THE OLD RED SANDSTONE OP THE ORKNEYS. 405 



two being in every respect similar, there are yet no recognisable and definite alternations 

 which could with certainty be used in dividing up the whole into an established 

 succession. This is true of the Orkney flags as a whole, as was pointed out by Messrs 

 Peach and Horne. They vary greatly, the principal types being a sandy flag, a clay 

 flag or mudstone, and a brittle calcareous or even bituminous flag. The sandy flags 

 never amount to pure sandstones, there being always a certain amount of clay and of 

 silky weathered and bleached mica, with very usually a calcareous cement between the 

 grains of sand. The clay flag is the purest and most abundant type. They are 

 relatively soft, fine-grained, and light grey in colour, except when darkened by organic 

 material. On their bedding planes the pale lustrous mica is often to be seen as a 

 shimmering film, while the microscope shows that in worn, tattered, and crumpled flakes 

 it is an important constituent of their mass. Sand in fine rounded grains and calcite 

 in greater or less abundance are constant constituents. Where, these softer beds occur 

 mixed with harder beds on a cliff face, they weather out rapidly into pale grey hollows, 

 and this is the origin of a frequently remarked feature of the Orcadian cliff scenery. 

 The calcareous and bituminous flags are the chief receptacles of the fossil remains 

 inclosed in these rocks. The fossil collector very soon learns that the best specimens are 

 obtained in a brittle, hard, usually slaty and thin-bedded rock, which rings to the 

 hammer like a piece of metal. This is in some measure due to the compactness and 

 impermeability which is conferred on these rocks by their abundant calcareous matter. 

 But there can be no doubt that, in turn, the presence of the organic remains facilitates 

 in some way the accumulation of carbonate of lime in the rock, as frequently around the 

 fossil is a well marked nodule, compact and hard, and evidently calcareous in nature 

 from the rapidity with which it weathers out, leaving the surrounding rock comparatively 

 unaffected. These are especially common in the dark flags among the sandstones of the 

 Eday series. The prevalent colour of these calcareous flags is dark blue-grey, and they 

 are fine-grained, and mostly free from the concretions so abundant in the more argil- 

 laceous rocks. In these latter they are so common that hardly a stone could be found 

 without some trace of them. Of all sizes, from that of a melon to less than a pea, and 

 of a remarkable and often grotesque variety of shapes, they show most clearly in the 

 weathered face of an old dry-stone dyke, or on the bare surface at the edge of the high 

 cliffs of the coast. From the manner in which they resist the weather, they are in 

 most cases probably siliceous — they are certainly harder and more difficult to break 

 than the rock surrounding them. Of these concretions the best known example is the 

 horse-tooth rock of Yeskenaby, to which Professor Heddle # and other authors have 

 devoted some attention. The rock itself occurs in situ at Borwick, near the great trap 

 dyke there. But this is merely an interesting variety of a phenomenon of universal 

 distribution throughout these flags. Their surfaces are often mottled and pitted with 

 innumerable little concretions, which it would be easy to mistake for coprolites or for 

 rain pittings. Not uncommonly these consist of pyrites and of marcasite, which on 



* Heddle, op. cit., pi. xiv. 



