342 Proceedings of the Boyal Physical Society, 



of the Upper Old Eed Sandstone of that island, and are 

 therefore to be considered of that age. 



Several dykes of basalt were observed among the islands. 

 They are most numerous and conspicuous on the west coast 

 of the Mainland from Brackness to Skail, but as they have 

 been so often described, it is unnecessary to refer to them in 

 detail. They have the same lithological characters, and behave 

 exactly in the same manner as the dykes in other parts of 

 Scotland, which have been regarded as the product of volcanic 

 energy in Miocene times. A noticeable feature about the 

 Orcadian representatives is, that they are usually divided up 

 the centre of the dyke by a line of vesicles. This is not an 

 uncommon feature elsewhere. 



Summary. — The descriptions we have given of the Lower 

 Old Eed Sandstone strata as represented in the Orkney 

 Islands, tend to confirm the conclusion previously arrived at 

 by Professor Geikie, that these flagstones, with the associated 

 arenaceous series, must be correlated with the higher sub- 

 divisions of the Caithness series. It is highly probable, there- 

 fore, when the ichthyology of Orkney is worked out in detail, 

 that the fossils will be identical with those derived from 

 the higher portions of the Caithness series. The great 

 development of the sandstone series in the northern isles is 

 of special interest, as it shows that the strata gradually 

 assume the arenaceous character which is so prevalent in 

 Shetland. 



Moreover, it is of importance to note that the coarse 

 silicious sandstones and marls, which are the highest repre- 

 sentatives of the Lower Old Eed Sandstone in Orkney, must 

 not be confounded with the massive red sandstones which 

 form the noble cliffs on the west side of Hoy. The latter rest 

 unconformably on the flagstones. It is evident, therefore, 

 that after the deposition of the Flagstone series, with its 

 associated sandstones and marls, the bed of this inland sea 

 was elevated so as to form a land surface. These strata 

 were subjected to a considerable amount of denudation ere 

 they were again carried below the water in Upper Old Eed 

 Sandstone times. 



