It is a habit peculiar to geologists to speak of millions of years as casually as politicians dispose of billions of dollars. For the latter, it does not seem to be real money, and I suspect that to us it is not really time. But to be comfortable with very long time is a habit which it took a century to develop.

There is, indeed, a need for much time. James Hutton thought that the earth was very old, with “no vestige of a beginning, no prospect of an end,” but the record of the rocks is ambiguous about time. The building of a large delta may take only a few centuries, but raising the Alps or the Sierra Nevada obviously demands more time. Early in the 19th century, a clear outline of the history of the earth had been established by the methods we have just discussed, but the time dimension remained elusive. Darwin's theory of evolution heightened the sense that much time had been involved in making humans out of single-celled algae, but no one could say just how much. It remained for the discovery of radioactivity in the last years of the 19th century to provide a firm time perspective of the geological past.