Great rocks, red as if burned by volcanic fire, hung from either side, almost over their heads, showing only a narrow strip of sky as he looked far up the dark chasm. And narrower yet seemed the pass as they kept on, until suddenly they came to a place where it was only possible to ride in single file. Here the leader halted and made the rest pass on. Mainwaring saw the girls go just before him, and then he followed, just as the crack of a rifle shot and a loud, ringing shout reached his ears. Only too well did he recognize the ring of the long-range rifle of the border king and the sound of Buffalo Bill’s voice. CHAPTER XXVI. IN NICK’S CAVERN. Mainwaring glanced back when he heard these sounds, and he saw the last man of the party, who rode just to the rear of where the leader sat in his saddle, reel and fall from his horse. At the same moment he saw Bill Harkness, the bandit chief, spur on into the narrow pass, bringing up the rear. As he did so he managed, by some contrivance which Mainwaring did not understand, to detach a huge mass of rock. This completely blocked up the road, so that pursuit was made impossible until it was cleared out of the way. Not being able to stop even had he desired to do so, Mainwaring had to keep on with the others a little farther, and then he found himself in the place he had already heard so much about--the far-famed “Nick’s Cavern,” the principal stronghold of the Death Riders, or of what now remained of that villainous band. It was an immense cleft, or yawning mouth, beneath the mountainside, which seemed to have been hollowed out by a river of fire, so lavalike were its curious and fantastic walls. The main cave was large enough for a regiment to have maneuvered in handsomely, while away off into the mountain ran dark halls and avenues. How far these went only a few of the outlaws, who had explored their secret recesses, knew. Mainwaring had no time to see more than that there was quite a large party of men there already, and that fires were burning, which not only served to partially light up the vast place, but gave several men and a few women the opportunity to do some cooking. A cheer greeted the arrival of the leader of the band, who seemed to be popular with most of his men. They greeted him as “Gallant Bill Harkness,” “Brave Bill,” and by other complimentary terms. The only answer which he made was a shrill whistle call, which brought all the men from the interior of the cavern to the spot where he had seated himself when he dismounted from his horse. As soon as they were all gathered around him he raised his hand to command silence and said: “Men! We have been followed closely by scouts, some Indians--Pawnees, I think--and a troop of cavalrymen from the fort. I’ve dropped the cliff rock in their path, but if they’re as spunky and clever as I think they are they’ll try to get us out of here.