"Not on this horse, please," remonstrated the man of science, as all three animals were urged to a fast trot.
The boys decided that as there was no one in sight, the Mexicans had left the valley unguarded for the night, and so did not hesitate to make all the speed they could. As a matter of fact, the valley was seldom visited except when a shipment of stolen cattle or ponies was required. It was, as the professor had said, a natural basin from which there was but one outlet, and that the boys were shortly to find.
For some time they rode along in the dark shadow of the rocky walls, which varied in height from about twenty feet to small precipices of a hundred feet or more.
"Say, it looks as if there wasn't any way out of this basin," began Ralph finally, in an impatient tone.
"There must be," replied Walt; "otherwise, how did they get the cattle and ponies into it?"
"Dropped 'em from a balloon, by the looks of it," rejoined Ralph, with a good-natured laugh at his own stupidity.
"Indeed, it looks as if such might have been the case," said the professor, "for all the visible sign there is of a pathway."
"Hold on! What's that there, dead ahead of us?" exclaimed Walt suddenly.
He had been riding a little in advance, and now drew rein abruptly and pointed to a darker shadow which lay against the gloom of the rock wall.
"Looks like a path," admitted Ralph.