It was a mistake to show Mary Polly Callahan as being the wife of Thomas Black the father of Lemuel Black.The parents of Lemuel Black, the father of George Walker Black, were Thomas Black (b. 1742 probably in Anson County, North Carolina) and Mary McKinnley (b. ~1746 in Pennsylvania).Lemuel Black who married Mary Ann Williams (the daughter of Matthew Williams and Barbara Walker) was not the son of Dr. Thomas Gillespie Black who married Mary Polly Callahan.However, some genealogical records show that Thomas Black and Mary McKinnley had a son Thomas Black who married a Mary Callahan.Whether these records are accurate in certainly in question but your Thomas Gillespie Black (b. 26 January 1772) may have some relationship with the George Walker Black lineage.The question is trying to find a way through all the errors in the genealogical records for the Black Families living in Oglethorpe County, Georgia during the time frame of interest.I have found that Thomas James Black was married to a Mary Callahan and in several cases some of this Black lineage were identified as being from Pennsylvania when they were much more likely from Pittsylvania County, Virginia.Many records show a Thomas Black married to a Mary Unknown and for some reason the name Mary Callahan was put in as a gap filler without looking at the dates involved. Some records show Lemuel Black's grandparents to be James Black and Jane Thomas while other records show them to be James Black and Elizabeth Rogers from Ireland.Same is true for the great grand parents of Lemuel as some records show them to be Robert Black and Ann Unknown from Ireland while others show them to be Samuel Russell Black and Agnes Glenn from Ireland. Just one more mystery to be solved.Hope this answers your question.