Also called gradualism. In 1830, British geologist Lyell (1797-1875) published Principles of Geology, which takes uniformitarianism as its predominant idea. Its meaning is: the geological processes that operate now to change the crust morphology have always operated during the whole period of earth history with the same intensity, and the past geological events can be explained by the phenomena and acting forces observed today. The classical concept is: “the present is the key to the past”. This theory does not mean that change is in constant speed, and catastrophes in local area cannot be ruled out.