During the latter days of our journey I noticed a curious change in the ground. It seemed now, in many places, to be like a soft, chalky limestone, which ran in pockets and seams between strata of very hard rock. I called Miela's attention to it once, and she pointed out a number of irregular shaped, small masses of a substance which in daylight I assumed might be yellow. These were embedded in the soft limestone. "Sulphur," she said. "Like that on your earth. There is much of it up here, I have heard." The Dark City occupied a flat plateau, slightly elevated above the surrounding country, and on the brink of a sheer drop of some six or seven thousand feet to an arm of the polar sea. Our problems now were very different from when we had laid siege to the Lone City. The conformation of the country allowed us no opportunity to approach closer than two or three miles to the barrage of light we must expect. We could not reach the city from these nearest points with our projectors. There were many lateral ravines depressed below the upper surface of the main plateau, and though the light-rays from the city, directed horizontally, would sweep their tops, we found we could traverse many of them a considerable distance in safety. But from the bottoms of them we could only fire our rockets without specific aim and our projectors not at all. Only by the most fortuitous of circumstances did we escape complete annihilation the first moment we appeared within range. We had no idea what lay ahead--although the guides we had brought with us from the Lone City informed us we were nearing our destination--and the scene remained in complete darkness until we were hardly more than five miles outside Tao's stronghold. Then, without warning, his lights flashed on--not only a vertical barrage, but a horizontal one as well--sweeping the higher points of the entire country around for a distance of twelve or fifteen miles. We were, at the moment, following the bottom of a narrow gully. Had we been on any of the upper reaches of the plateau we would undoubtedly have been picked out by one of the roving beams of light and destroyed. We camped where we were, and again for several days I attempted nothing, devoting myself to a thorough exploration of the country about us. The Dark City appeared impregnable. Beams of light from Tao's larger projectors were constantly roaming about the entire plateau that surrounded it, and every higher point of vantage from which one of ours could have reached them must have been struck by their rays a score of times a day.