Many of the stories are 150 years old.
The San Francisco Chronicle reports (http://bit.ly/1Pi8lG7) Twain wrote some of the letters and stories at the newspaper's offices when it was called the San Francisco Dramatic Chronicle.
Twain's job included writing a 2,000-word story, or "letter," every day and sending it off by stagecoach for publication in the Territorial Enterprise newspaper in Virginia City, Nevada.
He wrote about everything from San Francisco police to mining accidents, all with varying degrees of truth and humour.
Bob Hirst is editor of UC Berkeley's Mark Twain project and says the articles were found by combing through western
Information from: San Francisco Chronicle, http://www.sfgate.com