Wegener then added quickly in a footnote, “I wish to point out emphatically that this presentation is in many respects and of necessity rather schematic. For instance, in North America only the western ranges of the Cordillera are of Tertiary origin, while those in the East are older, and more so the farther east they lie. Naturally only the Tertiary folds can be linked to the separation from Europe.”14 This note, added as an afterthought, is an attempt to resolve an apparent contradiction in his argument: if North America drifted away from Europe later than South America from Africa, the mountains on its western margins should be younger, rather than older, than the Andes. Yet most of the Rocky Mountains of the United States and Canada are in fact substantially older, as are portions of the Basin and Range Province of the western United States, and both are hundreds of miles inland from the Pacific coast.
In turning to the next case, that of “Gondwanaland,” Wegener continues his argument concerning the Tertiary separation of great continental fragments. “If we also apply the views derived above concerning the correlation of folding with horizontal displacement to the Tertiary folds of the Himalayas, we arrive at a range of astonishing relationships.”15 The argument here is dense but can be summarized rather easily: India, which Sueß had described as a “fragment all the way around,” may now be interpreted as a northward-moving fragment of a very extensive Southern Hemisphere paleocontinent (Urkontinent), with the Himalayas representing the compression of about half of its former extent in its northern motion. If the Himalayas could be unfolded, they would then produce an India, the southern point of which would be adjacent to South Africa. Following his reading of Keilhack and others, Wegener rapidly sketched out the former extent of this continent, to include Africa, Madagascar, India, Australia, New Zealand, and Antarctica.
Wegener’s argument now embraced the following picture: there were once two great paleocontinents. The Southern Hemisphere continent, consisting of South America, Africa, India, Australia, New Zealand, and Antarctica, began to break up in the Mesozoic, probably in the Triassic. As Africa and India split apart, they both severed their connection with Australia, though the latter remained for some time in contact with the southernmost tip of South America and with Antarctica. In the Jurassic, the Antarctic block slid away from South Africa in the direction of the Pacific. In Wegener’s mind this would make the mountains of Graham Land and Victoria Land in Antarctica the results of the same kind of leading-edge crumpling that created the Andes. India, after its separation from Africa, moved northward, progressively crumpling itself against the southern margins of the Eurasian continent in the Tertiary and forming the Himalayas. South America began to break away from Africa at the end of the Mesozoic, splitting from south to north, with the Africano-Brazilian connection also being broken in the Tertiary period. It was at this same time that the Andes were created by the crumpling of the leading edge of the west-drifting continent.16
The Northern Hemisphere continent also began to split from south to north in the Tertiary, and with the opening of the Atlantic the progress of severing of the land connections was finally completed in the Quaternary ice age, with northern Europe, Greenland, and northern North America still close enough together at this point to create a comprehensive land surface for a more compactly situated Northern Hemisphere ice cap. Thus, the ensemble of former paleontological connections, later broken, and the worldwide appearance of great fold mountains in the Tertiary were to be united under the aegis of the displacement theory as complementary aspects of the same dynamic process.
Wegener’s reassembly and then separation of a Southern Hemisphere continent was constructed out of a paleontological record of former continuities now severed. Such a severing of connections, as he himself pointed out, could have come about from oceanic transgressions onto the continental surfaces at relatively shallow depth and need not have resulted from continental rifting and drifting. However, the question of the Southern Hemisphere ice age at the end of the Carboniferous and the beginning of the Permian presented an opportunity (of which Wegener had been made aware by Keilhack) that could tip the balance away from “transgressions” and toward splitting and drifting of continents. Incontrovertible glacial deposits, landforms, and other glacial phenomena occurred in southern India and Africa and northern Australia, as well as Madagascar. Even imagining Earth’s pole of rotation to have been displaced to a more likely location for the focal point of a great southern ice cap, such a move to a more plausible site left that pole in the middle of the ocean between western Australia and Madagascar and still required, Wegener noted, glacial deposits within 30°–35° of the equator.17
The Permian ice age has created up to the present a hopeless enigma for paleogeography. For these unmistakable ground moraines of an extensive inland ice cap, lying on a typically striated basement, are located in Australia, South Africa, South America, and above all in the Eastern India. Koken [Ernst Koken (1860–1912), author of Die Vorwelt (1893)] has indicated in a special essay, and illustrated with a map, that, given the present disposition of the landmasses, such a great extension of the polar ice cap is absolutely impossible. For even if one dismisses the South American findings as untrustworthy, which by now should be hardly permitted anymore, and were to place the pole in the most advantageous position imaginable, namely in the middle of the Indian Ocean, the furthest regions of the inland ice would still be assigned a geographical latitude of about 30–35°. In the face of such an extent of ice cover, scarcely a single part of Earth’s surface could have remained free of glacial phenomena. And in that situation the North Pole would have been in Mexico, where not a single trace of Permian glaciation is known. The South American remains would, moreover, come to rest almost on the equator.18
The question of the Southern Hemisphere ice age was, as Wegener noted, impossible to solve with the continents and the pole in their present positions. Even displacing the pole of rotation, without also displacing the continents, produced an ice cover so massive that no region of Earth could have remained unglaciated. The contradiction was so blatant and so severe that even an otherwise rather conservative geologist like Albrecht Penck had been moved to remark that the current dispositions of the Southern Hemisphere continents with regard to this glaciation made so little sense that “the motion of Earth’s crust in a horizontal sense be envisaged as a working hypothesis to be seriously taken into consideration.”19
Beyond this summary of the geological and paleontological evidence, there remained two additional large-scale data sets that Wegener wished to address: the differences between the Atlantic and Pacific sides of Earth, and the anomalous distribution of tropical, semitropical, and temperate species of fossils in high latitudes of the Northern Hemisphere. The difference between the two sides of Earth was something noticed by the relentless Eduard Sueß, who had left it unexplained, therefore providing an opportunity for Wegener to move against the contraction theory on yet another front. Sueß had characterized the Atlantic side of Earth as bordered by indented, rough, and broken coastlines, with fractured margins, while describing the Pacific side as ringed uniformly by fold mountains parallel to the shore. The Pacific was deeper than the Atlantic, its lavas had a somewhat different composition, and it was more heavily sedimented in its abyssal depths with radiolarian ooze and red deep-sea clay.20
Wegener had already dismissed (at the beginning of the paper) the notion that the Pacific was a scar left by the departure of the Moon: Schwarzschild had mathematically refuted George Darwin’s calculations supporting this hypothesis. Wegener argued instead that the Pacific was deeper simply because it was older, more isostatically compensated, cooler, and more rigid—thus its ability to crumple the leading edge of moving continents. The Atlantic margins, on the other hand, show the scars of their more recent rifting, and the Atlantic floor, having been uncovered more recently, was warmer and less dense than that of the Pacific; the Atlantic was therefore shallower and showed less extensive deep-sea sedimentation.21
Th
e last of Wegener’s six geological arguments, and perhaps the most interesting, was that concerning the displacement of the pole of rotation. Here he abandoned all remaining tentativeness: “Notwithstanding the great and justifiable caution with which one approaches in geology all hypotheses concerning the displacement of the poles, yet so much material has been produced lately from that point of view, that a great displacement of the poles may in any case be regarded as proven [nachgewiesen]. In the course of the Tertiary Period, the North Pole shifted from the vicinity of the Bering Strait over towards Greenland; the South Pole from South Africa toward the Pacific side of Earth.”22
It may seem that Wegener is here suddenly dropping geological argument and returning to geophysical argument by discussing the displacement of Earth’s pole rotation. This would be a source of confusion to Wegener’s readers in 1912 and thereafter: he consistently characterized the notion of the displacement of Earth’s pole rotation as a geological, rather than a geophysical, argument. It is hard to imagine something more geophysically significant than the displacement of Earth’s pole, yet from Wegener’s standpoint it was, and would remain, a geological hypothesis. Wegener classified his arguments not according to their topical focus but according to their evidentiary basis. The geophysical arguments for continental displacements were those based on data concerning Earth’s gravity field, the propagation of seismic waves, and the laboratory determination of the physics of the solid state under high temperatures and confining pressures: these were all geophysical data. The geological arguments for continental displacements (and for displacements of the pole of rotation) are those that are founded on geological data: petrology and mineralogy, paleontology, surface morphology, glacial deposits, and mountain ranges.
The evidence that appeared decisive for Wegener in the matter of pole shifts came from studies of successive layers of fossil plants in the Tertiary of Europe and the Arctic, with the shift back and forth among subtropical, temperate, and cold-climate plant remains. The study of very recent plant fossils had the immense advantage that one knew from their living near-relatives, in an entirely unambiguous way, what sort of climates they inhabited. This gave great force to the latitude-zone reconstructions of the geographical distribution of these same plants in the more distant past. Almost all of the authorities and sources quoted by Wegener in this section of his paper he obtained from Nansen’s scientific appendix to Auf Schneeschuhen durch Grönland, the same book he was reading and rereading in preparation for Greenland.23 Most notable among these paleobotanists was the work of Alfred Nathorst (1850–1921). Nathorst had plotted pole positions for the ice age and most recent parts of the Tertiary, and Nansen had highlighted and endorsed Nathorst’s work, as well as the puzzles it presented, as a presumptive reason for scientific expeditions to Greenland, of exactly the kind on which Wegener was about to embark. We may recall that Nansen, following Nathorst, had agreed that this sequence of plant fossils could not be explained by the normal rhythm of glacial and interglacial intervals within an ice age: tropical plants were simply too far north.24
Wegener imagined that in the course of the Tertiary, the North Pole shifted more than 30° from its current position and lay then in the Bering Strait. The pole of rotation then traveled back toward Greenland, overshooting its current position by perhaps 10°, and had moved back to its present position only at the end of the ice age. A Tertiary position of the North Pole in the Bering Strait would put the corresponding South Pole about 25° south of the Cape of Good Hope. Thus, the absence of glacial remains in the Southern Hemisphere in the Tertiary would be explained by the fact that this South Pole was at that time, if not at sea, then on the margin of the former South Pole continent, with much of the polar ice at that time being sea ice—as is characteristic of the North Pole regions in our own time. Similarly, in explaining the Permo-Carboniferous remains of the Southern Hemisphere, the placement of the Permian South Pole as much as 50° away from its current position (dictated to give a rational distribution of faunal remains) would have put the Permian North Pole beyond the Bering Strait in the Pacific, thus leaving no Permian glaciation in the Northern Hemisphere, which is consistent with what is observed.
Of the greatest importance for the understanding of the whole phenomenon is however the circumstance that the great displacements of the pole clearly are contemporary with the great displacements of the Continental platforms. Especially evident is the temporal coincidence of the best-verified pole displacement in the Tertiary, with the opening of the Atlantic Ocean. One may also perhaps be able to connect the (relatively insignificant) return journey of the pole since the ice age with the separation of Greenland and Australia. It seems in this connection as if the great continental displacements are the underlying cause of the displacement of the pole. The pole of rotation must in any case follow the pole of inertia. If this is altered by the displacement of the continents, then the pole of rotation must travel with it.25
Wegener’s hypothesis in this version includes both continental displacements and true polar wander. The displacements of the continents redistributed large masses across the surface of Earth and changed Earth’s pole of inertia. The pole of inertia is the pole (the axis of figure) defined by Earth’s center of mass, and if the surface masses redistribute, this pole will move to reflect this redistribution of masses and Earth’s new center of mass. The so-called Chandler wobble, or nutation, is the oscillation of the pole of inertia around the rotational pole, caused by Earth tides, atmospheric motions, and other variables. Wegener imagined, superimposed on this well-known phenomenon, a larger shift of the pole of inertia caused by the redistribution of very large continental masses, with the astronomical (rotational) pole following. Wegener did not imagine the whole “lithosphere” sliding over Earth’s interior, but individual continental fragments moving in different directions: South America, North America, and Greenland were moving to the west, while India and Australia were moving to the north.
Wegener’s argument here is quite dense, difficult to follow even using a good map, and typically baffling to readers not conversant with the relevant geography. What is important is that Wegener was creating a spatial/temporal grid of Earth, integrating continental positions, pole positions, and continental glaciations throughout geologic history, as an index to the distribution of latitude zones with appropriate vegetation. He was, however, trying to say in a scientific paper the sort of things usually said in a good-sized book, consequently with somewhat indifferent results.
The abandon with which Wegener pushed Earth’s pole this way and that across many degrees of latitude and longitude may seem reckless, but Wegener noted that the planetary astronomer Giovanni Schiaparelli (1835–1910) had in 1889 concluded that in a plastic Earth (the sort of Earth that Wegener had considered most likely: rigid for short-term stresses, but indefinitely deformable by long-term stresses) the pole of rotation might be displaced by any amount. Schiaparelli considered three cases: a completely rigid Earth, an Earth with “plastic” adjustment but some lag in its response, and “an earth sufficiently plastic not to lag appreciably behind.”26 Wegener favored the third of these alternatives, and he found these investigations extremely interesting, not least because Schiaparelli had argued that “in order to obtain a displacement of several degrees it would probably be necessary to move beyond those phenomena already revealed by the study of Earth’s crust.”27 By this he meant a resort to the sort of geophysical data produced many years after his argument—exactly the data on which Wegener depended. Schiaparelli had also asserted that “geological actions, sufficiently prolonged,” could at any time destroy the stability of the geographic poles and give rise to rapid movements of the pole of rotation: “once admitted, this will open new horizons for the study of the great physical revolutions that the crust of Earth has undergone.”28
Schiaparelli was an astronomer of note, and the quotation given above is indicative of just how many scientists—geologists, geophysicists, and astronomers—were willin
g at the turn of the twentieth century to consider displacements of Earth’s pole over rather large distances as an entirely plausible hypothesis to explain geological and paleontological phenomena. From Wegener’s standpoint, his own originality consisted largely in showing that pole displacements alone, however extensive, could not provide a sufficient explanation for the existing paleontological evidence; these had to be joined with actual displacement of the continents.
Attempts to calculate pole displacements caused by observed mass displacement had, up to the time of Wegener’s writing, concentrated on “only the very slight displacements that could be noted in earthquakes, for example,” and therefore “the conclusion was invariably reached that the produced pole displacements had to be immeasurably slight.”29 Hayford, whose North American gravity survey was so important to Wegener in establishing the thickness of the continents, had calculated that the shift of the crust along the San Andreas Fault during the San Francisco earthquake of 1906 could have shifted the pole of inertia only about 2 millimeters (0.08 inches), through the dislocation of a 40,000-square-kilometer (15,444-square-mile) block 118 kilometers (73 miles) thick, 3 meters (10 feet) to the north.30 Wegener noted that the displacements of continents with which he was concerned involved masses at least 100 times larger than those considered in this example and might cause pole shifts as large as 1° in 360,000 years, an order of magnitude that could shift the pole enough to produce the paleontological transitions from warm to cold climates and the reverse in the allotted time.31 Wegener estimated that it had taken roughly 10,000,000 years to open the Atlantic with this amount of annual displacement.
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