The Flaming Forest by James Oliver Curwood

The shock of the amazing discovery which Carrigan had made was as complete as it was unexpected. His eyes had looked upon the last thing in the world he might have guessed at or anticipated when they beheld through the window of St. Pierre's cabin the beautiful face and partly disrobed figure of Carmin Fanchet. The first effect of that shock had been to drive him away. His action had been involuntary, almost without the benefit of reason, as if Carmin had been Marie-Anne herself receiving the caresses which were rightfully hers, and upon which it was both insult and dishonor for him to spy. He realized now that he had made a mistake in leaving the window too quickly.

But he did not move back through the gloom, for there was something too revolting in what he had seen, and with the revulsion of it a swift understanding of the truth which made his hands clench as he sat down on the edge of the raft with his feet and legs submerged in the slow-moving current of the river. The thing was not uncommon. It was the same monstrous story, as old as the river itself, but in this instance it filled him with a sickening sort of horror which gripped him at first even more than the strangeness of the fact that Carmin Fanchet was the other woman. His vision and his soul were reaching out to the bateau lying in darkness on the far side of the river, where St. Pierre's wife was alone in her unhappiness. His first impulse was to fling himself in the river and race to her -- his second, to go back to St. Pierre, even in his nakedness, and call him forth to a reckoning. In his profession of man-hunting he had never had the misfortune to kill, but he could kill St. Pierre -- now. His fingers dug into the slippery wood of the log under him, his blood ran hot, and in his eyes blazed the fury of an animal as he stared into the wall of gloom between him and Marie-Anne Boulain.