Early estimates of the Earth's age were restricted by literal interpretations of Genesis, but the geological time scale had been greatly extended by the early nineteenth century. The development of stratigraphy encouraged the belief that the Earth's history included a sequence of distinct periods stretching back to a period long before the appearance of humankind. Calculations by nineteenth-century physicists imposed a limit of at most 100 million years on estimates of the Earth's age, and most geologists accepted this limit. It was only after the discovery of radioactivity that it became possible both to extend the scale of geological time and to develop techniques for providing exact dates.