Are you sure that he was born in Virginia in 1858 because there is a family tree on Ancestory.com which shows almost identical details (inclusive of Wife and Kids as being born Robert Augusta Thornhill, 1858 in Canton, Madison, Mississippi, USA. This Robert A. Thornhill died 21 July 1922 in Jefferson, Alabama, USA.
If this is the correct Robert A Thornhill then the following details are relevant.
Father = Addison Augusta Thornhill, b.8 Feb 1811 in Bedford County, Virginia, USA. He died 3 September 1902 in Madison, Mississippi, USA
Mother = Harried Thompson Joiner, b.16 Aug 1820 in South Carolina, USA. She died in ? May 1865 in Canton, Madison, Mississippi, USA
(Addison Augusta Thornhill had a second wife Mary Wilkinson b.1844 to d.1909)
This tree links back to Jordan FitzEssulf Thornhill, Constable of Wakefield born in West Yorkshire, England if it is indeed the correct tree.