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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 spifce 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 (1), 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 



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