Adam Clarke on Deuteronomy 33:15

The chief things of the ancient mountains - ומראש הררי קדם umerosh harerey kedem, and from the head or top of the ancient or eastern mountains, the precious things or productions being still understood. And this probably refers to the large trees, etc., growing on the mountain tops, and the springs of water issuing from them. The mountains of Gilead may be here intended, as they fell to the half tribe of Manasseh. And the precious things of the lasting hills may signify the metals and minerals which might be digged out of them.