314 DR JOHN S. FLETT 



impressions he received on this visit confirmed the opinion he had already formed, and 

 led him to place these beds definitely in the younger portion of the Old Red Sandstone 

 (22), (23). 



The subdivision of the Old Red Sandstone of Scotland into Lower, Middle, and Upper, 

 which Sir R. Mdrchison had advocated, was discarded by Sir Archibald Geikie (6) in 

 his well-known paper on the Old Red Sandstone of Western Europe (Part I.), which still 

 forms the principal source of information regarding the Orcadian Old Red Sandstone. 

 He proved definitely that the Upper rested in Hoy and elsewhere, with a marked 

 unconformability, upon an eroded surface of the Orcadian (7) ; but the Middle and Lower 

 subdivisions of Murchison he grouped into one. The evidence of the fossil fishes and 

 fossil plants points to their being distinct formations; and in its recent Memoirs (19) 

 the Geological Survey of Scotland has reverted to the threefold grouping of Murchison. 



In his paper, Sir A. Geikie does not express any decided opinion as regards the exact 

 horizon of the Shetland Old Red. He recognises that Murchison had relied mainly on 

 Hooker's determination of the Lerwick plants as Calamites in assigning these beds to 

 the topmost portion of the system ; and as this identification had been shown to be 

 dubious (32), the conclusion arrived at was hardly valid. The great resemblance of the 

 volcanic rocks in this series in Shetland to those in the Caledonian (Lower) Old Red of 

 Scotland, and the occurrence in Shetland of Estheria membranacea, known also in the 

 flagstones of Caithness and Orkney, were pointed out, and no doubt led him in sub- 

 sequent years to include the Shetland beds with his Lower Old Red Sandstone. At 

 any rate, this is the correlation that was ultimately accepted by him (8), (9). 



About the same time as Sir Archibald Geikie's paper appeared, Professor Heddle 

 published his geognosy of Shetland (13), in which brief space is given to the Old Red 

 Sandstone. In Dr Gibson's account of the Old Red Sandstone of the East of Shetland 

 (12) very careful descriptions of the lithology of the beds are given, but the lack of fossil 

 fishes is deplored. In the absence of more definite evidence, it is assumed that the 

 horizon of these beds is the same as that of the Caithness flags. 



In 1879 the first of a series of papers on the geology of Shetland by Dr Peach and Dr 

 Horne appeared in the Quarterly Journal of the Geological Society of London (28). This 

 included a description of the Old Red Sandstone, and was followed by two others. The 

 remarkable series of volcanic rocks was specially investigated (29). It is not too much to 

 say that, as the result of their work, amplifying and correcting the earlier descriptions of 

 Hibbert, Heddle, Geikie, and Gibson, the geology of Shetland, varied and complex 

 though it is, is better known than that of any part of Scotland which has not been 

 mapped by the Geological Survey. Four excellent maps of the geology of Shetland 

 have been published, one by Professor Heddle (13) and three by Dr Peach and Dr 

 Horne (28), (29), (30). 



To their accounts of stratigraphy of the Old Red Sandstone of Shetland, and its 

 relations to the older metamorphic rocks, little remained to be added. As regards the 

 age of these beds they maintained a conservative attitude, though acquiescing in Sir 



