31. an altar of whole stones--according to the instructions given to Moses (Ex 20:25; De 27:5).
      over which no man hath lifted up any iron--that is, iron tool. The reason for this was that every altar of the true God ought properly to have been built of earth (Ex 20:24); and if it was constructed of stone, rough, unhewn stones were to be employed that it might retain both the appearance and nature of earth, since every bloody sacrifice was connected with sin and death, by which man, the creature of earth, is brought to earth again [KEIL].
      they offered thereon burnt offerings unto the Lord, and sacrificed peace offerings--This had been done when the covenant was established (Ex 24:5); and by the observance of these rites (De 27:6), the covenant was solemnly renewed--the people were reconciled to God by the burnt offering, and this feast accompanying the peace or thank offering, a happy communion with God was enjoyed by all the families in Israel.

JFB.