You can read this book here in pdf - Electric Scotland

PREFACE
IT is not clear to what extent the earlier chroniclers were
misled by such errors in geographical direction as we see in
Ptolemy's map,
where Ireland extends to the N.N.W. of Cum-
berland, and northern Scotland is twisted round till north
has become east. Thus the relative position of Bede's
northern and southern Picts is uncertain.
Changes in the names of races add to the obscurity of
their boundaries. The Welsh of Strathclyde are not always
distinguishable from the Welsh further south, both being
called Britons, a name originally applicable to the Picts.
Scots settled in Pictland and in time ; part of Pictland
took the name of Scotia, and its inhabitants that of Scots.
The name of Picts became limited to those dwelling south
of the Forth. But no generic name is given to the district
south of Forth and Clyde and north of Solway and Tweed ;
and it is probable that the different races inhabiting this
area are not always very clearly distinguished.
When Ninian preached in Galloway he was, according to
Bede, a missionary to the Southern Picts. At that time
(before the Anglo-Saxon occupation of England) the Britons
had lost ground before the Picts and Scots Pictish ;
territory
was continuous from Galloway to Manau. Not till after the
Anglo-Saxons had driven the Britons northwards did Dumbarton
become a British stronghold. Then the Britons
established themselves in Strathclyde, during a period of
intense national feeling which may have given rise to the
Arthur legends. Some of the twelve victories ascribed to
Arthur may have been won in Scotland ; cf . the Historia
Brittonum, in M.G.H., AA., XIII, 199-200. (Cf. Henry of
Huntingdon, 48-49 ; William of Malmesbury, G.R., I, 11-12.)
Henceforward Pictland south of the Forth was divided
into Stirling and Manau on the one side, and Galloway on
the other.
'
After the formation of the Anglian kingdom of Northumbria
the Picts of Manau held their own long enough to