In some manner Saleratus Bill had discovered the young man's escape, and had already eliminated the other possibilities of his direction of flight. Bob shuddered at this evidence of the rapidity with which the expert trailer had arrived at the correct conclusion. He could not now skirt the mountain, as he had intended, for that would at once expose him in full view; he could not return by the way he had come, for that would bring him face to face with his enemy. It would avail him little to surrender, for the gun-man would undoubtedly make good his threats; fidelity to such pledges is one of the few things sacred to the race. With some vague and desperate idea of defence, Bob picked up a heavy branch of driftwood. Then, as the man drew nearer, Bob scrambled hastily over the smooth apron to the tiny beach that the eddies had washed out below the precipice.

Here for the moment he was hidden, but he did not flatter himself he would long remain so. He cast his eyes about him for a way of escape. To the one side was the river, in front of him was the rock apron with his enemy, to the other side and back of him was a sheer precipice. In his perplexity he looked down. A gleam of metal caught his eye. He stooped and picked up the half of a worn horseshoe. Even in his haste of mind, he cast a passing wonderment on how it had come there.

If Bob had not been trained by his river work in the ways of currents, he might sooner have thought of the stream. But well he knew that Saleratus Bill had spoken right when he had said that there were "no swimming holes" here. The strongest swimmer could not have taken two strokes in that cauldron of seething white water. But now, as Bob looked, he saw that a little back eddy along the perpendicularity of the cliff slowed the current close to the sheer rock. It might be just possible, with luck, to win far enough along this cliff to lie concealed behind some outjutting boulder until Saleratus Bill had examined the beach and gone his way. Bob was too much in haste to consider the unexplained tracks he must leave on the sand.

He thrust the branch he carried into the still black water. To his surprise it hit bottom at a foot's depth. Promptly he waded in. Sounding ahead, he walked on. The underwater ledge continued. The water never came above his knees. Out of curiosity he tapped with his branch until he had reached the edge of the submerged shelf. It proved to be some four feet wide. Beyond it the water dropped off sheer, and the current nearly wrenched the staff from Bob's hand.

In this manner he proceeded cautiously for perhaps a hundred feet. Then he waded out on another beach.

He found himself in a pocket of the cliffs, where the precipice so far drew back as to leave a clear space of four or five acres in the river bottom. Such pockets, or "coves," are by no means unusual in the inaccessible depths of the great box canons of the Sierras. Often the traveller can look down on them from above, lying like green gems in their settings of granite, but rarely can he descend to examine them. Thankfully Bob darted to one side. Here for a moment he might be safe, for surely no one not driven by such desperation as his own would dream of setting foot in the river.

A loud snort almost at his elbow, and a rush of scurrying shapes, startled him almost into crying aloud. Then out into the moonlight from the shadow of the cliffs rushed two horses. And Bob, seeing what they were, sprang from his fancied security into instant action, for in a flash he saw the significance of the broken horseshoe on the beach, the sunken ledge, and the secret of the horses' pasture. By sheer chance he had blundered on one of Saleratus Bill's outlaw retreats.

Hastily he skirted the walls of the tiny valley. They were unbroken. The river swept by tortured and tumbled. He ran to the head of the cove. No sunken ledge there rewarded him. Instead, the river at that point swept inward, so that the full force of the current washed the very shores.

Bob searched the prospect with eager eye. Twelve or fifteen feet upstream, and six or seven feet out from the cliff, stood a huge round boulder. That alone broke the shadowy expanse of the river, which here rushed down with great velocity. Manifestly it was impossible to swim to this boulder. Bob, however, conceived a daring idea. At imminent risk and by dint of frantic scrambling he worked his way along the cliff until he had gained a point opposite the boulder and considerably above it. Then, without hesitation, he sprang as strongly as he was able sidewise from the face of the cliff.

He landed on the boulder with great force, so that for a moment he feared he must have broken some bones. Certainly his breath was all but knocked from his body. Spread out flat on the top of the rock, he moved his limbs cautiously. They seemed to work all right. He backed cautiously until he lay outspread on the upstream slope of the boulder. At just this moment he caught the sinister figure of Saleratus Bill moving along the sunken ledge.

For the first time Bob remembered the tracks he must have left and the man's skill at trailing. A rapid review of his most recent a............