Stone roofs are found in some old Irish buildings, formed on the principle of the horizontal arch, or by each layer of stone overlapping and projecting within the layer placed below it till a single stone closes the top. A remarkable example of this type of stone roof is presented by the ancient oratory of Gallerus in the county of Kerry; and stone roofs of the same construction covered most of the old beehive houses and variously shaped cloghans that formerly existed in considerable numbers in the western and southern districts of Ireland, and more sparsely on the western shores of Scotland. In the Inchcolm oratory the stone roof is constructed on another principle--on that, namely, of the radiating arch--a form of roof still seen in some early Irish oratories and churches, whose reputed date of building ranges from the sixth or seventh onward to the tenth or eleventh centuries. The mode of construction of the stone roof of the Inchcolm cell is well displayed in the accidental section of it that has been made by the falling in of the western gable. One of Mr. Drummond's sketches (see woodcut, Fig. 9) represents the section as seen across the collection of flower-tipped rubbish and stones made by the debris of the gable and some accumulated earth. The roof is constructed, first, of stones placed in the shape of a radiating arch; secondly, of a thin layer of lime and small stones placed over the outer surface of this arch; and, thirdly, the roof is finished by being covered externally with a layer of oblong, rhomboid stones, laid in regular courses from the top of the side walls onwards and upwards to the ridge of the building. This outer coating of squared stones is seen in the external surface of the roof to the left in one sketch (see woodcut, Fig. 9); but a more perfect and better preserved specimen of it exists immediately above the entrance-door, as shown in another of Mr. Drummond's drawings (see woodcut, Fig. 6). [Illustration: Fig. 8. Interior of the building, showing splayed window in eastern gable, recess in interior of south wall, vaulted roof, etc.] [Illustration: Fig. 9. Exposed section of the arch of the vault.] The arch or vault of the roof has one peculiarity, perhaps worthy of notice (and seen in the preceding woodcut, Fig. 9). The central keystone of the arch has the form of a triangular wedge, or of the letter V, a type seen in other rude and primitive arches. Interiorly, a similar keystone line appears to run along the length of the vault, but not always perfectly straight; and the whole figure of the arch distinctly affects the pointed form.