Within the prescribed period, or if no period be prescribed within ten years after the expiration of the time limited by the special Act for the completion of the works, the promoters of the undertaking shall absolutely sell and dispose of all such superfluous lands in such manner as they may deem most advantageous, and apply the purchase money arising from such sales to the purposes of the special Act; and in default thereof all such superfluous lands remaining unsold at the expiration of such period shall thereupon vest in and become the property of the owners of the lands adjoining thereto, in proportion to the extent of their lands respectively adjoining the same.