fossilisation vs natural action

fossilisation and natural action both are nouns.

Word	Noun	Adjective	Verb	Adverb
fossilisation	Yes	No	No	No
natural action	Yes	No	No	No

As nouns, natural action is a hypernym of fossilisation; that is, natural action is a word with a broader meaning than fossilisation:

fossilisation: the process of fossilizing a plant or animal that existed in some earlier age; the process of being turned to stone
natural action: a process existing in or produced by nature (rather than by the intent of human beings)

Other hypernyms of fossilisation include action, activity, natural process.

fossilisation (noun)	natural action (noun)
becoming inflexible or out of date	a process existing in or produced by nature (rather than by the intent of human beings)
the process of fossilizing a plant or animal that existed in some earlier age; the process of being turned to stone