In 1804, Jameson succeeded Dr Walker as the third Regius Professor of Natural History at the University of Edinburgh, a post which he held for fifty years. During this period he became the first eminent exponent in Britain of the Wernerian geological system, or Neptunism, and the acknowledged leader of the Scottish Wernerians, founding and presiding over the Wernerian Natural History Society in 1808 until around 1850, when his health began to decline, together with the fortunes of the Society. Jameson's support for Neptunism, a theory that argued that all rocks had been deposited from a primaeval ocean, initially pitted him against James Hutton (1726–1797), a fellow Scot and eminent geologist also based in Edinburgh (but not in the university), who argued for the uniformitarian deistic concept of Plutonism, that features of the Earth's crust were endlessly recycled in natural processes powered by magmatic molten rocks.