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Alfred Harker (1859-1939) was a prominent petrologist who spent his career at St John's College, Cambridge, lecturing on and researching rock formations and related geological activity. He was elected a fellow of the Royal Society in 1902, and was president of the Geological Society from 1916 to 1918. He used his Cambridge lectures as the foundation for this book (first published in 1909), offering an introduction to the development of rocks and related volcanic activity. With more than one hundred diagrams of various aspects of geological formations, this work also provides a visual guide to the location and formation of igneous rocks. Over the course of the work, he covers the themes of vulcanicity, rock structure, crystallization, the role of magma and the principles of rock classification, giving a broad picture of the field of petrology around the beginning of the twentieth century.