For about a century and a half after the arrival of Columba, Iona continued in the position of the leading monastery of the north, extending her authority over many parts of Ireland, and bidding fair at one time to establish a similar influence throughout southern Britain, until a dispute about matters of comparatively trivial importance abruptly severed her connection with Northumbria. A difference of opinion had long existed between the clergy of the Picts, Britons, and northern Irish, on the one hand, and the followers of Augustine amongst the Anglo-Saxons, on the other, together with the ecclesiastics of southern Ireland, who had been latterly brought to conform to the practice authorised by the see of Rome. The proper time for the celebration of the Easter festival, and the orthodox form of the clerical tonsure, were the subjects in dispute; and though the former was clearly a question of astronomical calculation, and the tonsure was a badge of Paganism in the days of the primitive church, both points originated frequent and animated discussions, and were debated in the seventh century as important matters of faith. [Sidenote: A.D. 664.] Colman and his followers, at the celebrated conference of Whitby, had bade adieu to their adopted country sooner than relinquish the practice they had inherited from their predecessors; and fifty years later a similar tenacity in favour of “ancient customs” brought about the expulsion of the clergy of Iona from the territories of the Pictish sovereign. Nectan MacDeriloi, in consequence of a difference of opinion with his clergy about the points in question, dispatched envoys to Ceolfred, abbot of Wearmouth, requesting his decision upon the matters in dispute, and, at the same time, inviting the assistance of Saxon architects to build a stone church after the approved Roman model. The abbot responded to both appeals, and though every vestige of the stone church has long since disappeared, the letter of Ceolfred may still be read in the pages of the historian Beda. It strengthened and confirmed the king in his own convictions, and [Sidenote: A.D. 717.] he decided that his clergy should conform accordingly; but they were as firm in their resolution as Colman; and Nectan, summarily ordering them across Drumalban, appears to have transferred to his recent foundation (which seems to have been Abernethy) the pre-eminence amongst the monasteries of northern Britain, which had hitherto been the peculiar prerogative of Iona.[8]