A gol rush is a period of feverish migration of workers into the area of a dramatic discovery of commercial quantities of gold. Several gold rushes took place throughout the 19th century in Argentina, Australia, Brazil, Canada, Chile, New Zealand, South Africa, and the United States. Gold rushes helped spur permanent non-indigenous settlement of new regions and define a significant part of the culture of the North American and Australian frontiers. As well, at a time when money was based on gold, the newly-mined gold provided economic stimulus far beyond the gold fields. The history of gold rushes extends back to the Roman empire as well as the Spanish conquest of the Americas, and probably yet further back to Ancient Egypt and Saudi Arabia. The first known map is of a gold mine in Nubia, for example. The gold mining activities were described by Diodorus Siculus and Pliny the Elder.