MacDonald had turned again—­was listening—­and holding his breath.  Then he said, still with his face toward the mountain and the valley: 

“I may be mistaken, Johnny, but I think I heard—­a rifle-shot!”

For a full minute they listened.

“It seemed off there,” said MacDonald, pointing to the south.  “I guess we’d better get back to camp, Johnny.”

He started ahead of them, and Aldous followed as swiftly as he could with Joanne.  She was panting with excitement, but she asked no questions.  MacDonald began to spring more quickly from rock to rock; over the level spaces he began to run.  He reached the edge of the plain four or five hundred yards in advance of them, and was scanning the valley through his telescope when they came up.

“They’re not on this side,” he said.  “They’re comin’ up the other leg of the valley, Johnny.  We’ve got to get to the mount’in before we can see them.”

He closed the glass with a snap and swung it over his shoulder.  Then he pointed toward the camp.

“Take Joanne down there,” he commanded.  “Watch the break we came through, an’ wait for me.  I’m goin’ up on the mount’in an’ take a look!”

The last words came back over his shoulder as he started on a trot down the slope.  Only once before had Aldous seen MacDonald employ greater haste, and that was on the night of the attack on Joanne.  He was convinced there was no doubt in Donald’s mind about the rifle-shot, and that the shot could mean but one thing—­the nearness of Mortimer FitzHugh and Quade.  Why they should reveal their presence in that way he did not ask himself as he hurried down into the plain with Joanne.  By the time they reached the camp old Donald had covered two thirds of the distance to the mountain.  Aldous looked at his watch and a curious thrill shot through him.  Only a little more than an hour had passed since they had left the mountain to follow Joanne, and in that time it would have been impossible for their enemies to have covered more than a third of the eight-mile stretch of valley which they had found empty of human life under the searching scrutiny of the telescope!  He was right—­and MacDonald was wrong!  The sound of the shot, if there had been a shot, must have come from some other direction!

He wanted to shout his warning to MacDonald, but already too great a distance separated them.  Besides, if he was right, MacDonald would run into no danger in that direction.  Their menace was to the north—­beyond the chasm out of which came the rumble and roar of the stream.  When Donald had disappeared up the slope he looked more closely at the rugged walls of rock that shut them in on that side.  He could see no break in them.  His eyes followed the dark streak in the floor of the plain, which was the chasm.  It was two hundred yards below where they were standing; and a hundred yards beyond the tepee he saw where it came out of a great rent in the mountain.  He looked at Joanne.  She had been watching him, and was breathing quickly.