Several times they entered into wooded valleys full of smoke, and followed the traces of dragged logs across the ground until they came to gunpowder-mills. Here, tall whip-thin trees, the trunks hairy with miserable scrawny branches, * were cut and burnt endlessly until they became charcoal. This was taken to a water-powered mill to be ground to dust and mixed with the other ingredients. Men came out of these mills looking all drawn and nervous from never really knowing when they'd be blown up, and the Doctor supplied them with brimstone and saltpeter from the wagons. Teaching Jack that wars, like great rivers, had their wellsprings in numerous high remote valleys.