Structures of the same character, on the mainland of Orkney, were explored by Lieutenant F. W. L. Thomas, R.N., while engaged in the Admiralty Survey in 1848. In the course of his investigation of one of these at Savrock, about a mile to the westward of Kirkwall, and close to the sea-shore, some curious evidence was disclosed, showing the primitive arts of its builders, and their inability to overcome an obstacle requiring unusual skill or effective tools. In excavating the site for this subterranean dwelling they appear to have cleared away the soil till they reached the natural rock, which forms the floor of the vault. Pillars constructed at irregular intervals admit of the whole being covered by immense slabs resting on them, where the width is too great to be overarched at so slight an elevation by converging walls. A long passage leads from this chamber, floored, like it, with the natural rock. In one place, however, an irregular elevation of the strata occurs. Such an obstacle was either beyond the skill of the laborious architects, or at least demanded more exertion than they cared to expend on its removal; and the roof has accordingly been elevated so as to admit of free passage by ascending and descending over the superimposed rock. The passages, as in nearly all the structures of this class which have been carefully explored, are extremely straitened. Unfortunately this primitive dwelling supplied materials for building a neighbouring farm-house and offices before Lieutenant Thomas had an opportunity of exploring it; so that what remained was in a very imperfect and dilapidated state. Portions of the roof still entire, constructed of huge masses of unhewn stone--one of them measuring about five feet long--afforded abundant evidence that no amount of mere physical labour was grudged in the completion of the edifice, and seem to justify the probable assignment of it to a period prior to the introduction of metallic tools. In another of these subterranean buildings, however, situated on the Holm of Papey, Lieutenant Thomas observed some doubtful indications of the use of tools. "On the side wall, near the entrance," he remarks, "and about six feet from the floor, there is a neatly engraved circle, about four inches in diameter; there is also another stone, with the appearance of two small circles touching each other, cut upon it; but it is so common to find geometrical figures upon the Orkney flags, arising from a semi-crystallization of the pyrites which they contain, that I am unable to decide whether these are natural or not." The height of the passage where it remains perfect is only two feet seven inches; but nearly one-half of it is unroofed, and heaps of large stones lying scattered about afford evidence of the great extent of the building when complete. Within and around the area of this ancient structure abundant indications were discovered of its having been used as a dwelling-place.