412 MR JOHN S. FLETT ON 



sections shown me by Mr Peach. By his advice I searched carefully, on a subsequent 

 visit to the spot, for traces of the piped quartzites and other Cambrian rocks, but failed 

 to observe any. The presence of these pebble beds shows very clearly how great must 

 have been the physical changes which the area had undergone, before sediment so coarse 

 reached districts which had long been the seat of a deposit of the finest grain and the 

 most uniform nature. They are very local in distribution, no trace of the thick beds at 

 Heclabir being found among the yellow sandstones in other areas of Sauday, or indeed 

 anywhere in the district, except on the opposite shore of Eday, where their thickness is 

 quite trivial in comparison. 



The yellow beds of Eday and Sanday stretch southward into Shapinshay, where they 

 attain a much greater importance, forming, in fact, the whole thickness of the John o' 

 Groats series in that island. Here the outcrop forms the south-east corner, and is 

 bounded by a line running N.E. from the angle of the bay below the Established Church 

 on the south shore to the Bay of Crook on the east. The underlying flags seem to pas^ 

 up quite conformably and without any important break into a series of yellow current- 

 bedded sandstones, mixed with numerous thin beds of dark-coloured flags. Along the 

 east side the structure is simplest, the prevalent dips being S.E. and E.S.E., but elsewhere 

 the dips roll greatly, and the beds are evidently being constantly repeated. The yellow 

 sandstones overlying these mixed beds are very pure and massive, and cannot, with 

 any probability, be estimated at less than 400 to 500 feet. Only very rarely is a red- 

 coloured bed of clay to be seen ; but at more than one place there occur belts of flags 

 intercalated between yellow sandstones, and in some places 30 feet in thickness. These 

 flags may be the counterparts in this area of the flagstones which in Eday occupy a 

 similar position, and, like them, they contain the characteristic John o' Groats fossils, 

 one specimen of Tristichopterus alatus (Egert.) having been found by me at Store 

 Point in a coarse grey flag. It is among them also that the volcanic rocks * occur which 

 Peach and Horne described as the only evidence of contemporaneous volcanic action in 

 the lower Old Red of Orkney. They consist of a single lava flow, which, though much 

 weathered, is recognisable as an olivine diabase, and is distinctly vesicular at the top 

 surface, while it rests quite conformably on the underlying flag, which is considerably 

 baked and altered. f To their observations I have only a few to add. The interbedded 

 character of the volcanic rock is shown also by the occurrence at its south-western 

 corner of a bed of ash several inches thick immediately overlying it, while in several 

 places thin layers of sprinkled ash can be traced in the overlying flags a few inches 

 apart, and to a distance of 10 feet above the surface of the lava. This shows that 

 though the volcanic activity resulted apparently in only one outflow of lava, it con- 

 tinued for a time to produce occasional showers of ashes, which were spread out over the 

 sea-bottom, and mixed with the sediment accumulating there. At its base the lava 

 contains here and there a bit of an angular baked flag, but its upper surface is vesicular 



* Jameson, Mineralogy of the Scottish Islands, ii. 235. 

 t Peach and Horne, op. cit., pp. 9 and 13. 



