400 



MR JOHN S. FLETT ON 



of the island is probably a continuation southward of the rocks which occupy the centre 

 of the Eday syncline, and the eastward dipping flags of Shapinshay will then correspond 

 to those of Egilshay, Ferstness, and Westray, as the west dipping flags in Stronsay 

 correspond with those of Sanday. 



As will be evident from this brief summary, the North Isles of Orkney are composed 

 of two members of the Old Red Sandstone — the Rousay beds and the Eday sandstones. 

 The chief structural feature is the Eday syncline. The Rousay beds of Rousay and 

 Westray, dipping eastwards, pass under the sandstones, and emerge again with a west- 

 ward dip in Sanday and Stronsay, only to roll over again before they finally disappear 

 beneath the waters of the North Sea. The beds which in Rousay contain the type 

 fossils are, in all probability, the lowest of the flags of this area ; and although the 

 sections are frequently interrupted by the sounds which separate the islands, and by not 

 a few important faults, it is quite evident that the entire thickness of rock required to 

 explain the geological facts is by no means great. We have already stated 1000 feet as 

 the maximum required for the Rousay flags, and in no other island is so great a thick- 

 ness exposed. If we add to these the upper half of our estimate for the east side of the 

 Birsay and Evie series, we have a total thickness along this section of not more than 

 1500 feet. No more than an approximate estimate can possibly be formed in this 

 district, as the sections are so broken up by water, and no recognisable subdivisions can 

 be established, either lithologically or palasontologically, with which we might ascertain 

 the throw of the respective faults. 



Stromness. 



Kirkwall. 



Roseness. 



Section 3. — From Stromness, through Kirkwall, to Roseness (Holm). 



The West Mainland District. 



Owing to the prevalent north and south strike, the rocks of Rousay may be 

 expected to cross into Evie and Rendall, where they lie in very nearly horizontal 

 but slightly rolling folds, and from here to pass southwards into the district between 

 Finstown and Kirkwall. A similar conclusion is arrived at by the examination of 

 the rocks which stretch eastward from the Bridge of Waithe in Stenness (sect, 3). 

 Here we are among the rolling beds which mark the termination to the south of the 

 fault which runs along the side of the western anticline. These beds are undoubtedlv 

 to be placed, with their more northern representatives in Harray, among the upper beds 

 of the Stromness series. Further east in Stenness we find the effects of the western 

 anticline, though here little marked, and evidently dying out. Through most of 

 Stenness and throughout the Ward Hill of Orphir the dips are south-west. The anti- 



