'Alchemists say that veins of minerals in the earth are twigs and offshoots of an immense Tree whose trunk is the center of the earth, and that metals rise like sap – ' Sterling said, the firelight on his bemused face. Daniel was too tired at first to take the analogy – or perhaps he was underestimating Sterling. He assumed Sterling was prying for suggestions on where to look for gold mines. But later, as Sterling's coach was taking him off towards Charing Cross, he understood that Sterling had been telling him that the growth of money and commerce was – as far as Natural Philosophers were concerned – like the development of that mysterious subterranean Tree: suspected, sensed, sometimes exploited for profit, but, in the end, unknowable.