What metaphors are used in Voyagers to the West by Bernard Bailyn?

THe Iron Industry was both a symbol and a metaphor for slavery and indentured workers. Just before the Revolution, the American colonies were producing around one-seventh of the world's iron supply or about 30,000 tons every year. Over 250 ironworks existed, and they created a large demand for labor. The plants were built in isolation, often in heavily wooded areas.