Uniformitarianism is an idea that is opposed to catastrophism and attributed to James Hutton in Theory of the Earth (1785, 1795), and John Playfair, in Illustrations of the Huttonian Theory (1802). Basically, it states that throuought geologic time, the same processes that work geologically today have always existed and are still working today in the same intensity to account for all geologic change.
In contrast to the catastrophic view of geology, uniformitarianists contend that the same phenomena that is happening to the rocks today are the keys to past geologic processes.
Of course, as geologic phenomena became explicable in terms of physics and chemistry, this principle became established as a major philosophical tenet of geology and the controversy between uniformists and catastrophists ended. This came about as a result of observations by the efforts of English geologist Sir Charles Lyell. Some geologists discard the idea of uniformitarianism in favor of actualism, which is the more general concept that the laws of nature are invariant over time.
Although the principle of uniformity is correct according to physical laws, which have not changed over geologic time, the falling temperatures of the Earth have caused changes in its behaviour, evident in changes in ingneous activity and crust movement over time.