Bob, Pat, I've done some internet sleuthing since this morning and uncovered numerous newspaper articles, including an obit, of Showalter. I've altered the bio in the book to read as follows:

Name: Daniel Showalter
Civil War service: Lt. Col. 4th Cav, AZ Brigade
Born: c. 1829, Greene Co, PA
Died: March 4, 1866 Mazatlan, Mexico
Occupation: Miner in Mariposa County, CA. State legislator and secessionist. Ran a hotel in Mazatlan postwar.
Other: Arrested in CA in Dec. 1861 while leaving to go South, and imprisoned at Fort Yuma. Freed in May, 1862. Killed a fellow legislator in a duel in early 1861. Died of lockjaw as the result of a bar fight.
Buried: Mazatlan, in the “foreign cemetery”
References: Clendenen, Clarence C., “Dan Showalter—California Secessionist”, California History Magazine, v. 40, pp. 309-325. “Death of Col. Showalter”, Mariposa Free Press, March 10, 1866.

The cemetery may also be known as the "protestant" cemetery, Mazatlan. He was speaker pro tem of the California House prewar.