The implement employed by Nature in fashioning her underground caverns is usually water. Some mighty spring, deep within the earth's bosom, seeks an outlet for its accumulating current. It forces its way through whatever porous layer of rock comes in its path, and by persistent action, occupying ages of time, disintegrates and destroys it altogether. There is left, as a result of the subterranean stream's activity, a series of tunnels, widening out oftentimes into great rock chambers, and extending, in several well known instances, for many miles. Wherever water is the sole architect the lines that it carves, the forms it molds, are smooth, well-rounded; there are no jagged edges, sharp angles in the fairy palaces and intricate labyrinths that it leaves as specimens of its artistic method. The walls of the Guatavita tunnel, however, were eloquent of a totally different force employed in their making. The marks of an angry Titan were upon them; the Titan of Fire. They told of an elemental tragedy, swift and cataclysmic in its action. The deep scars in their surfaces, the rough crags and pinnacles jutting from them, were the epic characters in which the monster's struggle for freedom were written down for all posterity to study and wonder at. Thus, Leighton did not hesitate to attribute an igneous origin to the cave, and it was after a close examination of the earth and pebble-strewn floor that the smaller tunnel was chosen as the best for exploration. There were footprints in both tunnels, but in this one they were more numerous than in the other, where they had been made, according to Raoul, at the time dynamite had been used in the excavations. Comparing these footprints, those in the larger tunnel were evidently from ordinary shoes, while in the smaller they bore the impress of sandals. "Andrew's man in the toga is the one we want," remarked Leighton, a decision that added to Mrs. Quayle's agitation and did not appear to increase the schoolmaster's desire for adventure. The discovery of the imprint of sandaled feet, however, changed Doctor Miranda's attitude toward Andrew from banter almost to admiration. "It is true, what he say, this leetle fellow," he declared in astonishment. "He follow him here, the sandals--and he is alone. He is brave man, this Parmelee!" Raoul remained silent and Herran shrugged his shoulders skeptically. After all, it was difficult to believe, on the strength of a mere footprint, that the singular being described by the schoolmaster actually existed and had disappeared, like some wraith, in the depths of the cave. "That will be a hard path to follow," said Raoul finally. "I tried it--once." "What did you find?" "Nothing--a dead wall." "Mercy!" ejaculated Mrs. Quayle, not catching his meaning. "There was no danger that I could see," continued Raoul; "but there was hard traveling, and no result worth the effort." "Did you notice these footprints when you were here before?"