Douglas (occasionally spelled "Douglass") is a common surname of Scottish origin, thought to derive from the Gaelic "dubh glas", meaning "black stream".
 There are numerous places in Scotland from which the surname is derived.
 The surname has developed into the given name "Douglas".
 "Douglas" is a habitational name, which could be derived from any of the many places so-named.
 While there are numerous places with this name in Scotland, it is thought, in most cases, to refer to Douglas, South Lanarkshire, the location of Douglas Castle, the chief stronghold of the Lords of Douglas.
 The Scottish Gaelic form of the given name is "Dùbhghlas"; the Irish language form it is "Dúghlas", and "Dubhghlas", which are pronounced ] .
 According to George Fraser Black, in southern Argyllshire the surname is an Anglicised form of the surnames "MacLucas", "MacLugash" (which are derived from the Gaelic "Mac Lùcais").
Matheson is a surname derived from the patronymic form of a short form of the English "Matthew".
 This English personal name is ultimately derived from the Hebrew "Mattathia", which means "gift of God".
 An early record form of the surname "Matheson" is "Mathyson", recorded in 1392; this recorded name literally means "son of "Mathi""—"Mathi" being a pet form of "Matthew".
 Two different Scottish Gaelic surnames have been Anglicised "Matheson".
 One such surname is "Mac Mhathghamhuin" (Clan Matheson), which became Anglicised "Matheson" on account of its similar sound.
 This Gaelic surname is of an entirely different etymology than "Matheson", as the Gaelic "mathghamhuin" means "bear".
 Another Gaelic surname Anglicised "Matheson" is "Mac Matha".
 This Gaelic surname is derived from the patronymic form of a Gaelic form of "Matthew" (for example, the modern Scottish Gaelic "Mata" and "Matha" are cognates of the English "Matthew").
Douglas Castle was a stronghold of the Douglas family from medieval times to the 20th century.
 The first castle, erected in the 13th century, was destroyed and replaced several times until the 18th century when a large mansion house was built in its place.
 This too was demolished in 1938, and today only a single corner tower of the 17th-century castle remains.
 The castle was the former family seat of the Prime Minister, Sir Alec Douglas-Home.
 The castle was located around 1 km north-east of the village of Douglas, South Lanarkshire, in south-west Scotland.
 The remains are protected as a category C listed building.
The olallieberry (pronounced oh-la-leh, sometimes spelled ollalieberry, olallaberry, olalliberry, ollalaberry or ollaliberry) is the marketing name for the 'Olallie' blackberry released by the USDA-ARS (in collaboration with Oregon State University) that was a selection from a cross between the 'Black Logan' (syn.
 'Mammoth'), developed by Judge James Logan in California, and the youngberry, which was developed by Byrnes M. Young in Louisiana.
 According to Judge Logan, 'Black Logan' was from a cross between the eastern blackberry 'Crandall' and the western dewberry 'Aughinbaugh'. '
Youngberry' was from a cross of 'Phenomenal' x 'Austin Mayes'.
 'Phenomenal' in turn is a cross of the 'Aughinbaugh' western dewberry and 'Cuthbert' red raspberry and so has a very similar background to Logan's 'Loganberry' and shares a parent with his 'Black Logan'.
O'Dwyer is an Irish surname from the Gaelic "Ó Dubhuir", meaning "black".
 O'Dwyer in Irish is "O'Duibhir" (pronounced "O Dweer"), meaning grandson of Duibhir, an ancestor who, by tradition, lived sometime around the tenth century.
 The exact meaning of the ancestor's name is unknown, though 'black skirt' is one possibility, but the common modern consensus is that it means 'black and dun-coloured' which is thought to refer to Duibhir's hair and skin colouring.
Morrison is an English-language surname.
 The name is derived from a patronymic form of the English "Morris".
 The personal name "Morris" sometimes is a variant of "Maurice", and was quite popular in the Middle Ages.
 In Scotland the surname is sometimes also an Anglicised form of the Scottish Gaelic "MacGilleMhuire", a surname meaning "son of the servant of (the Virgin) Mary".
 The Scottish surname "Morrison" is also sometimes an Anglicised form of the Irish "O'Muirghesasain", meaning "descendant of "Muirgheas"".
 In Ireland the surname is sometimes an Anglicised form of the Irish "Mac Muiris", a surname derived from the patronymic form of "Muiris".
 The Irish personal name "Muiris" derives from the Gaelic adjective Mor or the Gaelic word Muir.
Delaney is an Irish surname derived from the Gaelic "Ó Dubhsláine", "Dubh" meaning black and "Slaine" for the River Slaine (Slaney).
 DeLaney is also of Norman origin.
 There is a branch of Dulaneys in the United States who trace back to a Thomas Delany.
 Thomas's son, Daniel, claimed to have been descended from Dr. Gideon Delaune, a Huguenot physician and theologian and founder of the Apothecaries' Hall.
 Hence, there are multiple discussions among genealogical circles as to the origin of Delaney since it can be anglicized Gaelic or anglicized French.
Eilean Dubh meaning 'Black Island' in Scottish Gaelic could refer to:
McElwain is a surname of Gaelic origin.
 It can be derived from either Scottish or Irish.
 The Scottish translation being the anglicized form of Gaelic Mac Gille Bheathain, meaning ‘son of the servant of (Saint) Beathan’ and also a personal name representing a diminutive of beatha ‘life’.
 The Irish form being the anglicised form of Gaelic Mac Giolla Bháin, meaning ‘son of the white-haired (i.e. ‘ash blond’) lad’.
 In Ulster this surname can be derived to be the Scottish form as opposed to the Irish form which would be more common in the rest of Ireland.
 Another anglicised version of the Irish "Mac Giolla Bháin" is the surname Kilbane.
McCann is an Irish surname derived from the Gaelic "Mac Cana", meaning "son of Cana".
 The "Cana" particle is a personal name meaning 'wolf cub'.

Question: Which castle was the stronghold of a family whose surname derived from the Gaelic term "dubh glas" meaning 'black stream'?
Answer: Douglas Castle