The book offers a conspectus of important developments in the earth sciences in the twentieth century. The introductory essay considers features of twentieth-century geoscience as a whole (and science more generally), including general trends within geology and its associated disciplines; and it reviews secondary literature on twentieth-century geology. The contributed essays span detailed discussions of particular issues, such as the application of the phase rule to metamorphic petrology; surveys of fields such as sedimentology and palynology; delineation of broad fields such as 'planetary geology', mathematical geology, and metamorphic geology; discussions of individual scientists' contributions (Norman Bowen, Arthur Holmes, Victor Goldschmidt, and Marie Tharp); an oral history account of the emergence of terrane theory; discussions of trends or problems within fields such as geomagnetism and stratigraphy; the social context of geology as regards palaeontological collecting and some of the Russian responses to the advent of plate-tectonic theory.