a jabbing flight of flashes pierced the darkness

  Suddenly a burst of five shots in quick succession warned Mahon that the Indian was alarmed. Recklessly the Sergeant looked through the window. From just beneath the sleepers that held the rails a jabbing flight of flashes pierced the darkness, pointing along the edge of the bank above the path leading up from Conrad's shack. A pause of only a moment--the Indian was filling his magazine--then another burst of the most rapid firing Mahon had ever heard from one rifle. Not a shot replied from the trees along the bank.   Mahon was puzzled. Was a big attack forming? Did the Indian see some threat of which those in the shack were unaware? Mahon issued sharp orders for increased vigilance. But why shoot in that direction to ward off concentrated attack?   The Indian's bullets continued to pour along the edge of the forest.   Mahon saw the idea. For some reason the bohunks were being driven temporarily to cover. Something--   The moon had moved a little over the top of the dark mass of trees. The grade was lit up. Mahon's eyes ran back and forward along the twin bands of dimly reflecting steel.   A man leaped to the top of the grade from the other side, swayed a little, and plunged forward toward the shack. With the moon full on him in that first moment he loomed unnaturally huge. In a bound Mahon reached the door and threw it open.