"If the sand-storm had hit this section, we'd been able to form some idee of how long ago them hoofs was planted there," said Pete; "but as it is, ther feller who wondered how ther apple got in ther dumpling didn't hev a harder problem than the nut we've got to crack." "There must have been several of them," said Jack, who had been gazing in the dust, which lay thick on the pathway to the summit of the mesa. "A dozen at least," nodded Pete. He tipped back his sombrero and scratched his ruffled hair, fairly at a standstill to account for what they had encountered. "Mightn't it have been prospectors?" asked Ralph. "Might hev bin, yes," agreed Pete; "but, fer one thing, my son, prospectors don't usually travel in dozens." "Hum--that's so," assented Jack, who at first had greeted Ralph's suggestion eagerly. "Look here!" cried Ralph suddenly, holding up a glittering object which he had just discerned in the bunch-grass at the base of the mesa. "What is it, my boy?" inquired the professor. Ralph extended the object for their inspection. "A strange coin," cried Walt. "Not so blamed strange, either," said Pete, picking it off the boy's palm and examining it. "It's a Mexican peso." "Then the men who were here were Mexicans?" cried Jack. "Not so fast, my boy," admonished Pete. "Might as well say that every feller who finds a Canadian dime in his pocket is a Kanuck. Say," he suggested suddenly, "suppose you boys jes' see if you can find any tracks around the base of the mesa." They scattered and looked carefully about them, but the bunch-grass grew in quite a broad belt all about, and no footmarks could be discerned. Nor did a careful examination of the grass show any broken or trampled blades, as would have been the case had ponies been there recently. "That decides it," announced Pete, after this last fact had been ascertained, "whoever made those foot-marks wasn't here recent, that's a fact. But who could they have been, and what brought them here?" "Maybe Indians," suggested Ralph sagely. "Yep, if Indians wore boots, which they don't," grinned Pete, while poor Ralph colored to the roots of his hair over the general laugh that arose at his expense. "I think," announced the professor finally, "that it would be our best plan to go ahead exploring the mesa. After all, there is nothing here that can hurt us. Those ruffians of Black Ramon's have been driven out of the country, and, anyway, they would not be likely to come here. As for Indians, their reservation is many miles to the north-east. Whoever was here, was either on a scientific quest, like ourselves, or else unfortunately lost in the desert." "Jes' ther same," grunted Pete, in a low voice that nobody overheard, "I'd like ter know what all this means: Big, shadowy shapes flitting around in ther night, and footsteps here in ther mornin'. It don't look right."