Mounting their horses, they picked their way down the steep face of the plateau and rode out across the bottom heading somewhat up stream until they came out on the river bank, where a little rocky beach shelving down into the water seemed to offer a pleasant spot for swimming. A few yards downstream rose the abrupt walls of one of the buttes, which looked as if it had been built up of many thin horizontal layers of sandstone. Its base was fringed with small brush and willow saplings and here and there a choke-cherry tree, well loaded with ripe fruit, of which the party decided to eat their fill when their swim was over. After their horses had drunk greedily of the fresh, sparkling water, their riders tied them among the saplings, threw off their clothes, and in a moment were laughing and splashing in the cold, clear stream, which, though too shallow to afford much swimming, was delightfully refreshing. They amused themselves for some minutes in picking up and throwing about the curious pebbles and larger stones, worn perfectly smooth and round by the water, which, owing to their resemblance to cannonballs, had given the stream its name. Presently Wallace waded out nearly to mid-channel,--not an easy feat, for the current was quite strong,--and there he found a hole six or seven feet deep. "Hello!" he shouted to his companions. "Watch me duck under and see how long I stay down." Lieutenant Dale and Al stopped motionless to watch him. Wallace crouched down in the water, then sprang erect as high as possible and, jumping forward, disappeared head first into the deeper pool. At the very instant when he turned over in the air his companions were electrified to hear the report of a musket from the base of the butte just below them, and as Wallace went out of sight they saw the bullet kick up a jet of spray apparently not two inches above his back. Wheeling round they saw a feather of smoke rise from the bushes at the further end of the butte, and without a word both of them dashed out of the river to the spot where their clothes lay. Each one of the three had his revolver with him, as always, and in less time than it takes to tell it Al and the Lieutenant, stark naked, had their weapons in their hands. Al heard a splash in the river below them. He sprang down to the water's edge and peered through the bushes. Not thirty yards away an Indian was riding his pony into the stream and Al raised his revolver and fired. The pony sunk to its knees and toppled over, flinging its rider into the water, but the warrior was up again in an instant and waded quickly back to the shore, where he disappeared behind the butte. At this moment Wallace rushed up and caught his revolver from its holster. "He's back of the butte," cried Lieutenant Dale. "We can head him off. You stay here and watch the river, Smith. Come on, Briscoe."