He took a swift glance all about him. In every direction lay the desert--glittering, far-reaching, lonely as the open sea. The only break in the monotony came to the south--on the border--where stretched the rocky, desolate ridge. "No one wouldn't come here without an object," reasoned Pete to himself, as they began the ascent of the narrow, tortuous trail, "now, what in thunder could that objec' hev bin?" CHAPTER V. THE HOLLOW ALTAR. "Magnificent indeed!" The words, falling from the professor's lips, echoed hollowly against the walls of the lofty, vaulted chamber in which the adventurers found themselves, after traversing a narrow passage leading inward from the causeway. The walls of this chamber, which must have been fully thirty feet in height at its greatest altitude, were formed of the soft rock, out of which it had been excavated apparently uncounted ages before. They were daubed with grotesque figures in faded, but still discernible, colors. Most of these figures had to do with scenes of violence, and in almost all of them the figure of what appeared to be an enormous rattlesnake, with human head and arms, predominated. Among the mural decorations were some that puzzled the professor considerably. They were crude drawings of men in what appeared to be intended for boats. The professor found these inexplicable. The very idea of boats in that arid spot seemed absurdly out of place. Why, then, should the mesa-dwellers have depicted them? Light was furnished to the chamber by an irregularly shaped hole in the roof above. Although there was plenty of illumination, it had yet been some moments before the adventurers, coming out of the brilliant sunlight outside, grew used enough to the gloom to make out their surroundings. When they did so, the first words uttered were those of the professor recorded above. Like some queer, long-legged bird, the man of science, with a giant magnifying glass held up to his eye, sped hither and thither on his long, angular limbs, inspecting minutely the drawings and crude attempts at decoration. Already he had out his tape-measure and sketch-book, making observations and recording measurements. Presently, however, he recalled himself from the first heat of his enthusiasm. "After all," he said, "we shall have plenty of time in which to explore this chamber, which seems to have been used as a council hall. Let us examine the remainder of this remarkable place." "You may well call it that, perfusser," grunted Pete. "It's remarkable fer the dust thet's in it, if nothing else. But what I'd like to know," he added to himself, "is jes' whar the owners of them footsteps vanished themselves to." Which brings us to a remarkable discovery, made a few moments before our party had entered the "Council Hall," as the professor called it.