Uniformitarianism and Unity

William Bowie settled on a theoretical position that accounted for isostasy and the jigsaw-puzzle fit of the continents but ignored the facts of historical geology. And yet, as we have seen, he interacted and corresponded with historical geologists, particularly with Charles Schuchert (1858–1942). Of all the American geologists who ultimately rejected the theory of drift, Schuchert was perhaps the one who engaged the problem the most seriously. As America’s foremost historical geologist, Schuchert was well placed to argue the case for or against drift, and he grappled with the question of continental connections for at least fifteen years. In the end, however, Schuchert, like Bowie, rejected continental drift. Just as Bowie argued against drift because of beliefs grounded in the exigencies of geodetic practice, Schuchert ultimately argued against drift because of beliefs grounded in the exigencies of geological practice. For Bowie, the practice was Pratt isostasy, for Schuchert, it was Uniformitarianism. Charles Schuchert rejected continental drift because he interpreted it to be incompatible with Uniformitarianism. However, he did not reject it because he could not see drift taking place, as might be supposed. Uniformitarianism has meant many things to many people, and to Charles Schuchert in the late 1920s, it meant—rightly or wrongly—an essentially steady-state earth, whose details were forever changing but whose large-scale patterns and relationships remained the same. And this seemed to him to deny the possibility of major changes in the configuration of the continents. Moreover—and perhaps more importantly—for Schuchert, as for most historical geologists, Uniformitarianism was a form of scientific practice, a means of doing historical geology. It was, in fact, the primary means of doing historical geology. For Schuchert, abandoning Uniformitarianism was nearly tantamount to abandoning historical geology altogether. Not surprisingly, he declined to do this. Like William Bowie, Charles Schuchert settled on a theoretical position that preserved his scientific practice. But whereas Bowie’s theoretical ideas had little staying power, the alternative that Schuchert embraced influenced a generation of geologists to believe that drift was not so much impossible as unnecessary.

The Collapse of Thermal Contraction

In 1901, Karl Zittel, president of the Bavarian Royal Academy of Sciences, declared that “Suess has secured almost general recognition for the contraction theory” of mountain-building. This was wishful thinking. Suess’s Das Antlitz der Erde was indeed an influential work, but by the time Suess finished the final volume (1904), the thermal contraction theory was under serious attack. Problems were evident from three different but equally important quarters. The most obvious problem for contraction theory arose from field studies of mountains themselves. As early as the 1840s, it had been recognized that the Swiss Alps contained large slabs of rock that appeared to have been transported laterally over enormous distances. These slabs consisted of nearly flat-lying rocks that might be construed as undisplaced, except that they lay on top of younger rocks. In the late nineteenth century, several prominent geologists, most notably Albert Heim (1849 –1937), undertook extensive field work in the Alps to attempt to resolve their structure. Heim’s detailed field work, beautiful maps, and elegant prose convinced geological colleagues that the Alpine strata had been displaced horizontally over enormous distances. In some cases, the rocks had been accordioned so tightly that layers that previously extended horizontally for hundreds of kilometers were now reduced to distances of a few kilometers. But in even more startling cases, the rocks were scarcely folded at all, as if huge slabs of rocks had been simply lifted up from one area of the crust and laid down in another. Heim interpreted the slabs of displaced rock in his own Glarus district as a huge double fold with missing lower limbs, but in 1884 the French geologist Marcel Bertrand (1847–1907) argued that these displacements were not folds but faults. Large segments of the Alps were the result of huge faults that had thrust strata from south to north, over and on top of younger rocks. August Rothpletz (1853–1918), an Austrian geologist, realized that the Alpine thrust faults were similar to those that had been earlier described by the Rogers brothers in the Appalachians. By the late 1880s, thrust faults had been mapped in detail in North America, Scotland, and Scandinavia.

The “Short Step Backward”

In 1922, Harry Fielding Reid, a founder of American seismology, encapsulated the American perspective on scientific method in a review of Wegener’s Origin of Continents and Oceans. “There have been many attempts to deduce the characteristics of the Earth from a hypothesis,” he wrote, “but they have all failed. There is the pentagonal system of Elie de Beaumont, the tetrahedral system of Green . . . [continental drift] is another of the same type.” The history of hypothetico-deductive reasoning in geology was a history of failure; progress was to be made another way. “Science has developed,” Reid concluded, “by the painstaking comparison of observations and, through close induction, by taking one short step backward to their cause; not by first guessing at the cause and then deducing the phenomena.” An obvious reading of Reid’s comment is that Wegener’s faulty methodology led him to faulty conclusions. No doubt Reid thought so. But another reading is the implicit suggestion that a different approach —a different presentation —might have elicited a more favorable response. Charles Schuchert’s comment that it was “wrong lor a stranger to the facts to generalize from them to other generalizations” suggested that if evidential support for drift could come from someone who was not a “stranger to the facts,” then Americans might be more disposed to entertain the theory. This thought had occurred a few years earlier to Reginald Daly and Frederick Wright. When Daly and Wright returned from South Africa in the autumn of 1922, each pondered the question of continental drift. Daly, whose proclivities ran to theory, developed a mechanical account of drift; Wright, an experimentalist, began to think about a possible test. The key empirical evidence was the alleged similarities between the Karroo formations in South Africa and age-equivalent rocks elsewhere in the world — evidence that had earlier motivated Suess’s idea of Gondwanaland and now supported Wegener’s theory of drift. But how similar were these rocks, really? Suess and Wegener had based their ideas on compilations of published literature; neither man had studied any of these rocks in person. In fact, there had never been a direct comparative study of the so-called Gondwana beds.

To Reconcile Historical Geology with Isostasy: Continental Drift

Alfred Wegener (1880–1930) first presented his theory of continental displacement in 1912, at a meeting of the Geological Association of Frankfurt. In a paper entitled “The geophysical basis of the evolution of the large-scale features of the earth’s crust (continents and oceans),” Wegener proposed that the continents of the earth slowly drift through the ocean basins, from time to time crashing into one another and then breaking apart again. In 1915, he developed this idea into the first edition of his now-famous monograph, Die Entstehung der Kontinente und Ozeane, and a second edition was published in 1920. The work came to the attention of American geologists when a third edition, published in 1922, was translated into English, with a foreword, by John W. Evans, the president of the Geological Society of London and a fellow of the Royal Society, in 1924 asThe Origin of Continents and Oceans. A fourth and final edition appeared in 1929, the year before Wegener died on an expedition across Greenland. In addition to the various editions of his book, Wegener published his ideas in the leading German geological journal, Geologische Rundschau, and he had an abstract read on his behalf in the United States at a conference dedicated to the topic, sponsored by the American Association of Petroleum Geologists, in 1926. The Origin of Continents and Oceans was widely reviewed in English-language journals, including Nature, Science, and the Geological Magazine. Although a number of other geologists had proposed ideas of continental mobility, including the Americans Frank Bursey Taylor, Howard Baker, and W. H. Pickering, Wegener’s treatment was by far the best developed and most extensively researched. Wegener argued that the continents are composed of less dense material than the ocean basins, arid that the density difference between them permitted the continents to float in hydrostatic equilibrium within the denser oceanic substrate. These floatin continents can move through the substrate because it behaves over geological time as a highly viscous fluid. The major geological features of the earth, he suggested — mountain chains, rift valleys, oceanic island arcs—were caused by the horizontal motions and interactions of the continents.

Introduction: The Instability of Scientific Truth

Scientists are interested in truth. They want to know how the world really is, and they want to use that knowledge to do things in the world. In the earth sciences, this has meant developing methods of observation to determine the shape, structure, and history of the earth and designing instruments to measure, record, predict, and interpret the earth’s physical and chemical processes and properties. The resulting knowledge may be used to find mineral deposits, energy resources, or underground water; to delineate areas of earthquake and volcanic hazard; to isolate radioactive and toxic wastes; or to make inferences and predictions about the earth’s past and future climate. The past century has produced a prodigious amount of factual knowledge about the earth, and prodigious demands are now being placed on that knowledge. The history of science demonstrates, however, that the scientific truths of yesterday are often viewed as misconceptions, and, conversely, that ideas rejected in the past may now be considered true. History is littered with the discarded beliefs of yesteryear, and the present is populated by epistemic resurrections. This realization leads to the central problem of the history and philosophy of science: How are we to evaluate contemporary science’s claims to truth given the perishability of past scientific knowledge? This question is of considerable philosophic interest and of practical import as well. If the truths of today are the falsehoods of tomorrow, what does this say about the nature of scientific truth? And if our knowledge is perishable and incomplete, how can we warrant its use in sensitive social and political decision-making? For many, the success of science is its own best defense. From jet flight to the smallpox vaccine, from CD players to desktop scanners, contemporary life is permeated by technology enabled by scientific insight. We benefit daily from the liberating effects of petroleum found with the aid of geological knowledge, microchips manufactured with the aid of physical knowledge, materials synthesized with the aid of chemical knowledge. Our view of life — and death — is conditioned by the results of scientific research and the capabilities of technological control.

The Depersonalization of Geology

Some historians have concluded that plate tectonics caused a change in the standards of the geological community, but the shift in standards of the American scientific community was not so much the result of the development of plate tectonics as it was a larger trend that helped to cause it. Geologists consciously chose to move their discipline away from observational field studies and an inductive epistemic stance toward instrumental and laboratory measurements and a more deductive stance. This shift helps to explain why geologists felt compelled to attend to the demands of geodesists even at the expense of their own data: it was the geodesists’ data, rather than their own, that seemed to be in the vanguard of their science. Geologists at the start of the twentieth century had high hopes for their discipline, and they were not disappointed. The Carnegie Institution’s Geophysical Laboratory became one of the world’s leading locales for laboratory investigations of geological processes, and work done there inspired scientists at other American institutions. At Harvard, for example, Reginald Daly joined forces with Percy Bridgman to raise funds for a high pressure laboratory to determine the physical properties of rocks under conditions prevailing deep within the earth. The application of physics and chemistry to the earth was also advanced at the Carnegie’s Department of Terrestrial Magnetism, where scientists pursued geomagnetism, isotopic dating, and explosion seismology.’ By mid-century, the origins of igneous and metamorphic rocks had been explained, the age of the earth accurately determined, the behavior of rocks under pressure elucidated, and the nature of isostatic compensation resolved, largely through the application of instrumental and laboratory methods. Similar advances occurred in geophysics and oceanography. The work that Bowie and Field instigated in cooperation with the U.S. Navy, and that scientists at places like Wood’s Hole and the Scripps Institution of Oceanography greatly furthered, had grown by the 1950s into a fully fledged science of marine geophysics and oceanography with abundant financial and logistical backing. This work —in gravity, magnetics, bathymetry, acoustics, seismology— relied on instrumentation, much of it borrowed from physics.

An Evidentiary and Epistemic Shift

World War II dashed the hopes and dreams of many, among them Richard Field. As President of the American Geophysical Union, Field was to host the International Geological Congress when it met in Washington, D.C. at the end of the summer of 1939, an event Field was greatly anticipating. Scientists were expected from around the globe, and the permanence —or nonpermanence —of ocean basins was high on Field’s list of topics for discussion. But as the meeting was about to begin, Adolf Hitler’s armies invaded Poland; many delegates turned around mid-voyage, and others who had already arrived in Washington quickly returned home. The following year William Bowie died; two years later Charles Schuchert died at the age of eighty-four, and Field was involved in a near-fatal car crash which effectively ended his scientific career. Bowie’s and Field’s scientific goals would be realized, however, albeit not by them. Together, they had assembled an advisory committee for gravity studies that included five subsections —navigation, geophysics, tectonics, oceanology [sic], and marine microbiology—with prominent members from acaclemia, and industry in the United States and abroad and from the U.S. Navy. Their ambitions went beyond gravity: Bowie and Field hoped to foster a global science of geophysics and oceanography to explore the three-fifths of the earth that scientists had scarcely visited by enlisting the material and financial support of the U.S. Navy and other navies. The U.S. Navy, for its part, was making plans to enlist geophysicists, both figuratively and literally. When war broke out, Harry Hess joined the naval reserve, as did many other young and aspiring geophysicists and oceanographers. In 1941, Hess was called to active duty and became the captain of an assault transport, the USS Cape Johnson. Among the ship’s tasks was the echo sounding of the Pacific basin; Hess subsequently became famous for the discovery of flat-topped sea-mounts, which he named guyots after the first professor of geology at Princeton, Arnold Guyot. I less published this discovery, but much geophysical work done during the war, and after, was classified.

Direct and Indirect Evidence

At the California Institute of Technology in the mid-1940s, a young Henry William Menard—later an expert on submarine physiography and director of the U.S. Geological Survey—learned about continental drift from Beno Gutenberg. For although most American earth scientists considered the question of drift settled, many Europeans did not. Among them was “Dr. G,” famous for his pioneering work on microseisms (the continual seismic disturbances that form the background “noise” of seismographs) and deep-focus earthquakes, who had come to Caltech from Germany in 1930. In 1939, he edited Internal Constitution of the Earth, part of a series entitled Physics of the Earth sponsored by the National Research Council. Gutenberg’s chapter, “Hypotheses on the Development of the Earth’s Crust and their Implications,” focused on the evidence for a plastic crustal substrate and “currents” within it. More than just an idea, he argued, subcrustal currents were necessary—in the past and at present — to account for both isostasy and horizontal crustal dislocations: “Many writers have expressed the belief that the strength of the interior of the earth prevents any currents today. The results of geophysical research, however, leave no doubt that such currents still exist. . . . [either] as a consequence of changes produced by disturbances at the surface [or as ] the primary cause of movement at the surface.” Gutenberg’s course at Caltech reflected these views. The strength of the crust was “enough to support [the] highest mountains,” he explained in class, but isostasy demonstrated that this strength “decreases downwards, and below 40 km or so plastic flow may occur.” This flow was implicated in both geological and seismological processes. Among the forces causing earthquakes, for example, Gutenberg suggested “elastic rebound as a release of shear due to sub-crustal flow and contraction of the crust & possibly differential movements in the crust from continental drift.” He noted that the energy release associated with earthquakes was “of the same order of magnitude as that due to temperature gradient,” which suggested that the most likely cause of plas tic flow was internal temperature differentials. One preexamination review sheet asked students for the meaning of isostasy and of “Wegener’s hypothesis.”

From Fact to Theory

If continental drift was not rejected for lack of a mechanism, why was it rejected? Some say the time was not ripe. Historical evidence suggests the reverse. The retreat of the thermal contraction theory in the face of radioactive heat generation, the conflict between isostasy and land bridges, and the controversy that Wegener’s theory provoked all show that the time was ripe for a new theory. In 1921, Reginald Daly complained to Walter Lambert about the “bankruptcy in decent theories of mountain-building.” Chester Longwell opined in 1926 that the “displacement hypothesis, in its general form . . . promises a solution of certain troublesome enigmas.” A year later, William Bowie suggested in a letter to Charles Schuchert that it was time for “a long talk on some of the major problems of the earth’s structure and the processes which have caused surface change. The time is ripe for an attack on these larger phases of geology.” One possibility is that the fault lay with Wegener himself, that his deficiencies as a scientist discredited his theory. Wegener was in fact abundantly criticized for his lack of objectivity. In a review of The Origin of Continents, British geologist Philip Lake accused him of being “quite devoid of critical faculty.” No doubt Wegener sometimes expressed himself incautiously. But emphatic language characterized both sides of the drift debate, as well as later discussions of plate tectonics. The strength of the arguments was more an effect than a cause of what was at stake. Some have blamed Wegener’s training, disciplinary affiliations, or nationality for the rejection of his theory, but these arguments lack credibility. Wegener’s contributions to meteorology and geophysics were widely recognized; his death in 1930 prompted a full-page obituary in Nature, which recounted his pioneering contributions to meteorology and mourned his passing as “a great loss to geophysical science.” Being a disciplinary outsider can be an advantage — it probably was for Arthur Holmes when he first embarked on the radiometric time scale. To be sure, there were nation alistic tensions in international science in the early 1920s— German earth scientists complained bitterly over their exclusion from international geodetic and geophysical commissions— but by the late 1920s the theory of continental drift was associated as much with Joly and Holmes as it was with Wegener.

Drift Mechanisms in the 1920s

The final chapter of the third edition of The Origin of Continents and Oceans was devoted to the dynamic causes of drift, and Wegener’s tone in these final fifteen pages was decidedly more tentative than in the rest. Frankly acknowledging the huge uncertainties surrounding this issue, he proceeded on the basis of a phenomenological argument. Mountains, Wegener pointed out, are not randomly distributed: they are concentrated on the western and equatorial margins of continents. The Andes and Rockies, for example, trace the western margins of North and South America; the Alps and the Himalayas follow a latitudinal trend on their equatorial sides of Europe and Asia. If mountains are the result of compression on the leading edges of drifting continents, then the overall direction of continental drift must be westward and equatorial. Continental displacements are not random, as the English word drift might imply, but coherent. This coherence had been the inspiration for an earlier version of drift proposed by the American geologist Frank Bursley Taylor (1860–1938). A geologist in the Glacial Division of the U.S. Geological Survey under T. C. Chamberlin, Taylor was primaril known for his work on the Pleistocene geology of the Great Lakes region. But his knowledge extended beyond regional studies: as a special student at Harvard, he had studied geology and astronomy; as a survey geologist under the influence of Chamberlin and G. K. Gilbert, he had published a number of articles on theoretical problems. One of these was an 1898 pamphlet outlining a theory of the origin of the moon by planetary capture; in 1903, Taylor developed his theoretical ideas more fully in a privately published book. Turning the Darwin–Fisher fissiparturition hypothesis on its head, Taylor proposed that the moon had not come from the earth but had been captured by it after the close approach of a cornet. Once caught, (lie tidal effect of the moon increased the speed of the earth’s rotation and pulled the continents away from the poles toward the equator. In 1910, Taylor pursued the geological implications of this idea in an article in the Bulletin of the Geological Society of America entitled “Bearing of the Tertiary Mountain Belt on the Origin of the Earth’s Plan.”