The County Assayer's Office was a government office in the county in which San Francisco resided in the 19th century. This office employed assayers whose job was to assay metals generated from mining operations.
After traveling back to 1893 San Francisco Earth, Lieutenant Commander Data visited a gentleman named Ches Whitley who worked as this office at this office. During his visit, Data claimed to be a geological engineer who was interested in locating information about mining operations during the 1850s. (TNG: "Time's Arrow, Part II")