THE OLD RED SANDSTONE OF THE ORKNEYS. 395 



the fact that at Stromness they rest upon the granite axis — that the Stromness beds 

 form the lowest zone of the Old Red Sandstone of the Orkneys. 



It is a matter of great difficulty to form a reliable estimate of the thickness of this 

 series in Orkney, as will be evident when we consider that its true base is nowhere seen, 

 and that its upper boundary must, in our ignorance of any but the general facts regard- 

 ing the distribution of fossils throughout the county, be of necessity an arbitrary one. 

 By far the best continuous section of these beds is that exposed along the shore from 

 the Ness of Stromness to Breckness, nearly three miles to the westward. The section 

 runs in a W. or W.N.W. direction, and during its whole course there is a continuous 

 exposure of the rocks at low water. They dip along the shore about W. 10° S., and 

 during the first half of the distance the average amount of dip is 15°. In the little 

 sandy bay beyond the churchyard the dip swings southwards, and is more gentle for a 

 little, but on the west side resumes its previous direction and amount. If we draw a 

 line perpendicular to the strike and measure the distance, it is almost exactly two miles, 

 and the thickness, allowing for an average dip of 12°, is about 2000 feet, which is exactly 

 the thickness estimated by Sir A. Geikie for a section parallel to this and a mile further 

 south, from the centre of Graemsay to the base of the Hoy Hills.* As a matter of 

 fact, as the flagstones at Ness rest on the granite conglomerate; and the rocks at Breck- 

 ness, if prolonged northwards along their strike, are seen to be on a level not greatly 

 differing from those which at Inganess rest on the west end of the same granite axis, 

 we are led to the conclusion that the western conglomerates must be on a much higher 

 level than those at the east end of the granite outcrop. . But the lowest rocks in this 

 district must be those which have been uplifted by the Tenston fault along the axis of 

 the West Mainland anticline. This fault is prolonged southwards through the Bay of 

 Ireland ; and if we carry the section backwards from Stromness to Bu Point, we find that 

 along this shore the rocks are so rolling that no great thickness is required to be added 

 to our estimate, the same beds being probably again and again repeated by means of 

 gentle folds. 



Results in substantial accordance with this are obtained by taking a section some 

 six miles to the northward, from the fault on Tenston Ness on the Loch of Harray, to 

 Skaill Bay on the west shore of the Mainland. The length of a section from Tenston 

 due west to the Atlantic is nearly four miles, and in the intervening country the dips 

 never vary greatly from a true W. In amount they differ, being 12° or more at 

 Lyking, at Voy nearly flat, at Sand wick manse 5°, at Rango 5°, at Skaill 3 to 7°. If 

 we accept 5° as an average, the thickness is 1760 feet. In this case the conditions are 

 not so satisfactory as in the preceding, the exposure of rock not being a continuous one. 



To this must now be added the rocks which lie between those of Breckness and 

 Skaill and the base of the Rousay series. That at both these places we are well within 

 the Stromness zone is evident from the fact that they are among the best known 

 localities for its type fossils. The district in the N.E. corner of the West Mainland 



* Sir A. Geikie, Old Red Sandstone, pt. i. p. 410. 



