Sheffield was moderately pleased. Mark had been doing better than expected. To be sure, he scarcely talked but that was not very serious. At least, he showed interest and didn't sulk. And he threw no tantrums.

Vernadsky was even telling Sheffield that last evening Mark had spoken to him quite normally, without raised voices on either side, about planetary crust analyses. Vernadsky had laughed a bit about it, saying that Mark knew the crust analyses of twenty thousand planets and someday he'd have the boy repeat them all just to see how long it would take.

Mark, himself, had made no mention of the matter. In fact, he had spent the morning sitting in his tent. Sheffield had looked in, seen him on his cot, staring at his feet, and had left him to himself.

What he really needed at the moment, Sheffield felt, was a bright idea for himself. A really bright one.

So far, everything had come to nothing. A whole month of nothing. Rodriguez held fast against any infection. Vernadsky absolutely barred food poisoning; Novee shook his head with vehement negativeness at suggestions of disturbed metabolism. "Where's the evidence?" he kept saying.

What it amounted to was that every physical cause of death was eliminated on the strength of expert opinion. But men, women, and children had died. There must be a reason. Could it be psychological?

He had satirized the matter to Cimon for a purpose before they had come out here, but it was now time and more than time to be serious about it. Could the settlers have been driven to suicide? Why? Humanity had colonized tens of thousands of planets without its haying seriously affected mental stability. In fact, the suicide rate, as well as the incidence of psychoses, was higher on Earth than anywhere else in the Galaxy.

Besides, the settlement had called frantically for medical help. They didn't want to die.

Personality disorders? Something peculiar to that one group? Enough to affect over a.thousand people to the death? Unlikely. Besides, how could any evidence be uncovered? The settlement site had been ransacked for any films or records, even the most frivolous. Nothing. A century of dampness left nothing so fragile as purposeful records.

So he was working in a vacuum. He felt helpless. The others at least, had data; something to chew on. He had nothing.

He found himself at Mark's tent again and looked inside automatically. It was empty. He looked about and spied Mark walking out of the camp and into the woods.

Sheffield cried out after him, "Mark! Wait for me!"

Mark stopped, made as though to go on, thought better of it, and let Sheffield's long legs consume the distance between them.

Sheffield said, "Where are you off to?" (Even after running it was necessary to pant in Junior's rich atmosphere.)

Mark's eyes were sullen. "To the air-coaster."

"Oh?"

"I haven't had a chance to look at it."

"Why, of course you've had a chance," said Sheffield. "You were watching Fawkes like a hawk on the trip over."

Mark scowled. "Everyone was around. I want to see if for myself."

Sheffield felt disturbed. The kid was angry. He'd better tag along and try to find out what was wrong. He said, "Come to think of it, I'd like to see the coaster myself. You don't mind having me along, do you?"

Mark hesitated. Then he said, "We-ell. If you want to." It wasn't exactly a gracious invitation.

Sheffield said, "What are you carrying, Mark?"

"Tree branch. I cut if off with the buss-field gun. I'm taking it with me just in case anyone wants to stop me." He swung it so that it whistled through the thick air.

"Why should anyone want to stop you, Mark? I'd throw it away. It's hard and heavy. You could hurt someone."

Mark was striding on. "I'm not throwing it away."

Sheffield pondered briefly, then decided against a quarrel at the moment. It would be better to get to the basic reason for this hostility first. "All right," he said.

The air-coaster lay in a clearing, its clear metal surface throwing back green high lights (Lagrange II had not yet risen.)

Mark looked carefully about.

"There's no one in sight, Mark," said Sheffield.

They climbed aboard. It was a large coaster. It had carried seven men and the necessary supplies in only three trips.

Sheffield looked at its control panel with something quite close to awe. He said, "Imagine a botanist like Fawkes learning to run one of these things. It's so far outside his specialty."

"I can run one," said Mark suddenly.

Sheffield stared at him in surprise. "You can?"

"I watched Dr. Fawkes when we came. I know everything he did. And he has a repair manual for the coaster. I sneaked that out once and read it."

Sheffield said lightly, "Well, That's very nice. We have a spare navigator for an emergency, then."

He turned away from Mark then, so he never saw the tree limb as it came down on his head. He didn't hear Mark's troubled voice saying, "I'm sorry, Dr. Sheffield." He didn't even, properly speaking, feel the concussion that knocked him out.