The discovery of geological time was one of the great achievements of the early nineteenth century. There were many factors that contributed to this development: the recognition of extinction in zoology; the discovery that fossils can be used to identify rock systems; the application of physical principles to the problem of a cooling earth. But one of the most important avenues of discovery came from the study of volcanoes, active and extinct, and from the recognition that many kinds of rocks had an igneous origin. It became apparent to many observers that volcanic action had played a major role in shaping the surface of the earth, and that this had been going on for a very long time.
The illustration for this section shows a range of volcanic craters and domes in the Auvergne region of France (see exhibit item 63).