Man's physical universe



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


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


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