The Earth as we know it today has been shaped by natural processes of the past. Uniformitarianism emphasizes the cumulative effects of processes operating at uniform rates. Catastrophists single out the importance of rare and unusual events. This article proposes a scale to measure the magnitude of rare events, and cites the inverse magnitude/frequency relation of naturally occurring events to formulate the philosophical basis for actualistic catastrophism. Mass extinction at the end of the Cretaceous is held up as an example that a catastrophe such a bolide impact could influence the course of biologic evolution. Improbability of such an event is no argument against such a hypothesis, because the improbable, even those with odds of once every billion years, must have taken place a few times in the geological history. The immensity of geological time is not only important because of the immensity of the cumulative effects, but also because the long duration provides a chance for the improbable to take place

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