THE OLD RED SANDSTONE OF THE ORKNEYS. 403 



ness is that of Sanday and Stronsay, while the sandstones of Deerness and Holm belong 

 to a syncline unrepresented in the northern section, except it be by the limited areas of 

 yellow and red sandstones in the island of Stronsay. 



The South Isles District. 



South of the Scapa faults not one of these features reappears, and the South Isles 

 of Orkney form a distinct district, with a well-developed structure of its own. It 

 consists of a geological basin, in the centre of which lie the higher beds, the sand- 

 stones.* They form the shores of Scapa Flow, from the Old Kirk of Orphir to near 

 Howquoy Head. They reappear in Hunda, the west of Burray, and the north-west 

 of S. Ronaldshay, here dipping west and north-west, and constitute also the north 

 end of Flotta. Around them pass the underlying flags of Orphir, Holm, the east of 

 Burray, the south-east of S. Ronaldshay, Swona, and the south of Flotta. In the 

 north, the junction is a fault ; and through South Ronaldshay and Burray it is evident 

 that several faults run north-east and south-west parallel to the strike of the rocks. Yet 

 in some places the succession is an interrupted one, as, for example, to the west of 

 Grimness Head and in the island of Flotta, In Burray the flags dip west, in S. 

 Ronaldshay north-west, in Flotta north, the strike thus sweeping gradually round. 

 Much broken up as the district is by the sea, it is yet sufficiently clear what the general 

 structure of the whole area must be. The Eday syncline is rapidly dying out in 

 Inganess Bay, and I could find no proof that the yellow sandstones pass across the 

 East Mainland near Kirkwall, to unite with those of Scapa Flow. Even should they 

 ultimately prove to be continuous, it is clear that the broad basin of the South Isles 

 cannot fairly be regarded as a continuation of the Eday syncline, which already at the 

 south end of Inganess Bay has narrowed to less than a mile in breadth, and has, further- 

 more, to cross the powerful dislocation of the east side of Scapa Bay. In all the features 

 of its structure, the South Isles area shows no point of comparison with that around 

 Kirkwall, still less with that of the North Isles of Orkney. 



The largest continuous area of these rocks is that of South Ronaldshay, which alone 

 I had time to examine in detail. It consists of two series, the grey flags of the south- 

 eastern district, and the yellow and red sandstones of the north-west. The general dip 

 throughout is N. to N.W., but the structure is by no means simple, as it is evident from 

 the coast sections that powerful dislocations cross the island from N.E. to S.W. On the 

 west side the flags extend from Brough Head to Barswick, much disturbed in manj^ 

 places ; and from thence to St Margaret's Hope, and for a mile further east along Water 

 Sound, the shore consists of yellow and red sandstones (faulted apparently in two places 

 at Barswick, where they are brought down against the flags, and at Sandwick). The 

 Hoxa promontory consists of an anticline of blue flags, and is bounded by a fault which 

 runs across the narrow isthmus. On the east shore, again (sect, 4), the dip is continu- 



* Peach and Horne, op. eit., p. 12. 



