THE OLD RED SANDSTONE OF THE ORKNEYS. 407 



The peculiar nature of this flagstone deposit is so strikingly new to the geologist 

 accustomed to the study of other districts that it cannot fail to suggest a consideration 

 of the question of its origin. Sir Archibald Geikie # has insisted strongly on the marked 

 difference between these and the sandstones which in other parts of Scotland are so 

 characteristic of the Old Red. This striking contrast in the nature of the strata points 

 to markedly dissimilar conditions of deposit. As we trace upwards the Old Red Sand- 

 stone of the Orkneys, we shall see that in process of time this type of sediment was 

 replaced by the more familiar one of yellow and red sandstones and red marls. There 

 can be no doubt that this was the result of marked changes in the physical geography 

 of the region ; and when we remember that at Cromarty beds of yellow sandstone 

 contain precisely the fossils of the flagstone beds around Stromness, and, beyond reason- 

 able doubt, were being formed at the same time, we see clearly the truth of Sir A. Geikie's 

 conclusion that the flagstones of Orkney are merely the result of certain peculiar con- 

 ditions of deposit. From their rippled and sun-cracked surfaces, they were certainly 

 originally laid down in shallow water ; and from the extensive area they now occupy, 

 they must in many cases have been laid down far from land. That this area was 

 tranquil I have shown to be probable, from the way the fine flags lie among the con- 

 glomerates of Stromness right against the old granitic shore. A similar mixture of 

 deposits is to be found at the present day only in the land-locked areas of our river 

 mouths and inland lochs. The other striking feature of these flags is the way in which 

 they combine materials in other formations confined to different rocks. All contain 

 sand, clay, and carbonate of lime in varied proportions, yet sandstones, limestones, or 

 true shales are never typically developed in this peculiar formation. 



III. The Eday Sandstones, or John o' Groats Series. 



The Rousay beds of Orkney, as described by many previous writers, pass upwards 

 conformably into an overlying series of yellow and red sandstones and marls, which 

 contain in many places the fossils which characterise the John o' Groats beds of 

 Caithness, and are to be regarded as on the same horizon with them. This is a very 

 different series, and much more varied than the Rousay beds of Orkney. An entire 

 change in the nature of the sedimentary deposits indicates a complete and comparatively 

 rapid change in the physical conditions of the area. The yellow sandstones, with their 

 flag beds grading upwards into red sandstones and marls, must have been the formation 

 of shallow areas of water, with currents sufficiently strong to introduce now and then 

 even layers of coarse gravel. The unvarying and monotonous Rousay beds, the deposit 

 of still, though comparatively shallow water, come suddenly to an end. It is interesting 

 to observe that these changes were accompanied by the outburst of volcanic action in a 

 district which had for ages been the seat of uninterrupted quiet sedimentation. In the 

 whole thickness of the Stromness and Rousay beds of Orkney there is no trace of 



* Sir A. Geikie, " Old Red Sandstone," p. 363. 

 VOL. XXXIX. PART II. (NO. 13). 3 Q 



