On their first stop ashore at St Jago in Cape Verde, Darwin found that a white band high in the volcanic rock cliffs included seashells. FitzRoy had given him the first volume of Charles Lyell's "Principles of Geology", which set out uniformitarian concepts of land slowly rising or falling over immense periods, and Darwin saw things Lyell's way, theorising and thinking of writing a book on geology. When they reached Brazil, Darwin was delighted by the tropical forest, but detested the sight of slavery, and disputed this issue with Fitzroy.