400 MR JOHN S. FLETT ON 



weathering give a rusty colour to the surrounding rock. When the flagstones weather, 

 the siliceous concretions, owing to their greater durability, stand out in high relief upon 

 the bedding planes, and give the rock often a curiously fretted and ornamented appear- 

 ance, and so numerous are they that frequently they resemble a solid mass of fretwork 

 or of repousee ornament upon the surface of the stone. On weathering the flags lose 

 also their prevalent pale or dark grey colours. Many of the dark calcareous flags around 

 Stromness weather with a creamy yellow crust, which resembles that of certain impure 

 carboniferous limestones. Yellow and different shades of brown are the prevalent tints 

 of the weathered stone. The changes are principally the removal of the lime in solu- 

 tion and the oxidation and hydration of the iron. It is the latter which stains the 

 rock, as is seen when we consider the source of the white colour which marks the 

 weathered flags in a peat bed, and which is due to the organic acids of the peat having 

 removed the iron from the rock. The decomposition gradually proceeding inward from 

 the surfaces and cracks, produces sometimes a curious effect on a seashore where a bee! 

 of calcareous flag is divided up by many joints into polygonal areas, around the out- 

 side of which is a soft, rusty, decomposed film, an inch or more in depth, while the 

 centre area is hard, grey, and comparatively fresh. The innumerable sun-cracked 

 and rippled surfaces were well described by Sir A. Geikie * in the flags around Thurso. 

 In thickness the beds vary from an inch up to perhaps 18 inches. In every district 

 of Orkney, flags of 2 or 3 inches thick and in large flags can be obtained for paving 

 purposes. A favourite kind at present is a coarse sandy flag in thick beds (0 inches), 

 obtained from Orphir. Thinner slabs, used formerly for roofing slates, are also of very 

 wide distribution. The thick beds are valued for building purposes, especially if the 

 bedding planes are smooth and the joints well marked. In the latter case they need no 

 dressing, as the builder places the smooth joint face, often covered with a fine layer of 

 glancing calcite, to the outside of the wall. In some places a variety of flag occurs, very 

 dark in colour and seemingly much crumpled, the minute laminae of which it consists 

 being contorted in every conceivable fashion. Such beds are of restricted distribution, 

 and usually markedly lenticular, as they thin out abruptly in no great distance. 

 They bear a superficial resemblance to certain curly oil shales in the Edinburgh district, 

 but when broken open they consist of an ordinary grey flag, the contorted layers being 

 often covered with a dark film. They are not due to earth movement and crushing, 

 as they occur in perfectly undisturbed rocks, and they probably result from peculiar 

 conditions of deposit, perhaps the escape of gases or the decomposition of organic matter 

 having produced their irregular internal structure. Where the flags are crossed by a 

 fault the disturbance is often very great, and quite out of proportion to the magnitude of 

 the dislocation. The rocks are bent and twisted, their surfaces slickensided and blackenol, 

 or a dark breccia produced, in which the flagstone particles glance with organic matter 

 till they resemble broken bits of coal. In some cases the fault is marked by a layer of 

 crushed rock powder, intensely black in colour, and mixed with calcite and iron pyrit^. 



* Sir A. GEIKIE, "Old Red Sandstone," Trans. Roy. Hoc. Edin., vol. xxviii. p. 393. 



