There are various important prehistoric structures, many of which pre-date the first written references to the islands by Roman and Greek authors. The Western Isles became part of the Norse kingdom of the "", which lasted for over 400 years until sovereignty for the Outer Hebrides was transferred to Scotland by the Treaty of Perth in 1266. Control of the islands was then held by clan chiefs, principal of whom were the MacLeods, MacDonalds, Mackenzies and MacNeils. The Highland Clearances of the 19th century had a devastating effect on many communities and it is only in recent years that population levels have ceased to decline. Much of the land is now under local control and commercial activity is based on tourism, crofting, fishing, and weaving.