5. Geology and Soil The geological formation of the Orkney Islands is, in its main features, of a very simple nature. If we except a comparatively small strip of land running northwestward from Stromness to Inganess in the West Mainland, and a still smaller patch in the neighbouring island of Graemsay, practically the whole county is underlain by the Old Red Sandstone formation. The two small areas above-mentioned—practically one, save for the intervening sea—are occupied by older crystalline rocks, consisting of fine-grained granite, and micaceous schist at times running into foliated granite, the whole traversed by veins of pink felsite. These formations are flanked on either side by a narrow band of conglomerate of the Middle Old Red Sandstone period, composed of the rounded pebbles of the underlying schist and granite. Of the Old Red Sandstone two divisions are found in the Islands—the Middle or Orcadian, and the Upper Old Red Sandstone. There are also in various parts but especially in Hoy and Deerness, local outcrops of volcanic material. Orkney thus constitutes practically a continuation of the north-eastern Highlands of Scotland, and, as in Caithness, flagstone is by far the most widely distributed rock. The Orcadian or Middle Old Red Sandstone is known to be a fresh water deposit, laid down by a large system of lakes which once extended over the north-eastern Highlands as far south as Inverness and Banff, and the same species of fossil fishes are found in the grey and black flagstones near Stromness as in the red sandstones on the south coasts of the Moray Firth. Leaving out of account the small area of crystalline rocks near Stromness, which are of age long anterior to any of the Old Red Sandstone deposits, the West Mainland parishes of Stromness, Sandwick, and Birsay contain the oldest rocks within the county, the Stromness flagstones with the fishes embedded in them being coeval with the Achanarras beds of Caithness. Next in point of age, come the flagstones of the East Mainland and the North Isles, corresponding in date and fossil remains to what are known to geologists as the Thurso Beds of Caithness. Following these in order of antiquity come the yellow and red sandstones and the dark red clays or marls which occur in the northern part of South Ronaldshay, and in Deerness, Shapinsay, and Eday. These are known as the John O’ Groats Beds, from their occurrence in that part of Caithness also, where indeed they were first exhaustively investigated. These three members of the Middle or Orcadian Old Red Sandstone are supposed to have a combined thickness of not less than 7000 feet, though this pile of sediment represents only the upper half of the formation, the lower beds of Caithness apparently not being represented north of the Pentland Firth. [Illustration: Berry Head, Hoy]