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Walter Alvarez could have told his father that it is hard to find any idea in the history of science more consistently and continuously spurned by authorities than the notion that meteorite impact has in any way affected the earth. The rejection stretches back to the dim beginnings not only of geology but of science itself. In the 1680s, William Whiston, mightily impressed by the great comet that had opened that decade, wrote that God had directed the comet at the earth and that its impact had produced both the tilt of the earth's axis and its rotation, had cracked the surface, and had allowed the waters to rise to create the biblical flood.6 Even though Whiston convinced no one, his idea so offended Charles Lyell, a founding father of geology, that nearly 150 years later he went out of his way to debunk Whiston's suggestion, writing that he had "retarded the progress of truth, diverting men from the investigation of the laws of sublunary nature, and inducing them to waste time in speculations on the power of comets to drag the waters of the ocean over the land—on the condensation of the vapors of their tails into water, and other matters equally edifying."7 Lyell's disciple, Charles Darwin, was equally convinced that catastrophes played no part in earthly events, writing: "As we do not see the cause [of extinction], we invoke cataclysms to desolate the world, or invent laws on the duration of the forms of life."8

A century later, little had changed. E. H. Colbert of the American Museum of Natural History, the dean of American dinosaur studies at the time, wrote: "Catastrophes are the mainstays of people who have very little knowledge of the natural world; for them the invocation of a catastrophe is an easy way to explain great events."9

Walter Bucher was an eminent American geologist and the country's leading authority on mysterious rock structures that geologists referred to as "cryptovolcanic." Here and there around the globe, rocks at the surface are broken into a set of concentric faults that form a bull's-eye pattern. Often the structures occur in sedimentary rocks hundreds of miles from the nearest volcanic lavas. Nevertheless, geologists, looking down and not up, could think of no other plausible origin for these structures than that they were created by gases exploding from invisible underground volcanoes. A few brave souls had the temerity to suggest that these features might actually be a kind of bull's-eye, marking the target struck by an impacting meteorite. In 1963 Bucher wrote in the definitive paper rebutting this view: "Before we look to the sky to solve our problems miraculously in one blow, we should consider the possibility that cryptoexplosion [cryptovolcanic] structures and explosion craters may hold important clues to processes going on at great depth below our feet, even if it threatens to lead us back to another 'traditional' concept, that of cooling of the outer mantle. Distrust in traditional thinking should not deter us from looking hard at all aspects of the problem. Doing so will probably yield more useful results than computing possible velocities of imagined meteorites."10 In other words, do not try to solve geologic problems by appealing to missing meteorites from space.

Tony Hallam, a distinguished British geologist, advised that "Environmental changes on this planet as recorded by the facies [rock types] should be thoroughly explored before invoking the deus ex machina of strange happenings in outer space. ... It is intuitively more satisfying to seek causes from amongst those phenomena which are comparatively familiar to our experience."11 A 1986 review article intended to sum up matters for students and teachers stated that "it is not necessary to invoke a meteorite impact to explain the K/T extinctions, and, in actuality, an impact does not explain those extinctions."12

The award for the most unlikely source of negative reaction goes to the New York Times, which in a 1985 editorial curiously titled "Miscasting the Dinosaur's Horoscope," declared that "terrestrial events, like volcanic activity, or change in climate or sea level, are the most immediate possible cause of mass extinctions. Astronomers should leave to astrologers the task of seeking the causes of earthly events in the stars."13 This prompted Stephen Jay Gould to fantasize in Discover magazine what might have been written in Osservatore

Romano of June 22, 1663: "Now that Signor Galileo, albeit under slight inducement, has renounced his heretical belief in the earth's motion, perhaps students of physics will return to the practical problems of armaments and navigation, and leave the solution of cosmo-logical problems to those learned in the infallible sacred texts."14

The attempt to debunk the Alvarez theory was not the first time the New York Times recommended that scientists come to their senses and follow its advice. In a 1903 editorial, the paper advised aviation pioneer Samuel Pierpont Langley "not to put his substantial greatness as a scientist in further peril by continuing to waste his time and the money involved in further airship experiments. Life is too short, and he is capable of services to humanity incomparably greater than can be expected to result from trying to fly."13 How fortunate that neither Langley nor the Alvarezes paid any attention.

When an author feels compelled to exorcise predecessors dead for over 100 years, when the paper that publishes "All the News That's Fit to Print" feels entitled to weigh in on its editorial pages, when theories are judged not on whether they meet scientific tests but on whether they are required or satisfying, it is obvious that the suggestion that earthly events have extraterrestrial causes leads otherwise sober-minded folks to give sway to their emotions. But why does an appeal to factors outside the earth produce such a negative reaction? Geologists, at least, have a reasonable answer: Starting with their first course in the subject, they have been taught that the earth simply does not change in response to sudden catastrophes. This notion of geology by catastrophe was disproven a century and a half ago; to resurrect it in the late twentieth century would be to return the science to its prescientific days.

No POWERS NOT NATURAL TO THE GLOBE

The key concept underpinning the geologists' view that slow change can accomplish everything is the vastness of geologic time. At some point in the not-so-distant future, ',000 years or '0,000 years from now, when fossil fuels and valuable ore deposits are gone, the permanent contribution of geology surely will be the concept of the limitless extent of geologic time—what John McPhee aptly calls "deep time." This is a major intellectual contribution equivalent to that of astronomy: the realization that the earth is not the center of anything, rather it is an inconspicuous planet revolving around one star among billions of stars, in one galaxy among billions of galaxies. But although we can observe other planets, stars, and galaxies, a human lifetime is so short that deep time surpasses understanding. We can fathom a few hundred years, even a few thousand, but we cannot comprehend the passage of millions and billions of years. A metaphor that well captures the different character of geologic time, again from McPhee, uses the old English yard, the distance from the King's nose to the tip of his extended finger, as the equivalent of geologic time. Apply to the regal digit one light stroke of a nail file, and the equivalent of human history disappears. Comprehension of geologic time must be accorded its rightful place as one of the great achievements of human induction. Its importance to an analysis of meteorite impact theory is that with time enough—with "time out of mind"— earth history can be fully explained with no need to appeal to catastrophes. To do so is to betray the key success of geology: recognition that within the vast length of geologic time, everything could be accomplished.

Because it is so foreign to our human time scale, the concept of deep time not surprisingly took several centuries to develop. Leonardo da Vinci was among the first to realize that the fossil shells found high in the mountains were the remains of animals that had once lived deep in the sea. Had he not been such a great painter, we would likely remember Leonardo as the outstanding scientist of his day. Another great advance came in the middle of the seventeenth century, when a Dane named Nicolaus Steno compared fossils encased in rock with the shark's teeth that sailors had brought him for study. He could see that the two were identical and reasoned that the teeth had somehow become enclosed in the rock; the teeth had existed before the rock had fully formed, therefore the teeth were older. This point seems elementary now but in its day was revolutionary, for it led Steno to realize that some materials of the earth were older than others and therefore that the earth, like a person, has a history that can be interpreted and understood.

Until two centuries ago, science was required to be consistent with the Book of Genesis. In the seventeenth century, James Ussher, archbishop of Armagle, working backward from the beginning of that book, allowing due time for the events described, calculated that the earth had been formed about 6,000 years before. Since all of the earth's history had to be fitted into such a short period, in this view, geologic processes must be rapid and catastrophe must be the rule. (Creationists today argue that the earth can be no more than 10,000 years old, even though the Sumerians were so advanced as to have a written language 6,000 years ago.)

But by the 1780s some had come to find catastrophism untenable, for it did not agree with the slow, inexorable processes of ero sion and deposition that they observed and analyzed. One such person was Scotsman James Hutton. A devout man, he believed that God had created the earth for the express benefit of mankind, and, since he could see the earth wearing away, became convinced that some process must restore it. Otherwise the continents would steadily erode into broad, uninhabitable plains—surely not what God had intended. Hutton sought and found evidence that sediments worn from the continents and deposited in the sea are subsequently hardened, heated, uplifted, and returned to the continents to start the process all over again. He viewed earth history as a series of endless cycles of decay and rejuvenation, with, in his most famous phrase, "no vestige of a beginning—no prospect of an end." His cycles required vastly longer periods of time than allowed by a strict interpretation of the Bible; indeed, they implied an "abyss of time."

It is curious that Hutton has wound up as the "founder of geology," for he started with theology rather than with the rocks, drew conclusions first and then sought evidence for those conclusions, and propounded a theory of endless cycles that is at best a vast oversimplification. Today we would hardly regard these as the mark of a great scientist. But Hutton has established his place in the pantheon of geology not for these reasons but because he enunciated a principle that was to become central to geologic thought and practice: "Not only are no powers to be employed that are not natural to the globe, no action to be admitted of except those of which we know the principle, and no extraordinary events to be alleged in order to explain a common appearance . . . we are not to make nature act in violation to that order which we actually observe . . . chaos and confusion are not to be introduced into the order of nature, because certain things appear to our partial views as being in some disorder. Nor are we to proceed in feigning causes, when those seem insufficient which occur in our experience.""

In other words, in explaining the earth, we are to call upon only those processes that we observe. Given time enough, they will do the job. This principle was to become the core concept of geology. Hutton summed up its central premise, in a phrase learned, if not thoroughly comprehended, by every beginning student of geology since: "The present is the key to the past." Like most slogans, this one has a deceptively simple appeal. A moment's thought reveals that since the time scale of human history is so short compared to deep time, important processes that act only rarely could have occurred long ago, but never since, so that there has been no chance for us to observe them. To the extent that they have not been seen, the present is not the only, and certainly not the complete, key to the past. How was this seemingly obvious point ignored? Largely because of the influence of Charles Lyell.

Born in 1797, the year that Hutton died, Lyell, through his Principles of Geology, became the most influential geological writer of all time.17 He was a lawyer who knew how to frame an argument, and his treatise, presented as a textbook, was in fact a "passionate brief for a single, well-formed argument, hammered home relentlessly," as Stephen Jay Gould has described it.18 Lyell believed with Hutton that God designed the earth for human beings, but that once He set the earth on its path, He never again intervened in its workings. Natural laws were invariant. The processes that we observe today, and only those processes, have been in operation since the beginning. Natural law and process are constant. In Lyell's philosophy there were no more things in heaven than there were on the earth; he needed no "help from a comet" to explain earthly processes."

Lyell also believed that the rate at which geologic processes acted was constant. He wrote, "If in any part of the globe the energy of a cause appears to have decreased, it is always probable that the diminution of intensity in its action is merely local, and that its force is unimpaired, when the whole globe is considered."20 He meant that violent upheavals in one part of the earth are offset and averaged out by quiescence elsewhere, leaving the overall rate of change over time the same. Drastic change therefore, like all politics, is local.

Given Lyell's belief that neither natural law, the kinds of processes that affect the earth, nor their rate, ever change, it is not surprising to find that he also believed that the earth has always looked as it does now, its history revealing no evidence of directional change. The pterodactyl is gone, true, but when climatic conditions are again favorable, it may return to "flit again through the umbrageous groves of treeferns."21 The earth always remains in the same state, neither progressing nor deteriorating.

We can divide Lyell's thesis into the constancy of law and process, and the constancy of rate and state. In what Gould has called "the greatest trick of rhetoric ... in the entire history of science,Lyell gave them all the same name—uniformity—thus obscuring the fundamental difference between them for well over a century. William Whewell, who reviewed the second volume of Lyell's book, lumped his two meanings together under the unwieldy term uniformitarian-ism. (He also coined the word scientist.) Whewell asked whether "the changes which lead us from one geological state to another have been, on a long average, uniform in their intensity, or have they consisted of epochs of paroxysmal and catastrophic action, interposed between periods of comparative tranquillity?"23 He predicted that the question "will probably for some time divide the geological world into two sects, which may perhaps be designated as the Uniformitarians and the Catastrophists."24 He was both wrong and right. The biblical cata-strophists of Lyell's day were clearly in the wrong and disappeared more quickly than Whewell predicted, but they have been replaced by today's neocatastrophists, the pro-impactors.

In order to understand how the earth works, and how geologists practice their science, the two types of uniformity have to be disentangled. The constancy of law and process, which Gould has called methodological uniformitarianism, describes not how the earth works, but how geologists ought to work. In common with other scientists, geologists reject supernatural explanations and employ known, simple processes before they turn to unknown, complicated ones. For example, today we can see streams eroding and depositing; it is only logical to assume that they have been doing so ever since liquid water appeared on the surface of the earth, and that many sedimentary deposits were formed by stream action.

Of course, this is not only the way that all science ought to work and does. It is nothing more than common sense, well expressed by William of Ockham in the fourteenth century: "One should not assume the existence of more things than are logically necessary." Throughout the history of science up to the space age, meteorite impact was simply a vague idea with very little to support it. Thus to endorse it, based on the knowledge available, was to violate Ockham's razor, as it has come to be called. But today just the opposite is the case. As we will see, scientists today are logically required to acknowledge that impact has happened numerous times.

All scientists reason from cases in which they can examine cause and effect to those in which only effect is evident. This is especially true in geology, where practically everything took place before we arrived. But nothing is special about methodological uniformity; it says only that geology is a science.

The classic example of the success of methodological unifor-mitarianism was the work of Swiss geologist and naturalist Louis Agassiz, who noted that modern glaciers high in the Alps could be seen to gouge rocks from their beds and to carry the dislodged pieces along, sometimes moving boulders as big as a carriage. When these glaciers melted back, the rocks over which they had passed were left polished and grooved; as they receded from their points of farthest advance, they were seen to leave behind ridges of rock debris. Agassiz then proceeded to find all of these features and more down the Alpine valleys, far below the snouts of present glaciers. He reasoned that glaciers must once have extended over a much greater range than they do today and proposed that there had once been a great ice age. This was not so hard to imagine when looking up a Swiss valley, but other scientists, finding glacial deposits far below the furthest extent of present ice sheets, extended the reasoning to conclude that huge ice sheets as much as a mile thick had advanced over much of the Northern Hemisphere.

Lyell presented the other type of uniformity—of rate and state, which Gould has called substantive uniformitarianism—as an a priori description of the way the earth works: Over the long span of earth history there has been no directional change, no progression. But substantive uniformity was tested and falsified in Lyell's own century, when it was learned that glaciers of vast size had advanced over the continents, that the seas at times had risen to drown the land and at other times had dried up, that mountain ranges had risen and been eroded away. Clearly, processes have operated at different rates and the earth has changed. The coup de grace to substantive uniformitarianism was the obvious progression shown by the fossil record, leading from one-celled bacteria in Precambrian rocks to modern Homo sapiens. But Lyell accepted evolution only in the 1866 edition of his Principles, and only then, Gould believes, because "it permitted him to preserve all other meanings of uniformity."25 Since he believed the rate of biological change always to be the same, Lyell was forced to conclude that the vast difference between the creatures that lived in the Cretaceous and those that lived in the Tertiary implied that the missing interval between them, which today we call the K-T boundary, represented as much time as all that has passed since. Today we know that time to amount to 65 million years, and the K-T boundary clay to represent to only a few thousand years at most.

To sum up, one type of uniformitarianism amounts to the statement that geology is a science; the second, which requires the adoption and maintenance of an a priori position regardless of the evidence, amounts to the statement that geology is not a science. Both cannot be true. But how then are we to account for the persistence of both in geological thought for nearly two centuries?

• The two types were so inextricably entangled that few students of geology ever realized that they were accepting "two-for-one." Since methodological uniformitarianism worked, the substantive variety tended to be accepted without anyone realizing that a fast one had been pulled.

• All catastrophism became equated with biblical catastrophism, to geologists an outmoded and shunned belief espoused only by scientific heretics.

• We are always attracted by a hero, and Lyell's writings had turned Hutton into the founder of geology. By espousing uniformitarianism, one stood tall beside the founding fathers.

• As uniformitarianism became dogma, it was re-espoused in each new geology textbook from Lyell to the present. Each generation of geologists learned uniformitarianism at its parent's knee, so to speak. Uniformitarianism had been around for so long that it never occurred to anyone to question it. [An exception who made public his doubts in his first scientific paper, written at age 25, is Stephen Jay Gould.26]

• To call upon catastrophe to solve difficult problems diminishes the skills of generations of intelligent, hardworking geologists. It is too easy—a cop-out.

• Finally, only after World War II did the most dramatic evidence opposing uniformitarianism—the scarred and magnetized seafloor, which supported the notion of drifting continents (or moving plates], and the record of impact on other bodies in the solar system—become known.

We can now understand how the Alvarez theory ran into trouble on two grounds. First, it was catastrophic and contradicted the venerable doctrine of uniformitarianism. Second (and worse], it appealed to an extraterrestrial process, seeming to belittle the hard-won scientific achievements of generations of earthward-directed geologists. Add to these two reasons the natural resistance met by new theories and we have gone a long way toward understanding why geologists were far from delighted with the new Alvarez theory.

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