Charles Lyell's Elements of Geology, published in 1838, was popular reading for an interested public, students, and amateurs, that described the basic rock types and the processes that made them. Lyell intended it as a supplement to his major work, Principles of Geology, which was published in three volumes from 1830-1833 for a more specialized audience. Principles contained detailed reasoning from evidence for uniformitarianism, the idea earlier outlined by the Scot James Hutton in the 1790s that the Earth's geological changes occurred through natural processes still operating today.