Their distribution too is interesting. No stone circles exist in the lowlands or south of the Frith of Forth and Clyde; and dolmens are rare in these regions, though this may arise from the extent to which cultivation is carried on there. Until, however, a statistical account is compiled, accompanied with a map, it is difficult to speak confidently on such a subject, but the general impression is that the lowlands are not, and never were, a region of megalithic remains; and if this is so, it is one of the many proofs that the dolmens are neither pre-Roman nor Celtic. At least we have no reason to believe that the Teutonic races who now occupy that country were settled there in the time of Agricola. But if the Celts or Picts who then inhabited that land had been in the habit of raising megalithic structures, we would have been more likely to find traces of them in that densely inhabited country than in the bleak uplands of Aberdeenshire, or the bare pastures of the Orkney Islands. The district of Scotland where these circles and rude stone monuments most abound is on either side of a straight line drawn direct from Inverness to Aberdeen, which is a locality where sculptured stones are also found in considerable numbers, but the rude stone monuments are not found in Angus or Fife, where their sculptured successors are most numerous. The district of the circles _par excellence_ in Scotland, however, is not on the mainland at all, but in the northern and western isles. The principal group is in the Orkneys; next in importance are those in Lewes. They are found in Skye and Kantyre. There are several in Arran, and thence the transition is easy to the Isle of Man, where they meet the English group in Cumberland. * * * * * The larger circles in the Orkneys are four in number; three of these stand on a long slip of land that divides the loch of Harra from that of Stennis. The fourth is at some little distance from the others, and separated from them by a narrow strait connecting the two lochs. Besides these there are several smaller earthen circles and numerous tumuli. The largest circle, known as the Ring of Brogar,[273] is 340 feet (100 metres) in diameter between the stones. These originally were sixty in number, ranging from 6 and 7 to 15 feet in height; outside the stones runs a ditch about 30 feet in width, and 6 in depth, but with no perceptible rampart on either side. Two causeways cross the ditch as at Penrith or Arbor Low (woodcuts No. 29 and 30) opposite to one another, but neither square with the axis of the spit of land on which the circle is situated, nor facing any of the four cardinal points of the heavens.