654 B. N. PEACH AND J. HOENE ON THE 



clay. Even on the cliffs of the island of Hoy, overlooking the 

 Atlantic, striated surfaces have been observed by Professor Geikie 

 at a height of 600 or 700 feet above the sea-level. 



The evidence now adduced regarding the ice-movement proves 

 beyond all doubt that the islands have been glaciated in one deter- 

 minate direction, independently of their physical features. A glance 

 at the strige map accompanying this paper (PI. XXYII.) shows the 

 remarkable uniformity of the ice-flow in the different islands. Here 

 and there, where local causes interfered with the general movement, 

 slight deflections are met with; but, on the whole, the prevalent 

 direction varies from W.N.W. to N.N.W. A careful examination 

 of the numerous striated surfaces convinced us that the ice-sheet 

 must have crossed the islands from the North Sea to the Atlantic. 

 Indeed no one who reflects for a moment on the physical features 

 of the islands could reasonably attribute the striations to a local 

 radiation of the ice. H we except Hoy, these scattered islands 

 contain no mass of elevated ground which is capable of giving rise 

 to a local ice-sheet. So far from this being the case, we shall have 

 occasion to refer to the absence of any indications of the existence 

 of local glaciers in most of the islands towards the close of the 

 Glacial period, a phenomenon which is doubtless due to this very 

 cause. On the contrary, when we view the persistent north- 

 westerly trend of the striations in connexion with the physical 

 features, when we consider that the glaciated surfaces along the 

 cliff-tops, as well as the roches moutonnees on the hill-slopes, prove 

 that the islands must have been overflowed by the ice, we cannot 

 resist the conclusion that the ice-movement during the primary 

 glaciation originated beyond the limits of Orkney. 



Fortunately the dispersal of the stones in the Boulder-clay amply 

 confirms the foregoing conclusions regarding the north-westerly 

 movement of the ice, while the presence of Scotch rocks in the same 

 deposit enables us to demonstrate that the ice-sheet which crossed 

 this group of islands must have radiated from the mainland of 

 Scotland. 



lY. BOULDEE-CLAY. 



This deposit is not spread over the general surface of the low 

 undulating tablelands in the form of a more or less continuous 

 covering. It occurs mainly round the bays, where it frequently 

 attains a considerable depth, while the inland districts are covered 

 with a thin clayey soil, due to the decomposition of the underlying 

 flagstones. We shall have occasion to describe one or two sections 

 of Boulder-clay which may be traced continuously along the shore 

 for half a milo, and which are quite undistinguishable from the 

 Lower Boulder-clay of Scotland. Occasionally thin patches of this 

 deposit are to be found on the cliff-tops, containing well-striated 

 stones and foreign rocks, clearly indicating that the islands must 

 have been overflowed by the ice. 



In the island of Westra the Boulder-clay is sparingly distributed, 

 but some excellent sections are to be met with round the bays in 



