It's hard to read but this tombstone marks the grave of Corporal Francis Marion Jones [source TNGenWeb], brother of my 3xgreat-grandfather Joseph Jones. Francis was born in Jefferson County, Tennessee in 1844. He enlisted in the Union Army on 6 May 1863, mustering into the 9th Tennessee Cavalry on 15 August 1863 at Camp Nelson, Kentucky.
Francis was captured and held at Morristown, Tennessee in present day Hamblen County by Confederate Major General Breckinridge. His prisoner of war record via Fold3.com is below:
Corporal Jones is buried in Cedar Grove Cemetery in Hamblen County, Tennessee, near his Thornhill cousins. It is likely that his mother Elizabeth Thornhill Jones is buried in one of the unmarked graves in this cemetery as well.