330 Proceedioigs of the Boyal Physical Society. 



rests unconformably, to the large development of the Flagstone 

 series, which is analogous to that of Caithness and to the 

 great succession of red and yellow sandstones of Hoy, which 

 graduate downwards into the flagstones. 



In 1878 appeared the first part of Professor Geikie's 

 treatise on the Old Eed Sandstone of Western Europe.* Tliis 

 valuable monograph was the first comprehensive attempt to 

 sketch the history of the deposits belonging to this formation 

 in Shetland, Orkney, Caithness, and the Moray Firth basin, 

 and to restore in outline the physical geography of the period. 

 In that portion of the memoir which refers to Orkney, he 

 pointed out, for the first time, the unconformity between the 

 massive yellow sandstones of Hoy and the flagstones. He 

 likewise called attention to the contemporaneous lavas and 

 tuffs which lie at the base of the Upper Old Eed Sandstone 

 of Hoy, and to the existence of volcanic " necks " from which 

 these materials had been discharged. He also controverted 

 the idea, advocated by Murchison, that the conglomeratic 

 strata which rest unconformably on the crystalline rocks at 

 Stromness, form the true base of the formation. He regards 

 them merely "as a local interruption of the Flagstone series, 

 due to the rise of an old ridge of rock from the surface of the 

 sheet of water in which these strata were accumulated." 

 Moreover, he correlates the Orkney flagstones with the 

 higher subdivisions of the Caithness series, which is so far 

 confirmed by the fossil evidence hitherto obtained. 



In the Miner alogical Magazine for December 1879, Pro- 

 fessor Heddle published a paper on the Orkney Islands, in 

 which he describes a well-marked trough which runs througli 

 the centre of the group of islands. The strata which occupy 

 the centre of the trough he describes as "loose arenaceous 

 freestones, with silicious granules sometimes so coarse as 

 almost to entitle them to the designation of grits." More- 

 over, he notes the important fact that these arenaceous strata 

 repose conformably on the ordinary blue flags of the islands. 



* Trans. Roy. Soc. Edin., XXVIII., 345. This memoir contains references 

 to other papers than those we have quoted on "The Old Red Sandstone of 

 Orkney." We have chiefly referred to those publications which treat of the 

 geological structure of the islands. 



