Principles of Geology (full title: Principles of Geology: being an attempt to explain the former changes of the Earth's surface, by reference to causes now in operation) is a book by the Scottish geologist Charles Lyell. Published in three volumes in 1830–33, it established Lyell's credentials as an important geological theorist and popularised the doctrine of uniformitarianism (first suggested by James Hutton). The central argument in Principles was that "the present is the key to the past": that geological remains from the distant past can, and should, be explained by reference to geological processes now in operation and thus directly observable.
Lyell's interpretation of geologic change as the steady accumulation of minute changes over enormously long spans of time was also a central theme in the Principles, and a powerful influence on the young Charles Darwin, who was given Volume 1 of the first edition by Robert FitzRoy, captain of HMS Beagle, just before they set out on the voyage of the Beagle. On their first stop ashore at St Jago, Darwin found rock formations which -seen "through Lyell's eyes"- gave him a revolutionary insight into the geological history of the island, an insight he applied throughout his travels. While in South America, Darwin received Volume 2, which rejected the idea of organic evolution, proposing "Centres of Creation" to explain diversity and territory of species. Darwin's ideas gradually moved beyond this, but in geology he was very much Lyell's disciple and sent home extensive evidence and theorising supporting Lyell's uniformitarianism, including Darwin's ideas about the formation of atolls.