My question pertains to the rules, and more specifically the ostensible violation of the rules on the modification of nouns into adjectives.
I sometimes experience difficulty of knowing when to adjective-ify nouns when (1) the adjective immediately precedes the noun in question and (2) the adjective version of the noun exists and is spelled differently.
Take the two examples I came across, surfing on the web:
a) "A journalistic career"
b) "A mathematics career"
Both phrases convey the same relative semantic idea and an almost identical construction bar one detail : in (a) the adjective version of journalist is deployed however in (b) the noun form of mathematics is left unaltered even though it functions as an adjective. The adjectival form of mathematics is obviously mathematical.
Is there a rubric or custom to distinguish when to adjective-ify nouns in these contexts? Or is it completely optional and both camps are valid?
Edit: another illustrative example : Is it correct to say "a gallery of cow pictures" or "a gallery of bovine pictures"?
Edit: My question is about the broader use and abuse of adjectives beyond the examples listed. Is there a hard-and-fast rule?