340 Proceedings of the Royal Physical Society. 



In the district of South Walls and the promontory of 

 Brim's Ness, in Hoy, the rocky cliffs exhibit the ordinary 

 grey flagstones. On the shore of the Longhope, between 

 Melsetter and the Inn, they pass under the red and yellow 

 sandstones ; and the same relation is observable on the coast 

 facing the Atlantic, a little to the south of Melsetter. On 

 a former visit of one of the authors with Professor Geikie 

 considerable difficult}^ was experienced in tracing the fault 

 which divides the Upper Old Eed Sandstones from the 

 arenaceous series associated with the flagstones, till the inter- 

 calation of the sandstones with the Lower series was realised. 



From what has been said it will be seen that Scapa Flow 

 occupies the site of a geological basin, towards which the rocks 

 dip on every side, and along the shores of which the highest 

 beds of the Lower Old Eed Sandstone exposed in Orkney are 

 to be met with as well as in Eday and Sanday. 



The beds in South Eonaldshay are exceedingly like those 

 exposed along the shores of Gills Bay on the opposite Caithness 

 Coast and in the intermediate island of Stroma, and are, in 

 all probability, their prolongations to the north-east, which is 

 the direction of their strike. We have thus a definite 

 horizon to start from, for the highest beds in Orkney are also 

 the highest in the Caithness series. 



It is worthy of note that the sandstones become coarser as 

 they are traced northwards. In South Eonaldshay there 

 are great masses of friable marly clays, intercalated with 

 the sandstones. In Eday and Sanday there are only about 

 ten feet of such strata, while the sandstones are very coarse 

 and even conglomeratic, and approach much more to the type 

 of the Lower Old Eed strata of Shetland. 



Igneous Rocks in the Lower Old Red Sandstone. — The only 

 contemporaneous rocks of this nature occur at the south- 

 east corner of Shapinshay, between Haco's ISTess and Foot, 

 where they form the coast line for about half a mile. They 

 are perfectly conformable with the flagstones. The upper 

 surface of the diabase is highly vesicular and amygdaloidal, 

 and exhibits all the characters of a regular lava flow. The 

 flagstones overlying it are not altered in the slightest along 

 the line of contact. The base of the volcanic series is not 



