shapes, all cut down into the solid rock with the sides perfectly
straight and smooth. Gerry took off his helmet and scratched his head.

"Now what do you make of that?"

"I know what it looks like to me," Steve said. "It looks just like the
foundations of a city--without the city. Those round pits are the
anchorages of the outer wall. Those square holes are the basements of
tall buildings. Only--somebody has lifted the whole city away."

"You're crazy!" Gerry growled. Steve shrugged.

"Maybe we all are! Anyway, I'm going to take a look into one of those
holes."

Steve walked quickly forward toward the nearest of the round pits.
Suddenly, just as he reached the very edge of the zone of bare rock,
there was a dull clash of steel. Something had seemed to pick Steve up
bodily and hurl him backward. He landed flat on his back on the ground,
his helmet bouncing off and rolling a few feet away.

"It hit me," he shouted.

"What did?"

"I don't know." Steve sat up and rubbed his head. "Y' know, Chief, it
really felt more as though I'd just walked squarely into a solid stone
wall."

"It has just occurred to me," Gerry said slowly, "that maybe that's
exactly what you _did_ do!"

Gerry walked forward cautiously, a foot at a time, one hand stretched
out before him. When he reached a spot on line with the place where
Steve had been stopped, his hand encountered something cool and firm and
smooth. It was like the surface of a highly polished stone wall. Or a
sheet of heavy and invisible glass. He ran both his hands over it. The
thing was continuous and solid. There was nothing visible to the eye,
and he could see far ahead of him across the strangely surfaced rocky
plain, but there was an impenetrable barrier blocking the path.

Stepping back a few feet, Gerry picked up a pebble and tossed it upward.
The stone bounced sharply back as soon as it came in line with the
invisible barrier. He threw the pebble higher and the same thing
happened. There was something mysterious and disquieting about the way
the stone would soar up into the clear air--and then sharply bounce
back from a point in space where nothing at all was visible.

"Magic!" Closana said nervously. Even the Earth-men of the landing party
had drawn together in a compact group, ray-tubes ready and eyes alert.

Gerry moved back a few feet farther, then hurled the stone forward and
upward as high as he could. This time the pebble did not bounce back. It
simply