The warning served to sharpen the boy's wits, and it was time. The river was rushing about ten miles an hour over a winding bed, where the bow could not see ahead for more than twenty or thirty yards, a space covered in a few seconds' time. Suddenly Harry gave a mighty back stroke, and Roger following suit almost instantaneously, the canoe was brought up with a jerk as though some mechanical brakes had been set. There was not much room to spare, for across the river a big tree had fallen, and behind it the ice had jammed, not enough to dam the water absolutely, but affording no possible passage for a canoe. A landing was made, though it was extremely difficult, and the canoes portaged past the obstruction, Rivers having found that the tree had jammed on a harsh and shallow rapid, over which they could not have taken the boats. Then the chief ordered two of the men to cut through the jammed tree so as to break the dam. "Why?" queried Roger of Bulson, as he was cutting and shaping a gigantic wooden crowbar for himself, while a couple of the other men were hacking through the tree; "why is it necessary to take all that trouble after we have got by?" "Supposing we got some distance down the river," was the reply, "where it wasn't easy to make a landing, and this jam broke above us and came pounding down the river, where would we be?" "But it wouldn't be going any faster than the stream, and we could keep ahead of it with paddles." "And if you came to a portage?" "That's true," said Roger, "I hadn't thought of that. We might get nipped between the ice behind and rocks in front." "You see," said Bulson, as he stepped on to the jam, "it's never wise to leave dangers at your heels." The tree having been cut through, all save a few inches, one of the choppers returned to the shore, while the other stood ready, watching Bulson. The latter, who was standing on the blocks of ice behind the tree, was studying their positions, how they were jammed, and what was the best way to free them without getting caught himself in the resultant turmoil. Presently he seemed satisfied for, inserting his huge crowbar between two pieces of ice, he yelled: "Cut!"