3'£f) MR JOHN S. FLETT ON 



A> will be shown in a subsequent part of this paper, they, fall naturally into two 

 subdivisions, a yellow series beneath and a red series above ; and it is the thin layers 

 of flag intercalated in the yellow sandstones which have furnished the fossils described. 

 A no less striking characteristic of these beds is the occurrence in them of that zono 

 of volcanic rocks of which the first mention was made by Professor Jameson.* 



The Rousay Beds. — The inquiry was next advanced into the beds which underlie 

 this zone, and were known to consist of a series of flagstones, presumably of great thick- 

 ness, and of wide distribution throughout the county. All efforts to break up this 

 series into recognisable subdivisions by means of belts of rock, with sufficiently well- 

 developed peculiarities to ensure their recognition in different districts, had hitherto 

 failed ; t and, from an extensive knowledge of these rocks, the present writer felt that 

 success was hardly to be hoped for in such an attempt. But should the distribution of 

 their fossils show that certain forms occurred only on particular horizons, this great 

 series could be broken up into zones, which could be identified wherever they occurred, 

 if only they contained a sufficient number of organic remains in a satisfactory state of 

 preservation. The base of the Eday sandstones was chosen as forming a well defined 

 horizon, from which it would be possible to work downwards into the flagstone series in 

 search of type fossils. These underlying beds were then followed from Eday, Westray, 

 and Sanday in the north to South Ronaldshay in the south ; the geological structure 

 being carefully mapped, and a record of the fossils observed in each district compiled at 

 the same time. The flagstones of these districts proved to be barren and unfossiliferous 

 compared with the well known localities, chiefly in the West Mainland of Orkney, from 

 which for many years fossils had been obtained in great numbers. Yet in every district 

 decipherable fragments were to be found ; and in some localities the fossils were quite as 

 satisfactory as in the better known beds of the West Mainland. By far the most common 

 were the sculptured bones and scales of Glyptolepis paucidens (Agassiz), which occurred 

 in every district examined, often in great profusion, and with them Dipterus valen- 

 cienesii (Sedgwick and Murchison), in every locality, and almost equally abundant. In 

 fact, both these fossils occur right up to the base of the Eday sandstones, though as yet 

 in Orkney not known with certainty to pass up into these overlying rocks. In Deerness, 

 Holm, and Eday the beds immediately below the sandstones are crowded with Dipterus 

 valencienesii (Sedgwick and Murchison), often in fine preservation, and covering the 

 surface of whole slabs of rock. After these in frequency comes Homosteus Milleri 

 (Traquair), of which the large and usually broken plates are often to be seen. Other 

 fossils were relatively few. In Crook Bay, Shapinshay, I found a Cheiracanthus, which 

 when submitted to Dr Traquair was determined to be Cheiracanthus Murchisoni 

 (Agassiz). At Dingieshowie, Deerness, at Kirkwall, and elsewhere, Osteolepis macrole- 

 pedotus (Ag.) is found. Diplopterus Agassizi (Traill) occurs in the East Mainland, 

 Estheria membranacea at Kirkwall, Rendall, and Westray. Coccosteus decipiens (Ag.) at 

 Kirkwall, Dingieshowie, and even in the sandstones at Deerness, as I learned from Mr 



* Up. cit. t Archibald Geikik, op. cit., p. 410. 



