Wegener’s drawing of the reconstructed continents of the Permo-Carboniferous period, with the reassembled continents on the left and the Pacific Ocean on the right, illustrating his view (in 1920) of the very early partition of Earth into land and water hemispheres. From Alfred Wegener, Die Entstehung der Kontinente und Ozeane, 2nd (completely revised) ed., Die Wissenschaft (Braunschweig: Friedrich Vieweg & Sohn, 1920).
On this Carboniferous Earth, with the continental land occupying one hemisphere and the Pacific Ocean the other, the “westerly migration” meant that the leading edge of the protocontinent would be the stretch of coastline from the Bering Strait to the southern tip of South America, and the trailing edge the margin extending from the southern tip of South America around Antarctica, Australia, Southeast Asia, and Japan to the Aleutian Islands, reaching the Bering Strait by the opposite great circle. Wegener imagined that to this point in Earth history, the entire Pangäa was capable of rotating to the west, and meridional rifting was in the initial stages.
The principal basis for this reconstruction was a compilation by Theodor Arldt, in his recently published Handbuch der Paläogeographie (1917).103 Arldt had made a compilation of land bridges postulated by twenty different paleogeographers for each geological era, in an attempt to develop a consensus view of when, in the geological past, the present continents had been connected and when they had not. The list includes Arldt himself and (notably) Carl Diener, Franz Koßmat, Melchior Neumayr, and the Americans Charles Schuchert (1858–1942) and Bailey Willis, the latter two both strong “permanentists.” Arldt’s table shows that, of those paleogeographers postulating land bridges in this era, all agreed that there were land bridges across the North Atlantic, across the South Atlantic, between Lemuria and Africa, and connecting all the segments of Gondwanaland.104
To produce a contrasting view, Wegener then jumped forward in time to the Eocene, an early epoch in the Tertiary period, and again drew a map of the continents, using the same tabular data. The map, reproduced here, is in an unusual projection, a transverse pseudocylindrical projection of the Mollweide equal-area map that had recently been developed to minimize distortion close to the poles. The projection is appropriate to the task but difficult for the unpracticed eye to interpret. Wegener used this map to show the geological continuities of mountain ranges, including the Tertiary, between North America and Europe and between South America and Africa. It is notable that even in the Eocene, which Wegener imagined to have begun fifteen million years ago (the current figure is fifty-six million years), the Atlantic split has barely begun and existed only as a shallow sea in its northern half, while Africa and South America were still connected directly.105
Wegener’s reconstruction of the continents in the Eocene (Quaternary) in an unusual projection, a transverse pseudocylindrical projection of the Mollweide equal-area map that had recently been developed to minimize distortion close to the poles. The projection is appropriate to the task, but difficult for the unpracticed eye to interpret. From Wegener, Die Entstehung der Kontinente und Ozeane (1920).
The next twenty or so pages in the book are taken up by a history of the splitting and drifting of the several portions of the original Pangäa, and they are so dense and so detailed that they must have baffled all but the most expert geographers. Wegener divided his argument into three sections: “The Atlantic Rift,” “Lemuria,” and “Gondwana-land.” The bulk of his argument, fifteen of the twenty pages, he devoted to the Atlantic, at once the most contested of the splits and the one for which he had the most evidence. There is a good deal of arguing here with unseen adversaries (above all, Diener), only occasionally mentioned by name. To read this material successfully, one needs access to a globe, an atlas, a gazetteer of place-names, a paleontological textbook, a geological text or at least some geological maps, and considerable time and patience. There is no doubt of his mastery of the argument, but it is an uneven mix of geological and paleontological data, geophysical theory, and speculative reconstruction which makes few concessions to the audience. The juxtaposition of solid geological data with speculative morphological reconstructions—especially of the complex rotations of various parts of Gondwanaland (Australasia, New Guinea, and New Zealand)—is jarring, unless one decides that Wegener is only trying to sketch what might be true, and on this point he is not always clear.
There are other problems as well. The section on Lemuria, the part of his argument about India’s northern movement and compression to form the Himalayas, is treated in a few pages, very speculatively, with but a single footnote. This is very odd, given the detailed critique of this part of his hypothesis by Andrée and Diener and others, critiques he had noted in his interleaved volume.106 The section on Gondwanaland is somewhat more detailed, but Wegener admits that the details of his reconstructions, apart from the connections among South America, Africa, and India around a conjectural Carboniferous South Pole, are almost purely hypothetical.
Wegener was never very comfortable with geological data and argumentation. He was a physicist, and for him geological argument was too cluttered by individual details. This speaks to a point deeper than his own discomfort: the appalling difficulty that geologists face in creating general theories for Earth out of geological argumentation. There is almost no economical way to present a global geological theory based on geological data. It is not that it cannot be done, but that the amount of detail required can be suffocating to the imagination. One can try to economize the empirical base, but beyond a certain threshold of generality, geologists are no longer interested and become radically more difficult to convince. It took the brilliance of someone like Sueß to reduce thousands of individual monographs into a few volumes of synthesis, and this took him more than twenty years. Wegener was overmatched by this particular task, and he had neither the time nor the will to pursue it.
Thus, when he turned to the question of the migration of the pole of rotation of Earth, there was a palpable sense of relief: not only was he back in the world of geophysics, but he was faced with the kind of problem that he knew best and most enjoyed: “As for the question of the positions of the pole in earlier geological periods, there is currently a mess [Verwirrung] that, even if it is historically comprehensible, no longer appears to be necessary, given the overall state of our knowledge.”107
Wegener was never happier than when he imagined he was cleaning up a mess in the published literature. When he worked with geological literature, he began with a mess but generally ended with a slightly improved mess. With geophysics, as in the case of displacements of the pole of rotation, the timeline was generally short, the number of players few, the principles reasonably clear, and authorities easier to come by. Not only that, but in physics there is an expectation that there will be an interplay between the theoretical principles and the collection of data, as well as a series of iterations of the theory showing the impact of these new data as they are collected.
Wegener was beginning to see how this worked, and he said so in his introduction to the section on displacements of Earth’s pole of rotation. The data we have, he wrote, are not very good for the Northern Hemisphere or for the Southern Hemisphere. The pole positions we establish in arguments like these are not final and are open to correction. No one is asking for mercy for being wrong, he said; what one wants instead is not to have, as geologists have repeatedly tried to do, a “death sentence” proclaimed on all attempts to map the movement of Earth’s pole, simply because the data aren’t as good as we would wish.108
He was also beginning to understand that men like himself appeared odd to working scientists remaining faithfully and clearly within the designated boundaries of their disciplines. When it came to geophysics, he was in excellent company discussing displacements of Earth’s poles: Laplace, Euler, Lord Kelvin, and Giovanni Schiaparelli (1845–1910). However, in correlating these displacements with the history of life, he was in decidedly less august company. His predecessors here were Paul Reibisch (1867–1934), an engineer w
ith an avocational interest in mollusks; Heinrich Simroth (1851–1917), a biological systematist working on slugs and snails; and Damian Kreichgauer (1859–1940), a physicist turned monk teaching general science in a monastery in the Netherlands.
When we begin to examine the details of Wegener’s chapter on Polwanderungen (migrations of the pole), we see a simple explanation of a geophysical process tied to a very complex set of data about plant and animal distributions. Wegener intended to use the evidence for shifting latitude zones collected by his predecessor “outsiders”—Reibisch, Simroth, and Kreichgauer—but to throw out their poorly founded geophysical theories. He would then use his own theory of pole migration, forced by continental displacement, and use a modified version of Köppen’s scheme of latitude zones to predict the position of the equator, the temperate regions, and the poles in every period from the Carboniferous down to the Quaternary.
This was a clear and concrete plan, but one cannot wonder that geologists (almost universally) took exception to it: it paid almost no attention to geology. Wegener used certain geological formations the way one might use fossils, to make an argument about shifting environments, but he had no intrinsic interest in them. Almost everything that interested geologists—the details of local structures, erosion and sedimentation, geosynclines, the creation of drainage networks and river valleys, ore genesis, mineralogy and petrology, volcanoes, igneous intrusions, metamorphism—received a brisk tangential treatment except where these matters played into Wegener’s history of displacements. Wegener even subtracted major aspects of geology and put them into geophysics: the fjords of Norway and the submarine canyons of the St. Lawrence, Hudson, and Congo Rivers were for him no longer erosional features but emergent rifts on the margins of continents. These continental margins did not acquire thick sequences of sediment by the weight of sediment pushing down the shelves. Rather, fracture and downfaulting of the margins of continental blocks (caused by their lateral displacement) created the space and elevation differences, which allowed sedimentation to proceed. Elevation was not itself a geological process, only apparently so, as the squeezing together of the crust and flotation by isostasy jointly took credit for this activity; thus, the erosion subsequent to such “uplift” was also geophysically dictated.
The description of the displacement of Earth’s pole is the same here as in both of Wegener’s previous attempts to explain it, albeit in an expanded version. Wegener had always followed the argument of Schiaparelli, who was himself working out consequences of a speculation by Lord Kelvin from the 1870s. Wegener quoted Kelvin in 1915 and again here, so it is probably worth repeating. Kelvin had said, “We may not merely admit, but assert as highly probable, that the axis of maximum inertia and the axis of rotation, always very near one another, may have been in ancient times very far from their present geographical position, and may have gradually shifted through 10, 20, 30, 40, or more degrees, without there being at any time any perceptible sudden disturbance of either land or water.”109
In a completely solid Earth, the pole can never move, other than a minor free oscillation. In a completely fluid Earth, the adjustment of the pole of figure to the pole of inertia would be instantaneous. The interesting case is in a viscous Earth, where the pole of figure can move but lags behind the pole of inertia for some period of time. Following Schiaparelli, Wegener imagines that there is a critical threshold of difference (geographical distance) between the pole of inertia and the pole of figure such that when this figure is exceeded, Earth, in order to reestablish its oblate spheroid to match the new distribution of mass, begins to flow. Once this flow begins, it may continue for quite some time, and if the forces causing the initial redistribution of mass continue, the pole of inertia will continue to move and the pole of figure will run to catch up to it.110 For small events and short time periods, Earth behaves as a solid. For large events and long time periods, Earth behaves as a viscous fluid.
From the standpoint of explanation, that was the easy part; the difficulty would be marshaling (in a comprehensible way) the thousands of paleontological data points that would indicate the position of the poles and the different climate zones in each geological era, and thus the path of the pole. In order to keep track of this, Wegener began to use the globe of Earth on which he had pasted tracings of the continents, to record the position of the equator and the poles in different periods, using small bits of colored paper attached with pins from his wife’s sewing kit.111
This was the work on which he and Köppen collaborated so intensely in the winter and spring of 1919/1920. Using the work of Reibisch and Kreichgauer to establish the position of the equator and the pole in the Carboniferous (these were in general agreement), Wegener and Köppen established that period of Earth history as a starting point. It was not, of course, the beginning of geological time, but it was the first period in which they felt reasonably certain of the data.
They chose to assume that Earth, in every period between the Carboniferous and the present, had a varied climate similar to the present. Köppen’s scheme for the climates of Earth in the recent past was too complicated for the distant past; they had not enough detailed data. They settled for a four-zone system of latitude bands with distinct fauna and flora. They started with a tropical rain zone between the equator and 20° of latitude, then a dry, desert zone between 20° and 30° of latitude, followed by a temperate rain zone from 30° to 60° of latitude, and finally a polar zone from 60° to the poles.
At the beginning of their investigation they focused on the history of the climate of middle Europe, for which the paleontological data were most dense, and they re-created, using these assemblages of plants and animals which indicated to Köppen distinct and latitude-dependent climates, the shift of latitude of this region of Earth throughout geological time, from just before the Carboniferous to the recent past. This was an engrossing exercise for both, and they were able to generate a curve showing that in the Devonian and Carboniferous through the Permian, central Europe had a tropical climate, in the Permian and the early Triassic it had a desert climate, and in the Jurassic it had been transformed into a temperate rain zone. By the Cretaceous it was passing again through a desert phase, and in the early Eocene it had for a time been (once more) tropical. Throughout the Tertiary there was a rapid and continuous motion of the climate from warm to cold: moving from an Eocene climate compatible with a latitude of 15° away from the equator to a climate in the Quaternary equivalent to latitude 60°+ north, then returning, in the recent past, to a temperate rain zone. They got the idea of graphing this as a continuous curve from Dacqué, though his attempt had been an impressionistic graph for the ups and downs of the climate of the whole Earth from warmer to cooler and back again.112
It was a successful effort, but they realized that to accomplish an accurate and verifiable position of the equator, and therefore of the poles, for each period of the geological past, they would have to repeat this labor for every major continental segment; it was the work of years, not months. For the first time in Wegener’s career he had run out of high-level syntheses that could accomplish the task of collating the individual data he needed. Much of the work in the decade 1900–1910 was still appearing only in 1919–1920 because of publication delays caused by the war. Even an experienced synthesizer and textbook writer such as Emanuel Kayser could barely keep up with the flood of new information, as he noted in the preface to his 1918 edition of his text.113
It dawned on Wegener early in the spring of 1920 that he would finish this book, but when it came out it would not be “finished.” He had enough data for a rough estimate of the pole positions, and by mapping the movement of the North Pole, he could get an estimate for the location of the South Pole. Knowing the latitude of Germany in different periods, and estimating the amount of its shift in latitude which was due to displacement rather than pole migration, he could sketch a paleoequator, using interpolated data for the other continents as a check. It was an extremely rough pass, and he knew i
t and said so. On nearly every page of this long chapter he repeated the warning that the results were provisional and subject to revision and that he had depended trustingly on different authorities for different places in different periods.114
In spite of the uncertainty attending this technique, Wegener did manage to produce pole positions for every period from the Devonian down to the recent, with a total displacement of about 65° since the Carboniferous, and with several important oscillations in between. Wegener was not able to produce a map with an accurate paleoequator, and his summary chart gives only positions of the North Pole, the South Pole, and Germany—the poles in terms of latitude and longitude; for Germany, latitude only.115
From the standpoint of physics research it was a good first pass through the data, but from the standpoint of the way geology and paleontology were “supposed to be done” it was sketchy, conjectural, and premature. This is not to say that it was not well sourced and intricate. Yet it was (and is) even more difficult to follow than the geological data in the chapter on continental displacements. It required familiarity with geographical place-names, geological time divisions, and plant and animal species in the present and the past, and its compression works decidedly against it: Wegener was trying to summarize in about thirty pages of text what others had struggled to compress into books of 400–600 pages. It was just too much.
It is in the final sections of the book that we see the most remarkable transformation of Wegener’s argument. These two chapters, on the causes of continental displacement and on their measurement by astronomical position finding, take up only about ten pages, but they are together a radical revision of his entire hypothesis. Not only is the content different, but so is the status of the investigation. It is no longer the solution to a long-standing problem, but very much a long-term investigation and a work in progress which will lead to such a solution in the future.
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