650 B. IT. PEACH AND J. HOUNE ON THE 



many minor undulations, yet on the whole there is a gradually 

 descending series towards the western headlands of Rowsa and 

 Westra. 



In Strousa and Sanda the arenaceous series and the underlying 

 flagstones are repeated by a series of faults, which are laid down on 

 the map. 



The south-east corner of Shapinshay is occupied by these sand- 

 stones, 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 Plow 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, 

 Plota, South E-onaldshay, 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 Mow 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 Eed 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 Eed strata consist wholly 

 of flagstones, save the conglomeratic beds, which repose unconform- 

 ably 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 pro- 

 minent tableland, trenched by a series of deep narrow valleys, 

 which are 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 

 hiU-slopes. 



The greater portion of the island is occupied by coarse false- 

 bedded sandstones, which are but the counterpart of the Upper Old 

 Eed 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 oi the island 



* " The Old Eed Sandstone of Orkney," by B. N. Peach and J. Home. 

 Trans, of the Phys. Soc. Edinb. vol. v. 1880. 



