His mental decline continued apace until on 14 December 1887 he got lost in the Forest of Mamore while out walking near the Bishop of Argyll's home in Ballachulish.
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He travelled several times to the continent and often visited the Bishop of Argyll, his friend Alexander Chinnery-Haldane, in Ballachulish, Oban, a place which he loved.
The Pope handed the matter over to be examined by Bertrand du Pouget, Bishop of Ostia, but Aonghas died before the matter was solved.
A small hill immediately to the east of the main street of the village was the site of Killespickerill, which was built in 1228 as the seat of the Bishop of Argyll.
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Perhaps soon after or soon before, Thomas inspected charters of Paisley Abbey on behalf of Laurence de Ergadia, Bishop of Argyll.