One major shift, immediately evident, is Wegener’s decision to move the geodetic argument—the actual measurement of displacement—closer to the front of the book, just after the historical introduction and the summary chapter entitled “Character of the Displacement Theory, and Its Relationship to Previously Dominant Ideas Concerning Alterations of Earth’s Surface in Geologic Time.”24 In the first edition (1915) and also in the second edition (1920) the measurement data and claims for continental displacement had been given in the last chapter. In the third edition (1922) they were in the final chapter of “presentation of the evidence” (Beweisführung), just before the extensive and more speculative “elucidations.”
Wegener wanted very much to claim that continental displacement had finally been measured, and therefore he began by saying, “We begin the presentation of the evidence with the proof [Nachweis] of contemporary continental displacement through repeated astronomical position-finding, on the grounds that recently (by this method) the first real proof of the contemporary displacement of Greenland—predicted by the displacement theory—has been furnished, and because this sort of quantitative corroboration will probably be considered, by the majority of scientists, to be the most reliable and exact evidence in favor of the theory.”25
This initial paragraph somehow captures the flavor of the entire book. The long, convoluted sentence, the shift back and forth between the notion of proof and that of simple evidentiary corroboration, and the hopeful sense that a recent result would tip the balance in favor of measurement of drift all reflect the problems that Wegener was facing. Reviewing the geodetic argument, we can see that Wegener’s problem was one that always seems to bedevil scientists looking for “long geophysical time-series.” Whatever technique was being employed (whether in 1823 or in 1927) always seemed to seek its answers just at the instrument’s limit of resolution. The reduction of systemic and personal errors always seemed to fall short of the desired quantity: the probable or possible error in every case exceeded the quantity being measured.
These geodetic measurements in Greenland involved the methods of lunar distances, lunar culminations, and stellar transits. They were recorded in successive decades by chronometer, telegraphic time signal, and finally radio signal, with constantly evolving zenith telescopes and transits. The stations from which they were made were imperfectly mapped and, in the case of the 1871 Germania Expedition, not known accurately within a kilometer. While the instruments employed in the course of a century of observations were increasingly sophisticated, only the 1927 measurements—made with a radio time signal and a zenith telescope with an impersonal micrometer that could measure the culmination independent of the observer’s reaction to it—would even begin to meet a reasonable level of accuracy by the standards of the 1920s.
Wegener continued to press the question of measurement because he had always thought that it was the only true means to establish continental displacements as a fact rather than a hypothesis. Moreover, from the time of his graduate student years and his PhD dissertation, the painstaking “stitching together” of measurement protocols with different error margins, different measuring techniques, and different systems of reduction had been his stock-in-trade; in short, he believed completely in the ability of scientists to calibrate and make accurate observations with precision instruments.
In the next chapter, “Geophysical Arguments,” a different series of problems appears. The original geophysical premise was that the continents were made of lighter material than the ocean floor in which they floated, and that they moved through the ocean floor like icebergs through water. The principle of isostasy governed the flotation, and the difference between short-term behavior of the subcrust (rigidity) and its long-term behavior (fluidity, viscosity) allowed the continents and oceans to react by fracture to short-term stresses and by displacement and flow to long-term stresses.
Now Wegener had to confront the complications and uncertainty in the analysis of the behavior of the crust and mantle (the Sial and Sima) brought forward by seismology, gravity, and magnetic surveys far more sophisticated than those available early in the century. While continuing to make the same argument about continental flotation which he had made in the first edition, he was forced to admit that the ocean floors perhaps had a partial layer of Sial—whatever that was—on them, since no one knew any longer precisely what rocks (not just densities but actual mineral content) dominated the composition of the “Sial” and the “Sima,” nor was there any possibility of a consensus on the viscosity of Earth at any depth.26 It is not that anything fundamentally disconfirmed his ideas, but that the forward march of geophysics, geochemistry, seismology, gravimetry, and solid mechanics had deprived his theory of the certainty attendant on its initial simplicity. On every topic there was now a range of opinions rather than a “yes” or a “no.” Wegener was resolute in presenting these complexities and uncertainties, while maintaining that the general principles involved still pointed toward displacement and away from permanence.27
For instance, Wegener was able to argue that it was a matter of little consequence that the coefficient of plasticity or “stiffness” of Earth was that of steel, with a coefficient of 1011 in the mantle and 1012 in the core, since this stiffness, detected through seismology, is a measure of instantaneous response. On the other hand, the coefficient of viscosity—the parameter that would govern the ability of Earth to flow on long timescales, was estimated to be anywhere from 1013 to 1021—a variation from scientist to scientist across nine orders of magnitude. Wegener was (as was his habit) simple and direct: “Under forces applied over geological time scales, the earth must behave as a fluid; for example, this is shown by the fact that its oblateness corresponds exactly to its period of rotation. But the critical point in time where elastic deformations merge into flow phenomena depends precisely on the viscosity coefficient.”28 In 1928 no one had empirical knowledge of what this was, either for Earth as a whole or for any particular layer of Earth.
There is a complex and highly interesting history attached to this debate, and one hopes that someday someone will write it. Suffice it to say that in the 1920s the debate over the states of matter (solid, liquid, gas) and their behavior under high temperature and confining pressure was increasingly framed in terms of the behavior of rocks at great depth. That is, fundamental questions in solid mechanics were being debated based on evidence and speculation concerning the interior of Earth, rather than on laboratory experimentation.
As Adrian Scheidegger has pointed out, of those branches of “continuous displacement theory” which were more or less well developed, it was the theory of “plasticity” that was most pertinent to discussions of displacements observed in Earth’s crust. However, this theory of plasticity described the (observed) behavior of a metal during cold-working, and this is why there is always reference to the elasticity of steel in such discussions. Again, the results do not apply to inhomogeneous materials under high confining pressures many thousands of times greater than those available in a laboratory and acting on timescales millions of times longer than a human life.29
This shifting balance of certainty and uncertainty which Wegener encountered in the various subfields of earth science in 1927–1928 is even more evident in the next chapter, “Geological Arguments.” Faced with geophysical uncertainty in his arguments—where certainty had prevailed before—Wegener was more and more forced to rely on geological, paleontological, and paleoclimatic evidence. Yet it was on the geological evidence that he had been most thoroughly battered by his critics. Most of the favorable geological evidence for displacement was from the Carboniferous, and there was not much of that—especially after his critics had finished picking away at the details; there remained the secure evidence of glacial deposits from a Carboniferous ice cap leaving remains on widely separated Southern Hemisphere continents, as well as the Glossopteris flora, some sediments, and some trend lines of mountain ranges.
In 1928, however, Wegener found hi
mself in possession of a cornucopia of positive, detailed, globally distributed, and powerfully confirmatory geological evidence of exactly the sort of continental connections, and subsequent disruptions, that were the content and basis of his entire displacement theory. Foremost among these were the results of an extended reconnaissance in South America, funded by the Carnegie Institution of Washington and carried out in 1923 by Alexander du Toit (1878–1948).30
Du Toit was one of South Africa’s leading geologists. He had been a member of the South African Geological Survey since 1909 and of its irrigation department since 1920 (the latter post gave him more flexibility in his fieldwork). He went to South America in 1923 specifically to test Wegener’s displacement hypothesis from the standpoint of the field data in South America in comparison to the rocks he had worked on in South Africa for fifteen years. Because his funding sponsors were in the United States, he had to restate his objective in a way that did not mention Wegener directly, but the intention was always there.31 Du Toit spent five months in the field between late June and late November 1923, in Argentina, Chile, Uruguay, and Brazil, everywhere accompanied by leading South American geologists and paleontologists. He covered a 45° arc of latitude in a 10° wide band of longitude along the eastern coast of South America.32
He presented his fieldwork in 1927 as A Geological Comparison of South America with South Africa and tried in the descriptive sections to strike as neutral a tone as possible (once again a requirement of his sponsors in the United States). Nevertheless, the book as a whole amounted to a full endorsement of Wegener’s continental displacements as the “working hypothesis” with the most explanatory power. At the end of du Toit’s synthesis of the stratigraphical and paleontological data he had collected and analyzed along the eastern coast of South America and in parts of the Andes, he wrote,
That these two continents were intimately connected during several geological epochs will, I venture, be acknowledged after the perusal of the preceding pages, though the manner of such union would admittedly be speculative. In preparing this review an attempt was made first of all to write the historical account, irrespective of any hypothesis as to the manner of such union or of the ultimate mode of separation of the land-masses, though it became evident, as the data were assembled, that they pointed very definitely in the direction of the displacement hypothesis, and that they could most satisfactorily be interpreted in the light of that brilliant conception.33
He then reiterated his conviction of the superiority of Wegener’s displacement hypothesis over all other explanations of the similarity between South America and Africa, in a chapter entitled “Bearing on the Displacement Hypothesis.” Du Toit here wrote,
Such points of resemblance have now become so numerous as collectively almost to exceed the bounds of coincidence, while they are, moreover, confined not to one limited region nor to one epoch, but implicate vast territories in the respective land-masses, and embrace times ranging from pre-Devonian almost to the Tertiary. Moreover, these so-called “coincidences” are of a stratigraphical, lithological, paleontological, tectonic, and climatic nature.… If … the two land-masses are pictured as having moved closer together … a great number of observations and deductions are now found to be brought into apparent harmony and these possible “coincidences” are disposed of in the simplest fashion.
This is precisely what the displacement hypothesis effects, thereby providing a simple explanation of many otherwise puzzling observations. The fact that many eminent scientists have cast doubt upon its geophysical possibility should not be permitted to cloud the issue.34
Moving along to his conclusions, du Toit argued that it was of particular importance that his readers recognize the decisiveness of this geological evidence, “for those arguments based on zoo-distribution are incompetent to do so [i.e., decide], being as a rule equally, though more clumsily, explicable under the orthodox views involving lengthy land connections afterward submerged by the oceans.”35 Du Toit, however, concluded,
Such an analysis [of the existing data], it can be affirmed, does not favor the notion of one or more relatively narrow connecting links or “land bridges” lasting down to the early Mesozoic, but on the contrary supports the presumption of some continuous land area embodying those sections of Gondwanaland that are now represented in these two continents. It furthermore strongly favors the admittedly revolutionary idea that geographically these two portions were appreciably closer in the past; indeed, the evidence is, I think of sufficient weight to warrant such a viewpoint being adopted as a working hypothesis at any rate … the displacement hypothesis, if not an actual explanation of the phenomena, would at least seem to contain more than a germ of the truth, despite its revolutionary and heterodox nature and apparent lack of agreement with geophysical considerations.36
Wegener recognized du Toit’s book immediately for what it was: the most influential geological support he had ever received. He quoted from it at length in his “Geological Arguments,” though he remarked, “If we wanted to cite every detail in the book which favors the theory we would have to translate it [the book] from start to finish.”37 Wegener then quoted du Toit’s comment (given above) that all the data in South America pointed—from the beginning of his investigation—toward the displacement hypothesis; Wegener modestly left out du Toit’s characterization of his work as “a brilliant conception.”38
Wegener reproduced du Toit’s map of the reassembly of South America with Africa, fully agreeing with him that a gap of 400–800 kilometers (250–500 miles) between coastlines was needed to accommodate present-day differences. “I am bound,” wrote Wegener, “to agree completely with this point. Not only must there remain room between the two coastlines for the shelves that extend in front of them, but possibly even for the material composing the Mid-Atlantic Ridge [Bodenschwelle].” Wegener concluded this discussion thus: “I must admit that the reading of du Toit’s book made an extraordinary impression on me, as I had scarcely dared to expect a geologically perfect match between these two continents.”39
If this were the only geological support Wegener obtained between 1924 and 1928, it would still have been a stunning endorsement of his original conception and, moreover, would still have validated the very transatlantic correspondences scoffed at by British, German, and North American geologists. But it was not the only such support, for Wegener had by now read and digested Emile Argand’s Tectonique d’Asie. Here again was geological support for his displacement hypothesis from an internationally well-known, if admittedly controversial, supporter. Wegener quoted Argand extensively, giving his principal results for the unified character of the great folds extending from the Alps through the Himalayas to the marginal ranges of East Asia.40 He concluded by quoting Argand as follows: “The elegance with which the displacement theory explains these significant facts, which were not known when the theory was first advanced, is certainly a strong point in its favor. Strictly speaking, none of these facts really proves displacement theory or even the presence of sima [as a medium in which the mountains float] but they all fit in excellently with both ideas to an extent that makes them highly probable.”41
Wegener’s redrawn map from Alexander du Toit’s A Geological Comparison of South America with South Africa (Washington, DC: Carnegie Institution of Washington, 1927). Wegener was quite willing to leave the large “gap” between the continents in du Toit’s resonstruction. From Alfred Wegener, Die Entstehung der Kontinente und Ozeane, 4th (completely revised) ed. (Braunschweig: Friedrich Vieweg & Sohn, 1929).
If there were no comparable supporters for Wegener’s theory of the separation of the North Atlantic continents (North America from Europe) or for the separation of Madagascar and India, he could point nevertheless to an abundant literature on the East Indies, especially the Sunda Archipelago and the Celebes, produced and published by Dutch geologists, with increasing reliance on Wegener’s displacement theory as the most likely explanation for their complex geological features.
From the time of Molengraaf’s work on coral reefs in 1916, Wegener had always had supporters among those whose fieldwork covered the East Indies and Australasia. Van der Gracht, in the published version of his address to the American Association of Petroleum Geologists in New York in 1926, the “symposium” volume that had finally appeared in early 1928 (and that Wegener had seen), remarked that the explanation Wegener was able to give for the Sunda Archipelago’s otherwise bewildering tectonic structure “is the reason why the Dutch geologists (Molengraaf, Brouwer, Wing Easton) who worked in the East Indies, are invariably favorably inclined to Wegener’s hypothesis. I have also visited this area: the evidence is indeed striking.”42 To this roster of Dutch supporters Wegener was able to add G. L. Smit Sibinga (1895–1963), who had concluded that “a working hypothesis based on the fundamental ideas of Taylor and Wegener” was superior to any other mode of approach in explaining the complex geological facts of this region.43
One can imagine the pleasure and satisfaction that Wegener took in this transformation of his theory from an outlandish supposition to a real working hypothesis for geologists. In 1912 this had been the sum of his ambition, and whatever ground he had had to relinquish in 1920, 1922, and 1924 in terms of specific proposals and ideas, he could see, as he compiled (in 1928) the evidence in the various categories from the component sciences, that he was exactly where he had always wanted to be. He had everything except the absolute proof of ongoing displacement in the present, and until that absolute proof appeared, the most that his theory of continental displacements could achieve was the status of a working hypothesis. It was now a working hypothesis in Europe, in Africa, in South America, in Asia, and in Oceania, though still short of the mark in the United States and Canada.
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