Arthur Holmes, who lived from 1890 to 1965, was one of the greatest geologists of the first half of the 20th century. He was particularly successful at applying developments in physics to geological problems. Holmes worked on fundamental problems—the origins of igneous rocks and mountains, mantle convection, the age of the Earth, and continental drift—and he played central roles in major controversies about the latter two of these topics. His Principles of Physical Geology was a wonderful introduction to geology. Although historians have discussed Holmes' work on continental drift, they have paid little attention to his dating of the Earth. Discovery of the Earth's age was a major scientific accomplishment. Holmes also devoted much more attention to dating questions than to continental drift. Over 50 of his papers were about dating, while fewer than 10 dealt with continental drift.