All appropriations by the Legislature for the construction of buildings, structures, highways and bridges shall constitute continuous carrying accounts for the purposes designated by the Legislature in such appropriations. The State Controller is authorized to carry forward all such appropriations to the succeeding fiscal year, provided the construction shall have been begun by the letting of a contract or contracts or by actually starting the work during the year for which the appropriations were made. Any balance remaining after the completion of the object of the appropriations shall revert to the General Fund in the State Treasury or to the fund from which it was apportioned under existing provisions of law.