II. Geological Structure.

The geological structure of the islands is comparatively simple. From Stromness on the Mainland northwards to Inganess there is an axis of ancient crystalline rocks on which the representatives of the Old Red Sandstone rest uncouformably. These crystalline rocks consist of a fine-grained granite and a grey micaceous flaggy gneiss, which occupy a strip of ground about four miles in length and about a mile in breadth. They arc prolonged southwards in the island of Graemsa. With this exception the whole of the Orkney-Islands are occupied by Old Red Sandstone strata. In the island of Hoy representatives of both the upper and lower divisions of this formation are met with, and here they are separated by a marked unconformity; but in all the other islands the beds belong to the lower division.

Throughout the islands there is a remarkable uniformity in the character of the strata belonging to the lower division. They consist mainly of hard blue and grey calcareous flagstones, which are so typically developed in Caithness. Fortunately, however, the highest beds of the Orcadian flagstone scries are totally different in character from those just described, being composed of coarse siliceous red and yellow sandstones and marls. The sandstones are full of false-bedding, and frequently conglomeratic, containing pebbles of granite, quartzite, gneiss, and other crystalline rocks.

The distribution of this arenaceous series has an important bearing on the question of the ice movement. On referring to the map accompanying this paper (Pl. XXVI.), it will bo seen that it forms a well-marked zone, running nearly north and south through the centre of the group. The relations which these siliceous sandstones bear to the flagstones are best seen in Eda, where they cover the greater part of the island, and where they form smooth flowing hills upwards of 300 feet in height. The sandstones lie in a syncline, the axis of which runs north and south, and on both sides of the island they rest conformably on the flagstones. In the islands which lie to the west and north-west of Eda, viz. Fara, Westra, Papa Westra, Egilsha, and Rowsa, the strata consist wholly of blue and grey flagstones, which are inclined at gentle angles. Though there are page 650 many minor undulations, yet on the whole there is a gradually descending scries towards the western headlands of Rowsa and Westra.

In Stronsa and Sanda the arenaceous series and the underlying flagstones are repeated by a scries of faults, which are laid down on the map.

The south-east corner of Shapinshay is occupied by these sandstones, where they are associated with a dark green slaggy diabase, which forms part of an ancient lava-flow. They reappear on the south shore of Shapinshay Sound, and cross the Mainland in a narrow strip from Inganess Head to Scapa Bay. They are continued also along the north-west shore of Scapa Flow as far as Orphir Kirk, and they likewise extend along the eastern shore to Howquoy Head, near St. Mary's. These sandstones and marls are brought into conjunction with the flagstones of the Mainland by two great faults, which we have traced on the ground; but in Cava, Fara, Flota, South Ronaldshay, and Burra they graduate downwards into the flagstones, and are regularly interbedded with them. As the result of careful mapping of the coast-sections in the southern islands, we have come to the conclusion that Scapa Flow occupies the centre of a geological basin, towards which the strata are inclined on almost every side, and round whose shores the highest members of the Lower Old Red Sandstone in Orkney are to be found. We have elsewhere stated our reasons for believing that the Orcadian flagstones, with the conformable sandstones and marls, are the equivalents of the higher subdivisions of the Caithness series^*.

It ought to be clearly borne in mind that to the north-west of the great fault which extends from Houton Head eastwards by Scapa to the bay east of Work Head, the Old Red strata consist wholly of flagstones, save the conglomeratic beds, which repose unconformably on the crystalline axis, north of Stromness.

The physical features as well as the geological structure of Hoy are somewhat different from those which obtain in the other islands. Instead of a low undulating tableland, terminating seawards in a bluff cliff or sloping downwards to a sandy beach, which is the dominant type of Orcadian scenery, the island of Hoy forms a prominent tableland, trenched by a series of deep narrow valleys, which arc occasionally flanked by conical hills upwards of 1500 feet high. These narrow valleys must have been admirably adapted for nourishing a series of local glaciers towards the close of the Glacial period, as is evident from the long moraines now strewn over the hill-slopes.

The greater portion of the island is occupied by coarse false-bedded sandstones, which are but the counterpart of the Upper Old Red Sandstones at Dunnet Head, in Caithness. Near the base of this division occur some contemporaneous volcanic rocks, which are admirably exposed on the noble cliff in the north-west of the island

^* " The Old Red Sandstone of Orkney," by B. N. Peach and J. Horne Trans, of the Phys. Soc. Edinb. vol. v. 1880.

page 651 and at the base of the Old Stan of Hoy. The whole series rests unconformably on the flagstones; and in the south-west portion the upper division is brought into conjunction with the lower by a fault which extends from Melsetter to the coast-line opposite Risa island^*.