Slate bottom with metal edges,I'm guessing it's older than the 70's.Probably more like the 60's.I remember my parents having tanks like that when I was in elementary school(60's).How much is it worth?????The thing is that everything is worth money to someone.The trick is finding that person.If I were you I would get the saltwater out of there as soon as possible,and clean it up real good.Then take some pics and post it on ----.Set a rather high reserve so that it doesn't get sold too cheap.This way you can get a feel for it's value.