Thanks - I see where Margaret Black fits in now....

1865 MARRIAGE THOMAS HARRIS (age 24 - born 1841?) AND MARGARET BLACK (age 18)
Father Robert Harris (Ag Lab)
Mother Elizabeth Sinclair
address 29 Brown Street, Glasgow
occ: Iron Moulders Labourer
1867/1868 BIRTH OF HUGH HARRIS (aged 29 on his marriage Cert)
is there a birth cert? presumably would name Thomas and Margaret Black as parents.
1897 MARRIAGE HUGH HARRIS
Father Thomas Harris (Iron Moulder deceased)
Mother Margaret Dunlop formerly Harris originally BLACK
address 412 Garscube Road, Glasgow
1886 - DEATH THOMAS HARRIS
Iron moulder aged 43 (born 1843?)
Informant George Harris (Brother)(129 Garbraid St Glasgow)
Father Samuel HARRIS (farmer - deceased)
Mother Sarah Harris originally Haggin (deceased)
address: 541 Garscube Road, Glasgow
So - Margaret Harris married Thomas Black in 1865, and had a son Hugh (in 1867/8) who marries in 1897. Hugh names his parents Thomas & Margaret. Thomas names his parents Robert and Elizabeth.
BUT in 1886- There is a death cert for a Thomas Harris - who was also married to a Margaret Black. informant is his brother George - GEORGE gives Thomas' parents names as Samuel and Sarah. NB the age on this is two years younger than on the marriage cert

POSSIBLILITES...
a) there are two Thomas Harris and they both married a Margaret Black (BUT you only found ONE marriage certificate. Seems unlikely but not impossible. Is there another death cert anywhere?
b) They are the same Thomas Harris but he gave wrong names for parents on marriage cert. These same names appear on son Hugh's marriage cert so seems more likely to be correct.
c) They are the same Thomas Harris but GEORGE gave the wrong parents names on the death cert.
....WHY would he do this? Perhaps Thomas Harris is not really his full brother, perhaps they had different parents and George didn't know?
NB: ADDRESS is given as541 Garscube Road (although not same number house as that shown for Hugh's marriage(at 412).
d) Registrar was having a senior moment and just wrote it all down wrong!

It is all very odd. It's look like Robert and Elizabeth is more likely to be right as that is what Thomas himself believed - but there is certainly some confusion on George's part about who Thomas' parents were.
Is there a marriage for a Samuel Harris and Sarah Haggin?
Do you have birth cert for George Harris (the brother?)
Milly
