The Principle of the Uniformity of Nature is widely acclaimed as the philosophical foundation upon which the edifice of Historical Geology has been erected. As most commonly stated, this principle is given the form of a metaphor, “The present is the key to the past,” and perhaps it is this poetic phraseology that has made Uniformity at once so appealing and at the same time so ambiguous. Some geologists maintain that this maxim is a substantive statement about present and past configurations of the Earth, while others assert that it refers only to the logic and method by which geologists attempt to reconstruct the past. Gould (1965)1 distinguishes these viewpoints as substantive uniformitarianism and methodological uniformitarianism, respectively.
Proponents of substantive uniformitarianism are inclined to interpret geologic history in terms of changes that are more even and cyclic than episodic or variable. Among the advocates of this viewpoint, however, there is no agreement as to whether Uniformity is to be called an axiom, a postulate, an hypothesis, or a law. Those who oppose substantive uniformity tirelessly point out that the present condition of the Earth, with its high-standing continents and remnants of vast continental glaciers, must be quite atypical of average conditions through geologic time, so that the present cannot be a very reliable key to the past. These opponents have no difficulty in classifying substantive uniformity; it is a discredited scientific hypothesis, an a priori assumption, or an anti-historical dogma.
Advocates of methodological uniformitarianism seem to view the classic formulation