Historical chronology, human or geological, depends... upon comparable impersonal principles. If one scribes with a stylus on a plate of wet clay two marks, the second crossing the first, another person on examining these marks can tell unambiguously which was made first and which second, because the latter event irreversibly disturbs its predecessor. In virtue of the fact that most of the rocks of the earth contain imprints of a succession of such irreversible events, an unambiguous working out of the chronological sequence of these events becomes possible.

History, human or geological, represents our hypothesis, couched in terms of past events, devised to explain our present-day observations.

[The] first postulate of the Principle of Uniformity, namely, that the laws of nature are invariant with time, is not peculiar to that principle or to geology, but is a common denominator of all science. In fact, instead of being an assumption or an ad hoc hypothesis, it is simply a succinct summation of the totality of all experimental and observational evidence.