Huronian, the name applied to a vast system of rocks occurring in the neighbourhood of Lake Huron. They rest unconformably upon the Laurentian (q.v.), and arc older than the Potsdam or Lower Cambrian. They are from 10,000 ft. to 20,000 ft. in thickness, and consist of quartzites, greenish chloritic slates with conglomerates containing pebbles of Laurentian rocks, limestones - one band, north of the lake, being 300 ft. thick - and intercalated greenstones. No fossils have as yet been detected in them.