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XIII.—On the Age of the Old Red Sandstone of . 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 , 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 , in the extreme south, to Rovey Head, a little north of , and comprising also the islands of , Noss, and . 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 Head in , 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 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 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 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 , 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. On 316 DB JOHN S. FLETT the west side of Noss Sound a small volcanic neck, described by PEACH and HORNE (29), occurs, with a thin bed of ash. Faulting also is visible here, and the beds are often steeply inclined. In Noss the easterly dip again prevails, and in the great cliff on the east side of this island a fine section of thin flaggy sandstone and grey shales is exposed, exceedingly well stratified, and resembling closely many of the cliff features of Orkney and Caithness. In view of the persistent easterly dip, often at fairly high angles, the whole thick- ness of this series must be several thousand feet; but the evidence of faulting along the shores of the Sounds is sufficient to render exact estimates impossible. The fish beds in Cullingsburgh Voe are rather above the middle of the Bressay Sandstones. The fossils occur in a thin-bedded, flaggy, grey micaceous sandstone, and the plates are black in colour and well preserved. With them thin black impressions of plants are exceedingly common. The strata were evidently laid down in shallow water, close to land; and the general facies of the rocks and of the fauna is in harmony with the supposition that they were fresh-water deposits. Fossiliferous bands must occur in Bressay besides that in which our excavations were made, as we found a fish fragment in a beach stone on the east side of Cullingsburgh Voe, and another in a loose rock to the west of the houses of Cullingsburgh. Professor HEDDLE (20), (10) tells us that he saw " specimens of small fishes, apparently acanthoides, embedded in a fine-grained muddy sandstone ; they were stated to occur in a quarry north of Gardie, in Bressay." There is no reason to doubt this record ; but, unfortunately, the fishes can no longer be traced (if they were preserved). I made a careful search in all the quarries near Gardie, but could see no remains of fossil fishes. It is clear that the Shetland Old Red Sandstone is by no means so barren as has hitherto been supposed ; but to obtain good fossils, great skill and patience, with some measure of luck, will be required.

THE FAUNA OF THE BRESSAY SANDSTONES.

The fish-remains obtained in these beds have been determined by Dr TRAQUAIR to belong to Asterolepis (sp.) and Holonema (sp. nov.); possibly there are also fragments which may be referred to a species of Holoptychius. Of these genera, the first is typically Upper Old Red; it occurs also in beds assigned to that series at Nairn. Holonema is a genus founded by NEWBERRY (26) for remains from the Chemung beds (Upper Devonian) of North America. Holoptychius is a very characteristic Upper Old Red genus. So far, then, the evidence of the fish fauna points clearly to the Shetland beds belonging to the Upper part of the Old Red system. It is not, however, absolutely satisfactory when closely examined. The Holonema is a new species ; the Asterolepis also is probably new, though as yet not definitely named; and caution is necessary in classifying Old Red faunas in such cases. As an example of this we may quote the typical Lower Old Red genus Cephalaspis, which occurs in the Middle Old Red of ON THE AGE OF THE OLD RED SANDSTONE OF SHETLAND. 317

Caithness (35) and the Upper Old Red of Canada (36). Coecosteus also is known from the whole range of the Devonian or Old Red Sandstone succession (19). It is easy, however, to exaggerate the importance of exceptional cases. More important is the fact that Asterolepis is represented by a species as yet undescribed in beds immediately under- lying the John o' Groats zone of the Orcadian Old Red in Orkney (4). This indicates that if the Shetland beds be Upper Old Red, they have close relationships to the Orcadian. Similarly, there are grounds for believing that the Nairn beds (18) which contain Asterolepis are older than the Elgin, Dura Den, Jedburgh, and Kiltorcan Sandstones, in which Bothriolepis and Holoptychius nobilissimus occur. The latter must represent the highest portion of the Old Red Sandstone, as in places they pass up conformably into the base of the Carboniferous. The only other animal remains as yet obtained from the Shetland Old Red Sandstone are Esthena membranacea (Pacht) and plant fragments. Of these, tlie former was first recorded by Sir R. MURCHISON (22), and was described by Professor RUPERT JONES (31). This species is abundant and well preserved in the Orcadian Old Red of Caithness and Orkney. It occurs also in the Devonian of Livland (Livonia) in Russia, but has not been found in the Upper Old Red Sandstone of any part of Great Britain (except Shetland). This is sufficient to indicate that the Shetland beds have close relationships with the Middle Old Red Sandstone.

THE FOSSIL FLORA OF THE SHETLAND OLD RED SANDSTONE.

The fossil plants found near Lerwick have been described by many palseobotanists. They are in a most unsatisfactory state of preservation, but (with the exception of Sir JOSEPH HOOKER) (15) all who have examined them have noted their resemblance to the hardly less imperfect plant-remains so numerous in the Middle Old Red of Caithness and Orkney. Recognising that little was definitely known about their real nature, I had specimens forwarded to Dr KIDSTON, in order to obtain the latest and most authoritative opinion regarding them. He assures me that, whatever may be their botanical affinities, they have nothing in common with the Lower Old Red flora of Forfarshire and Perth- shire, or the Upper Old Red floras of Kiltorcan and Roxburghshire. So far as they are determinable, they resemble rather the plants of the Orcadian Old Red. This opinion, though vague, is of value, as confirming the evidence provided by the crustaceans (Estheria) and one of the genera of fishes (Asterolepis).

LITHOLOGY.

The lithological similarity between the Old Red Sandstone of Shetland and that of Caithness and Orkney has been remarked on by various writers. It is best seen in the cliff exposures when viewed from a distance, as the thin-bedded flags of Bressay and Noss, where eroded by the sea, yield cliffs very like those of Orkney. Closer examination, 318 DR JOHN S. FLETT however, hardly strengthens the comparison. The Shetland beds are coarser, more micaceous, and less uniform than those of Orkney. Clearly they were formed by the denudation of a slaty and gneissose series, like that which forms the axial ridge of Shetland. Evidence of this sort does not carry much weight; but we may note that the Old Red Sandstone of Shetland certainly resembles the Caithness beds in litho- logical character more closely than the Upper Old Red Sandstone of Hoy, Ross-shire, and the districts south of the Moray .

SUMMARY.

In speculating on the age of the Shetland beds, the character of the vertebrate fauna is by far the most reliable index ; and next to it we may place the evidence of the plants. The former points unmistakably to Upper Old Red conditions, the latter to close affinities with the Orcadian. The conclusions which at the present time we are justified in arriving at may be summarised as follows :— (1) The Old Red Sandstone beds of Shetland belong to a distinct zone of that system, different from any other yet identified in Britain. (2) By the evidence of its fossil fishes, it is most naturally placed in the Upper division. (3) Both the fauna and the flora indicate affinities with the Orcadian or Middle Old Red of Caithness and Orkney, consequently it may be the lowest or one of the lowest zones of the Upper Old Red Sandstone.

LITERATURE.

(1) BOUB, AMI, Essai Geologique sur I'Ecosse, p. 114 (n.d. probably 1820). (2) FLEMING, J., Mem. Wemerian Soc, vol. i., p. 174 (1808). - FLETT, JOHN S., " On the Discovery in Orkney of the John o' Groat's Horizon of the Old Red Sandstone," Proc. Roy. Phys. Soc. Edin., vol. xiii., p. 255 (1896). (3) „ „ " The Old Red Sandstone of the Orkneys," Trans. Roy. Soc. Edin., vol. xxxix., p. 383 (1898). (4) „ „ Ibid., p. 409 (1898). (5) GBIKIE, Sir A., Memoir of Sir R. Murchison, vol. i., p. 139 (1875). • (6) „ „ "The Old Red Sandstone of Western Europe (Part I.)," Trans. Roy. Soc. Edin., vol. xxviii., p. 345 (1878). "The Old Man of Hoy," Geol. Mag., p. 49 (1878). „ Geological Sketches, p. 76 (1882). (8) ., „ Text-Book of Geology, p. 1010 (1903). (9) ,, „ Geological Map of Scotland (published by Bartholomew, Edinburgh). (10) „ „. Trans. Roy. Soc. Edin., vol. xxviii., p. 418 (1878). (11) „ „ Text-Book of Geology, p. 1012 (1903). (12) GIBSON, GEORGB, The Old Red Sandstone of Shetland (1877). - . . ON THE AGE OF THE OLD RED SANDSTONE OF SHETLAND. 319

(13) HEDDLE, M. F., Mineralogical Magazine, vol. ii. (1879). (14) HIBBERT, SAMUEL, A Description of the Shetland Islands, p. 160, etc. (1822). (15) HOOKER, JOSEPH D., "Note on the Fossil Plants from the ," Quart. Jour. Geol. Sue, vol. ix., p. 49 (1853). JAMBSON, R., Outline of the Mineralogy of the Shetland Isles, p. 15, etc. (1798). (16) { „ „ The Mineralogy of the Scottish Isles, vol. ii., p. 195, etc. (1800 and 1813). (17) KIDSTON, Ii., Catalogue of Palmozoic Plants in the British Museum, pp. 233 and 238 (1886). (18) MACKIE, W. S., Sand Grains, Elgin (1894). (19) Memoirs of tlie Geological Survey of Scotland : " The Geology of Lower Strathspey." (20) Min. May., vol. ii., p. 156. (21) MURCHISON, Sir RODERICK I., "Note on the Age and Relative Position of the Sandstone containing Fossil Plants in the Shetland Isles," Quart. Jour. Geol. Soc, vol. ix., pp. 50 and 50 bis. (1853). (22) ,, ,, " On the Succession of the Older Rocks in the Northernmost Counties of Scotland, with some Observations on the Orkney and Shetland Islands," Quart. Jour. Geol. Soc, vol. xv., p. 353 (1859). (23) „ „ Siluria (4th ed.), p. 259(1867). (24) Nature, Nov. 23, 1899, vol. lxi., p. 80. (25) NEILL, PATRICK, A rTour through some of the Islands of Orkney and Shetland, p. 72, etc. (1806). (26) NBWBERRY, ,T. S., Palaeozoic Fishes of N. America, p. 92 (1889). (27) PEACH, C. W., Quart. Jour. Geol. Soc, vol. xxxv., p. 811 (1879). (28) PEACH, B. N., and HORNE, J., " The Glaciation of the Shetland Isles," Quart. Jour. Geol. Soc, vol. xxxv., p. 778 (1879). (29) „ „ " The Old Red Volcanic Rocks of Shetland," Trans. Roy. Soc Edin., vol. xxxii., p. 359 (1884). (30) „ „ " The Geology of Shetland," in Tudor's The Orkneys and Shetland (1883). , RUPERT JONES, T., Quart. Jour. Geol. Soc, vol. xv., p. 404. I „ ,, On Fossil Estherise and their Distribution, ibid., vol. xix., p. 140 (1862). 1 .i » "Monograph of the Fossil Estherise," Palseontographical Society, p. 14 (1862). • „ „ Geol. Mag., 1890, pi. xii., and 1891, p. 50. (32) SALTER, J. W., " On some Remains of Terrestrial Plants in the Old Red Sandstone of Caithness," Quart. Jour. Geol. Soc, vol. xiv., p. 72 (1858). (33) SEDGWICK, A., and MURCHISON, R. I., "On the Structure and Relations of the Deposits contained between the Primary Rocks and the Oolitic Series in the North of Scotland," Trans. Geol. Soc, ser. ii., voL iii., p. 125 (1828). (34) SHIREPP, Agriculture of the Shetland Islands (1814). (35) TRAQUAIR, R. H., "On Cephalaspis magnifica (Traq.)," Proc. Roy. Phys. Soc. Edin., vol. xii., p. 269 (1893). (36) ,, „ " Notes on the Devonian Fishes of Scaumenac Bay and Campbelltown in Canada," Geol. Mag. [3], vol. vii., 1890, p. 16. (37) TUFPNELL, HENRY, " Notice of the Discovery of Fossil Plants in the Shetland Isles," Quart. Jour. Geol. Soc, vol. ix., p. 49 (1853).