The answer to your question depends on WHERE in Marin County the person lived.
If in the San Rafael area and Catholic, then Mt. Olivet Cemetery located at 270 Los Ranchitos Road, San Rafael, CA 94903; if not Catholic, the person would have been buried in Mt. Tamalpais Cemetery, 2500 5th Avenue, San Rafael, CA 94901.
If in Sausalito, the person would most likely have been buried in what was then called Fernwood Cemetery located at 301 Tennessee Valley Road, Mill Valley, CA 94941. About 1972 it was renamed Daphne Fernwood Cemetery. It has since been sold but your letter will reach them even if the name has been altered.
If the person lived in western Marin the chances are that they may have been buried in Bolinas Cemetery or in Olema or Tomales. Tomales had both a Catholicand a Presbyterian church and cemetery. You can contact the Church of the Assumption, Shoreline Highway, Tomales, CA 94971.
If there is a particular person you would like to have looked up, I have a book of burials. Please let me know which cemetery seems most likely.