It is probable that both the island groups of Orkney and Shetland were visited at a very early period by wandering clerics of the Irish Church, whose missionary efforts contributed so much to the diffusion of Christianity in Scotland. But we have no record of an earlier visitation than that of the companions of St. Columba, although there are indications that between that time and the colonisation of the islands by the heathen Northmen, these Irish clerics were no strangers in any of the island groups. The Irish monk Dicuil, who wrote his treatise “De Mensura Orbis Terrarum” in or about the year 825, states that “thirty years before that time some clerics had told him that they had lived in an island which they supposed to be Thule, where at the summer solstice the sun only hid himself behind a little hill for a short time during the night, which was quite light; and that a day’s sail towards the north would bring them from thence into the frozen sea.” This island is obviously Iceland. He then states that there are many other islands in the northern British sea, which lie at the distance of two days and two nights from the northern islands of Britain, in a straight course, and with a fair wind and a full sail. “One of these,” he says, “a certain honest monk told me he had visited one summer after sailing a day, a night, and another day, in a two-benched boat.” These appear to be the Shetland Islands. Dicuil further states that “there are also some other small islands, almost all divided from each other by narrow sounds, inhabited for about a century by hermits proceeding from our Scotia;[4] but as they had been deserted since the beginning of the world, so are they now abandoned by these anchorites on account of the Northern robbers; but they are full of countless sheep, and swarm with sea-fowl of various kinds. We have not seen these islands mentioned in the works of any author.” Here the reference to the “small isles separated by narrow sounds” is distinctive of the Faroes, of which the long narrow sounds are the peculiar physical feature; while the statement that they are full of countless sheep, taken in connection with the fact that the Northmen named them “Sheep-isles” (Fær-eyiar), establishes the identity of the group which Dicuil describes. The Faroes were colonised by “the Northern robbers,” led by Grim Kamban, in 825, the very year in which Dicuil was writing.