You likely already have yr answerBUT. In the north of the county there were gifts of land to the monastery of Lindisfarne. These passed to the Durham Bishopric when Cuthbert's body, via ChesterLe Street got to the oxbow island where Durham cathedral was set up. These land rights and others became part of the Durham Palatinate ( a sort of semi autonomous area) (medieval monarchs wanted the Bishop (early on a very temporal power)to look after the North against the Scots. Not only land rights but court functions went with the Palatinate. The Bishop early on had rights of execution.
The Palatinate was wound down in the 1800s in a gradual way.They did this partly by removing powers and partly by making Queen Victoria the Palatine. That was the easy thing to do and things got back into the hands of the Sovereign and hence Parliament and it's usual functions of courts etc.
As this went on you had a situation where in Bedlington near me, some people still pursued contentious legal cases through the still extant Bishops courts. It was cheaper, more local, and less bother than to use the now empowered courts of Chancery in London, vide Dickens on the Chancery Courts.
So Bedlington, and the so called 'Islandshire" (places near Lindisfarne) were for a long time, juridically DURHAM and appeared on maps as Durham. I discovered when looking for my Logan and Havery ancestors that even Norham was so to speak, Durham. Gradually there position was regularised and in about 1855, (a date you mentioned ?) the Palatinate and trough it the grants to the predecessors of the Bishop of Durham all went.
Now some lighter stuff. I am now looking at a Lamberton Toll marriage certificate given by Andrew Lyons for a marriage on 28th May 1870. This was of course an 'Irregular Border Marriage'. The bride and groom were Jane Havery of Spittal and Alexander Anderson of (Ross Cowdrait or Burnmouth) They were my great grandparents and I have photos of both. Jane in one pic is with her neighbours (?) they wearing little capes, aprons, and horizontally pleated skirts. A type dress for fisher wives. Jane is holding my grandfather whom I knew. He said they eloped to Laverton. I doubt it there were very many 'marriages' there.
As usual they took some friends to testify to the marriage (it was a marriage by repute and their friends had that repute. So there were two witnesses , Robert Johnstone and NOTE This , Agnes Paulin.
The business of the county borders was an organic process as you will gather, not a smooth rational creation.
