The theme of the origin of mountains by compression was here for Wegener the through line uniting all aspects of the displacement theory, just as it had been the through line uniting all aspects of the contraction theory. And here, without any special announcement, Wegener undertook the most dramatic expansion of his theory over the 1912 version. His argument proceeds thus. In considering the characteristic phenomena associated with the free displacement of continental blocks, it is best to think of them as if they were oceanic islands; this is as it was in the first version. Their motions, however, can now be divided into two groups: one in which displacement through or over the Sima actually takes place, and one in which a block is passively carried along by a current flowing through the Sima (Simaströmung). The larger blocks—those the size of South America, for instance—are more likely to travel against the Sima, producing a range of characteristic phenomena. Smaller blocks, such as Madagascar and India, are carried passively by these currents in the Sima. Moreover, these currents flowing in the Sima are geologically active even when not carrying continental blocks. For instance, the compressional folding of most of the coast of East Asia is the result of the underthrusting of the continental margin by a current of mantle material (Sima) flowing to the north.
The dynamism and fluidity of the displacement theory in this version (1915) stand in marked contrast to Wegener’s earlier and later pictures of continental displacement. Once again, one sees the powerful influence of Hans Cloos in urging Wegener to consider the idea of streaming in the Sima as a component of the theory.70 The notion of such streams in the subcrust had been hypothesized by an Austrian geologist, Otto Ampferer (1875–1947), in 1906, as an important part of the folding of the Alps.71 The American geologist Bailey Willis had proposed that the marginal ranges of Asia had been created by the underthrusting of the continent by the mobile ocean floor moving toward the continent in a process he described as “gravitational creep.”72 What Wegener was proposing was quite different from both Ampferer and Willis in its scope and details, but the inclusion of the idea, in general, was clearly designed to bring the displacement hypothesis closer to current geological conceptions and to smooth the way for its serious consideration.
On this highly mobile globe, the larger continents move across and through the subcrust, while fragments of former continental masses are carried by a variety of currents moving in this same hot and fluid substrate; all the characteristic phenomena of the continental outlines and margins are generated from the motions themselves. On the leading edge of the largest continental blocks, mountains are thrown upward, because the plastic crust can hold the deformation caused by the resistance of the viscous fluid Sima, while the latter material cannot permanently deform. As the continents move, the subcrust plows underneath them, emerging on the other side. Wegener’s favorite example of this ensemble of phenomena was the Drake Passage between South America and Australia. The fragmentary archipelagoes of Tierra del Fuego on the one hand and Graham Land on the other are pieces of the leading coast of South America and Antarctica torn away by the resistance of the Sima. The South Orkney Islands and South Georgia and the South Sandwich Islands are fragments torn away from Antarctica; the Falkland Islands, on the east coast of South America, represent former sections of the west coast of South America torn away and carried in the Sima stream completely underneath the continental tip, only to emerge at the surface again on the other side.73
Wegener concluded his discussion of the mechanics of displacement by noting that all questions of displacement had to be considered relatively. It was, for instance, not absolutely certain that South America was drifting and pushing against the Sima of the Pacific; it was equally possible that the Pacific Sima was streaming against the continent, although not capable of pushing it along, as might have happened with a smaller continental fragment. Moreover, the morphology of continental margins, some of which have mountains on more than one side, indicates that there must have been, in Earth’s history, back-and-forth motions of continental blocks, such that the long-term development of the continental crust is not a matter of one-sided motion, with a single leading edge always susceptible to deformation. Rather, the long-term result of continental motions was the emergence of ever-thicker continental blocks as Earth’s surface moved from a time of universal oceanic cover to the emergence of higher and higher continental platforms; most of this elevation was concealed as deep “continental roots” caused by the lateral pressure of the fluid but still viscous Sima against the stronger but still plastically deformable continental blocks.
The choice of the word “mechanics” to describe all this was technically correct, but it was bound to be misleading to the audience and thus unfortunate. By “mechanics” Wegener means only what any physicist would mean, a comprehensive description of the interaction of the parts of the system. But “mechanics,” or “mechanism,” is often an abbreviation in geological discourse for “causal mechanism,” that which brings something about. Thus, in modern geotectonics, the mechanism of continental displacement is the motion of tectonic plates, and the mechanism of plate motion is supposed to be convection in the mantle (what Wegener would have called Sima).
When encountering Wegener’s succeeding chapter entitled “Likely Causes of Displacement,” it becomes evident why he made this choice to speak of mechanics in one chapter and causes in another. He considered it in 1915, as he did in 1912, too early to talk about the causes of displacement.74 Yet because several of his critics had identified the lack of a presumptive cause (as he reports here) as a drawback in the presentation of his theory, he reluctantly decided to remove this objection by suggesting several possible presumptive causes without choosing among them. By separating out mechanics from causes, making these two things distinct, he hoped to blunt criticism of the theory. He mentioned here in passing the possibility of tides within Earth, some magnetic action because of the discrepancy between the magnetic pole and the pole of rotation, and the possibility of a so-called pole-fleeing force in which masses close to the poles would, by Earth’s rotation, be forced toward the equator because their center of buoyancy was higher than their center of gravity. None of these were demonstrated or demonstrable.
He then added a few highly interesting speculations that the cause of such motions on Earth might be “cosmic” since observational astronomy permits us to see phenomena on the surfaces of Mars and Jupiter which suggest (and which our observations of sunspots show us) that planetary surfaces and even the surface of the Sun are capable of motion. For planets above a certain size—perhaps Mars, and certainly Jupiter—displacement of surface elements by currents beneath the surface might be a real possibility.75 He concluded by pointing out (and the distinction is important) that we might be able to establish the reality of continental motions by measuring them astronomically, and yet we might never know their cause.76
Notably, one well-documented motion on Earth which Wegener refused to invoke as a cause of continental displacement was the wandering of Earth’s pole of rotation, which Wegener took to be independently established by paleontological evidence. Wegener argued, in this edition as in 1912, that displacements of the pole had to be invoked to explain the distribution of the Permo-Carboniferous glaciation in the Southern Hemisphere, as well as the fossil record of Tertiary land plants in the Northern Hemisphere. The causal connection between continental displacement and polar wandering (if there were any) must run the other direction: the displacement of large continental fragments, as well as the significant transfers of mass across the surface of Earth, would be most likely causes of pole wander, as the axis of rotation moved to correspond to the axis of inertia.77
For all this attention to geophysics, mechanics, possible causes, and polar wander, Wegener still believed that the fundamental evidence for the existence (as opposed to the plausibility and possibility) of continental displacements was geological and paleontological. The matching geological formations across thousands of miles of abyssal oceans, the foss
ils of closely related shallow-water and terrestrial species on different sides of the Atlantic, and the provenance of the Gondwanaland flora in widely separated Southern Hemisphere continents (along with evidence there of massive continental glaciation in the Carboniferous and Permian) were the facts that pointed directly to continental displacement. He announced, at the beginning of his chapter entitled “The Atlantic Ocean,” that “this and the following chapter [on Gondwanaland] contain the most important evidence for the displacement theory.”78
Cloos had been helpful at every stage in the recasting and expansion of the argument and increasing Wegener’s sensitivity to addressing major anomalies, in hopes of winning a geological audience. But it was in the areas of geology and paleontology that Wegener had sought out his guidance in the first place. It was precisely the most recent literature produced since 1912 and the literature on regions of the world where Cloos had worked (or had plans to work) that figured in the revision of the chapters on transatlantic continuities and on the reconstruction of Gondwanaland. From Cloos he learned of very recent work, presented at the International Geological Congress in Toronto in 1914 and not yet published, establishing very close connections between the Sierra south of Buenos Aires and the mountains of the cape in South Africa.79
Cloos also introduced Wegener to a geological resource of which he had been previously unaware, the Handbuch der Regionalen Geologie, an extensive international collaboration that had just begun to appear in many large volumes and would appear continuously for another decade. In these works, all the stratigraphic, paleontological, economic geology and petrology, as well as the geological structure and morphology of all regions of the world, were to be gathered together under the editorship of recognized experts from every major geological community in Europe and North America.
Of most immediate use for Wegener’s theory were the volumes by Paul Lemoine on West Africa (1913) and Madagascar (1911) and those by Patrick Marshall on New Zealand (1911, 1912).80 These were very helpful to Wegener in strengthening his argument for the former connection of Africa to South America, the connection of Madagascar to the coast of East Africa, and the bewildering geological connections stretching from Indonesia to New Guinea and Australia and south to New Zealand. The material on Australia, New Zealand, and New Guinea, which constitutes the major addition to Wegener’s consideration of Gondwanaland, was provided directly by Cloos, and his knowledge of these areas contained much information not yet published, and some known to Cloos only from his field experience and conversations with other workers in this region.
It was also Cloos who urged Wegener to be cautious about his advocacy here, even if he could not blunt Wegener’s editorializing entirely. After completing an extended argument about the former unity of Australasia, Wegener concluded, “The uncertainty of such reconstructions cannot be eliminated by the citation of geological sources.”81 Nevertheless, he continued, not only does the bulk of the material harmonize with the displacement theory, but also it harmonizes so much better with the displacement theory than with the theory of sunken land bridges that it provides the most striking evidence for the former.82
Throughout the process of revision and extension of his theory, Wegener was in good spirits and surprised by the speed with which he could work. In early February, one month into the project, he had written to Köppen, “With the continental displacements I am making good and unexpected progress. This is unbelievably fascinating work. I already have a draft of about 50 pages, though this is only about half [of what I plan].”83 In the same letter, he praised Cloos for his help and also passed along another piece of information: “The geologist Dacqué (Munich) has sent me the page proofs of his book on paleogeography, in which he takes a stand in favor of my theory.”84
The unexpected support of Munich paleontologist Edgar Dacqué (1878–1945) had much to do with Wegener’s elevated mood. Dacqué had taken his PhD with the great paleontologist Karl von Zittel and had the previous year (1914) been named extraordinary (i.e., associate) professor of paleontology at the University of Munich, in conjunction with his appointment to superintend the paleontological collections of the Bavarian State Museum. The proofs he sent to Wegener were those of his major work: Grundlagen und Methoden der Paläogeographie (Fundamentals and methods of paleogeography).85 Dacqué was very explicit in the introduction that this was a treatise not in paleogeography but on paleogeography.86
Like Wegener’s work, Dacqué’s was theoretically innovative and comprehensive, addressing directly such questions as continental motion, pole wander, and “continental permanence.” It was a massive and beautifully produced book of more than 500 pages, with attractive and well-produced graphics and maps and an exhaustive index. Dacqué was at this time a rising star in his field. As had Wegener, he had written a PhD thesis with a historical slant: a history of evolutionary theory from antiquity down to the present.87
Dacqué had read Wegener’s 1912 paper and been immediately persuaded by the power of this “profound synthesis.”88 He provided in his book extensive summaries of various aspects of Wegener’s hypothesis and found Wegener’s synthesis the best existing approach to the question of faunal and floral distributions, especially for Gondwanaland, more so than those of the great geologists of the previous generation: Sueß, Neumayer, and others.89 In addressing the question of continental and oceanic permanence, he made a novel interpretation of Wegener’s findings. If it was clear that the continents were not permanent (in place, outline, or size), Wegener’s ideas gave a new meaning to the concept of oceanic permanence: the deep sea was always there because it was a distinct Earth shell, whether any given part of it happened to be covered by continental fragments at any one time.90
Dacqué’s book placed Wegener on an equal footing in the citation cascade with Sueß, Émile Haug (1861–1927), Koken, Rudzki, Neumayer, Krümmel, Bailey Willis, and Albrecht Penck. It treated his ideas with the utmost seriousness and made a point of their superiority to other dynamic theories of crustal motion, especially the novel and influential ideas of Damian Kreichgauer (1859–1940). Kreichgauer, in Die Äquatorfrage in der Geologie (1902), had produced paleogeographic maps for every geological period, mapping (with climate and fossil evidence) the motion of the equator across the face of Earth throughout geologic history, and demonstrating that polar motion was the best way to account for all the faunal and floral distributions in each period.91 His qualitative theory of how this took place was based on the notion that the fluid Earth produced an upward pressure on the outer crust, accounting for volcanic activity on the one hand and making the crust highly mobile on the other. It was evident to Dacqué, as it had been to Wegener, that this was in complete contradiction of the principle of isostasy, a concept with which Kreichgauer seems to have been unfamiliar.92 There was much for Wegener to absorb in Dacqué’s book, too much for the time he had allotted to the writing of this short book. The impact was there, however, even though Wegener had time to make only a few references to it in his manuscript.
If the work of writing the book had been fascinating, the results were less than satisfying. Wegener noted in the preface, written in early March 1915, that the “crucial experiment” of longitude measurements had been abandoned with the outbreak of war, and that he had undertaken this revision during a convalescent leave after being wounded at the front. Because of the disadvantageous circumstances in which it was written, he remarked, “the presentation is marred by many imperfections, that might have been avoided in more peaceful times.”93 Nevertheless, he was able to bring together some new evidence and also to give a more detailed formulation of the number of points “that in the first presentation [1912] were misunderstood largely as a consequence of their brevity.”94
Given that in March only half of his six-month convalescent leave was over, we might inquire why he did not spend more time polishing and improving the work if indeed he found the results so unsatisfactory. The answer to this lies in the simple fact that continental displacements, f
or all their fascination, were a sideline to his own work in atmospheric physics, work that he had been pursuing concurrently with the writing of this book. Even before he had begun drafting the new version of his argument concerning displacements, he had begun working extensively on the problem of lunar tides in the atmosphere.
The concurrent work Wegener pursued in the spring of 1915 on lunar tides was quite different from the qualitative work on displacements and refreshingly complicated. He wrote to Köppen on 11 January that he had begun working on the problem of lunar tides they had discussed during his convalescence in Hamburg the previous October and November. He wrote again the very next day, to say that he was really getting deeply into the problem.95 Wegener often wrote a note to Köppen merely to share his excitement at the way his work was going; this feeling about scientific work was a strong bond between the two men.
The problem had to do with inconsistent observations of lunar atmospheric tides taken at different latitudes at different times. At low latitudes, near the equator, there was a clear pattern of semidiurnal atmospheric tides matching (in phase with) the oceanic tides at those locations. Such tides in the atmosphere could be measured in wavelike deviations of some hundredths of a millimeter in barometer readings. This semidiurnal tidal fluctuation was commonly observed in barometers on board ships, for ships must rise and fall above the surface of the solid Earth as the tidal bulge lifts the water beneath them and allows it to recede. Yet, interestingly, such fluctuations could also be observed on tropical ocean islands and on continental surfaces at low latitudes, and such fluctuations could not be attributed to tidal bulging of the solid Earth; they had to record actual tidal action on the atmosphere itself. This was all fine, consistent, easily understood. The problem was that at higher latitudes such as Hamburg and Berlin, the semidiurnal fluctuations were sometimes present, sometimes offset from the ocean tides by some length of time, and sometimes entirely absent, leaving no clear barometric signature.
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