N.C. Gen. Stat. § 54-66
All mortgages pledged to secure the payment of the bonds issued hereunder shall be deposited and left with the State Treasurer. The land mortgage association may, with the approval of the State Treasurer, remove such mortgages from the custody of the State Treasurer, substituting in place thereof other of its mortgages, or money or State of North Carolina bonds or certificates of deposit, endorsed in blank, issued by State or national banks located in North Carolina, farm mortgage bonds issued under the provisions of the Federal Farm Loan Act approved July 17, 1916, or obligations of the United States government, in an amount equal to or greater than the amount unpaid upon the notes secured by the mortgages withdrawn.
N.C. Gen. Stat. § 54-66