GLACIA.TION OF THE OEKNEY ISLANDS. 649 



the existence of roches moutonnees with striated surfaces, Boulder- 

 clay, and valley moraines in the islands in an article which ap- 

 peared in ' Nature ' *. This article was written in reply to a letter 

 by Samuel Laing, Esq., M.P., in which he asserted that there is no 

 evidence that Orkney had participated in the general glaciation of 

 Britain f. So far from there being any lack of evidence regarding 

 the glaciation of these islands, we hope to show that they contain 

 abundant proofs of having undergone severe glacial conditions. Our 

 observations, however, completely confirm Mr. Laing's statement 

 that there are no raised beaches in the islands indicating changes of 

 the relative levels of sea and land since glacial times. 



II. Geological Stettctuke. 



The geological structure of the islands is comparatively simple. 

 From Stromness on the Mainland northwards to Inganess there is 

 an axis of ancient crystalline rocks on which the representatives of 

 the Old Eed Sandstone rest unconf ormably. These crystalline rocks 

 consist of a fine-grained granite and a grey micaceous flaggy gneiss, 

 which occupy a strip of ground about four miles in length and 

 about a mile in breadth. Thej are prolonged southwards in the 

 island of Grraemsa. With this exception the whole of the Orkney 

 Islands are occupied by Old Red Sandstone strata. In the island 

 of Foy representatives of both the upper and lower divisions of this 

 formation are met with, and here they are separated by a marked 

 unconformity ; but in all the other islands the beds belong to the 

 lower division. 



Throughout the islands there is a remarkable uniformity in the 

 character of the strata belonging to the lower division. They consist 

 mainly of hard blue and grey calcareous flagstones, which are so typi- 

 cally developed in Caithness. Fortunately, however, the highest beds 

 of the Orcadian flagstone series are totally different in character from 

 those just described, being composed of coarse siliceous red and 

 yellow sandstones and marls. The sandstones are full of false- 

 bedding, and frequently conglomeratic, containing pebbles of granite, 

 quartzite, gneiss, and other crystalline rocks. 



The distribution of this arenaceous series has an important 

 bearing on the question of the ice movement. On referring to the 

 map accompanying this paper (PI. XXVI.), it wiU be seen that it 

 forms a well-marked zone, running nearly north and south through 

 the centre of the group. The relations which these siliceous sand- 

 stones bear to the flagstones are best seen in Eda, where they cover 

 the greater part of the island, and where they form smooth flowing 

 hills upwards of 300 feet in height. The sandstones lie in a syncline, 

 the axis of which runs north and south, and on both sides of the island 

 they rest conformably on the flagstones. In the islands which lie 

 to the west and north-west of Eda, viz. Fara, Westra, Papa Westra, 

 Egilsha, and Rowsa, the strata consist wholly of blue and grey 

 flagstones, which are inclined at gentle angles. Though there are 



* ' Nature,' vol. xyi. p. 414. t * Nature/ vol. xvi. p. 418.. 



