Section 5. Severance tax on coal -- trust fund. The legislature shall dedicate not less than one-fourth (1/4) of the coal severance tax to a trust fund, the interest and income from which may be appropriated. The principal of the trust shall forever remain inviolate unless appropriated by vote of three-fourths (3/4) of the members of each house of the legislature. After December 31, 1979, at least fifty percent (50%) of the severance tax shall be dedicated to the trust fund.