Conrad never tells us exactly when the San Tomé mine was founded. It seems to have always been there, as befitting the overarching symbol of human desire, greed and idealization that it is. "The early days" mentioned here (before the War of Independence), are presumably the days of Spanish dominion, and we can read this line as a specific reference to the methods of the Conquistadors in America. But there is a broader symbolic quality here, evoking the "early days" of mankind as a whole, when great empires arose unabashedly on the backs of slave labor. This broader quality is subtly enhanced by placing Mrs Gould as our witness, since she herself represents the timeless spirit of human compassion. See her introduction as "an old resident" and also the line about her having "seen it all from the beginning."