Tlie Old Red Sandstone of Orkney. 337 



it is easy to account for the Eday sandstones being brought in 

 by the aid of a comparatively small fault. It is important to 

 note this dislocation, for these beds are continued across the 

 channel to Inganess on the Mainland, whence they stretch 

 across to Scapa, and are extended on to near Orphir Kirk, 

 being cut off from the flagstones along their whole northern 

 boundary by a large fault with a downthrow to the south. 



An interesting feature connected with the patch of sand- 

 stones and the associated flags at Haco's Ness, is the occur- 

 rence of interbedded volcanic rocks, clearly proving the 

 existence of volcanic action in Lower Old Eed Sandstone 

 times in Orkney. The lithological character of these rocks, 

 as well as their appearance under the microscope, will be 

 described under a separate heading. 



On the Mainland, the arenaceous series just described as 

 occurring in Shapinshay, and which is likewise represented 

 in Stronsay, Sanday, and Eday, is well developed. But 

 before describing the relations of the two groups as repre- 

 sented on the Mainland, we shall refer briefly to the develop- 

 ment of the Flagstone series in the western part of the island. 

 The unconformity between the flagstones and the axis of 

 crystalline rocks at Stromness has been frequently described. 

 Our observations tend to confirm the conclusions already 

 arrived at by Professor Geikie, that the conglomeratic strata 

 which repose on the gneissic rocks merely indicate a local 

 base. It is quite true that the brecciated flagstones are 

 mainly derived from the underlying crystalline rocks, but the 

 conglomeratic character disappears within a short distance of 

 the gneiss. Professor Geikie has alluded to the fact that the 

 general dip of the flagstones in Hoy is to the north and north- 

 west, and consequently the flaggy strata, which rest uncon- 

 formably on the gneissic ridge, are probably higher in the 

 series than those in Hoy. This evidently points to a gradual 

 subsidence of the area during the deposition of the Flagstone 

 series. 



The strata represented in the north-western portion of the 

 Mainland are evidently the southern prolongations of the 

 flaggy series which we have already described as occurring in 

 Westray. They are admirably exposed on the shore between 



