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uncountable
(chiefly geology) The scientific principle that natural processes operated in the past in the same way and at the same rates that they operate today. quotations
There has been much puffy stuff written about whether Lyell's uniformitarianism permitted variations in intensity of causes, or whether he applied his logic in a consistent way, and whether he assumed indefinite stretches of geological time.
2004, Richard Fortey, The Earth, Folio Society, published 2011, page 20