The Old Red Sandstone Volcanic Rocks of Shetland. by BN PEACH and J. HORNE, of the Geological Survey of Scotla

( 359 ) XXI.—The Old Red Sandstone Volcanic Rocks of Shetland. By B. N. PEACH and J. HORNE, of the Geological Survey of Scotland. (Plates XLV. and XLVI.) CONTENTS. PAGE Introduction, 359 5. Necks on Bressay and Noss, . 377 I. GEOLOGICAL STRUCTURE OP THE VOLCANIC 6. Summary of Events indicated by ROCKS, 361 Volcanic Phenomena of Shetland A. Contemporaneous Lavas and Tuffs, . 361 Old Red Sandstone, 378 1. Porphyritic Lavas and Tuffs of North- mavine, . .361 II. MICROSCOPIC CHARACTERS. 379 2. Diabase Lavas and Tuffs of Aithsting 1. Porphyrite LavasA and Sandsting, .... 364 2. Diabase Lavas, lBasic Rock8> . 379 3. Probable Horizon of the Lavas and 3. Intrusive Diabase | Tuffs of Aithsting and Sandsting, 365 Rocks, ' 4. Lava of the Holm of Melby, . 366 4. Intrusive Sheets— . 381 5. Bedded Lavas and Tuffs of Papa Rooeness Sheet, . 382 Stour, 366 Sandsting Sheet, . 382 6. Band of Tuff in Bressay, . 367 Papa Stour Sheet, . 382 B. Intrusive Igneous Bocks, .... 368 5. Dykes— Acidic Rocks. 3 83 1. Intrusive Sheet of Binary Granite Binary Granites . 383 in Northmavine, .... 368 and Quartz-fel- 2. Intrusive Sheet of Granite in Sand- sites, . 383 sting, 370 Rhyolites, . 384 3. Intrusive Sheet of Spherulitic Fel- Summary of Results, . 386 site in Papa Stour, . 371 4. Dykes— 373 APPENDIX.—Table of Chemical Analyses of a. Binary Granites, . 373 eight Specimens of Shetland Old Red 6. Quartz-felsites, . 373 Volcanic Rocks, by R. R. TATLOCK, c. Rhyolites, .... 374 F.R.S.E., 387 d. Diabase Rocks, . .376 Perhaps the most interesting feature connected with the Old Red Sandstone formation in Shetland is the evidence of prolonged volcanic activity in those northern isles. The great development of contemporaneous and intrusive igneous rocks, which gives rise to some of the most striking scenery in Shet- land, is all the more important when compared with the meagre records in the Lower Old Red Sandstone of Orkney and the Moray Firth basin. Not till we pass to the south of the Grampians do we find evidence of a far grander display of volcanic action during this period, in the sheets of lava and tuff in the Sidlaws and Ochils and in the great belt stretching from the Pentlands south-westwards into Ayrshire. The relations of the Shetland igneous rocks are admirably displayed in the various coast sections, especially in the mural cliffs of Northmavine and some of the Western Islands. From these records, though they have been subjected to much denudation, it is possible to con- struct a tolerably complete sketch of the volcanic history of this formation, as developed in that region. VOL. XXXII. PART II. 3 M 360 B. K. PEACH AND J. HORNE ON THE No previous attempt has been made to furnish a chronological account of the Old Red volcanic phenomena of those northern isles. In HIBBERT'S admirable volume * there are various references to the granite masses of the Mainland and the amygdaloidal claystones in the south-west of Northmavine. He also refers to the porphyritic and amygdaloidal rocks in Papa Stour, which were likewise described by Dr FLEMING.! In various papers published in the Mineralogical Magazine,^ Dr HEDDLE notes the existence of interbedded and intrusive igneous rocks of this age in Shetland, with descriptions of the minerals obtained from them. The first attempt, however, to connect these Old Eed volcanic rocks with their representatives south of the Grampians, was made by Dr ARCHIBALD GEIKIE, the present Director-General of the Geological Surveys. In 1876 the geological structure of Papa Stour, which is almost wholly com- posed of volcanic rocks, was solved by him in company with Mr B. N. PEACH > and, as the result of that traverse, an account of the geology of that interesting island was given in his celebrated paper on " The Old Red Sandstone of Western Europe," published in the Transactions of this Society.§ Though unable to visit the volcanic rocks on the north side of St Magnus Bay, he ventured to suggest thata the amygdaloidal claystones referred to by HIBBERT would turn out to be merely a repetition of those in Papa Stour,—a suggestion which has been amply verified by subsequent investigations. During our successive visits to Shetland, which were undertaken mainly with the view of examining the glacial phenomena of the group, we were induced to pay close attention to the distribution and geological structure of the Old Red Sandstone rocks, on account of the important bearing which they have on the ice-carry during the glacial period. A brief sketch of the development of the contemporaneous and intrusive igneous rocks was given in the paper which we communicated to the Geological Society in 1879.|| But since that paper was read we have twice visited the islands in the course of our holiday rambles, in order to work out in greater detail the volcanic history of that period. Our last visit was specially devoted to the investigation of an interesting series of rhyolites, which have not hitherto been described, though at certain localities they have a remarkable development. A large number of microscopic sections have been prepared and examined, while detailed chemical analyses of the typical volcanic rocks have been made for us by our friend Mr R. R. TATLOCK, F.R.S.E., one of the public analysts for Glasgow. We now propose to lay the results of these investigations before the Society. * HIBBERT'S Shetland Isles, pp. 341, 474, 484, 491. f Mem. Wernerian Soc, vol i. p. 162. J Mineralog. Mag., vol. ii. pp. 160, 170 et seq., 253 ; vol. iii. p. 32. § Trans. Edin. Roy. Soc, vol. xxviii. p. 345. || Quart. Jour. Geol. Soc, vol. xxxv. p. 786. OLD RED SANDSTONE VOLCANIC ROCKS OF SHETLAND. 361 I. GEOLOGICAL STRUCTURE OF THE VOLCANIC EOCKS. The records of volcanic activity are mainly confined to the west and north- west portions of the Mainland and the islands adjoining the western seaboard. They may be grouped in two divisions,—first, the contemporaneous igneous rocks, comprising the lavas and tuffs which were erupted and spread over the sea-floor during the accumulation of the sedimentary deposits; second, the intrusive igneous rocks, which were injected at a later date, probably towards the close of the Old Red Sandstone period in Shetland. The result of the chemical analysis of typical examples of these divisions clearly proves that the former belongs wholly to the basic series, while the latter includes both acidic and basic rocks. A. Contemporaneous Lavas and Tuffs. Beginning first with the interbedded volcanic rocks, the best development of them is to be found in the south-west part of Northmavine, between Stenness and the mouth of Rooeness Voe. No finer sections could be desired than those exposed along the storm-swept cliffs of the Grind of the Navir. Here and there the observer sees narrow " gios " which have been excavated in the tough lavas and ashes and occasionally a subterranean passage or tunnel, com- municating with the surface by a funnel-shaped aperture, from which, during storms, a column of spray issues with the advancing tide. The best examples of this latter phenomenon are to be seen at the " Holes of Scraada." The tract of ground occupied with this series of ancient lavas and tuffs measures about six miles in length from Stenness to Ockren Head. It is evi- dent, however, that they must originally have covered a larger area, from the isolated fragments which have escaped denudation, in the islet of Doorholm, and Esha Ness Skerry. In the southern part of this tract, between Stenness and Hamna Voe, the terrace-shaped features which are characteristic of volcanic areas are so apparent, that the eye can easily follow the successive out- crops of the lavas and tuffs. This area is almost entirely occupied with contemporaneous volcanic rocks, there being but few intercalations of sedimentary deposits. Along the east side, between Rooeness Voe and Brei Wick, the lavas and tuffs are bounded by a great sheet of granite and quartz-felsite, which will be described in a subsequent page. On the south bank of Rooeness Voe, rather more than a mile from Ockren Head, the relation between the two is admirably exposed in a steep grassy " gyo." At this locality the slaggy porphyrites, which form a cliff about 300 feet high, with a beautifully slicken-sided surface, are brought into conjunction with the pink granitoid rock by a fault. Owing to the covering of peat, we were unable to trace this fault across the 362 B. K PEACH AND J. HORNE ON THE peninsular tract. On the shores of St Magnus Bay at Brei Wick, the interbedded and intrusive igneous rocks are not found in such close proximity; the junction between the two being concealed by a sandy beach. From the admirable coast sections there is little difficulty in determining the geological structure of the volcanic masses. They form a great syncline, the centre of which is occupied by a coarse volcanic breccia or tuff and from underneath this breccia there crops out a series of slaggy diabase-porphyrites, with occasional beds of red ashy sandstones and flags. Such is the general arrangement of the strata, though the succession is occasionally disturbed by faults of greater or less magnitude. The order of succession is best displayed in the cliffs bounding St Magnus Bay, and we shall therefore describe first of all the section between Brei Wick and Stenness. On the west side of Brei Wick Bay, which is the eastern limit of the interbedded volcanic rocks, the following section is visible. 4 3 FIG. 1.-—Section in Brei Wick Bay, Northmavine.

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