Fifth.- In all other cases, namely, in all cases of claims and disputes respecting land gained by alluvion or by dereliction of a river or the sea, which are not specifically provided for by the rules contained in this Regulation, the Courts of Justice, in deciding upon such claims and disputes, shall be guided by the best evidence they may be able to obtain of established local usage, if there be any applicable to the case, or, if not, by general principles of equity and justice.