Continental Drift, the theory that continents move slowly about the earth's surface, changing their positions relative to one another and to the poles of the earth. In the past the theory has been discussed but not generally accepted, most geologists believing the continents to be fixed in place and subject only to vertical movements, such as those observed during mountain uplift. In recent years, however, a sound body of evidence in support of a modified form of the drift theory has been found. Ideas are becoming precise and unified, with emphasis on a moving, evolving ocean floor. The new theory is called plate tectonics.

Early Theories Soon after the Atlantic Ocean had been mapped, about three hundred years ago, it was noticed that the opposite coasts had similar shapes, but it was not until the middle of the 19th century that accurate maps were published demonstrating that the two coasts could be fitted together quite closely. Some geologists then suggested that the fit of the coasts was not an accident--that the continents were once joined and had subsequently drifted apart. None of the suggestions were taken seriously.

In 1912, however, the German meteorologist Alfred Wegener investigated the fit of the Atlantic coasts more carefully than had his predecessors and grouped all the continents together into one great land mass, which he called Pangaea. He supposed that the mass began to break apart about 200 million years ago. He also showed that some geological features on the opposite coasts could have fitted together, and that there were many striking similarities between the fossil plants and reptiles on the opposite coasts, particularly the coasts of Africa and South America. If the continents were pushed together, the geological, fossil, and other lines of evidence would join together accurately in the way that lines of print on a torn newspaper would