410 MR JOHN S. FLETT ON 



and are separated by a fault by the upper Old Red Sandstone, which extends over the 

 most of the remainder of the island. 



The Yellow Sandstones and Flags of the John o' Groats Series. 



Starting at the northern extremity of their area in Orkney, we find that in Eday 

 these beds occupy a comparatively small area and are of very limited development. 

 At the Kirk of Skaill, on the eastern shore of Eday, a belt of yellow sandstones 

 immediately overlies the top flags of the Rousay series. These are followed by 

 a thin zone of red marls, which in turn are overlaid by thin-bedded calcareous flags, rich 

 in fossil remains, of which Dipterus macropterus was the only one I found in satisfactory 

 preservation. Above these we find a series of yellow and red beds (with thin layers of 

 conglomerate), which form a gradual transition to the red and brown sandstones and 

 marls so largely developed in the centre and north end of the island. The whole 

 thickness of this series is not over 100 feet, and it is, in fact, their most insignificant 

 development in any part of Orkney. Were it not for the very convincing sections 

 elsewhere obtained, it would be impossible to regard these beds as other than a merely local 

 facies of the basal series of the red beds. Messrs Peach and Horne # give the following 

 estimated thickness : — 



Red and yellow sandstones — 

 Flagstones, 40 feet. 

 Reddest shales, 15 feet. 

 Hard white sandstone, 20 feet. 

 Gray calcareous flagstones. 



— the last being the underlying Rousay series, as I regard them, as they contain no 

 trace of John o' Groats fossils of the group of flagstones interbedded with the sand- 

 stones, while Dipterus valencienesii and Glyptolepis paucidens are not infrequent 

 in them. These yellow beds and flags stretch across London Bay, and emerge again at 

 Millbounds, where the section is very similar to that described. 



On the west side of the syncline the same beds crop out again just to the east of Fers- 

 ness, where they furnish the chief supply of yellow freestone used for building purposes 

 in Kirkwall and throughout the islands. A hundred yards to the east of the pier the 

 yellow beds come in gradually below the red, which here dip about E.S.E. Among 

 them occur again a belt of thin flags and an insignificant red series. The section, in 

 fact, repeats in every respect that to the east, and D. macropterus is found in the flags 

 to the west of the pier, but here the thickness must be somewhat greater, as the 

 average dip is about 20°, and the area of shore occupied is about 400 yards. At 

 Warness, again, to the south-west corner of the island, the underlying flags, with here 

 and there a yellow bed, pass up into a yellow sandstone series, 70 to 80 feet thick, over- 



* Peach and Horne, op. cit., p. 5. 



