Gresham's law
The US Treasury was put on a strict hard money standard, doing business only in gold or silver coin as part of the Independent Treasury Act of 1846, which legally separated the accounts of the federal government from the banking system. However the fixed rate of gold to silver overvalued silver in relation to the demand for gold to trade or borrow from England. Following Gresham's law, silver poured into the US, which traded with other silver nations, and gold moved out. In 1853 the US reduced the silver weight of coins, to keep them in circulation, and in 1857 removed legal tender status from foreign coinage.