A geologist is a person whose work, or serious study, is geology: the rocks, the soil, and how the Earth has changed since it was formed. They may walk hills with a hammer, read layers in a cliff like pages in a book, look for water, oil, or minerals, or warn people about earthquakes and landslides. In a university they are the “rock people,” not the “map and country people.” Though actually, in everyday talk people sometimes say geologist for anyone in boots looking at the ground, including engineers who only need the ground to be safe for a building. Strictly, a geologist is trained in that science. Please do not call a geographer a geologist — that mix-up is extremely common, in English and in Chinese.