( 313 ) XIII.—On the Age of the Old Red Sandstone of Shetland. By John S. Flett, M.A., D.Sc. (Read March 18, 1901. MS. received May 26, 1908. Issued separately July 8, 1908.) In spite of its remote situation, the Old Red Sandstone of Shetland attracted a con- siderable amount of attention from geologists during the last century. It is exposed in excellent coast sections, which often yield very beautiful cliff scenery; and, in addition to being the most northerly of the stratified rocks of Great Britain, it includes a rich succession of volcanic and intrusive rocks which are of great interest and variety. The axis or backbone of the Shetland archipelago consists of gneiss, mica schist, slate, and lime- stone, with epidiorites, serpentine, and talc schists. On each side of this there is an area of Old Red Sandstone; that on the east extending from Sumburgh Head, in the extreme south, to Rovey Head, a little north of Lerwick, and comprising also the islands of Bressay, Noss, and Mousa. On the west side of Shetland the Old Red Sandstone Series is much altered, probably by the heat of the granite and other intrusive rocks, so that they often have the appearance of quartzite, and were for a long time regarded as belonging to the metamorphic series. In 1879, however, PEACH and HORNE (28) showed that, in places, they contained fossil plants which indicated that they belonged to the Old Red Sand- stone formation. The earliest accounts of the Old Red rocks of Shetland are to be found in the descriptive works of JAMESON (1G), NEILL (25), BOUE (l), SHIREFF (34), FLEMING (2), and HIBBERT (14). These writers were all of the Wernerian school, and described the conglomerates, sandstones, and flags as " secondary," resting on the " primitive" or metamorphic group. Of these accounts the best are those of HIBBERT and of FLEMING ; the latter in particular deserves mention, as being the first to record the occurrence of fossil plants in the Lerwick Sandstones. In 1853 an important advance was made by the description of some fossil plants from South Ness, Lerwick, by Dr (afterwards Sir) JOSEPH D. HOOKER (15). He referred them to two species of Calamites. This paper was communicated to the Geological Society of London, and was accompanied by a note by Sir RODERICK MURCHISON (21), in which he stated his conviction that " the sandstone of Lerwick is of the same age as the rocks of Elgin, Burghead, Tarbet Ness in Ross, and Dunnet Head in Caithness, all of which Professor SEDGWICK and myself described as constituting the uppermost member of the Old Red Sandstone, and as overlying the Caithness flagstones, with their numerous ichthyolites." MURCHISON, accompanied by SEDGWICK, had already visited Caithness, Ross, and Cromarty, and was familiar with the Old Red Sandstone of these districts (33). He subse- quently proceeded again to Caithness, and thence to Orkney and.Shetland (5). The TRANS. BOY. SOC. EDIN. VOL. XLVI. PART II. (NO. 13). 48 314 BR 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. MURCHISON 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 Oalamites 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 ON THE AGE OF THE OLD RED SANDSTONE OF SHETLAND. 315 A. GEIKIE'S relegation of them to the Lower division of the system (which, of course, included the Orcadian). In this opinion they were supported by C. W. PEACH (27), who re-examined the fossil plants described by HOOKEU, and found that they exhibited close affinities to those obtained in the Old Red of Caithness and Orkney. In 1898, after spending part of several years in investigating the Old Red Sandstone of Orkney (3), I determined to visit Shetland and make a search for fossil fishes which would establish the position of the sandstones and shales of these islands relatively to those of Caithness and Orkney. Six weeks were spent in a scrutiny of all the best ex- posures on the east side of the mainland from Sumburgh Head to Lerwick, and in the islands of Bressay and Noss. The results, though unsatisfactory, were not entirely dis- appointing, as indecipherable fragments of fishes were obtained in Bressay, at Lerwick, Sandwick, and the east side of Quendale Bay. Further search was determined on ; and to meet the expenses of quarrying, a grant was applied for from the Royal Society of London (Government Grant Committee), which was conceded. Consequently, in 1899, with the consent of the late Mr HAMILTON, of Unst, an opening was made in the beach on the east side of Cullingsburgh Voe in Bressay, and our expectations were soon con- firmed by the discovery of scattered plates of undoubted fossil fishes belonging to new species. These fishes were handed to Dr TRAQUAIR for identification, and a preliminary notice was inserted in Nature to announce the discovery of a new zone of the Old Red Sandstone of Scotland (24). THE FISH-BEARING BEDS OF BRESSAY, AND THEIR POSITION IN THE OLD RED SANDSTONE OF SHETLAND. As recognised by all who have described this area, the structure of the district around Lerwick and Bressay is exceedingly simple. A little west of Lerwick, coarse conglomerates are faulted against the metamorphic series. They dip towards the east, and are succeeded at the town of Lerwick by reddish and grey sandstones, often current-bedded, and sometimes containing large rounded pebbles of quartzite, granite, etc. At the point south-east of Lerwick known as the Nabb, grey micaceous sandstones occur, full of plant- remains, and containing also the small crustacean Esiheria membrancea (22). On the opposite shore of Bressay Island the first beds met with are brownish and grey sand- stones, often conglomeratic, and sometimes brecciform, with occasional grey and reddish shales. A series of faults or crush belts run nearly north and south along this side of the island from Maryfield to Ham, setting the beds frequently on end, and converting them into breccias and crush conglomerates. In crossing Bressay the dip of the rocks is consistently east or south-east, varying from ten to thirty degrees. The commonest rocks are grey, micaceous, thin-bedded sandstones, with coarser, less micaceous, gritty seams, often current-bedded. The sandstones contain rounded clay galls, and their sur- faces are often covered with blackened fragments of plants and shreds of fine shale. Small faults are frequently seen in the coast sections, mostly running parallel to the strike.