simple reading of the Act would explain:

-An Act to provide a national Currency, secured by a Pledge of United States Stocks, and to provide for the Circulation and Redemption thereof.

Sec. 15.  And be it further enacted, That every association, after having complied with the provisions of this act preliminary to the commencement of banking business under its provisions, shall transfer and deliver to the Treasurer of the United States any United States bonds bearing interest to an amount not less than one third of the capital stock paid in;  which __bonds shall be deposited with the Treasurer of the United States__, and by him safely kept in his office until the same shall be otherwise disposed of, in pursuance of the provisions of this act.

Section 16.  And be it further enacted, That upon the making of any such transfer and delivery, the association making the same shall be entitled to receive from the Comptroller of the Currency circulating notes of different denominations, in blank, registered and countersigned as hereinafter provided, equal in amount to __ninety per centum of the current market value of the United States bonds__ so transferred and delivered, but not exceeding the par value thereof, if bearing interest at the rate of six per centum, or of equivalent United States bonds bearing a less rate of interest;  and at no time shall the total amount of such notes, issued to any such association, exceed the amount at such time actually paid in of its capital stock.

Sec. 41.  And be it further enacted, That every such association shall at all times __have on hand, in lawful money of the United States__, an amount equal to at least twenty-five per centum of the aggregate amount of its outstanding notes of circulation and its deposits;

"lawful money" ie. United States notes, commonly reffered to as "greenbacks"