In August 1927, the New York Herald Tribune published a two-part literary manifesto by Virginia Woolf. ‘Poetry, Fiction and the Future’ called for the invention of a new form of writing. Established literary modes, thought Woolf, were ‘not enough’ to meet the demands of ‘an age like ours’. Realism was out. Lyric poetry, too. Neither could encompass the ‘monstrous, hybrid, unmanageable emotions’ of which the modern ‘mind’ was ‘full’:

That the age of the earth is 3,000,000,000 years; that human life lasts but a second; that the capacity of the human mind is nevertheless boundless; that life is infinitely beautiful yet repulsive; that one’s fellow creatures are adorable but disgusting; that science and religion have between them destroyed belief; that all bonds of union seem broken, yet some control must exist.

It was to ‘this atmosphere of doubt and conflict’, Woolf wrote, that she and her fellow writers had now to respond.

It might surprise us to see the long age of the Earth taking first place in Woolf’s list of modernity’s horrors. Young Earth biblical literalism, to today’s mind, is the stuff of medieval (or born-again) superstition. But in 1927 that most commonplace of facts – that our planet existed for billions of years before we did – had only recently been settled.

A couple of months before Woolf sat down to write ‘Poetry, Fiction and the Future’, H.G. Wells’s publisher Ernest Benn printed the second edition of The Age of the Earth (1913): a short academic pamphlet by the English geologist Arthur Holmes. The Woolfs had a copy of it on their bookshelves. No. 102 in Benn’s Sixpenny Library, this revised edition was compact enough to fit inside a jacket pocket, but the revelation it contained was enormous. Planet Earth was between 1.6 and 3 billion [thousand million] years old.

It is not often in science – not often in life – that such a definitive answer comes to so vexed a question. So we can forgive Holmes the smugness of his opening line: ‘With insatiable curiosity’, he writes, ‘men have been trying for thousands of years to penetrate [this] carefully guarded secret.’ Holmes, it is true, was the first to succeed.

Still, ‘thousands of years’ is rather an overstatement. Granted, human beings have been speculating about the age of the Earth since antiquity at least, but answers have tended to be near at hand, according to the ponderer’s religious cosmology. Aristotle believed, like Vedic, Taoist and Jain scholars before him, that the Earth was without a beginning or an end. Plato, meanwhile, thought the Earth had been created, albeit in a now very distant past. In this he concurred with Mesopotamian, Ancient Egyptian, Zoroastrian and Jewish thought. And this finite view of the Earth won the day, becoming a tenet of the three main Abrahamic religions.

‘In the beginning God created the Heaven and the Earth.’ Then, in 1650, James Ussher, the Archbishop of Armagh, put a number on it. According to calculations he had made on the basis of data from the Old Testament and various Hebrew and Roman calendars, announced Ussher, God must have made Earth on Tuesday 23 October 4004 BC. At midday, to be precise. That settled the question – in circles scientific and theological – for a good century and a half. William Whiston’s suggestion, in A New Theory of Earth (1696), that Genesis’s six days might represent six calendar years (pushing the date of creation back to 4010 BC) was met, therefore, with scorn and ridicule.

Yet his dissent proved an alarum of the revolution to come. A presbyterian ‘Flood Geologist’, Whiston was struggling to reconcile his own biblical literalism with the physical laws he knew to be true and with the geological and fossil record he saw around him. ‘The Obvious or Literal Sense of Scripture’, he wrote, ‘is the True and Real One.’ But only ‘where no evident Reason can be given to the contrary.’

Ironically, in their quest for physical proof of the biblical Flood – the guiding purpose of A New Theory of Earth – Whiston and his fellow Flood Geologists were stumbling upon ample reason to doubt Ussher’s timescale. Marine fossils were emerging from Alpine mountaintops; strata presented grand geological shifts in bas relief. Perhaps a single, catastrophic deluge was responsible, perhaps not. As Whiston himself had written: ‘That which is clearly accountable in a natural way, is not without reason to be ascribed to a Miraculous Power.’

Belief in that ‘Miraculous Power’ was still strong. Even as tentative a suggestion of gradual geological change as Whiston offered in A New Theory of the Earth opened him up to charges of heresy. William Nicholls (1664-1712) wrote that Whiston’s ‘chief fault’ was that his Theory ‘stuck more to Mr. Newton’s than to Moses’ [theories]’. For the Calvinist John Edwards (1637-1716), Whiston’s too-loose interpretation of biblical writ smeared ancient Jews and early Christians as a pack of ‘Idiots and Blockheads’ ready to accept ‘any Story of a Cock and Bull’.

On the eve of the Age of Enlightenment, such unbending attitudes were already in retreat. With John Locke’s and David Hume’s empiricism in their arsenal, the next generation of geologists would be the first to think seriously about a ‘natural way’ the Earth’s features could have formed. That left the Ussherian timescale in tatters.

The ‘father of modern geology’ was James Hutton (1726-97): an Edinburgh medical doctor who turned gentleman farmer upon inheriting a clutch of Berwickshire farms in his mid-twenties. As John McPhee notes in Basin and Range (1981), Hutton had seen ‘Hadrian’s Wall running across moor and fen after sixteen hundred winters in Northumberland’, and ‘[n]ot a great deal had happened to it’. Ranging his acres of newly acquired farmland, Hutton began to develop a hypothesis. And in 1764 he embarked upon a years-long tour of Scottish geology to test it.

After a quarter of a century of careful observation, Hutton was finally ready to unveil his Theory of the Earth (1785) to the Royal Society of Edinburgh (RSE). It set out exhaustive proof, from his years looking at rocks, at strata and at the Earth’s topography, that the features of the landscape on which the modern world stood had been shaped over millennia by ancient rivers and seas. The same processes that formed the Earth and its geological features, he explained, were still observably in action today. The evidence led Hutton to one irresistible conclusion: ‘Time, which measures every thing in our idea, and is often deficient in our schemes, is to nature endless and as nothing.’

Thus began the ‘Golden Age’ of Scottish and English Geology. A generation later, Charles Lyell (1797-1875) concurred with Hutton in his Principles of Geology (1830) that the ‘views of the immensity of past time’, to which the study of rocks and fossils granted access, were ‘too vast to awaken ideas of sublimity unmixed with a painful sense of our incapacity to conceive a plan of such infinite extent’. This sublime disorientation at time’s possible magnitude fast became the hallmark of this Golden Age. Its most characteristic example is the mathematician and Church of Scotland minister John Playfair’s (1748-1819) account of a 1788 trip to Berwickshire with Hutton. At the Siccar Point rock promontory near Jedburgh, Hutton showed Playfair a juncture where sandstone strata collide at different angles, which is known today as ‘Hutton’s Unconformity’. Unconformities like these, explained Hutton, indexed the sedimentation of rock over millions of years, and the equally slow actiones of tectonic uplift and erosion. Listening to his friend speak, wrote Playfair, his ‘mind seemed to grow giddy by looking so far into the abyss of time’.

Gone were the religious censors of a century before. The Golden Age men’s theories belonged to the Enlightenment mainstream. Charles Darwin (1809-92), a self-professed disciple of Lyell’s, brought his copy of Principles of Geology on the voyage of The Beagle in 1831. On the Origin of Species (1859) instructs anyone who is able to read Lyell’s work and not come away ‘admit[ting] how incomprehensibly vast have been the past periods of time’ to ‘at once close this volume’. The ‘future historian’, Darwin correctly predicted, would ‘recognise’ Lyell for ‘having produced a revolution in natural science’.

Yet like all revolutions, this one spawned rival factions. Prior to the discovery of radioactivity, the belief of geologists’ and evolutionary biologists’ that the Earth must be hundreds of millions of years old at least was dogged by an absence of (in Holmes’s word) ‘material’ evidence. The Age of the Earth spelt the end of a centuries-long row that divided the physical sciences. On one side were geologists, biologists and palaeontologists, who had long believed the Earth was far older than man could imagine. Opposite them were geochemists, mathematicians and physicists, who dismissed that belief as romantic superstition; for over a century they were on the winning side.

The Golden Age geologists were creatures of the Enlightenment. Yet they thought and wrote in an idiom that struck many of their enlightened fellow scientists as rather too reminiscent of Aristotelian eternalism. Hutton concludes his ‘Theory of the Earth’, after all, by announcing the ‘result […] of our present enquiry’ as: ‘we find no vestige of a beginning – no prospect of an end.’ The geologists’ proposition was an essentially negative one: the Earth was not created 5,800 years ago. Hutton never ventured to guess how old the Earth was; all he and his colleagues had, as John McPhee put it, ‘was their new and expanding insight that they were dealing with time in quantities beyond comprehension’. It would be another century and a half, as we know, before anyone could measure the age of a rock, in the billions of years. Yet Hutton ‘sensed something like it,’ writes McPhee: he ‘sensed the awesome truth,’ as he stood before crags, cliff faces and unconformities and thought. His imagination stepped in where empirical observation stopped short.

Lyell laid even greater stress than Hutton on the incompleteness of the geological record, and the imaginative leaps that geologists would have to be bold enough to make as a consequence. ‘Signs’, he insisted, can ‘convey to our minds more definite ideas than figures can do, of the immensity of time.’ A geologist must take an object in the present – a rock, a mountain, a continent, a planet – and imagine its development backwards through time, all the way back to its point of origin. Imagination was as much his instrument as reason.

It is easy to see, when we read these accounts, why many Victorians came to regard geology as a pseudoscience. To imagine time could be infinite, scientists rightly pointed out, was akin to claiming the Earth was a perpetual motion machine. As Lord Kelvin (1824-1907) reminded Lyell in 1863, such a model ‘violates the principles of natural philosophy’ in ‘exactly the same manner’ as ‘to believe that a clock constructed with a self-winding movement may fulfill the expectations of its ingenious inventor by going for ever’.

Over the decades that followed this polemic, the ‘father of Thermodynamics’ led a counter-expedition to find the true age of the Earth. With him were the astronomer George Darwin (son of Charles), the physicist Hermann von Helmholtz, and a coterie of their largely English and German colleagues. ‘British popular geology’, they held (and Kelvin wrote), had made ‘a great mistake’ in imagining that the Earth could have been ‘going on for ever in the present state.’

While they were irrefutably correct on that count, Kelvin and his followers were wrong to take Lyell’s remark that ‘the time during which organic life has existed on earth is practically infinite’ literally. Comments to this effect were concerned less with the Earth’s age than with the human imagination’s limits. As Playfair wrote of the trip to Siccar Point, he ‘became sensible’, as he gazed at the rocks, ‘of how much farther reason may sometimes go than the imagination can venture to follow’.

Even Hutton’s famous conclusion – ‘no vestige of a beginning – no prospect of an end’ – was basically rhetorical. A statement to the same effect from an abstract to the treatise Hutton read to the RSE in 1785 is clearer: ‘as there is not in human observation proper means for measuring the waste of land upon the globe, it is hence inferred, that we cannot estimate the duration of what we see at present, nor calculate the period at which it had begun’. Therefore, ‘with respect to human observation, this world has neither a beginning nor an end.’ Hutton’s point was not that the Earth was infinite, but that no evidence of its origin could at present be observed, and (in true empiricist fashion) that it was therefore unsound to assume that there had been any one point of origin.

Even as Kelvin scoffed, geologists were working on empirical methods to calculate the figures of which their minds could not conceive. Three years prior to Kelvin’s attack on Lyell, the English geologist Joseph Phillips (1800-74) had attempted to date stratified rocks from the Ganges basin by calculating the current rate of sedimentation and working backwards. He came up with a figure of around 96 million years, but concluded (correctly) that the effect of erosion made his method essentially useless, since this figure was manifestly ‘much too short’.

Then, in 1866, the Irish geologist Samuel Haughton (1821-97) used the cooling rate of basalt to calculate that at least 2,298 billion years must have passed between the formation of the oceans and the beginning of the Tertiary period. But by the time he announced his results, Kelvin’s 1862 estimate that the Sun was only between 100 and 500 million years old had already gained widespread acceptance. Geology’s standing in the scientific community had fallen to such a low point that few took notice of Haughton’s estimate, which turned out to be remarkably close to the truth.

The Earth, according to Kelvin, was probably only around 400 million years old: a figure he revised down to 100 million in 1868, 50 million in 1876, 20 million in 1881, then back up to 24 million in 1897. In 1899, the Irish physicist John Joly (1857-1933) divided an estimated total mass of sodium in the world’s oceans by the rate at which he’d seen rivers deposit sodium in the ocean, and came up with a figure of 99 million years for the age of the ocean. And that, it seemed, was that.

At the dawn of the 20th century, then, physics and mathematics appeared between them to have reigned in the imaginative excesses of geology. In a mournful paper of 1909, the Oxford Professor of Geology, William Johnson Sollas (1849-1936), conceded that the ‘only approach to Geology’ now lay ‘through the other sciences’ and that ‘endeavours to enter [it] in some other way’ had been ‘responsible for much of the pseudoscience that has been perpetrated in [geology’s] name.’ Kelvin died in 1907 victorious.

So what an irony it is, in retrospect, that it was Kelvin’s beloved physics that proved him wrong in the end. In 1896, in two separate labs in France, Henri Becquerel, Marie Curie and Curie’s husband Pierre proved the existence of radioactivity. Three years later, the Cambridge-based New Zealander Ernest Rutherford (1871-1937) introduced the concept of radioactive half-life: the time it takes for half of the starting number of radioactive atoms in a sample of material to decay. This was to be the tool the geologists had been waiting for. At last, they would prove for certain what had for centuries been a purely theoretical matter.

Rutherford seemed, at first, to beat them to it. In 1905, now a professor of physics at McGill University, he dated a rock sample by comparing its relative levels of uranium and helium. It was 500 million years old, he announced: five times Joly’s best guess for the age of the ocean. ‘Doomsday Postponed!’ declared the New York Times. Rutherford was awarded the Nobel Prize in 1908.

But another professor of physics, Robert Strutt (1875-1947), at Imperial College London, detected a flaw in Rutherford’s method. Rutherford’s rock could only have contained a fraction of the total helium its decaying uranium had produced, as most would have escaped into the air as a gas. It couldn’t possibly be as young as 500 million years old. So Strutt set out to develop a better method of radioactive dating. And in 1910 he hired a research assistant from his undergraduate course on radioactivity: a final-year geology student called Arthur Holmes.

Under Strutt, Holmes invented the more reliable new method of lead-uranium dating. He had originally enrolled at Imperial, aged 17, as a physicist. Based on his first experiments in this method, in 1911 he drew up a geological timescale, spanning the Carboniferous (340 million years ago) Period all the way to the Precambrian (1-1.64 billion years ago). But graduation and a graduate job with a mining company in Mozambique beckoned. Holmes parted for Africa before his scale was published.

The gold, copper and tin his employers sought there proved elusive. Holmes passed his six months in Mozambique following the latest developments in nuclear science in the European scientific press with the fervour of a prisoner reading news from the outside. Inspired by the dramatic granite peaks all around him, his mind soon turned back to the work he’d left behind. As Cherry Lewis details in her book on Holmes, The Dating Game (2000), he abandoned camp frequently in search of thorium and radium, and recruited a number of Mozambican lab assistants to help him process samples. By the time he was shipped back to England with malaria, the 21-year-old Holmes had his eyes trained firmly on a singular ambition. He re-enrolled at Imperial as a doctoral student.

Back at Imperial, he launched straight back into the research he had begun as an undergraduate, further refining his lead-uranium dating method. The first edition of The Age of the Earth announced the dramatic result of those early experiments: one of Holmes’s samples was 1.6 billion years old. It caused ripples of excitement in geological circles. H.G. Wells, who had studied geology at Imperial in the 1880s, was among those who took early notice of Holmes. He wrote in his bestseller The Outline of History (1920) that the Earth was ‘a vast repository of time, containing in its crust the records of nearly two billion years of life and death; and yet it is less than a million years ago that the first hominids appeared upon the Earth’. Wells sent an autographed first edition to Woolf’s husband, Leonard.

Yet geologists were wary of drawing too strong a conclusion. Holmes’s method of lead-uranium dating was controversial. He, a mere student, had no real authority. And geologists’ fingers had been burnt just a decade before, when they whipped themselves into a frenzy over Rutherford’s helium dating, which turned out, in the end, to be a red herring.

But even as Holmes was writing his first edition, Rutherford’s former research assistant, Frederick Soddy (1877-1956), was developing the key that would unlock his ‘carefully guarded secret’. Soddy had left Canada to become a lecturer at Glasgow. There, with his assistant Ruth Pirret (1874-1939), in 1913 he discovered isotopes: different forms of one element with the same chemical properties but different atomic weights. In his 1921 address upon winning the Nobel Prize for Chemistry, Soddy described his discovery as the simple observation that atoms can have ‘identical outsides but different insides’.

This was it: the final piece of Holmes’s puzzle. And he knew it. Armed with an estimate of uranium’s half-life and now able to differentiate between ‘ordinary lead’ and lead daughter-isotopes of uranium, based on their atomic weights, Holmes could count backwards in time to find the point at which the uranium had begun to decay. With a wink at Pascal – or perhaps at Kelvin – he likened radioactive elements ‘to clocks kept going by springs that gradually unwind themselves’:

But while a clock records time in ticks that are heard and pass away, the radio-active elements are engaged in keeping a more material register of time, after the manner of an hour-glass, the accumulating materials being helium and lead. Every radioactive mineral may thus be regarded as a natural chronometer, registering time by the atoms of helium and lead that are unceasingly produced within it year after year.

Here, at last, was an irrefutable answer to his ‘bold question’, which he could unveil in Benn’s 1927 edition.

He crowned that volume with an epigraph from Walt Whitman’s ‘Song of the Open Road’ (1856): ‘The Earth is rude, silent and incomprehensible at first—/ Be not discouraged—keep on.’ Now aged 41 and head of the Geology Department at Durham, Holmes surely intended this missive, at least in part, to go to his younger self. The near-fatal bout of malaria that had forced him back to Imperial from Mozambique had also exempted him from military service. By the time of the 1927 reprint read by Woolf, Holmes had spent 14 years refining and re-refining his methods in the light of new science. Now he had it in black and white: an answer to the question that had animated his life.

The Whitman epigraph is a dedication, too, to generations of geologists who came before him: the men whose works he had pored over as a child in Gateshead, and who had inspired him, aged 18, to switch from physics to geology. For more than a century they had been smeared as charlatans by physicists and mathematicians who laid sole claim to reason. Yet they’d kept on.

The vast majority of geologists (and with them biologists) never accepted the physicists’ low estimates. As the American geologist Thomas Chrowder Chamberlin (1843-1928) told the 1922 meeting of the American Philosophical Society in Philadelphia, it had ‘long been the view of biologists that the evolution of life required much more than [the] 100,000,000 years’ proposed by the ‘conservative school of geologists’ led by Joly and Sollas. ‘Astronomical opinion’ had also been ‘trending towards the view that long periods [were] necessary for certain typical phases of celestial evolution’. In reference to his own sojourn into astrophysics – the 1905 Chamberlin-Moulton Planetesimal Hypothesis – Chamberlin added that ‘a period of the order of two or three billion years’ seemed to him to be ‘the most probable’.

So the definitive proof that Holmes developed between 1913 and 1927, that the Earth must be several billion years old, was less new scientific knowledge (as, say, Becquerel and the Curies’ discovery of radioactivity had been) than it was a confirmation that the system of knowledge to which Holmes referred as the ‘philosophical aspect of geology’ had a basis in material reality. The modern estimate of the Earth’s age at around 4.54 billion years dates from a 1956 paper on ‘The Age of Meteorites and the Earth’ by the American geochemist Clair Cameron Patterson, who developed Holmes’s uranium-lead dating method into lead-lead dating, which measures ratios of primordial lead in stone meteorites relative to radiogenic leads. No more reliable method has ever been found.

Prior to the invention of radiometric dating then, ‘the imagination’ (returning to John Playfair’s terms) had had a far better grasp of deep time than had ‘reason’. The world is 4.54 billion years old. And a ‘human life lasts but a second’. Yet ‘the capacity of the human mind is nevertheless boundless’. This is what Holmes bequeathed to modernity, as much as his lauded scientific discoveries: a humanist repudiation of Victorian science’s claim on truth. Kelvin died believing he had discovered the age of the Earth. He and his acolytes claimed their mathematical proofs were irrefutable, but Holmes refuted them. Victorian certainty gave way to modern ‘doubt and conflict’. And as daunting as that was, it was a liberation, too. The Age of the Earth was testament to the modern doctrine Woolf would set down in A Room of One’s Own (1929), two years later: ‘There is no gate, no lock, no bolt that you can set upon the freedom of my mind.’