He was arranging the two blankets that were to cover him, when he heard a rumbling noise far over his head. At first he thought it was distant thunder echoing along the ridges, but the wisps of cloud were too light and thin to indicate any storm. He saw the horses and mules rise in alarm, and then not one but several of them gave out shrill and terrible neighs of terror, a volume of frightened sound that made young Clarke's heart stand still for a moment. The sound which was not that of thunder, but of something rolling and crashing, increased with terrific rapidity, stopped abruptly for a moment or two and then a huge dark object shooting down in front of his eyes, struck the ground with mighty impact. It seemed to him that the earth trembled. He sprang back several feet and all the horses and mules, rearing in alarm, crouched against the cliff. A great bowlder lay partly buried. It had rolled from the edge of the cliff high above, and he divined at once that the Sioux had made it roll. They had climbed the stony mountains enclosing the defile, and were opening a bombardment, necessarily at random, but nevertheless terrible in its nature. While he hesitated, not knowing what to do, a second bowlder thundered, bounded and crashed into the chasm. But it struck much farther away. The Little Giant came running at the sound, leaving Boyd on guard at the mouth of the pass, and as he arrived a third rock struck, though, like the second, at a distance, and he knew without any words from Will, what the Sioux were now trying to do. As he looked up, a fourth crashed down, and it fell very near. "So that's thar trick?" exclaimed the Little Giant. "Simple ez you please, but ez dang'rous ez a batt'ry o' cannon. Look out, young William, thar's another." It struck so close to Will that he felt the shock and ran back to the shelter of the overhanging cliff, where, driven by instinct, the horses and mules were already crowding. Nor did the Little Giant, brave as he was, hesitate to follow him. "When you're shot at out o' the sky," he said, "the best thing to do is to go into hidin'. One ain't wholly under cover here, but it ud be a long chance ef any o' them rocks got us." "What about Jim, watching at the mouth of the pass?" "He won't stir until he hears from me. He'll set thar, unmoved, with his rifle ready, waitin' fur the Sioux jest ez ef he expected them to come. I'll slip back an' tell him to keep on waitin', also what's goin' on in here." "Skip fast then! Look out! That barely missed you! They're sending the rocks down in showers now."