There was no object near him behind which an enemy might lie concealed; the gray floor of the desert within many hundred miles of him was smooth and flat and obstructionless. Far away, half a mile, perhaps, he saw a thrusting knob of rock, with some cactus fringing it. From where he sat in the saddle it seemed that the rock might be the peak of a mountain reaching upward out of the sea of sand and desert waste--but it was barren on sides and top, and would afford no concealment for an enemy, except at its base. And even the base was not large enough to conceal more than a few men. The rider gazed long at the rock, but could detect no sign of movement near it. He had turned from it, to look again into the western distance, when Purgatory whinnied lowly. Flashing around in the saddle, the rider again faced the rock. And he saw movement there now. The distance was great, but the clarity of the atmosphere brought a moving object distinctly into his vision. The object was a man, and, like a huge fly, he was crawling rapidly up the sloping side of the rock, toward its peak, which flattened abruptly at the summit. The man bore a rifle. The rider could see it dragging from the man's hand; and in a flash the rider was out of the saddle, throwing himself flat behind a low ridge of sand, his own rifle coming to a rest on a small boulder as he trained its muzzle upon the man, who by this time had reached the summit of the rocks in the distance. The rider waited, nursing the stock of the rifle, his eyes blazing, while Purgatory, seemingly aware of an impending tragedy, moved slowly away as though understanding that he must not expose himself. The rider waited, anticipating the bullet that would presently whine toward him. And then he heard the report of the man's rifle, saw that the smoke streak had been directed downward, as though the man on the summit of the rock were shooting at something below him. The rider had been pressing the trigger of his own weapon when he saw the smoke streak. He withheld his fire when he divined that the man was not shooting at him; and when he saw the man on the rock shoot again--downward once more--the rider frowned with embarrassment. "Don't even know I'm here!" he mused. "An' me gettin' ready to salivate him!" He got to his knees and watched, curiosity gleaming in his eyes. He saw the man on the rock fire again--downward--and he noted a smoke spurt answer the shot, coming upward from the base of the rock. The rider got to his feet and peered intently at the rock. And now he saw another man crouching near its base. This man, however, was not the one the man on the summit of the rock was shooting at, for smoke streaks were issuing from a weapon in that man's hand also, but they were horizontal streaks. Therefore the rider divined that the two men must be shooting at another who was on the far side of the rock; and he ran to Purgatory, speaking no word until he had vaulted into the saddle. Then he spoke shortly.