660 B. N^. PEACH AjSTD J. HOENE ON THE 



has an important bearing on the questiom of the extension of the 

 ice in the North Sea. Some smaller blocks of gneissose rocks occur 

 in the neighbourhood. A few boulders of conglomeratic sandstones 

 occur in Eda, which may be purely local. 



On the Mainland blocks of white and reddish-grey sandstone are 

 strewn on the hill-slopes north of Pinstown and on the moory 

 ground south of Maes Howe, which have been derived from the 

 north-west shore of Scapa Plow ; and so also along the west coast, 

 between Brak Ness and Inganess, north of Hoy Sound, boulders of 

 granite and gneiss are met with on the flagstone area to the west of 

 the axis of crystalline rocks. 



YII. Conclusion. 



The evidence now adduced regarding the glacial phenomena of 

 Orkney is of the utmost importance in solviag the question of the 

 extension of the ice in the North Sea. We have already referred 

 to the remarkable uniformity in the trend of the ice-markings 

 throughout the islands, which, with certain exceptions, vary from 

 W.N.W. to N.N.W. From the manner in which these striations 

 maintain their persistent north-west trend, irrespective of the 

 physical features of the country, it is evident that the agent which 

 produced them must have acted independently of the islands. 



Nay, more, the dispersal of the stones in the Boulder-clay leaves 

 no room for doubt that the ice-sheet must have crossed the islands 

 from the North Sea to the Atlantic. It is no doubt true that the 

 lithological varieties of the Orcadian rocks are not so numerous as 

 in Shetland, and hence the corroborative evidence of the north- 

 westerly movement is not so abundant. Still in those cases where 

 the geological structure of the ground permitted us to test with 

 certainty the direction of the ice-carry, we were driven to the con- 

 clusion that the ice-flow must have been towards the Atlantic. In 

 Westra the Boulder- clay sections contain striated blocks of red and 

 white sandstone, which have been derived from Eda, and it is 

 particularly observable that they diminish in number as we 

 move towards the north-west. In Shapinshay blocks of the slaggy 

 diabase from the south-east corner of the island occur in the Boulder- 

 clay near Galtness ; and so also in the Mainland, the red and 

 white sandstones which cross the centre of the island are repre- 

 sented in the moraine profonde on the shore between Houton Head 

 and the Loch of Stennis. Yet, again, to the west of the axis of 

 crystalline rocks at Stromness, smoothed blocks of gneiss and 

 granite are fouud in considerable abundance. 



Eortunately, however, we have additional evidence which enables 

 us to demonstrate, not only that the ice-movement must have been 

 from the North Sea towards the Atlantic, but, what is of still 

 greater moment, that the ice which glaciated Orkney must have 

 come from Scotland. In the numerous sections of Boulder-clay 

 described in this paper we have had occasion to refer to the occur- 

 rence of smoothed and striated stones of dark-grey limestones fuU 

 of plant-remains, oolitic limestone, calcareous breccia, chalk, chalk- 



