Given the following context:

In August 1560, parliament assembled in Edinburgh and legislated that the Scottish church would be Protestant, the Pope would have no authority and that the Catholic mass was illegal. Scottish cathedrals now survived only if they were used as parish churches and as Elgin had been fully served by the Kirk of St Giles, its cathedral was abandoned. An act of parliament passed on 14 February 1567 authorised Regent Lord James Stewart's Privy Council to order the removal of the lead from the roofs of both Elgin and Aberdeen cathedrals, to be sold for the upkeep of his army, but the overladen ship that was intended to take the cargo to Holland capsized and sank in Aberdeen harbour. In 1615, John Taylor, the 'Water Poet', described Elgin Cathedral as "a faire and beautiful church with three steeples, the walls of it and the steeples all yet standing; but the roofes, windowes and many marble monuments and tombes of honourable and worthie personages all broken and defaced".Decay had set in and the roof of the eastern limb collapsed during a gale on 4 December 1637. In 1640 the General Assembly ordered Gilbert Ross, the minister of St Giles kirk, to remove the rood screen which still partitioned the choir and presbytery from the nave. Ross was assisted in this by the Lairds of Innes and Brodie who chopped it up for firewood. It is believed that the destruction of the great west window was caused by Oliver Cromwell's soldiers sometime between 1650 and 1660.At some point, the cathedral grounds had become the burial ground for Elgin. The town council arranged for the boundary wall to be repaired in 1685 but significantly, the council ordered that the stones from the cathedral should not be used for that purpose. Although the building was becoming increasingly unstable the chapterhouse continued to be used for meetings of the Incorporated Trades from 1671 to 1676 and then again from 1701 to around 1731. No attempt was made to stabilise the structure and on Easter Sunday 1711 the central tower gave way, demolishing the nave....

answer the following question:

What structure did the council order the stones from the cathedral not be used for? 

boundary wall