The tenuous vapor, too, seemed to swirl his way. It made his lungs smart and his eyes water. He still breathed in painful gasps, but even that short period of rest had done him some good. But it was the heat that drove him to his feet again. He crawled painfully to the crest of the next hill. He looked back. This was the highest hill he had come upon and he could see most of the purple range in the deep, deep dusk. Now he was more than halfway through the hills. He had barely a quarter-mile to go, northward. But east and west the range of purple hills was a ceaseless, undulating mass of lifts and hollows, of ridges and spurs of all imaginable colorings. And at the tips of most of them were wisps of curling gray. From his position he could see a long stretch of the hills not hidden by the surrounding darkness. Back along the way he had come, the army ants now swept up into the range of hills. Scouts and advance-guard parties scurried here and there. They stopped to devour the creatures inhabiting the surface layers. But the main body moved on inexorably. The hills, though, were alive, not upheavals of the ground but festering heaps of insanely growing fungus, hollowed out in many places by tunnels, hiding-places, and lurking-places. These the ants invaded. They swept on, devouring everything.... Burl leaned heavily upon his club and watched dully. He could run no more. The army ants were spreading everywhere. They would reach him soon. Far to the right, the vapor thickened. A thin column of smoke arose in the dim half-light. Burl did not know smoke, of course. He could not conceivably guess that deep down in the interior of the insanely growing hills, pressure had killed and oxidation had carbonized the once-living material. By oxidation the temperature down below had been raised. In the damp darkness of the bowels of the hills spontaneous combustion had begun. The great mounds of tinderlike mushroom had begun to burn very slowly, quite unseen. There had been no flames because the hills' surface remained intact and there was no air to feed the burning. But when the army ants dug ferociously for fugitive small things, air was admitted to tunnels abandoned because of heat. Then slow combustion speeded up. Smoulderings became flames. Sparks became coals. A dozen columns of fume-laden smoke rose into the heavens and gathered into a dense pall above the range of purple hills. And Burl apathetically watched the serried ranks of army ants march on toward the widening furnaces that awaited them.