Innes, Cosmo. “A Letter to the Earl of Buchan from Monsignore Carlo Erskine, Domestic Prelate to Pope Pius VI., With Notices of Papal Bulls Issued to Scotland”. Proceedings of the Society of Antiquaries of Scotland, vol. 2, Nov. 1858, pp. 139-42, doi:10.9750/PSAS.002.139.142.