THE CRANBERRY Wilderness in West Virginia is now permanently protected against development and, a greater threat, coal mining. Well known to naturalists and hikers, this splendid expanse of wooded land, part of the Monongahela National Forest, now becomes the largest wilderness preservation on the East Coast. President Reagan signed the bill last week, for which he deserves applause. But greater applause goes to the many people in Congress, and in the conservation movement, who successfully pushed the legislation to passage in the final days of the lame-duck session.

It would not have been possible without an ingenious buy-out of the coal rights, engineered in the Senate. While the state of West Virginia owns the surface of the forest, the CSX Corp.--the corporate parent of the Chesapeake and Ohio Railway-- owned the coal under it. In return for the coal, Congress has given CSX credits that it can use in bidding for mining rights on other federal land. This device mitigates the impact on the federal budget, and sets a useful precedent for the preservation of other wilderness areas.

But Mr. Reagan vetoed a companion bill to establish similar protection over part of the Osceola Forest in Florida. Like the Cranberry Wilderness, the Osceola swamps contain valuable minerals--in this case, phosphate. Four companies had applied for leases to mine it, and here again the Senate offered bidding rights in exchange. Those rights, the administration said, might be worth as much as $200 million--a real cost to the government, although deferred. Last week, the secretary of the interior, James Watt, moved to resolve the matter differently by denying the lease applications. There is no possibility, he held, that the mining companies could meet the legal requirement of restoring the surface of the swamps. Since that decision makes the lease applications worthless, the administration saw no reason to offer the mining companies an expensive buy-out. That, the president said, is why he vetoed the bill.

Most conservationists find it difficult not to view anything involving Mr. Watt with the deepest suspicion. They fear that the mining companies may now go to court, overturn Mr. Watt's decision, and proceed to go after the phosphate. Our own view is that the administration is working in good faith. But it would be heartening to see them give further evidence of it by vigorously supporting a revised Osceola bill that adds a permanent ban on mining there.