Diminutive dry-built stone cells or oratories, with sloping or curved walls, having the roof closed in with overlapping stones, converging towards the centre, and covered with flag-stones, are still found in the remote islands. One oratory also exists at Inchcolm where the stone roof is supported by a true arch, as in some of the latest Irish examples. It should, however, be pointed out that huts of similar construction to the above are known to have been erected and inhabited in recent times in the Outer Hebrides.[43] The hermitages above referred to, although belonging to this oldest type of structure, may thus possibly not be the oldest buildings in the country. At a later time the rude monastic cells and hermitages were followed by the churches established by the missionaries from Iona. The Scottish churches erected by the Columbans were, like those of Ireland, of extreme simplicity, and generally of small dimensions. They consisted of a simple oblong chamber, with a single door and a single small window. The walls were often built without mortar, and the wall apertures were finished with undressed stones. These structures were sometimes covered with a plain barrel vault, and sometimes with rafters and thatch. The jambs of the doorway incline inwards and have straight lintels; the windows are either square-headed or rudely arched. Until the Romanesque influence is felt, not a trace of any kind of ornament is to be found on these churches. Latterly, a few details resembling Norman work are introduced. In other examples of this type the details are more advanced. The door jambs are upright and are covered with semi-circular arches, and the windows are also similarly treated. The buildings, however, possess few features to enable the date of their erection to be determined. They may possibly have all been erected during a long course of years at different times in different localities, according to local circumstances; but it is natural to suppose that those of the more refined type are the latest.