“The abundance of Things, and our great facility in making them available, obscures a fundamental truth which we need to see very clearly if we are to have any hope of meeting the main problem confronting Western civilization today. That basic truth is that the Things which are so abundant, even the gold and silver itself, rest upon the land and are derived from it.

Regardless of any techniques which may be developed to extract more from the land, there is a limit beyond which we can not go; and if our techniques speed up the process of utilization and destruction, as they are now doing, they hasten the day when the substance on which they feed and on which a swollen population temporarily subsists will approach scarcity or exhaustion. Then the scholars will look back on the age when the Golden Door opened, and men marched out to the Great Frontier to create the greatest boom that the world has known; they will make myths and legends about it, and in poetry and literature express their poignant yearning for New Frontiers. They will see the frontier as the great factor in the age called modern, see it clearly as the lost factor which they would so love to find.”

Walter Prescott Webb
“The Great Frontier“
1951

Update:
I should add that the Great Frontier of which he speaks is that beginning with the Age of Exploration; from the 1400s or so through today and who knows how much longer.