Well, while I couldn't find a Mason Hartley, I can tell you that a battalion of cavalry often refered to as 2nd Ark Cav due to their earlier enlistments in that unit existed in 1863-64 as the 18th Ark Cav Battalion but most commonly referred to as McMurtrey's Battalion. McMurtrey was also from the area that is now Cleveland Co. and is buried near the intersection of Hwy 79 and 25 in Rison. However, there are no service records for this unit. Many of them still refered to themselves as members of the 2nd Ark Cav and they did serve with their old Commander, Col Slemmons on the Missouri Raid. That late in the war, records were poorly kept and nothing exists after Feb 1864. Probably most of his records if they existed would have been captured at Longview Prairie on Mar 29, 1864 when the wagon train was totally captured. Keep looking, never know when some tidbit will fall into your lap.