting the atmosphere with a poisonous sub-

stance: lead.

Patterson fought for years to get his fi ndings into the public arena.

The leaded gasoline industry responded with the same deceptive and

dishonest techniques that Big Tobacco and Big Oil would later use to

sow doubt about the dangers of their products. But Patterson’s impec-

cable science overcame their smoke and mirrors. His research was key

to the passage of the Clean Air Act of 1970. In 1995, Patterson won the

Tyler Prize for Environmental Achievement. His letter of nomination

said in part, “Patterson has never slanted statements of his results to

accommodate or placate special interests, either within the scientifi c

community or outside. . . . The lesson has been given to other scien-

tists that, if they have the vision, their . . . work has the potential to

immediately affect the wellbeing of the world.” 12

After Patterson’s 1956 paper, scientists went on to measure the ages

of many other meteorites, of a number of rocks from the Moon, and

F I G U R E . Geologic mescale Source : G. B. Dalrymple, Ancient Earth, Ancient Skies: The Age of Earth and Its Cosmic Surroundings (Stanford, Calif.: Stanford University Press, ).

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D U C K S O U P • 61

of countless rocks on Earth. Along the way, they extended the “iso-

chrone” that Houtermans had invented, allowing them to obviate

the two main pitfalls of age dating: the possible presence of original

daughter atoms and the possible loss or gain of atoms.

As one example of the success of the methods, let us consider a

meteorite recently found lying on the ice at the La Paz Icefi eld in

Antarctica. Five different teams measured its age using four different

parent-daughter pairs. Each gave the same result, three billion years,

to within a few percent. Had one or another of the assumptions that

underlie age dating been violated, these different methods could not

have given the same result. (This meteorite is especially interesting

because it came from the Moon, blasted off by the impact of another,

much larger meteorite.)

Brent Dalrymple has tallied the ages of specimens from the Moon

returned by the Apollo missions, some of which give ages as old as

Patterson’s meteorites. As he reports: “Even the most conservative in-

terpretation of the age data . . . leads to the conclusion that the Moon’s

age must equal or exceed 4.5 billion years.” 13

As we will see in part 3, nothing has happened to the Moon since

its formation except meteorite impact and the upwelling of basalt lava

in the lunar seas, or maria . Our heavenly companion has no wind or

water to erode and reshape its rocks, no internal fi res to reset its atomic

clocks. Earth is a different story. So many geologic events have beset

our living planet that it would seem most unlikely that we could ever

fi nd a rock or mineral whose atomic clocks retain their original age. We

even believe that the outer surface of the primordial Earth was molten.

Before the clocks of radioactivity could have retained their true ages,

the Earth would have had to cool and solidify. Nevertheless, Australian

scientists have found a number of terrestrial zircons with ages of over

four billion years and one that dates to 4.4 billion.

Few measurements in science escape revision for more than half

a century. Though scientists have made more precise measurements

than Patterson was able to do, his discovery stands unaltered. That the

Earth, the Moon, and the meteorites are 4.5 billion years old is one of

the ground truths of science.

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