In 1912, Alfred Wegener proposed the idea that the continents have moved and offered a respectable mass of supporting evidence for it. It is appropriate, then, that the theory of continental drift was most widely known as “Wegener's hypothesis” during the more than fifty years of debate that preceded its ultimate acceptance by most earth scientists. In brief ,Wegener's hypothesis stated that, in the late Paleozoic era, all of the present continents were part of a single giant land mass, Pangaea, that occupied almost half of the earth's surface. About 40 million years ago, Pangaea began to break into fragments that slowly moved apart, ultimately forming the various continents we know today. However, until then 1960s, most scientists were reluctant to accept this. There are several reasons. First, while the period during which Wegener's theory was propounded and debated saw rapid developments in many branches of geology and an explosion of new knowledge about the nature of the earth, little of this evidence seemed to support Wegener. For example, data drawn from the new science of seismology, including experimental studies of the behavior of rocks under high pressure, suggested that the earth has far too much internal strength and rigidity to allow continents to “drift” across its surface. Measurements of the earth's gravitational field made by some of the early scientific satellites offered further evidence in support of this view as late as the early 1960s. Second, and perhaps most significant, Wegener's theory seemed to challenge one of the most deeply-held philosophical bases of geology―the doctrine of uniformitarianism, which states that earth history must always be explained by the operation of essentially unchanging, continuous forces. Belief in the intervention of unexplained, sporadic, and massive shaping events―known as catastrophism―was considered beyond the pale by mainstream geologists. Wegener was not, a catastrophist―he did not suggest that some massive cataclysm had triggered the breakup of Pangaea―but his theory did imply a dramatic change in the face of the earth occurring relatively late in geologic history. Such a belief, viewed as tainted with catastrophism, was abhorrent to most geologists throughout the first half of this century.