Steinfield shook his head vigorously from side to side, causing the two tufts of white hair that jutted from the sides of his otherwise smooth head to wave about in a frenzy. "No. We can tell that it's about as old as the rest of the Solar System. What I mean is—it hasn't been where it is very long."

He caught Hunt's shoulder and half turned him to face a wall chart showing a sectional view through the Lunar center. "You can see it on this. The red shell is the original outer crust going right around—it's roughly circular, as you'd expect. On Farside—here—this blue stuff sits on top of it and wasn't added very long ago."

"On top of what used to be the surface."

"Exactly. Somebody dumped a couple of billion tons of junk down on the old crust—but only on this side."

"And that's been verified pretty conclusively?" Hunt asked, just to be doubly sure.

"Yeah . . . yeah. Enough bore holes and shafts have been sunk all over Farside to tell us pretty closely where the old surface was. I'll show you something over here . . ." A major section of the far wall comprised nothing but rows of small metal drawers, each with its own neatly lettered label, extending from floor to ceiling. Steinfield walked across the room, and stooped to scan the labels, at the same time mumbling to himself semi-intelligibly. With a sudden "That's it!" he pounced on one of the drawers, opened it, and returned bearing a closed glass container about the size of a small pickle jar. It contained a coarse piece of a light gray rocky substance that glittered faintly in places, mounted on a wire support.

"This is fairly common KREEP basalt form Farside. It—"

"'Creep'?"

"Rich in potassium—that is, K—rare earth elements, and phosphorus: KREEP."

"Oh—I see."

"Compounds like this," Steinfield continued, "make up a lot of the highlands. This one solidified around 4.1 billion years ago. Now, by analyzing the isotope products produced by cosmic-ray exposure, we can tell how long it's been lying on the surface. Again, the figure for this one comes out at about 4,100 million years."

Hunt looked slightly puzzled. "But that's normal. It's what you'd expect, isn't it?"