THE OLD RED SANDSTONE OF THE ORKNEYS. 385 



county consisted of the flagstones and sandstones of the lower series. The distribution 

 of these two members was described, and sections given to show their relation to one 

 another. In their paper on the Glaciation of the Orkney Islands # a map was published, 

 which reappears in the chapter contributed by them to Tudor' s The Orkneys and Shetland^ 

 and leaves little to be desired so far as regards a knowledge of the distribution of the 

 different lithological types which constitute the Old Red Sandstone of the Orkneys. The 

 structure of the county, they regarded, with Professor Heddle, as, in the main, a syncline 

 which runs from Eday to South Eonaldshay, broken in the Mainland by two great 

 faults which cross it and follow the shores of Scapa Bay. In the centre of this syncline 

 lie the sandstones which form the uppermost member of the lower series, while the flag- 

 stones form the rest of the district, with the exception of the area occupied by the upper 

 Old Red Sandstone in the island of Hoy. They showed also that in Shapinshay, among 

 the yellow sandstones of the lower Old Red, occurred a belt of contemporaneous volcanic 

 rocks, consisting of a single outflow of a diabasic lava.+ 



I. — The Pal^eontological Subdivisions of the Orcadian Old Red Sandstone. 



The Eday Sandstones. — So far, those geologists who had endeavoured to make out 

 the structure and succession of the Orcadian Old Red Sandstone had relied mostly on 

 the different types of sedimentary rocks to establish their conclusions, without reference 

 to the fossils the rocks contained. But in 1896, in a paper read to the Royal Physical 

 Society of Edinburgh, § the present writer showed that among the yellow sandstones of 

 the lower Old Red Sandstone of Deerness, Orkney, occurred three fossils not previously 

 recorded from Orkney, and known only to occur in the John o' Groats sandstones of 

 Caithness, viz., Dipterus macropterus (Traq.), Tristichopterus alatus (Egert.), and 

 Microbrachius Dicki (Traquair). In this way the opinion, already expressed by pre- 

 vious authors, || that the sandstones which conformably overlie the flagstones in 

 Orkney were the northern representatives of the similar beds at John o' Groats, Caithness, 

 was confirmed by palseontological evidence. During the following summer investigation 

 was made whether the sandstones in other districts of Orkney, to which had been 

 assigned the same position, contained the same suite of fossils, with the result that in 

 several of the localities examined (in Shapinshay, Inganess Bay, and Eday) one or 

 other of them was proved to occur, and it was established that they constituted the 

 type fossils of a palaeontological zone of the Orcadian Old Red Sandstone, which was 

 at the same time distinguished by the lithological characters of its rocks. This may, in 

 consequence, be designated the zone of Tristichopterus alatus (Egert.), or, from the 

 locality in Orkney in which they have been principally studied, the Eday sandstones. 



* Qiwrt. Jour. Geol. Soc. Lond., vol. 36. 



t London, 1883. 



% The occurrence of this basalt was noted by Jameson, Mineralogy oj the Scottish Isles, ii. 235. 



§ Proc. Roy. Phys. Soc. Edin., vol. xiii 



|| Peach and Hornb, op. cit. Sir A. Geikie Old Red Sandstone, p. 409. 



