Science of Discworld III

builds a new layer, like a skin, on its surface. This ‘rim’, as it is called, is roughly the same age as the surrounding rock; the inner core is far older.
    Now the process may repeat. The core of zircon, with its new rim, may be pushed up with the surrounding rocks to make a new mountain range. When those mountains weather, the zircon may return to the depths, to acquire a second rim. Then a third, a fourth … Just as tree rings indicate the growth of a tree, so ‘zircon rims’ reflect asequence of mountain-building and erosion. The main difference is that each ring on a tree corresponds to a period of one year, whereas the rims on the tiny zircon crystal correspond to geological cycles that typically last hundreds of millions of years. But, just as the widths of tree rings tell us something about the climate in the years that are represented, so the zircon rims tell us something about the conditions that occurred during a given geological cycle.
    By one of those neat coincidences that Paley would interpret as the Hand of God but nowadays we recognise as an inevitable consequence of the sheer richness of the universe (yes, we do see that those statements might be the same), the zirconium atom has the same electric charge, and is much the same size, as an atom of uranium. So uranium impurities can easily sneak into that zircon crystal. This is good for science, because uranium is radioactive. Over time, it decays into lead. If we measure the ratio of uranium to lead then we can estimate the time that has elapsed since any given part of the zircon crystal was laid down. Now we have a powerful observational tool, a geological stopwatch. And we also have a simple prediction that gives us confidence in the hypothesis that the zircon crystal forms in successive stages. Namely, the core should be the oldest part of the crystal, and successive rims should become consistently younger, in separate stages.
    A typical crystal might have, say, four layers. The core might date to 3.7 billion years ago, the next to 3.6 billion years, the third to 2.6 billion years, and the last one to 2.3 billion years. So here, in a simple ‘stone’, we have evidence for geological cycles that last between 100 million and one billion years. The order of the ages agrees with the order in which the crystal must have been deposited. If the general scenario envisaged by geologists were wrong, then it would take only a single grain of sand to disprove it. Of course that doesn’t confirm the huge geological cycles: those are deduced from other evidence. Science is a crossword puzzle.
    Zircons can teach us more. It is thought that the ratio of twoisotopes of carbon, carbon-12 and carbon-13, may distinguish organic sources of carbon from inorganic ones. There is carbon in the Isua formation, and the ratio there suggests that life may have existed 3.8 billion years ago, surprisingly soon after the Earth’s surface solidified. But this conclusion is controversial, and many scientists are not convinced that other explanations can be excluded.
    At any rate, for the Isua zircons we know that it is not an option for them to have ‘lain there for ever’. Stones are far more interesting than they might seem, and anyone who knows how to read the rocks can deduce many things about their history. Paley believed that he could deduce the existence of God from the complexity of an eye. We can’t get God from a zircon, but we can get vast geological cycles of mountain-building and erosion … and just possibly, evidence for exceedingly ancient life.
    Never underestimate the humble stone. It may be a watch in disguise.
    Paley’s position is that what you see is what you get. The appearance is the reality. His title Natural Theology says as much, and his subtitle could scarcely be plainer. Organisms look designed because they are designed, by God; they appear to have a purpose because they do have a purpose: God’s. Everywhere Paley looked, he saw traces of God’s handiwork; everything around him was evidence for the Creator.
    That kind of ‘evidence’ exists in such abundance that there is no difficulty in accumulating examples. Paley’s central example was the eye. He noted its similarity to a telescope, and deduced that since a telescope is designed, so must an eye be. The camera did not exist in his day, 3 but if it had existed, he would have found even closer similarities. The eye, like a telescope or a camera, has a lens to bringincoming light