390 MR JOHN S. FLETT ON 



accordance with the facts already known regarding their distribution in the other 

 districts in which they occur. A mutually exclusive occurrence of this nature can only 

 be regarded as due to the disappearance of one series of forms before the arrival or 

 evolution of the other, and clearly establishes that the successive stages of the deposition 

 of the Old Red Sandstone of the Orkneys were accompanied by changes in the fauna 

 which inhabited the waters in which the rocks were being formed. 



II. — The Structure of the Orkneys. 

 I. Strornness Beds. 



To the geologist who endeavours to unravel the structure of the Orkneys, a magnifi- 

 cent opportunity is afforded by the excellent and numerous coast sections. So com- 

 pletely is the country cut up by sounds and bays, that at no place can there be any 

 doubt as to the general structure ; and even in the larger areas of land, as in the West 

 Mainland, wherever cultivation is to be found, dwelling-houses and stone dykes have 

 been built, and one is, as a rule, at no difficulty in finding stone quarries within a com- 

 paratively short distance of one another. If we add to these the many opportunities 

 provided by the inland lochs and streams for an examination of the underlying rocks, it 

 will readily be understood how it is possible, in a comparatively short time, to map with 

 satisfactory detail very considerable areas of country. Only in a very few places do 

 superficial accumulations of boulder clay or peat moss conceal the relations of the rocks 

 beneath, through any extensive tract of land. Wherever the flagstones are present, the 

 structure may almost be said to be writ large on the face of the country. As has been 

 frequently observed by writers on the scenery and geology of Orkney, the hills have 

 then markedly terraced contours, the harder beds of flag resisting erosion and forming a 

 terrace, while the softer beds between, by their more rapid decay, form miniature 

 escarpments. These terraces are everywhere present in flagstone districts of Orkney, 

 and to the experienced eye at once reveal the secret of the underlying structure. In 

 some places, as in Rousay and in Westray, they form so noticeable a feature of the 

 landscape, as to remind one at once of the terraced volcanic districts of many parts, 

 both of Eastern and of Western Scotland. That they are preglacial in origin is proved 

 by the glacial striations with which they are often covered,* and no doubt they have 

 suffered during that epoch a considerable amount of rounding and obliteration ; their 

 fine development on the west side of Rousay and of Westray is thus a relic of the old 

 preglacial Orcadian landscapes, which owes its preservation to the fact that the ice 

 movement being from east to west, the west side of these hills was spared the intense 

 erosion to which the rest of the country was being subjected. 



The Strornness beds of Orkney, although, as a matter of fact, probably the least 

 extensively developed of any of the subdivisions of the lower Old Red Sandstone, have, 



* Peach and Horne, Proc. Roy. Phys. Soc, Edin., 1880, p. 3. 



