can't drive night and day. We can. And there's a long way to go. We'll catch him!"

But Sattell had a head start. Five miles from the beginning of the chase, the track they followed swung to the right and down a rolling hillside. They followed. And a seamless highway built of stone, patently artificial, came out of the hillside and stretched away across country.

It was forty feet wide. And here, in some dust that had drifted across it at some spots, they saw the trail of Sattell's car. At other places, even for most of the way, the winds had kept the roadway clear.

Jerry increased his speed. Borden thought to look at the road behind them.

Ellen, understanding, said, "No, the poor creatures couldn't keep up. They were running after us as if their hearts were breaking, but they couldn't make it."

Ten miles farther on, the highway was overwhelmed by wind-drifted sand. The trail of Sattell's fleeing car went up over the sand dune. They went after it. Half a mile farther, the highway was clear again. It swung south, headed out across the desert.

They did not catch sight of Sattell or his car.

For a stretch of twenty-five miles the arrow-straight road was raised above the average level of the sands, and it was wind-swept. Then it went into a low range of rust-colored hills. Here they saw signs again of Sattell's passing. The streaked, rounded furrow of his vehicle's peculiar tread in wind-blown sand across the road.

On the far side of the hills they thought they had overtaken him when they saw the glint of golden metal a little off the highway.

They stopped. Borden and Jerry approached the spot, weapons ready. It was a ground car, past question, one like their own, but it had not been newly wrecked. That disaster had happened generations ago. The car had literally been pulled in half. It had been gripped by something unthinkably powerful and wrenched in two. The metal, strained and stretched before it broke, showed what had happened.

There were bones nearby. Not skeletons. Bones. Individual bones. Not gnawed. Not broken. Simply separated by feet and yards of space.

VI

Some ten miles farther on they came to the first of the forts, a great, towering structure of rocks piled together across the road. It was a parapet sixty feet high, 

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