"Many expeditions have been fitted out for the purpose, my boy," was the rejoinder, "but so far no trace has ever been found of it, and it is, no doubt, like the lost river of which Pete was telling us, a mere myth." "I didn't say it was a miff," protested Pete. "I jes' said I didn't believe it." The remainder of that afternoon was spent in making more measurements and sketches of the interesting mesa, and the boys, on their own account, conducted a search for a possible entrance to the lost river. But, as may be supposed, they found none. "I guess as romance-seekers we are not a success," said Jack, as at sun-down they prepared to quit. "Just think, what a proud bunch we'd have been if we could say we--The Border Boys--discovered the lost river of the mesa dwellers." "We might be a sorry bunch, too," amended the practical Walt. "I tell you, Jack, I don't want anything to do with lost rivers, especially when they are underground." "Walt, the spirit of adventure is lacking in you," laughed Jack. "You'd never make a Don Quixote----" "A donkey who?" asked Walt innocently. "Oh, you're the limit," chuckled Ralph, going off into a roar of laughter at the ranch boy's expense. That evening the animals' pasture was changed to the opposite side of the mesa, where they could find fresh grass. The camp, however, was left as it was. After supper watches were assigned, as usual, the latter part of the night guardianship falling to Coyote Pete and Jack once more. When, soon after midnight, Walt and Ralph Stetson aroused them, there was nothing much to report except that One Spot had engaged in a spirited kicking match with his brethren. Outside of that, all had been, to quote Walt: "Quiet along the Mesomac." "We'll patrol round the whole mesa," said Coyote Pete, as he and Jack shouldered their rifles, "meeting by the stock on the other side." After a few words more, the two sentries strode off into the darkness in different directions, meeting, as arranged, by the stock. Neither had anything to report, and in this way they kept up the night watch for an hour or more. They had met for the sixth time by the tents containing their sleeping comrades, when from the opposite side of the mesa came a shrill neigh of terror, followed by sounds of wild galloping and snorting. "Something's up!" shouted Pete, as, with his rifle in readiness and followed closely by Jack, he tore around the mesa to ascertain the cause of the trouble. As the two sentries emerged into view of the spot in which the stock had been tethered, they came upon a spectacle which, for a moment, caused them to recoil as abruptly as if a deep canyon had suddenly opened up before them. CHAPTER VII. WHAT CAME ACROSS THE DESERT.