The huge concave of the rocky wall towering above them threw the woman's scream far into the vast profound of the can. It came sharp to the man's ear, yet terminated abruptly; as when two rapidly moving trains pass, the whistle of one is heard shrill for one moment only to be cut short on the instant. Brief as it was, however, the sound was sufficiently appalling; its suddenness, its unexpectedness, the awful terror in its single te, as well as its instantaneity, almost stopped his heart. With the indifference of experience and long usage he had been riding carelessly along an old pre-historic trail through the can, probably made and forgotten long before the Spaniards spied out the land. Engrossed in his thoughts, he had been heedless alike of the wall above and of the wall below. Prior to that moment neither the over-hanging rock that curved above his head r the almost sheer fall to the river a thousand feet beneath the narrow ledge of the trail had influenced him at all. He might have been riding a country road so indifferent had been his progress. That momentary shriek dying thinly away into a strange silence changed everything.