Chapter 1: The Mill of Exquisite Workmanship
If an elderly but distinguished scientist says that something is possible he is almost certainly right, but if he says that it is impossible he is very probably wrong.
-- Arthur C. Clarke
A Gentle Confrontation For geology, the twentieth century began on May 5, 1904, in a polite encounter between two physicists -- one from the Victorian era and one whose work would launch twentieth-century science. On that day, geology started the long process of casting off the fetters of unfounded assumptions, uncritical deference to authority, and inability to measure and quantify.
The occasion was the meeting of the Royal Society of London, which had selected Ernest Rutherford (1871-1937), a New Zealand-born physicist and professor at McGill University in Montreal, to give its prestigious Bakerian Lecture. The audience of nearly eight hundred people included the cream of British science. They had come to hear the brilliant young scientist explain the results of his experiments in the new field of radioactivity, a name coined a few years earlier by the French physicist and Nobelist Marie Curie (1867-1934). Few in the audience could have fully understood the brave new world of transmutating atoms that Rutherford described. His science must have seemed closer to alchemy. For the most eminent scientist in the audience, Lord Kelvin, Rutherford's discoveries had special import: they cut the ground from under Kelvin's life's work. As Rutherford told it:
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 conflicted 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.
Kelvin began to study the shape and age of the earth as a teenager. He kept doggedly at it for more than sixty years, right up until his death three years after Rutherford's speech. With rare determination, Kelvin continually refined his estimate of the age of the earth. By the end of the century, he had reduced his original figure of 100 million years to 20 million, a span that most geologists thought too small to accommodate all that they had seen recorded in the rocks. Just when the geologists had enough, Rutherford came to their aid.
Close to the time of his speech to the Royal Society, Rutherford was walking the McGill campus. In his pocket he carried a small black object, a specimen of a uranium oxide mineral called pitchblende. Meeting a colleague, Rutherford said, "Adams, how old is the earth supposed to be?" The answer came back at Kelvin's earlier figure of 100 million years. "I know," said Rutherford quietly, "that this piece of pitchblende is 700 hundred million years old."
Kelvin had calculated the age of the earth starting from the unquestioned assumption that both the sun and the earth had begun hot and had been cooling ever since. Rutherford did things differently. He took the piece of pitchblende into his laboratory, measured its radioactivity, and calculated its age directly. His figure of 700 million years turned out to be wrong; but considering that it was the first absolute age measurement and that he had made assumptions as well, it was not wrong by much.
Kelvin's assumptions would always remain unprovable. As the years passed and he adjusted his calculations again and again, his answer diverged further and further from the true age of the earth. In contrast, Rutherford's experiments steadily improved his assumptions, or converted them into facts. As a result, his answers became ever more accurate. Kelvin personified nineteenth-century science; Rutherford invented twentieth-century science.
By revealing that physicists could no longer explain natural phenomena entirely through the principles of mechanics and heat, radioactivity set the stage for the atomic physics and quantum mechanics of the twentieth century. But on that day in May 1904, the people gathered to hear Rutherford's lecture had no way of knowing what lay even a few years ahead. That did not stop Kelvin from pontificating. In a letter to his wife after his lecture, Rutherford wrote that he admired Lord Kelvin's confidence "in talking about a subject of which he has taken the trouble to learn so little." Kelvin even carried around, and was quick to show off, a small piece of radium that glowed in the dark. He never realized that the radioactive rays emanating from his amusing toy aimed straight for the heart of his assumptions.
Tonguestones Though some Eastern religions were exceptions, not until the last two hundred years did people in the West think of the earth as having an age. The early Hindus believed in great cosmic cycles that repeated every 4,320,000 years; all the cycles taken altogether totaled 1,972,949,094 years. When Christian theologians eventually began to consider the age of the earth, they turned to the Bible. Archbishop James Ussher (1581-1656) invented the most famous and long-lasting biblical chronology. In contrast to the time scales based strictly on the Bible, Ussher also used astronomy and history. Eventually, he calculated that the earth was born on the "entrance of the night preceding the twenty-third day of October in the year of the Julian calendar 710." This translates to October 22, 4004 b.c. Thus, in Ussher's chronology, the earth is so young that its age measures in human generations. Science being too rudimentary to provide evidence to the contrary, well into the twentieth century Bibles still cited Ussher's date as the birth date of the earth.
Before the Enlightenment, the origin of fossils and sedimentary rocks was a mystery. Rather than being the remains of creatures that once lived, people thought that fossils were "Sports of Nature" -- tricks planted to fool them or to test their faith. This view began to change with the discoveries and writings of a Danish anatomist and cleric named Niels Stensen, who Latinized his name to Nicolaus Steno (1638-1686).
Since antiquity, the island of Malta has produced the finest specimens of "tonguestones" -- flattened, blade-shaped objects found embedded in rocks or lying on the surface. Some said the stones resembled a human tongue and possessed mysterious but useful properties, such as enhancing sexual prowess or controlling flatulence. Others thought them the perfect Sport of Nature, of no use whatsoever.
One day, sailors brought Steno the head of a Great White Shark to dissect. He noted that the shark's teeth, though smaller than the Maltese tonguestones, were in every other way identical to them. Steno deduced that the tonguestones were, or had once been, the teeth of sharks. But how had solid rock come to encase the tonguestones? In asking this seemingly innocent question, Steno laid his hand on the cloak of mystery that had always shrouded the age of the earth, and drew it back just enough to reveal the awful abyss beyond. The tonguestones disclosed, not an earth formed instantaneously and fully as we find it, but the passage of time. Sensing the chasm, unable to reconcile science and religion, Steno was eventually to recoil.
In considering how one solid could come to be inside of, or on top of, another, Steno concluded: "At the time when any given stratum was being formed all the matter resting upon it was fluid, and, therefore at the time when the lowest stratum was being formed, none of the upper strata existed." Such reasoning led him to deduce that a shark's tooth must be younger than the rock that encases it. Thus, rocks record not only divine creation, in which Steno believed, but the passage of time and the lives and deaths of creatures. But then biblical creation and earth history had to be reconciled. Steno could not do so, and he chose religion. He summed up his credo: "Beautiful is that which we see, more beautiful that which we know, but by far the most beautiful is that which we do not comprehend."
Hourglasses Soon those who, unlike Steno, could set aside the dogma of the church, at least temporarily, began to try to calculate how much time might have passed since the earth formed. They used the principle of the hourglass. Over time, some quantity changes at a known or assumed rate, as when sand falls from the upper to the lower cone of an hourglass. If one knows how much sand is in the top and in the bottom of the hourglass, and the rate at which the sand falls through the constriction, one can calculate the length of time the process has been going on.
The accumulation of sedimentary rocks provided one of the first geological hourglasses. If one knows both the thickness of a sequence of rocks and the rate at which they accumulated, one can calculate how long the sequence took to form. Although the calculation is simple, it masks assumptions and difficulties that plague all the hourglass methods.
One obvious assumption is that one knows or can estimate the rate of accumulation; another is that the rate has been constant. The only non-circular way to estimate the past rate of sediment accumulation is to assume that today's rate equals the long-term average. But today's sedimentation rate could be higher or lower than the average, throwing off the result. (In fact, today's rate appears to be several times higher.)
Another assumption inherent in any hourglass method is that all of the sand that fell into the bottom cone of the hourglass is still there. We know that this is not true for sedimentary rocks, because they almost always contain gaps left where erosion has removed rocks. In that case, the total measured thickness of sedimentary rocks, and therefore the calculated age, will be low by some indeterminate amount. But at least the accumulation method gives a minimum age.
Instead of measuring how rapidly something builds up, one can measure how rapidly something else declines. If one knew how much erosion had lowered the land surface, and if one knew the rate, a simple calculation would reveal how long the erosion had been going on.
Unprovable assumptions underlie both the accumulation and the erosion hourglasses, yet early scientists had no other way of calculating ages. The temptation to fine-tune the hourglass of erosion led even Darwin into a calculation that he came to regret.
Telliamed and Buffon The first to attempt to measure the age of the earth was a French diplomat and traveler, Benoît de Maillet (1656-1738). De Maillet assumed that a universal sea had once covered the earth but had since shrunk, stranding formerly coastal towns high above sea level. He estimated the rate of sea level decline at 3 feet in 1,000 years. At that rate, to perch a formerly seaside town at 6,000 feet would take 2 million years. But since the earth is obviously much older than its towns, de Maillet arbitrarily raised his estimate for its age to 2 billion years.
Well aware that such an immense figure would incur the wrath of the church, de Maillet presented his conclusions in the guise of a dialogue between a French missionary and an Eastern mystic named Telliamed (de Maillet spelled backwards). The manuscript remained unpublished until a decade after de Maillet's death. Then the priest to whom he entrusted his work moved the decimal point to the left. Since de Maillet had shifted the decimal point to the right, he could hardly have complained. But the correction did little good; Voltaire, among others, denounced de Maillet as a heretic.
Though his assumptions were wrong, de Maillet did show that, starting from observation and measurement rather than from the Bible, one could calculate an age for the earth. That age might be measured not in the scores of years by which a human lifetime is counted, nor even in the thousands of years of Ussher, but in millions and billions of years.
Georges-Louis Leclerc, comte de Buffon (1707-1788), left us a forty-four-volume treatise on natural history, and the aphorism, "Style is the man himself." Where de Maillet calculated, Buffon experimented. He postulated that a collision with a comet caused the sun to eject a long streak of matter that eventually condensed into the planets. The still molten protoplanets, spinning rapidly on their axes, in turn flung off smaller globules that became their moons. His theory led Buffon to an analogy between the once molten earth and a molten sphere of metal.
Buffon built iron spheres of different sizes, heated them up, measured how long it took them to cool, and extrapolated to a ball of iron the size of the earth. He got ages as great as 3 million years, but lowered his published result to 75,000 years. Even that proved too much for the theological faculty of the Sorbonne, who instructed Buffon to reduce his estimate.
"You Can't Win"; "Eventually, You Lose" Over the eighteenth and nineteenth centuries, descriptive geology slowly progressed. Steno's principle of superposition and the pioneer English geologist William Smith's discovery of index fossils allowed geologists to construct their standard geologic column -- the ideal section of rock in which every known formation is shown in its correct thickness and position. The thickness of Paleozoic and younger sedimentary rocks clearly totaled tens of thousands of feet. This was far more than could have accumulated in the few thousand years to which biblical chronology limited earth history. Thus, by the middle of the nineteenth century, though geologists suspected that the age of their planet must measure in millions of years, they had no way of knowing just how many millions.
With the publication of Sir Charles Lyell's Principles of Geology in 1830-33, geologists got more millions than they bargained for. Lyell rejected the catastrophism of floods, impacting meteorites, and violent upheavals. He argued persuasively that it is unscientific to claim as an agent of geologic change any process that we cannot observe operating today. A Cambridge professor, William Whewell (1794-1866), gave Lyell's theory the unwieldy name "uniformitarianism."
Lyell claimed not only that natural laws and processes are constant, but that erosion, deposition, and the like have always operated at the same rate. He asserted that constancy in one part of the globe offsets change in another, leaving the whole the same. If the processes that shape the earth, and the rate at which they operate, are constant, then over the long run the appearance of the earth must also be constant. Lyell's philosophy not only required immense amounts of time, it offered no way of limiting time. For Kelvin, Lyell's unlimited draft on the bank of time violated the laws of nature.
The first 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 first law states, "You can't win" -- one cannot get more energy out than one puts in. The second law, jointly discovered by Kelvin and the German mathematical physicist Rudolf Clausius, states that every thermodynamic process flows in only one direction and afterwards cannot be returned to its original state. Without the intervention of an external device, heat will not flow from a lower to a higher temperature. The message of the second law is, "Eventually, you lose."
In describing geologic time as infinite and the earth as unchanging, Lyell claimed that the earth is a perpetual motion machine, one that can not only win the energy battle but go on doing so forever. But the first and second laws prove that a perpetual motion machine is impossible. Kelvin carried the battle to the geologists, charging 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." For "Natural Philosophy," read physics. Geology would have to bow to physics and Kelvin would see that it did.
Colossus Kelvin got an early start in thinking about the earth. In 1840 as a sixteen-year-old university student, he wrote an essay called "On the Figure of the Earth." Though the paper won the University at Glasgow medal, Kelvin never published it. Perhaps he was never satisfied -- for sixty-seven years he continued to refine his essay, giving it a last polishing only two months before his death. Stephen Jay Gould has written that Charles Lyell "doth bestride my world of work like a colossus"; but for half a century, Kelvin bestrode the entire world of science, dominating biologists and geologists alike.
By limiting the age of the earth and the sun, Kelvin would put right the great mistake of geology. Whatever the limit turned out to be, it would be less than Lyell's infinity of time and would return geology to concordance with the second law. Since Kelvin could not measure the age of the earth and the sun directly, he had to estimate their ages from theory, which required that he have a model. He began with the nebular hypothesis for the origin of the solar system, in which he had excellent company.
Newton's laws of motion and gravitation revealed the fundamental "glue" that holds the clockworklike apparatus of the solar system together, but they did not explain how the system began. In 1755, Immanuel Kant (1724-1804) postulated that the solar system started as a cloud, or nebula, of particles. Newtonian gravitational attraction drew the particles into clumps, which, growing larger by accretion, eventually became the planets. Kant, as a philosopher, did not realize that his model failed to explain why the planets revolve around the sun in the same direction and in the same plane.
Four decades later, the great French polymath Pierre-Simon Laplace (1749-1827) improved Kant's model. Laplace's theory began with the sun cooling and contracting. Since angular momentum is maintained, the shrinking, whirling sun had to spin faster, increasing the centrifugal force on its outer regions. At some point, the centrifugal force that tended to fling material outward just exceeded the gravitational force that tended to draw it in, pinching off blobs of solar material and launching them as planets. The planets recapitulated the process to create their moons. By Kelvin's day, the nebular hypothesis had become the starting point for any system of cosmogony. The theory was so deeply engrained that nearly everyone, Kelvin included, forgot that it was only a model, and one not subject to experiment or proof.
To derive the age of the earth starting with the primordial nebula, scientists needed a mathematical construct. The French mathematician Joseph Fourier (1768-1830) had worked out the theory of heat conduction in a solid body. He showed that by knowing how rapidly temperature increases with depth inside the earth, one can calculate how long it would have taken a body the size of the earth to cool to its present surface temperature. Fourier calculated an age so far out of line with current views that he did not even bother to write it down: 200 million years. At age seventeen, the precocious Kelvin had already learned Fourier's theory well enough to correct a mistake in its use made by a professor at the University of Edinburgh. Years later, when he wished to correct the mistake of geology, Kelvin again turned to Fourier.
At first, the idea that as meteorites fall into the sun, Icarus-like, the gravitational energy they lose converts to heat, attracted Kelvin. But calculations showed that the energy released by falling meteorites was inadequate to explain the sun's abundant heat and light. In the absence of any other source, the sun's energy must be that left over from its birth. The second law then requires that the sun be running down. If the sun's energy is waning, it must have been hotter in the past and will one day be cooler, so cool as to be unable to warm and illuminate the earth. No sooner had Kelvin helped to discover the second law of thermodynamics (at the age of only twenty-eight) than he used this reasoning to set out the fundamental conviction that was to guide his work for the rest of his life:
Within a finite period of time past the earth must have been, and within a finite period of time to come the earth must again be, unfit for habitation of man as at present constituted, unless operations have been, or are to be performed, which are impossible under the laws to which the known operations going on at present in the material world are subject.
In a famous 1862 article in the popular Macmillan's Magazine, Kelvin described 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 cannot continue to enjoy the light and heat essential to their life, for many millions of years longer, unless sources now unknown to us are prepared in the great storehouse of creation.
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.
In the last several words of each of the two quotations, Kelvin wisely ceded that he and his contemporaries did not yet know everything. He explicitly recognized that future discoveries of laws, operations, and sources of energy might invalidate his assumptions and conclusions. The prospect of unknown sources of light and heat was especially prophetic, for a "storehouse of creation" unknown to Kelvin did exist: radioactivity. But its discovery and significance lay four decades in the future. Never once, not even after the discovery of radioactivity, did the obstinate Kelvin heed his own wise counsel.
In 1862, Kelvin turned from the sun to the earth and to Fourier's theory. His first calculation gave the same age for the earth as Fourier: 100-200 million years. In 1865, Kelvin wrote a paper entitled "The 'Doctrine of Uniformity' in Geology Briefly Refuted," as though, in one "brief" paper, physics could do away with the underlying premise of geology and have geologists willingly submit. As the historian of science Stephen Brush has observed, "Apparently Kelvin was infected by the common fallacy that an established scientific theory can be immediately overthrown by citing a single devastating argument against it." Nothing could be further from the truth.
Throughout his long life Kelvin returned repeatedly to the age of the earth, incorporating the latest heat flow and other new data. Though his method remained the same, his answer fell with "harmonic regularity": "By 1868...100 million years. In 1876...50...and in 1881...20 to 50 million years. Finally, in 1897...24 million years." Biologists and geologists, who required ever-expanding periods for their theories, must have feared that Kelvin was going to leave them no time at all.
Lord Kelvin's Opponents From the first, Kelvin's opponents provided easy targets. Lyell made no effort to estimate the earth's age and indeed did not seem to conceive of the planet as having an age. He claimed that although it continually loses heat to space, a thermoelectric engine in the earth's interior allows it to remain at the same temperature indefinitely. To give him credit, Lyell came up with this idea before Clausius and Kelvin discovered the second law. But like Kelvin and others we will meet, when he had the chance to change his mind, Lyell never took it. Against the adamantine second law of thermodynamics, Lyell offered a perpetual motion machine, a prospect as unlikely as his view that extinction is not forever and that, one day, the wings of the pterodactyl may again flap over a forest primeval.
In the 1868 address to the Geological Society of Glasgow that began his attack on the great mistake of British geology, Kelvin directed his argument not at the living and formidable Lyell, but at the long-dead John Playfair, interpreter of the inarticulate James Hutton. As an example of geological reasoning, Kelvin chose Playfair's sixty-six-year-old statement: God "has not permitted in his works any symptoms of infancy or of old age, or any sign by which we may estimate either their future or their past duration." Playfair appeared to be saying that time is limitless and any attempt to measure it is pointless. To the discoverer of the second law, this was scientific heresy.
Those two champions of evolution, Charles Darwin (1809-1882) and Thomas Huxley (1825-1895), fared no better. Lyell's Principles had so impressed Darwin that he took a copy aboard HMS Beagle and later wrote, "He who can read Sir Charles Lyell's grand work and yet does not admit how incomprehensibly vast have been the past periods of time, may at once close this volume." Lyell provided Darwin with the time he required.
Only once did Darwin attempt to calculate a geologic age, and there he fell into a trap. He chose to estimate how long it had taken to erode a large dome in Kent, southern England, called the Weald. Darwin estimated that "for a cliff 500 feet in height, a denudation of one inch per century for the whole length would be an ample allowance." He then concluded, "At this rate, the denudation of the Weald must have required 306,662,400 years; or say three hundred million years." Everyone scoffed at the notion that one valley could be several times older than Kelvin's earth. Before long, Darwin was referring to "those confounded millions of years."
Thomas Huxley, Darwin's fierce and stubborn "Bulldog," had by all accounts routed the "shallowly-eloquent" Bishop Wilberforce in their famous debate of 1860. Wilberforce patronized Huxley by asking whether it was through his father or his mother that he had descended from an ape. Huxley countered that he would not be ashamed of an ancestor who was an ape but would be of one who used gifts of eloquence in the service of falsehoods. Now, nine years later, Huxley's opponent was not the condescending cleric, but Lord Kelvin, the man acknowledged by even his opponents as the leading British scientist.
Huxley and Kelvin did not confront each other directly. In his 1868 address, Kelvin fired the first shot, calling out the error that geologists had made, and were continuing to make, in claiming that the age of the earth had no limit. The latest edition of Lyell's Principles had just appeared; again Lyell ignored what Kelvin regarded as fundamental laws of nature. Now Lyell even admitted that he called for a perpetual motion machine.
As president of the Geological Society of London, Huxley may have had little choice but to answer Kelvin. In 1869, Huxley tried to do so, but found himself in the same weak position in which many geologists over the next one hundred years were to find themselves: unable to refute an apparently superior quantitative argument from a physicist. Huxley could not understand, much less counter, Kelvin's mathematics, so he had to fall back on "mother-wit" and his considerable rhetorical skills. Huxley pounced on Kelvin's selection of the long-dead Hutton and Playfair as his targets, arguing that geologists had long since modified the overly rigid uniformitarianism of the two founding fathers. "Catastrophes may be part and parcel of uniformity," Huxley claimed.
Huxley's most telling point, and the most quoted statement from his speech, came after a long attack on Kelvin's many assumptions. 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 fineness; but, nevertheless, what you get out depends upon what you put in." But Kelvin would not let Huxley off with an appeal to mere mother-wit: "The very root of the evil to which I object is that so many geologists are contented to regard the general principles of natural philosophy, and their application to terrestrial physics, as matters quite foreign to their ordinary pursuits." Since his opponents could not refute his impeccable mathematics,and since they could offer no better alternative, Kelvin prevailed. Huxley's address was the last time for several decades that anyone would challenge Kelvin. He had shifted the ground of debate about the earth. No longer could Lyell's limitless time be endorsed, nor his unchanging earth.
Swept Away Geologists set about trying to measure the length of geologic time by their own methods, part of a general effort during the second half of the nineteenth century to base their science more on measurement than description. But after laboriously calculating the age of the earth by one of their hourglass methods, geologists required an external reference to certify their result as reasonable. Yet only one reference existed: the apparently exquisite mathematical calculation of the leading scientist of the day, Lord Kelvin, who had determined the age of the earth to be 100 million years. To use a geological method and reach the same conclusion as Kelvin not only validated one's scientific acumen, it confirmed the stature of the discipline of geology. The combination proved irresistible.
Though not many occupied themselves with the age of the earth, those who did "produced an amazing variety of methods and an even greater homogeneity of results." No matter what assumptions and approaches they used, the hourglass calculators wound up agreeing with Kelvin. Two examples illustrate the fragility of their assumptions and the malleability of their results.
T. Mellard Reade (1832-1909) tried the hourglass of erosion, employed to his regret by Darwin. Reade assumed that the crust of the earth under the seafloor has the same composition and thickness as the crust under the continents. He assumed that both the surface area of the earth undergoing erosion and the rate of erosion have been constant. His calculations, which appeared in the 1870s, produced an age of 600 million years, which Reade viewed as a minimum and which he initially defended against Kelvin's much lower limit.
When the Challenger oceanographic expedition of the 1870s, sponsored by the British Admiralty and the Royal Society, found that the crust of the seafloor is not sedimentary, but is mainly basalt, Reade had to adjust his calculation. He also decided, arbitrarily, to assume that the area undergoing erosion is the same as the area that receives the eroded sediments. Next, he introduced a correction to recognize that sedimentary material is recycled: sedimentary rock erodes into sediment that hardens into sedimentary rock that erodes again, and so on. When the adjustments were over, Reade's result had shrunk to 95 million years, allowing him to say that the earth's age is "somewhere between 100 and 600 million years," thus preserving his original figure while still allowing Kelvin's.
Samuel Haughton (1821-1897), professor of geology at Trinity College, Dublin, made the most bizarre and revealing series of calculations. He first concluded that all the time before the Tertiary amounted to over 2 billion years, far above Kelvin's limit. Haughton then invented a peculiar and incomprehensible method of using fossils to estimate the rate of decline of the earth's temperature. This led him to just the opposite conclusion: The time since the Miocene epoch is greater than all the time that preceded it -- the Paleozoic era, the Mesozoic era, and all the pre-Miocene part of the Cenozoic era put together (cf. Figure 1.1). Haughton maintained this position even though it is obvious that far more sedimentary rock predates the Miocene than postdates it. He then went on to assume a much larger area over which sedimentation takes place, and a much greater thickness of accumulated sedimentary rock, than did others. His assumptions, uncorrected, gave an age of about 1.5 billion years. Following in the footsteps of Telliamed and his priest, as the final step in his calculation, Haughton capriciously divided by ten.
Haughton's 150-million-year result entered the literature and remained, his arbitrary methodology forgotten, as yet another confirmation of Kelvin's infallibility. Geologists accepted any result that came close to Kelvin's, no matter how contrived, as further corroboration that geology could stand beside physics. Galileo, Steno, Telliamed, and Buffon bowed to God; the Victorian geologists bowed to Lord Kelvin.
But the geologists were not the only ones to pay homage. George Howard Darwin (1845-1912), second son of Charles, was a protégé of Kelvin and later a professor at Cambridge. He served as president of the Royal Astronomical Society and president of the British Association. Like his contemporaries, George Darwin subscribed to the nebular hypothesis, according to which the moon had started out close to the earth and has since gradually receded to its present location. One skilled at mathematics might be able to work backwards and find out when the moon had begun its retreat from the proto-earth -- the birthday of the solar system.
Darwin calculated that for the earth and moon to achieve their present separation would have taken 56 million years. He cautioned, "The actual period, of course, must have been much greater," saying that his calculation "is only a wild speculation, incapable of verification." Unfortunately, Darwin could not resist noting that his result fell within the range established by Lord Kelvin. Like other estimates of Earth's age in the second half of the eighteenth century, George Darwin's estimate of 56 million years entered the literature as yet another confirmation, and by a rigorous and independent method, of Kelvin's accuracy. No one remembered Darwin's caution.
As certainty replaced mere confidence, Kelvin and his followers squeezed the stratigraphers even more. One sycophant proclaimed in 1876 that it is "utterly impossible that more than ten or fifteen million years can be granted." Geologists could scarcely fail to feel the pinch, nor resent the tone, according to which it was neither God nor Nature, but Kelvin, who granted geologic time.
Just as British geologists began to muster the courage to stand up to Kelvin, support for him came from a new source, the founding director of the U.S. Geological Survey, Clarence King (1842-1901). King had advocated the use of quantitative methods in geology and had the private funds to set up his own laboratory and practice what he preached.
Kelvin had assumed that the earth had been initially molten and at a temperature of 3,900 degrees Celsius. King started there, but having more recent information about the melting point of rocks and the distribution of temperature within the earth, he could extend and refine Kelvin's calculations. King settled on 24 million years as the age of the earth.
In an 1897 address, his last on the subject that had preoccupied him throughout his long scientific life, Lord Kelvin pronounced himself in agreement with King: 24 million years was just right. In a tone of triumphant certainty, Kelvin proclaimed that King had reconciled the age of the earth, as determined from solar heat, with that calculated from terrestrial heat. Kelvin announced that the reconciliation "suffices to sweep away the whole system of geological and biological speculation demanding an 'inconceivably' great vista of past time, or even a few thousand million years, for the history of life on earth." But new advances would sweep Kelvin's conclusions away.
The Bank of Time Over the course of the second half of the nineteenth century, geologists had gradually accommodated themselves to Kelvin's 100-million-year age for the earth. They found ingenious, and sometimes disingenuous, ways of confirming it. Huxley's appeal to "mother-wit" exposed the inadequate arsenal of biologists and geologists in the face of Kelvin's weaponry of exquisite calculations. They had no choice but to capitulate. The first leading British geologist publicly to accept Kelvin's limited time scale was Sir Archibald Geikie (1835-1924), director of the Geological Survey of Scotland and later director general of the Geological Survey of the United Kingdom. In paper that appeared in 1871, just after the Huxley-Kelvin debate, Geikie endorsed Lord Kelvin's 100 million years. "We have been drawing recklessly upon a bank in which it appears there are no further funds at our disposal," Geikie wrote. "It is well, therefore, to find that our demands are really unnecessary."
But it was Kelvin's demands that eventually proved too much. No sooner would the Victorian geologists accede to his latest, always lower, limit than Kelvin would lower it again. Opposition grew, and as century's end approached, geological opinion began to turn. In 1892, Geikie reversed himself, declaring that "some assumption has been left out of sight." Expressing his frustration with the overbearing Kelvin, Geikie fumed, "It is difficult satisfactorily to carry on a discussion in which your opponent entirely ignores your arguments, while you have given the fullest attention to his."
Other geologists joined Geikie in stressing Kelvin's unfounded assumptions. One noted that "The utmost any physicist is warranted in affirming is 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 proof that there is no other source." In other words, ignorance is no foundation for certainty.
The criticism from closest to home came from Kelvin's former assistant and partner, John Perry (1850-1920). As Kelvin relentlessly reduced the possible duration of geologic time while escalating his professed certainty, Perry came to believe that it was his "duty to question Lord Kelvin's conditions." He showed that only a slight change in Kelvin's assumptions could produce quite a different answer for the age of the earth. In a sincere but naive statement, Perry wrote to a supporter of Lord Kelvin that "as soon as one shows that there are possible conditions as to the internal state of the earth," Kelvin's case was undone. Kelvin met Perry partway by conceding that based on terrestrial heat alone, the age of the earth could be as great as 4,000 million years. But still the sun's heat limited the age of the solar system, and therefore the age of the earth, to a few score million years. Only two years later, Kelvin endorsed King's 24 million figure. But Kelvin's mill of exquisite workmanship was about to find sand in its gears -- sand from the ultimate hourglass.
When the American geologist Thomas Chrowder Chamberlin (1843-1928) read the text of Kelvin's speech, he took offense at Kelvin's authoritarian language: "half an hour after solidification" and "certain truth." Chamberlin was in a strong position to challenge Kelvin. He had been president of the University of Wisconsin and was chair of the geology department at the University of Chicago. Years earlier, Chamberlin had pointed out the disadvantages of rigid adherence to a single explanation. He advocated instead "multiple working hypotheses," in which minds are kept open to different interpretations for as long as possible.
Together with Forest Ray Moulton (1872-1952) a young astrophysicist, Chamberlin had begun to work out an alternative to the nebular hypothesis of Kant and Laplace. He and Moulton thought that instead of a hot nebula flinging off the planets, they may have accumulated from cold, tiny, "infinitesimal planets," or "planetesimals."
Chamberlin and Moulton challenged the premise on which Kelvin based all his calculations. That the earth had once been completely molten, Kelvin had described as a "very sure assumption." Chamberlin "beg[ged] leave to challenge," arguing that the earth may have formed slowly from cold planetesimals and therefore may never have melted entirely. Consistent with his philosophy, Chamberlin did not claim that he was right and Kelvin wrong, only that either could be right.
Chamberlin also attacked Kelvin's age for the sun, arguing that no one really knew the source of the sun's energy. His words were prophetic:
Is present knowledge relative to the behavior of matter under such extraordinary conditions as obtain in the interior of the sun sufficiently 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 affirm either that the atoms are really elementary or that there may not be locked up in them energies of the first order of magnitude. With great prescience, Chamberlin had set the stage for the ideal hourglass. But first, geologists were to have a last go with their methods.
The Salt Clock In 1715, Edmund Halley (1656-1742), discoverer of the eponymous comet, proposed that an hourglass of salt would reveal the age of the oceans. If one knew the rate at which streams and rivers dissolve salt from rocks and deliver it to the oceans, and if one knew the total amount of salt in the sea, one could calculate how long the process had been going on, which would equate to the age of the oceans. In Halley's day, no one knew either, so his idea had to be rediscovered in the 1870s. John Joly (1857-1933), professor of geology and mineralogy at Trinity College, Dublin, made the method his own (using sodium rather than sodium chloride). By the end of the nineteenth century, scientists knew that the oceans contain about 1.42 × 1016 metric tons of sodium and that streams deliver about 1.43 × 1018 metric tons of sodium per year. Thus, the age of the oceans, which Joly equated with the age of the earth, is 99 million years. Joly attempted to correct for the presence of original sodium in the primordial oceans, which lowered his estimate to 89 million years. This number Joly defended for the rest of his life, thirty-three years into the new century, and long after it had become clear that radioactivity had obviated the Kelvin time scale.
As enthrallment with Kelvin began to wane, geologists recognized that their methods, although they gave older and therefore more acceptable ages than Kelvin's, included so many assumptions that they were bound to be inaccurate. One scientist listed four possible sources of error in the stratigraphic clock:
- The total accumulation of sediment occurs in no one spot;
- Sediment accumulates at greatly different rates;
- The duration of time represented by unconformities (gaps) is unknown;
- Sedimentary particles are recycled through erosion, deposition, and erosion again.
An even more fundamental assumption was that the current rates of geologic processes are close to the average rate over all of geologic time, and that those rates have not varied. Finally, since at the time only relative rock ages from fossils could be established, and since Precambrian rocks contain no macrofossils, geologists of the nineteenth century had no way of knowing that Precambrian time makes up almost 90 percent of geologic time. That alone was to reduce their calculated ages to only a fraction of the true age of the earth.
Joly's salt clock contained an additional, unsuspected flaw. When we repeat his calculation using modern data for the sodium content of rivers and oceans, we obtain an "age" of about 68 million years, not far below Joly's later calculations. But we know that the earth and the oceans are vastly older than that. If Joly's method is not wrong because of the poor quality of data put into the equation, why is it wrong? Unknown to Joly, geological processes have carried some of the sand in the bottom of his hourglass back to the top. This error caused him to derive neither the age of the earth nor the age of the oceans, but of something else entirely.
Joly and his contemporaries incorrectly assumed that all the salt delivered to the oceans remains dissolved in seawater. In the 1960s, scientists discovered that as erosion adds chemical elements to the oceans, and deposition removes them, seawater reaches chemical equilibrium. After that, the more of an element added, the more removed, leaving the average concentration everywhere the same. In particular, much of the sodium that reaches the oceans evaporates or joins sedimentary rocks, which are later exposed to erosion and whose sodium returns to the sea. It turns out that Joly had measured, not the age of the oceans, but the average length of time that a sodium atom, delivered to the sea, remains dissolved.
As the nineteenth century closed, the era of flawed methods began to close as well. An hourglass of inexorable precision was about to replace all the previous hourglass methods. Elaborate calculations of the age of the earth based on untestable assumptions were to go the way of the gas lamps that lit British streets. The settling of scientific disputes by rhetoric was likewise to decline, a victory for reason if a defeat for eloquence. The twentieth century awaited with methods for measuring the age of the earth that were to brook little debate. All the efforts of nineteenth-century scientists to measure and limit the age of the earth, Lord Kelvin's included, came to naught. Sadly, he and his contemporaries had not been able even to prepare the ground.
Copyright © 2001 by James Lawrence Powell