What is plain white on the outsie and a riot of green, blue, red, purple, white and black depicting, perhaps, the location of Thornhurst, Pa., on the inside?

Answer: The envelope bearing a letter from the U.S. Geological Survey.

The agency hasn't hired a designer to give its stationary a dash of style. What it has done is find a way to reduce its huge surplus of maps -- about 18 million of them -- that are costing thousands of dollars to store in warehouses in Arlington and Denver.

In the past, the agency has tackled its surplus problem by shredding the maps and recycling the paper.

But, according to Director H. William Menard, "People are very sentimentally attached to the maps. They were distressed we were disposing of them that way. We decided to experiment with making them into envelopes."

Menard, a former professor of geology at the University of California's Scripps Institution of Oceanography who came to the Geological Survey two years ago, said the unusual envelopes now cost about 60 percent more to make than conventional ones. But he said the gap will be closed when the agency increases its present order from 50,000 envelopes to 1 million.

"It's been a successful experiment," Menard said. "We're going to continue it."

Menard said the most common reaction from people who have received the recycled envelops has been: "'Thank you for saving the trees.' That's pretty much my reaction."

The agency has whittled its stock of maps down from more than 115 million to 100 million. It has to keep a large number on hand, Menard said, because the country is divided up into 40,000 quadrangles, ranging from 7 by 5 miles to 15 by 10 miles in size. He said that if only 2,500 copies of each quadrangle are maintained to cover requests, this adds up to 100 million -- the number the Geological Survey now has in stock.

The detailed maps are used in government and business, and by hikers and prospectors.

"They're printed on good paper," Menard said. "They're designed to last out in the field, even to be rained on."

Or be delivered by the U.S. Postal Service.