Thomas H. Clark has been called one of Canada’s great scientists of the 20th century and is probably the only person on this list to have a mineral (Thomasclarkite) named in his honour. The geology of the Saint-Laurent Lowlands was his major lifetime work and laid the foundations for all subsequent mapping and research studies in the area. He authored or co-authored more than 100 scientific publications. He also co-wrote, with C.W. Stearn, the textbook The Geological Evolution of North America, which quickly became the standard text in university-level geology. He was director of the Redpath Museum from 1932 to 1952 and had previously personally collected many of the museum’s fossils. Clark always said he was happiest when teaching in the Department of Geology (now Earth and Planetary Sciences) and was considered a brilliant pedagogue. Amazingly, he was active at McGill until 1993, retiring at the age of 100. Among his honours were the Logan Gold Medal from the Geological Association of Canada (1971) and the Prix Grand Mérite of the Association Professionnelle des Géologues et Géophysiciens du Québec (1993). If only for having served McGill for some 70 years, he should be the Greatest McGillian; but his formidable combination of research, teaching and writing also makes him a worthy candidate.