The supreme court shall consist of not less than seven justices who shall be selected as provided by this article. All cases shall be heard with not fewer than four justices sitting and the concurrence of a majority of the justices sitting and of not fewer than four justices shall be necessary for a decision. The term of office of the justices shall be six years except as hereinafter provided. The justice who is senior in continuous term of service shall be chief justice, and in case two or more have continuously served during the same period the senior in age of these shall be chief justice. A justice may decline or resign from the office of chief justice without resigning from the court. Upon such declinat ion or resignation, the justice who is next senior in continuous term of service shall become chief justice. During incapacity of a chief justice, the duties, powers and emoluments of the office shall devolve upon the justice who is next senior in continu ous service.