Hi Bonnie, I can't find a Mt. (or Mount) Pleasant Cemetery in Placer Co.There's a list of cemeteries at www.cagenweb.com/placer/cemeteries.htm . It also lists the contact info for the five cemetery districts in Placer Co.
I did a Google search and there is a Mount Pleasant Valley in Placer Co. That might be helpful info when talking to the cemetery board people.
It's also not in findagrave.com.
The Placer County Genealogical Society, www.pcgs.pcgenes.com/ also has a list of cemeteries. They're not listed on-line but if you contact them, they might be able to shed more light on this.
Death records weren't required in California on a state-wide basis until 1905. Most counties have earlier ones but I don't know if they'll have one as early as 1857. Contact the Placer County Clerk-Record-Registrar of Voters, www.placer.ca.gov/Departments/Recorder/Vital%20Records.aspx. They will charge if they find the record or not.
Being that it's from 1857, it might be in the Online Archives of California. Successful Searching!