172 THE EARTH AS MAN'S ABODE The History of the Earth Is Divided into Eras, Periods, and Epochs on the Basis of Major Topographical Changes. Each era consisting of millions of years represents a time of widespread mountain-building; each period represents local disturbances and uplifts of relatively short duration; an epoch is a subdivision of a period. Each period begins with increasing submergence of lands and closes with the retreat of the sea. Fig. 55. Summary of the earth's history in a clock's face. (Courtesy of Chester A. Reeds, the University Society, and the American Museum of Natural History.) The study of the history of the earth has required a tremendous amount of careful piecing-together of fragments of evidence. The broad outline is fairly well established in much more detail than is possible to present in this book. The authorities differ as to the length of some periods by a million years or more; but a million years is but a moment in geological time, and all figures are rough estimates at best. The following synopsis of the earth's history represents more or less a cross section of modern geologists' interpretations of the data they have gathered.
THE HISTORY OF THE EARTH 173 Synopsis of the Earth's History Starting with the oldest eras Estimated Time