John Calvin Commentary Isaiah 34:3

"Their slain also shall be cast out, and the stench of their dead bodies shall come up; and the mountains shall be melted with their blood." — Isaiah 34:3 (ASV)

Their slain shall be cast out. By this circumstance, he shows that it will be a great calamity. For if a few persons are slain, they are committed to the earth; but when so great a multitude is slain at one time that not enough people remain to bury them, there is no thought of interment, and therefore the air is polluted by the stench of their carcasses.

Hence it is evident that God is sufficiently powerful to lay low innumerable armies. Perhaps also, the Prophet intended to heighten the depiction of God's judgment, because shame and disgrace will be added to the slaughter of the nations, so that they will be deprived of the honor and duty of burial.

And the mountains shall melt on account of their blood. Another figure of speech is employed to show more fully the extent of the slaughter, for the blood will flow from the mountains as if the very mountains were melted, just as when the waters run down violently after heavy showers and sweep away the soil along with them. Thus also, he shows that there will be no means of escape, because the sword will rage as cruelly on the very mountains as on the field of battle.