The huge concave of the rocky wall towering above them threw the woman's scream far into the vast profound of the cañon. 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 note, 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 cañon, 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 nor 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. The man was riding a sure-footed mule, which perhaps somewhat accounted for his lack of care, and it seemed as if the animal must also have heard and understood the meaning of the woman's scream, for with no bridle signal and no spoken word the mule stopped suddenly as if petrified. Rider and ridden stood as if carved from stone. The man's comprehending, realizing fear almost paralyzed him. At first he could scarcely force himself to do that toward which his whole being tended—look around. Divining instantly the full meaning of that sudden cry, it seemed hours before he could turn his head; really her cry and his movement were practically simultaneous. He threw an agonized glance backward on the narrow trail and saw—nothing! Where there had been life, companionship, comradeship, a woman, there was now vacancy. The trail made a little bend behind him, he could see its surface for some distance, but not what lay beneath. He did not need the testimony of his eyes for that. He knew what was down there. It seemed to his distorted perceptions that he moved slowly, his limbs were like lead, every joint was as stiff as a rusty hinge. Actually he dropped from the mule's back with reckless and life-defying haste and fairly leaped backward on his path. Had there been any to note his progress, they would have said he risked his own life over every foot of the way. He ran down the narrow shelf, rock strewn and rough, swaying upon the unfathomable brink until he reached the place where she had been a moment since. There he dropped on one knee and looked downward.