On Monday night at approximately 10:30 pm, a sheriff's deputy conducted an enforcement stop on a vehicle that had a mechanical violation in the 500 block of Redwood Highway Frontage Road in Mill Valley.  A search of the vehicle produced a loaded semi-automatic firearm with two additional magazines that were also loaded.  Gabriel Michael Holmes of Mill Valley was arrested for carrying a loaded concealed handgun in a vehicle and was transported to the Southern Marin Sub-Station.  Holmes was released on a citation to appear in court.