One striking circumstance in the Inchcolm oratory--viz., its vaulted or arched roof, has been already sufficiently described; and, in describing it, I have stated that the arch is of a pointed form. In many of the ancient Irish oratories the roof was of wood, and covered with rushes or shingles; and most of them had their walls even constructed of wood or oak, as the term _duir-theach_ originally signifies. But apparently, though the generic name duir-theach still continued to be applied to them, some of them were constructed, from a very early period, entirely of stone; and of these the roofs were occasionally formed of the same material as the walls, and arched or vaulted, as in the Inchcolm oratory. In speaking of the construction of the primitive larger churches of Ireland, Dr. Petrie states, that their "roof appears to have been constructed generally of wood, even where their walls were of stone;" while in the oratories or primitive smaller stone churches, "the roofs (says he) generally appear to have been constructed of stone, their sides forming at the ridge a very acute angle."[88] The selection of the special materials of which both walls and roof were composed, was no doubt, in many cases, regulated and determined by the comparative facility or difficulty with which these materials were obtained. At no time, perhaps, did timber exist on Inchcolm that could have been used in constructing such a building; whilst plenty of stones fit for the purpose abounded on the island, and there was abundance of lime on the neighbouring shore. Stone-roofed oratories of a more complex and elaborate architectural character than that of Inchcolm still exist in Ireland, and of a supposed very early date. We have already found, for instance, Dr. Petrie stating that "we have every reason to believe" that the stone-roofed oratories known as St. Kevin's House at Glendalough, and St. Columba's House at Kells, "were erected by the persons whose names they bear,"[89] and consequently that they are as old as the sixth century. These two oratories, are, as it were, two storeyed buildings; for each consists of a lower and larger stone-arched or vaulted chamber below, and of another higher and smaller stone-arched or vaulted chamber or over-croft above. The old small stone-roofed church still standing at Killaloe, and the erection of which Dr. Petrie is[90] inclined to ascribe to St. Flannan in the seventh century[91] presents also in its structure this type of double stone-vault or arch, as shown in the following section of it by Mr. Fergusson.[92] When treating of the early Irish oratories, Mr. Fergusson observes, "One of the peculiarities of these churches is, that they were nearly all designed to have stone roofs, no wood being used in their construction. The section (Fig. 11) of the old church at Killaloe, belonging probably to the tenth century, will explain how this was generally managed. The nave was roofed with a