Though modern geologists rejected a strict interpretation of Lyell's uniformity of state, by the 1950s most of those in North America had come to believe that at least the outer appearance of the earth, with its continents and ocean basins, had not changed dramatically—cer-tainly continents had not drifted. The notion that seafloors spread out to plunge beneath continents, that the ocean basins are geologically young, that the continents have never been in the same place twice— all proved hard for those raised on uniformitarianism to accept.
After a generation to get used to plate tectonics, geologists have incorporated it into uniformitarianism. Because we can measure with laser beams, satellites, and global positioning systems the almost imperceptible movement of continents and the spreading of the sea-floors, and can use the data to project backward to determine what the surface of the earth used to look like, the present can still be said to be the key to the past. Indeed, the way in which plate tectonics shows how older crust is buried in the mantle and recycled into new crust is reminiscent of Hutton's endless cycles. But Hutton and Lyell would certainly have rejected continental drift as impossibly antiuniformitarian. In any event, to say that one can infer the past positions of continents from their present positions and measured rates of motion is to appeal only to methodological uniformitarian-ism, which, as we have seen, is only to say that geologists proceed scientifically.
But meteorite impact as a force on the earth takes us into a new realm. Since we have never observed a large meteorite striking the earth, yet the existence of terrestrial craters tells us that they have, we cannot understand earth history by relying solely on processes that we can observe today. In short, the present is not a reliable key to the past. Just the opposite: To understand the role of impact cra-tering, we have to invert Hutton's aphorism and realize that, in the case of an event so rare as to fall outside human experience, the past must provide the key to understanding the present and the future.
By the time the Alvarez theory appeared in 1980, the space age had brought overwhelming evidence that impactors of every size had hit every object in the solar system countless times. We could of course stretch definitions to recognize the ubiquity of impact and claim that it amounts to a kind of uniformity, but this is equivalent to saying, "catastrophism is uniformitarian," an abominable oxymoron that would empty both words of meaning. As Ursula Marvin has pointedly said, "To regard the cataclysmic effects of impact as uniformitarian is an exercise in 'newspeak.'"2'
Walter Alvarez drew the right conclusion about the proper place for uniformitarianism in geological thinking: "Perhaps it is time to recast uniformitarianism as merely a sort of corollary to Ockham's Razor, to the effect that if a set of geological data can be explained by common, gradual, well-known processes, that should be the explanation of choice, but that when the evidence strongly supports a more sudden, violent event, we will go where the evidence leads us."28
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