1891-S Morgan Silver Dollar. "S" mint mark, denoting the San Francisco Mint.
Morgan Silver Dollars ran from 1878 to 1904, and then again in 1921. The obverse depicts a profile portrait representing Liberty, modeled by Anna Willess Williams, while the reverse depicts an eagle with wings outstretched. The mint mark, if present, appears on the reverse above the "o" in "Dollar".
In 1898, Congress approved a bill that required all remaining bullion purchased under the Sherman Silver Purchase Act to be coined into silver dollars. When those silver reserves were depleted in 1904, the Mint ceased to strike the Morgan dollar. The Pittman Act, passed in 1918, authorized the melting and recoining of millions of silver dollars. Pursuant to the act, Morgan dollars resumed mintage for one year in 1921. The design was replaced by the Peace dollar later the same year.