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"Earth Day may be a turning point in American history," Gaylord Nelson told a Denver crowd of 4,000 last week. "It may be the birth date of a new American ethic that rejects the frontier philosophy that the continent was put here for our plunder, and accepts the idea that even urbanized, affluent, mobile societies are interdependent with the fragile, life-sustaining systems of the air, the water, the land." Nelson's mood may have been a bit too euphoric. Still, even though some of the ecological enthusiasm engendered by Earth Day may fade, the earth itself is not likely to let anyone forget the problem for long.