geologist whose studies of the igneous rocks (igneous rock) of Montana revealed many previously unknown varieties. In 1889 he served as an assistant with a U.S. Geological Survey party in Yellowstone Park and later in Montana. He joined the faculty of Yale University in 1892 and became professor of physical geology in 1897. In Quantitative Classification of Igneous Rocks (1903), Pirsson, along with the U.S. geologists Whitman Cross, Joseph Iddings, and Henry Washington, presented a totally new classification system and new terminology for igneous rocks. Pirsson's most important books are Rocks and Rock Minerals (1908) and Textbook of Geology (1915), which by 1929 was the most widely used geology textbook in the world.