Watching Jake Barto next morning, I saw that the little image in his hand pointed right across the center of that cloud-topping mountain. That meant we had to go around it, for we were not equipped for such climbing, nor would there have been any sense in it. Jake figured on circling to the left, and I was glad, for I for one wanted no parts of that disk ship that Polter and Noldi had seen in the other direction. Jake ignored me. He was unpredictable! It was a long mountain, and we traveled along one side, toward the north, figuring on crossing to the east wherever a pass appeared. After a time a faint trail showed, and we followed it. It drew us higher, until we were moving perilously along a ledge of rock, with precipitous walls above and a sharp drop below. Higher and higher, above the tree-line now, the path went on, and there were signs of travel along it that worried me. Polter was in the lead, and as we rounded a shoulder of rock, gave a cry of wonder. We hurried after, to see the trail breaking over a low crest of the mountain, and leading now downward. This shoulder of rock outthrust here marked the place where the trail we were following crossed the ridge of the mountain crest at its lowest point. But it also marked something else, which was what had caused Polter's cry. A line of dust across the trail and along the near-bare rocks stirred and lifted and fell fitfully, as if the air was barred passage by some invisible wall, and there were the skeletons of birds that had flung themselves against the invisible wall and died, falling there. There was the skeleton of a goat half across the trail; and at one side, what had once been a man! All these dead--and the bones could be seen here and there along the far line of the dust--had gone so far and no farther. Polter had stopped fearfully ten feet from the clearly marked line--and I for one had no desire to add my skeleton to the others. For a few minutes none of us had anything to say, then reason reasserted itself, and I pressed past Polter, knowing that the thing was an illusion born of coincidence and wind currents. Some baffling current of wind around the mountain formed here a wall of air cleavage, and the skeletons were merely coincidence. I pushed up to the strange line of lifting and falling dust, a little roll showing the magic of invisible force, and pressed on, as if to cross. Behind me a cry gave me pause. I turned, looking for that cry's source, for it seemed to me the cry was the girl I had rescued from Barto. That saved me, for the little horse behind me pressed on across the strange line--and faltered, gave a horse-scream of terror, fell dead before me.