There was a California CDV that was sold at auction a year or so ago possibly identified as the Eureka Blue Stars ca. 1880's (if memory serves me). I agree with the mining theory as the fans seem to be standing on slag heaps, and the baseball diamond looks like it was strip mined to bedrock, or a good sized cut. Not saying it is California, but it may be another possibility.

Here's the link to the California CDV with players in uniforms. Obviously the players in your photo & this one have different colored ballcaps: http://www.net54baseball.com/showthr...=183660&page=3