Look what I found !!

This has been repealed but ....

Title 12, Chapter 2, Section 152 of the U.S. Codes

-CITE-
12 USC Sec. 152 01/24/94

-EXPCITE-
TITLE 12 - BANKS AND BANKING
CHAPTER 2 - NATIONAL BANKS
SUBCHAPTER IX - FORMATION OF ASSOCIATIONS TO ISSUE GOLD NOTES

-HEAD-
Sec. 152. Lawful money reserve of associations issuing gold notes;
receiving notes of other associations

-STATUTE-
Every association organized under section 151 of this title shall
at all times keep on hand not less than 25 per centum of its
outstanding circulation, in gold or silver coin of the United
States; and shall receive at par in the payment of debts the gold
notes of every other such association which at the time of such
payment is redeeming its circulating notes in gold coin of the
United States, and shall be subject to all the provisions of title
62 of the Revised Statutes: Provided, That, in applying the same to
associations organized for issuing gold notes, the terms ''lawful
money'' and ''lawful money of the United States'' shall be
construed to mean gold or silver coin of the United States; and the
circulation of such associations shall not be within the limitation
of circulation mentioned in title 62 of the Revised Statutes.

-SOURCE-
(R.S. Sec. 5186.)
Found this from Franklin Sanders, the Moneychanger.