The Federal Reserve Act 1923 would have legalised the Dollar as we know it today I'd imagine... That's the lawful currency of the day since that act...It actually says it in the first paragraph...
http://en.wikipedia.org/wiki/Federal_Reserve_Act
Printed copy · Anthology Volume 4, page 577Printed copy from The Tender For Law Anthology, Volume 4, page 577 (printed 13 June 2014). Added by this site; not part of the Facebook export.