The Royal Arch degree is not native to Scotland, but seems to have been introduced from both Irish and English sources, often Military Lodges, towards the middle of the eighteenth century. The earliest reference to the degree is at Stirling in 1743.
These Military Lodges introduced many other degrees beyond the Craft, and when the regiments moved on, Lodges in the vicinity sometimes continued to work them. Thi situation continued until the end of the eighteenth century.
However the early Secret Societies Acts caused the Grand Lodge of Scotland,in 1800, to issue a warning to it's Lodges against the working of any degrees other than those of Entered Apprentice, Fellow Craft and Master Mason. Many Lodges heeded the warning and the additional degrees associated wth the Royal Arch or Templar masonry had to be worked in assemblies seperate from the Lodge. The feeling grew amongst the Brethren that these assemblies should be legitimised in some way. A few obtained Charters from the Grand Encampment of Ireland. Others petitioned the Templar Grand Body in England and in 1810, under the patronage of the Duke of Kent, the Royal Grand Conclave of Scotland was chartered and Alexander Deuchar appointed it's first Grand Master.
The Earl of Sussex was supportive, and advised Deuchar to make every effort to persuade the Grand Lodge of Scotland to adopt a similar stance, and take the Royal Arch under it's wing. however, after an initially encouraging reply from the Grand Secretary of the Grand Lodge of Scotland, the matter became inextricably bogged down in Committee.
Eventually a meeting of interested Chapters could be delayed no longer and, in August 1817, representatives of 34 Chapters met in Edinburgh and the Supreme Grand Royal Arch Chapter of Scotland was erected and consecrated. Charters were issued, and the new Grand Body grew slowly but steadily, and gradually all bodies working the Royal Arch degree in Scotland came under it's control.
Extracted from Supreme Grand Chapter of Scotland.