The same natural boundary which separated the eastern from the western tribes afterwards divided the kingdom of the Strathclyde Britons from that of the Angles; at a subsequent period, the province of Galweia from that of Lodoneia in their most extended sense; and now separates the counties of Lanark, Ayr, and Dumfries from the Lothians and the Merse. Galloway in its limited sense was not more clearly separated by its mountain barrier on the north from Strathclyde, than were the Pictish from the British races by the same chain, and the earlier tribes of the Selgovæ and Novantæ from the Damnii. In the other two great compartments situated on the north of the Firths of Forth and Clyde, two great mountain chains and two large rivers formed the principal landmarks in the early history of the social occupation of these districts. These two principal mountain chains were in fact the great central ridges from which the numerous minor chains proceed, and the rivers flow in opposite directions, forming that aggregate of well-watered glens and rocky defiles which characterise the mountain region of Scotland, till its streams, uniting their waters into larger channels, burst forth through the mountain passes, and flow through the more fertile plains of the Lowlands into the German Ocean. [Sidenote: The Mounth.] The first of these two great mountain chains was known by the name of the Mounth, and extends in nearly a straight line across the island from the Eastern Sea near Aberdeen to the Western Sea at Fort-William, having in its centre and at its western termination the two highest mountains in Great Britain—Ben-na-muich-dubh and Ben Nevis. [Sidenote: Drumalban.] The second great chain, less elevated and massive in its character, but presenting the more picturesque feature of sharp conical summits, crosses the other at right angles, running north and south, and forming the backbone of Scotland—the great wind and water shear, which separates the eastern from the western districts, and the rivers flowing into the German Ocean from those which pour their waters into the Western Sea. It is termed in the early records of Scottish history Dorsum Britanniæ, or Drumalban—the dorsal ridge or backbone of Scotland. It commences in Dumbartonshire, and forms the great separating ridge between the eastern and western waters from south to north, till it terminates in the Ord of Caithness.