Under these circumstances, it would seem that there was really but one course for them to pursue, which was, to remain where they were until night, and then trust to the assistance of darkness for them to get away without being discovered. Such, we say, was the course dictated by common sense and prudence, and yet a most unexpected and novel cause now appeared to prevent it. There are few of our readers, perhaps, who are not aware of the rapidly increasing distress caused by an unusual sound in the ears, when it is continued for any length of time. The screaming of a car-whistle, or the near booming of a cannon, after a while becomes unbearable, and will drive mad the person who has not become accustomed to it. In the excitement of the occasion, and the danger from the Blackfeet, the lads were insensible to any discomfort from the overwhelming roar of the falls; but now, when standing directly beneath them and reflecting that they would probably have to wait for hours, they became aware of a growing uneasiness--a nervous restlessness, rapidly changing into distress, which convinced both that they could not maintain their position and retain their senses at the same time. It would not be long before the time would come when they would be willing to incur any risk to escape the intolerable anguish of this uproar. Harry was ready to make the leap at once, but Little Rifle had hopes of accomplishing the result by another plan. It will be remembered that he had started to explore what seemed to be a cavern, the opening of which was but a short distance away; but he had withdrawn from the attempt under the conviction that there was more imminent need of him elsewhere. He was now ready to take up the work again, and motioning for Harry to follow, he moved toward the dark, forbidding depth of what might be an interminable cavern. To give himself the free use of his arms, he had strapped his rifle to his back, and he now groped cautiously along, like a man feeling his way in a dark room. Of course the darkness rapidly deepened, until in a few minutes it became impenetrable. The floor of the cavern was slippery, and the utmost care was necessary to prevent a fatal catastrophe. When their eyes became of no further use, both sunk upon their hands and knees, and crept along in this fashion, Harry keeping so close that he could touch the foot of Little Rifle, who used one hand, as an elephant uses his trunk in venturing upon a treacherous support. Two noteworthy facts impressed themselves upon the lads. The floor of the cavern was becoming freer from dampness, and after a time was absolutely dry--a most gratifying change, for they had had so much of water during the last few hours, that it seemed to both that a day or two of scorching thirst would be acceptable, as a means of relief.