I thought I would add this documentation for the sake of completion:
Mark apparently had six children with Isabella Erskine; they were not married.
Isabella, born1834, Polmont, was the daughter of William Esrkine/ Isabella McIntyre.
Mark/ Isabella Erskine had the following children in Polmont: Arthur 1858-1935 (Mary Walker 1882); Christina 1861; James 1862 (Helen Wright 1885); Charlotte 1864; Christina 1869; Isabella Erskine Gray 1874 (Charles Miller 1892).
The children used either Gray or Erskine as a surname.
Arthur Gray marriage record: Married 30Dec 1883, Shieldhill Polmont. Arthur Gray 24, Mossend Shieldhill, coal miner, bachelor. Parents, Mark Gray, labourer (reputed father)/ Isabella Erskine, outdoor worker. Mary Walker 19, East Shieldhill Polmont, brickwork worker, spinster. Parents, Robert Walker, coal miner/ Elizabeth Thomson. Witnesses, James Erskine. Isabell Walker.
Arthur Gray (Mary Walker) death record: Died 1Feb 1935, Learigg Shieldhill, coal miner, age 77. Spouse, Mary Walker. Parents, Mark Gray, coal miner (dec)/ Isabell Erskine, outdoor worker (dec). Died of cerebral hemorrhage, 7 days. Informant, Arthur Gray, son.
James Erskine marriage record: Married 31Dec 1885, Mossneuk Polmont. James Erskine 23, Polmont, coal miner, bachelor. Parents, father not noted/ Isabella Erskine, house keeper. Helen Wright 17, Muiravonside, dairymaid, spinster. Parents, George Wright, coal miner/ Margaret White. Witnesses, George Wright/ Catherine Gibson.
Isabella McIntyre Erskine birth record: Born 15June 1874, Mossneuk Polmont, illegitimate. Mother, Isabella Erskine, seamstress.
Note that Isabella was born 4 years after Mark had married Margaret Nimmo.
Isabella Erskine death record: Died 2Jan 1922, Mossneuk Shieldhill Polmont, single, age 79. Parents, William Erskine, coal miner (dec)/ Isabella McIntyre (dec). Died of chronic bronchitis, cardiac failure. Informant, John Erskine, grandson.