It was not the Roman mission which finally succeeded in converting the North and the Midlands. That success was due to the Scottish and Pictish Church. At the end of the sixth century, Columba, an Irish missionary, crossed over to the solitary rock of Iona, where he established an abbey on the Irish model, and quickly evangelised the northern Picts. From Iona, some generations later, went forth the devoted missionaries who finally converted the northern half of England. The native churches of the west, cut off from direct intercourse with the main body of Latin Christendom, had retained certain habits which were now regarded by Rome as schismatical. Chief among these were the date of celebrating Easter, and the uncanonical method of cutting the tonsure in a crescent instead of a circle. Augustine, shortly after his arrival, endeavoured to obtain unity between the two churches on these matters of discipline, to which great importance was attached as tests of submission to the Latin rule. He obtained from Æthelberht a safe-conduct through the heathen West-Saxon territories as far as what is now Worcestershire; and there, "on the borders of the Huiccii and the West-Saxons," says Bæda, "he convened to a colloquy the bishops and doctors of the nearest province of the Britons, in the place which, to the present day, is called in the English language, Augustine's Oak." Such open-air meetings by sacred trees or stones were universal in England both before and after its conversion. "He began to admonish them with a brotherly admonition to embrace with him the Catholic faith, and to undertake the common task of evangelising the pagans. For they did not observe Easter at the proper period: moreover, they did many other things contrary to the unity of the Church." But the Welsh were jealous of the intruders, and refused to abandon their old customs. Thereupon, Augustine declared that if they would not help him against the heathen, they would perish by the heathen. A few years later, after Augustine's death, this prediction was verified by Æthelfrith of Northumbria, whose massacre of the monks of Bangor has already been noticed.