actualism The theory that present-day processes provide a sufficient explanation for past geomorphological phenomena, although the rate of activity of these processes may have varied. The theory was first clearly expressed in 1749 by G. L. L. deBuffon, who proposed an age for the Earth of about 75 000 years, and was developed by James Hutton (see uniformitarianism). As ‘actualism’, it came to be generally accepted as a result of the much more convincing arguments advanced by Sir Charles Lyell (1797–1875) in his Principles of Geology (published 1830).

actualism The theory that present-day processes provide a sufficient explanation for past geomorphological phenomena, although the rate of activity of these processes may have varied. The theory was first clearly expressed in 1749 by G. L. L. Buffon (1707–88), and was the essential principle of uniformitarianism as presented in 1830 by C. Lyell (1797–1875).