The urns were facing the tower at last. Gordon dropped his bow, put out a hand, burying his fingers in the smooth flesh of Karola's nude shoulder. "All over, all over. Jolly good fight while it lasted." "Thor, Thor," Karola whimpered. In another instant, the urns would thunder out their destructive fury. But the moment lingered into minutes, and still the urns were silent. A wondering babble broke from the throats of the androids. Some of them bent and stared within the urns, where tiny green flames flickered. Those green flames should have annihilated the last of the outlaw settlement. Yet they did not. Karola looked at Peter Gordon. "Do you think--Thor--?" * * * * * Aava knew he was on the rock bridge now. Thor knew that Aava knew, and still he dug and battered his axe upward. He had a depression sculpted from the roof. A few more blows and-- The axe dug in. Thor pulled it loose. He heard Aava, then. A blast of titanic heat, of power unimaginable, came roaring up at him. Thor leaped outward, away from the bridge. For a moment he hung a hundred feet above the jagged floor of the cave. In that instant, Aava hurled himself upward, filling the cave with radiance and intolerable heat. Thor threw wide his arms, closed them on a stalactite dropping its thin rock formation from the roof. His legs spraddled the drooping stone, hugging it. Aava raged, biting and burning at the stone bridge, seeking his quarry. Sullenly, he dropped back within the quartz oval. Thor almost missed the bridge, leaping back for it. His hands scrabbled at the loose shale, sliding and slipping, before his fingers tightened on a rough projection. With insane might, he flung himself and his axe again at the depression. Before Aava gathered himself once more, he had to do it. The axe dug in. When he pulled it loose, a few flecks of sand slid with it. The thin grains showered downward, running in a steady stream. "Earthling, stop! The sand must not come down on me. Stop and--" The voice of Aava rose to a shrill crescendo, battering at his ears. But Thor worked on. His axe arm lifted. The crack widened. Tons of sand hung above that thin roof, on delicate balances. By opening the roof even so slightly as he had done, he was destroying that balance. An incredible weight of sand was waiting, waiting-- Aava rose in all his might and splendour, to seal the crack. And the sand fell. Thor reeled back, battered by thundering deserts. He hung on what was left of the rock bridge, staring. Upreared in green iridescence, showered by falling tons of sand that formed a tan curtain around him, Aava writhed. His great bulk was twisted into strange convolutions, distorted grotesqueries of liquid movement. A great spray of fire lapped out and upward to seal the gap through which the sand streamed downward. It rose against the falling tons, and was pressed back and down.