The architecture of the tumulus, though offering some indications of great value, hardly possesses any features sufficiently marked to fix its date with certainty. Externally it is a truncated cone (woodcut No. 87), about 92 feet in diameter, by 36 feet in height, and is surrounded at a distance of about 90 feet by a ditch 40 feet wide, and 6 feet deep, out of which the earth seems to have been taken which was required to form the mound. Internally it contains a chamber slightly cruciform in plan, measuring 15 feet 4 inches, by 14 feet 10 inches, and, when complete, probably 17 feet in height. On each of three sides of the chamber is a sepulchral loculus, entered by a small opening 3 feet from the ground. The largest of these, that on the right as you enter, is 7 feet by 4 feet 6 inches, and the central one 5 feet 6 inches by 4 feet 6 inches. Each of these was closed by a single stone carefully squared, so as to fit the opening. The passage leading into the central chamber was 3 feet wide by 4 feet 6 inches in height, and originally closed, apparently by a doorway at 2 feet 6 inches from the chamber. Beyond this it is lined by two slabs 18 feet long, reaching nearly to a recess, which seems arranged as if to receive the real door which closed the sepulchre, probably a large stone. Beyond this the passage still extends some 20 feet to the present entrance, but is of very inferior class of masonry, and how much of it is modern is not clear. The first thing that strikes any one on examining this mound is that it certainly is the lineal descendant of the great cairns on the banks of the Boyne, but separated from them by a very long interval of time. It is not easy to determine what interval must have elapsed before the side chambers of those tombs merged into the "loculi" of this, or how long it must have been before their rude unhewn masses were refined into the perfectly well-fitted masonry of this one. Some allowance must, however, be made for the difference of material. The old red sandstone of the Orkneys splitting easily into self-faced slabs, offers wonderful facilities for its use, but still the way in which the angle-buttresses of the chamber were fitted, and the cells finished, and the great slabs line the entrance, all show a progress in masonic science that must have required centuries, assuming, of course, that they were built by the same people. But was this so? So far as we at present know, these islands, when conquered by Harold Harfagar in 875, were inhabited by two races called Pape and Peti. The former were generally assumed to have been colonies of Irish missionaries and their followers, who settled here after the conversion of the Picts by St. Columba in the middle of the sixth century. The Peti, it is also generally assumed, were the Pechts, or Picts.[281] It will not be easy to ascertain now whether they were so or not, as, according to Bishop Tulloch, they were so entirely exterminated by the Northmen,