Albert Barnes Commentary Psalms 65:6

"Who by his strength setteth fast the mountains, Being girded about with might;" — Psalms 65:6 (ASV)

Which by His strength sets fast the mountains—Fixing them firm on their foundations. This is an exhibition of vast strength or power on God's part, as if He fixed them so firm that they could not be moved—as if He handled with ease those vast masses of matter, with all their rocks and forests, and caused them to repose steadily and calmly on their foundations. We have few more exalted conceptions of God's power than to imagine Him lifting a vast mountain with ease, letting it down where He pleases, and settling it so firm that it cannot be moved.

Being girded with power—That is, He seemed to be surrounded or encompassed with power, as a man girds himself up when he wishes to put forth a great effort of strength.