Wegener believed that his theory of continental drift resolved many of the discrepancies in the geological theories of his day: the fossil homologies, the distribution of glacial deposits, and the origin of mountain belts. In The Origin of Continents and Oceans, however, he gives as the starting point for his theory the apparently unrelated observation of the jigsaw fit of the continents.16 He was not the first to make this observation, but he was among the first to infer from it the previous existence of a supercontinent. Wegener imagined Africa slotting into the curve in the eastern margin of South America, South America sliding into the margin of North America, and so on, until he had combined the modern continents into a single landmass, the supercontinent Pangaea. The explanatory power of this construction became apparent to Wegener when some years later he stumbled upon a discussion of homologies in the geological record.17 In Pangaea, he realized, homologous fossil assemblages from South Africa and South America sat side-by-side and scattered Permian glacial deposits converged near the South Pole. Other fossil, structural, and paleoclimatic homologies could likewise be reconciled. In Pangaea, these anomalies were no longer anomalous. They made perfect sense.
This evidence convinced Wegener of the past existence of a supercontinent. However, to argue that the continents were once part of a single landmass required that Wegener abandon the assumption that relative positions of the continents were fixed. This did not trouble him. He argued that the continents were like great ships moving through plastic material of the ocean crust.18 If the continents could move, he reasoned, their present positions did not conflict with the previous existence of Pangaea. Intriguingly, moving continents also provided a way to address the origin of mountain belts. Large mountain ranges in the interiors of the continents, typified today by the Alps and the Himalayas, could form where tracts of continental crust collided.19 Because continents were too buoyant to sink, the crust in the zone of compression would thicken, elevating the surface to produce a mountain range. Coastal ranges, on the other hand, formed as a result of the resistance of the ocean floor to the drifting continents.20
Wegener thus argued in lectures and four editions of his book that the modern continents had once been part of the supercontinent Pangaea. At some point in the distant past this supercontinent had broken apart, leaving the individual continents to move through the viscous material of the ocean floor. Unlike speculation about land bridges and sunken continents, the theory of continental drift did not violate the principle of isostasy or ignore homologies in the rock record. Its main flaw in Wegener's mind was that it lacked a mechanism, a deficiency for which he tried to compensate by speculating that a tidal or Coriolis force could perhaps drive the motion of the continents.21 But he was wedded to neither mechanism; future research could address that question. Finally, and not unimportantly, Wegener provided within his theory a testable hypothesis. Continents that had moved in the past were likely to be moving in the present, and if such movements were on the order of a few meters per year, astronomical observations might be precise enough to discern them.22
If geologists could accept the idea of moving continents, continental drift was a theory of unique explanatory power. When coupled with isostasy, drift theory provided explanations for the four phenomena described above. Unfortunately, the hurdle posed by mobile continents proved difficult to surmount.
Go To A Revolution Deferred