But only for two strides.  For there had come to him a warning.  He swung on his heel and waited.  Again he heard the light rumble of shale, and before that had died away a sinister click.  Alert in every fiber, his gaze swept the bluff—­and stopped when it met a pair of beady eyes peering at him over the edge of the precipice.

The two pair of eyes fastened for what seemed like an eternity, but could have been no longer than four ticks of a clock.  Neither of the men spoke.  The outlaw fired first—­wildly, for the arm which held the rifle was cramped for space.  Keller’s revolver flashed an answer which tore through Irwin’s teeth and went out beneath his ear.  With a furious oath the man dropped his weapon and flung himself upward and forward, landing in a heap almost at the feet of the detective.

“Don’t move!” ordered the latter.

Brad writhed forward awkwardly, knew the shock of another heavy bullet in his shoulder, and catching his foe by the legs dragged him from his feet.  Keller’s revolver was jerked over the edge of the precipice as he let go of it to close with the burly ruffian.

Both of them were unarmed save for the weapons nature had given them.  The detailed purpose of the struggle defined itself at once.  Irwin meant by main strength to fling the detective into the gulf that descended sheer for five hundred feet.  The other fought desperately to save himself by dragging his infuriated antagonist back from the edge.

They grappled in silence, save for the heavy panting that evidenced the tension of their efforts.  Each tried to bear the other to the ground, to establish a grip against which his foe would be helpless.  Now they were on their knees, now on their sides.  Over and over they rolled, first one and then the other on top, shifting so fast that neither could clinch any temporary advantage.

[Illustration:  THEY GRAPPLED IN SILENCE SAVE FOR THE HEAVY PANTING THAT EVIDENCED THE TENSION OF THEIR EFFORTS. Page 340]

Yet Keller, with a flying glance at the cliff, knew that he was being forced nearer the gulf by sheer strength of muscle.  Irwin, his jaw shattered and his shoulder torn, was not fighting to win, but to kill.  He cared not whether he himself also went to death.  He was obsessed by the old primeval lust to crush the life out of this lusty antagonist, and his whole gigantic force was concentrated to that end.  He scarce knew that he was wounded, and he cared not at all.  Backward and forward though the battle went, on the whole it moved jerkily toward the chasm.

The end came with a suddenness of which Larrabie had but an instant’s warning in the swift flare of joy that lit the madman’s face.  His foot, searching for a brace as he was borne back, found only empty space.  Plunged downward, the nester clung viselike to the man above, dragged him after, and by the very fury of Irwin’s assault flung him far out into the gulf head-first.