assumption that present- day processes are the same as those
that operated in the geologic past.
But Hutton’s geologic imagination went still further. In his
1789 treatise Theory of the Earth, he made the even more daring
generalization that this particular unconformity recorded just
one iteration in an endless cycle of rock accumulation, uplift,
erosion, and renewal on Earth, extending backward into the dim
mists of time. Hutton’s singular intuition about Deep Time— a
radical rewriting of Earth’s past— opened the intellectual doors
through which modern geology and biology could emerge.
Without Hutton and his champion, Charles Lyell, who raised
uniformitarianism to orthodoxy a generation later in his mas-
sive, rhetorically virtuosic Principles of Geology, Charles Darwin
would not have had his insight about the power of time to shape
26 Ch a pter 2
organisms through natural selection. (Lyell’s exhortations about
the antiquity of the Earth echoed in Darwin’s head during his
five years on the HMS Beagle expedition; the first volume of
Principles of Geology was perhaps the most important book in
the small library he brought with him). But Hutton’s appealing
vision of a world in an infinite, repeating loop was in some ways
a chimera, an abstraction that excused itself from the harder,
messier work of reconstructing the particulars of Earth’s biog-
raphy. In Greek, there is a useful distinction between time as
something that simply marches on— chronos— and time that is
defined within a narrative— kairos. Hutton gave us the first glim-
mers of planetary chronos, but the task of calibrating it, and add-
ing kairos, has consumed geologists for the past two centuries.
Early attempts to transcribe the geologic record into an ac-
count of Earth’s history were based on the idea that certain
rock types had formed worldwide at distinct times in the past.
Crystalline rocks like granites and gneisses were considered the
original or “Primary” rocks, while stratified ones such as lime-
stones and sandstones were “Secondary.” Semicohesive gravel
and sand deposits were “Tertiary,” and loose, uncemented sed-
iments were “Quaternary” (the latter term persists, quaintly,
on the modern geologic timescale, and “Tertiary” survived into
the late twentieth century). But there was no basis for knowing
whether the ages of these rock varieties were truly the same
from place to place.
In the early 1800s, the first preliminary sketches for a well-
calibrated chart of deep time were made possible by the as-
tute observations of a canal- digger named William Smith, who
noted that certain types of fossil shells occurred in the same
sequence in strata all across England (see figure 3). These index fossils— as distinctive to specific geologic periods as pillbox
hats and bell bottoms are to cultural eras— made it possible
An atlas of time 27
F I G U R E 3 . The concept of index fossils
to draw connections between layers that are not spatially con-
tinuous, first in Britain, then across the Channel into France.
Amateur fossil collectors like the celebrated Mary Anning of
Lyme Regis— immortalized in the “She sells seashells” tongue
twister— were essential to the early stages of assembling the
geologic timescale. Old ideas that rock strata were global in
nature and recorded the same events worldwide had to be
abandoned; the planet’s long history turned out to be far more
complex than Hutton ever imagined. But decades of laborious
mapping and collecting, classifying and cataloging, lumping
and splitting ultimately led to the global correlation of sedi-
mentary sequences from all over the world.
The result is the geologic timescale most familiar to the pub-
lic: going backward from the present, the Cenozoic Era with its
28 Ch a pter 2
multifarious mammals, the Mesozoic and its redoubtable rep-
tiles, the Paleozoic with murky coal- swamps, gasping lungfish,
and scuttling trilobites. The rich profusion of fossil life- forms
allowed each era to be further subdivided into periods, periods
into epochs, epochs into ages. But beneath the lowest shelly
layers in the Paleozoic rocks, below strata of the Cambrian
Period, the rocks fell silent; no fossils could be found. It appeared
that life had sprung suddenly into existence in the Cambrian, a
vexing mystery that greatly troubled Darwin. Without visible
fossils, the one tool that Victorian geologists had for demarcating
geologic time, these oldest rocks were a knotty skein that could
not be untangled, so they were simply shelved under the name
“Precambrian.” It would take a century before geologists would
recognize that the Precambrian Earth teemed with life— and that
Precambrian time represents almost 90% of Earth’s history.
I think of the second half of the nineteenth century as the
dark ages for geology.
After Hutton’s transcendental vision of a self- renewing
Earth, Lyell’s inspirational treatise on how the new science
of geology would make it possible to “trace the events of in-
definite ages,” and Darwin’s brilliant synthesis of biological
and geologic observations, internal and external forces con-
spired to slow the intellectual momentum. Among these forces
was the indomitable physicist William Thomson, Lord Kelvin
(1824– 1907), who began to take an interest in geology soon
after the publication of Darwin’s On the Origin of Species. As
the high priest of thermodynamics, Kelvin rightly attacked the
Hut tonian idea of an infinitely old Earth— a kind of per petual
motion machine— as a violation of his second law. But his par-
ticularly ferocious attack on Darwin for an unsophisticated es-
timate of the minimum age of the Earth in the first edition of
Origin suggests that the motivations were not entirely scientific.
An atlas of time 29
Darwin somehow sensed, without any knowledge of the ac-
tual mechanism of heredity, that evolution by natural selection
would have required hundreds of millions to billions of years to
produce the observed diversity of living and fossil life- forms.
His intuition about the magnitude of geologic time was truly
remarkable, but it was undermined by his inclusion in Origin of
a single poorly judged attempt at quantification. Like Hutton,
Darwin used erosion as a metric of elapsed time. Greatly under-
estimating the power of English rivers to sculpt the landscape,
he suggested it had taken a single valley, the Weald, about 300
million years to form (a value too large by a factor of at least
100). Since the rocks that formed the valley walls were still
older, yet among the youngest in the region, Darwin surmised
that the Earth itself could be a thousand million (billion) years
old or more. His conclusion was— astonishingly— correct, but
this one argument, in a book that is otherwise a paragon of
carefully wrought exposition,
was naïve and easily demolished.
Starting in the early 1860s, Kelvin published a series of papers
in which he used the most advanced physics of the day to esti-
mate the age of the Earth based on assumptions about the rate
of conductive cooling of the planet and the lifespan of the Sun.
Between 1864 and 1897, his determination of Earth’s age shrank
from a few hundreds of millions of years to just 20 million years.
As the time Kelvin would allot to geology continued to contract,
a few frustrated geologists attempted to reclaim the question and
made independent estimates by summing the thicknesses of all
known strata from Cambrian to recent time, then dividing the
total by an assumed sedimentation rate. This approach yielded
ages of hundreds of millions to billions of years, but the large
uncertainties involved made the results easy to dismiss. A small
number of younger physicists who were able to follow Kelvin’s
calculations began to question his framing suppositions— which
30 Ch a pter 2
would indeed be proven wrong decades later— but they were
reluctant to incur the wrath of the leading scientist of the day.
A brave chemist, John Joly (who would later invent color pho-
tography), suggested that the sodium content of seawater was a
proxy, or stand- in, for the age of the Earth. His (also erroneous)
assumption was that the sea had become progressively more
saline over time as rivers delivered dissolved elements from rocks
on land to the sea. By using typical values of sodium dissolved in
river water, Joly estimated Earth’s age at 100 million years, gain-
ing back some of the ground geologists had lost to Lord Kelvin.3
In his later years, Darwin called Kelvin his “sorest trou-
ble.” Darwin died in 1882, haunted by uncertainties about his
own lifework, which he felt in his marrow must be correct.
Twentieth- century physics would finally rebut Kelvin’s argu-
ments, but Kelvin’s true purposes were made plain in a speech
he made on the occasion of his election as president of the
British Association for the Advancement of Science: “I have
always felt that the hypothesis of natural selection does not
contain the true theory of evolution, if evolution there has been
in biology. . . . Overpoweringly strong proofs of intelligent and
benevolent design lie around us . . . , and teaching us that all
living things depend on one everlasting Creator and Ruler.”4
PA U S I N G F O R T E A W I T H C H A R L E S
The question of the duration of geologic time probably mat-
tered more deeply to Darwin than to any other person in his-
tory, and every time I think about the intellectual dissonance he
must have suffered in the last decades of his life, I feel a surge of
empathy for him. On the 200th anniversary of Darwin’s birth,
I organized an all- day reading of On the Origin of Species at our
university library, with dozens of faculty, staff, and students
An atlas of time 31
each taking a turn reading aloud for 20- minute stints, with
breaks every hour for brief discussion.
The event took place in the period- appropriate, wood-
paneled venue of the rare- books room. We served tea and
scones with marmalade, and a few people even showed up in
Victorian- era dress. Although I knew this would be an intel-
lectually engaging event, I hadn’t anticipated it would also be
an emotionally moving experience. Over the course of the day,
the cumulative effect of hearing Darwin’s words spoken aloud
was overwhelming. Through the voices of men and women,
scientists and musicians, philosophers and economists, young,
middle- aged, and old, Darwin’s own very human voice could
be heard— his delight in the minutest details of the natural
world, his earnest thoroughness as a scientist (several people
fell asleep during the long sections on pigeon breeding), his
personal timidity and reluctant role as a revolutionary, and,
most affectingly, his wracking self- doubt and preemptive de-
fensiveness against the attacks that he fully anticipated. Origin
is a humbly argued, methodical, (and quite often tedious) ex-
plication of an idea that Darwin was convinced must be right
but also knew would be subject to savage critiques. He did not,
however, seem to think the question of geologic time would be
one of the scientific objections. In Chapter 9, he wrote: “He
who can read Sir Charles Lyell’s grand work on the Principles
of Geology, which the future historian will recognise as having
produced a revolution in natural science, yet does not admit
how incomprehensively vast have been the past periods of time,
may at once close this volume.”
By the end of the reading marathon, it seemed as if Darwin
had been in the room with us, and I had a strong, irrational
wish to speak with him. I recalled the painting of an elderly
Darwin that hangs in the National Portrait Gallery in London.
32 Ch a pter 2
It depicts a hunched, sad- eyed man who, it seemed to me, was
almost physically cramped by the intellectual limits of his day. I
yearned to convey to him how marvelously his simple idea has
flowered and itself evolved, informing countless new fields of
inquiry, and to share with him scientific news that would have
eased his troubled mind: Earth is old.
R O C K S K E E P T I M E
In addition to the injury done to Darwin, the controversy over
the age of the Earth caused lasting damage to geology. When
the conclusions of physics seemed incompatible with the in-
creasingly detailed documentation of Earth’s long history, some
geologists declared that geology had to break with other sci-
ences and pursue its own methods as a wholly independent
field of inquiry. This aggravation at the impasse with physics
is understandable, but it would unfortunately influence the
way generations of geologists were educated, and it set the
dis cipline back by decades. Distaste for physics and distrust
of those not trained as geologists contributed, for example,
to geology’s long, obstinate denial of the evidence for moving
continents. In 1915, a German meteorologist, Alfred Wegener,
presented carefully documented evidence that Earth’s land-
masses had once been united in a supercontinent, Pangaea. But
Wegener’s lack of geologic credentials (combined with Amer-
ican and British antipathy toward all things German during
World War I) made his ideas anathema within geologic circles
until the plate tectonic revolution of the 1960s.
But in the first years of the twentieth century, a revolution in
physics would finally provide the tools to lead geology out of the
Victorian labyrinth in which it had become lost. Only a decade
after the accidental discovery of the phenomenon of radioactivity
An atlas of time 33
by Henri Becquerel in 1897, it would already be used to deter-
mine the age of rocks. By 1902, the work of Marie Curie in Paris
and Ernest Rutherfor
d at Cambridge had shown that radioactive
decay was a kind of natural alchemy in which some elements (for
example, uranium) spontaneously emit energy as they transmute
to other elements (e.g., lead), at a consistent rate proportional
to the remaining amount of the first element. We would now say
that certain elements— which are always defined by the number
of protons in their nucleus— have different subtypes called iso-
topes, with varying numbers of neutrons, and that some of these
parent isotopes decay to daughter isotopes of other elements.
But the structure of the atom was not even known at that time;
the nucleus wasn’t discovered by Rutherford until 1911, and the
concept of isotopes emerged several years after that.
In 1905, Rutherford demonstrated that radioactivity was an
exponential decay process and immediately recognized its po-
tential as a natural clock that could be used to determine the age
of uranium- bearing rocks. But it was a precocious 18- year- old
physics student at Imperial College, Arthur Holmes, who
undertook the project of determining the first absolute geo-
logic dates.5 Starting in 1908 (the year after Lord Kelvin died),
Holmes began seeking appropriate rock samples and separat-
ing minerals, especially zircon, that were known to contain
uranium (U) but no lead (Pb) at the time of crystallization.
He then needed to find the relative concentrations of uranium
and lead in the mineral and used Rutherford’s radioactive decay
law, which quantified radioactivity as a function of time, to find
how many years had elapsed since the mineral crystallized.6
The math is remarkably simple; the only numbers required
are (1) the daughter: parent (Pb:U) ratio, which grows as a rock
ages, and is independent of the (unknowable) original amount
of parent material (see table 1); and (2) the decay constant for
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