ts'i'^ ^ifc **>-i'^->: ■•av^ V. OforncU InitJcrBitg ffiibrarg atljaca, Wem ^atk BOUGHT WITH THE INCOME OF THE SAGE ENDOWMENT FUND THE GIFT OF HENRY W. SAGE 1891 Cornell University Library QE 264.W68I8 1907 The geology of Islay, including Oronsay 3 1924 003 986 498 Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924003986498 MEMOIRS OF THE GEOLOGICAL SURVEY. SCOTLAND. THE GEOLOGY OF ISLAY, INCLUDING ORONSiY AND PORTIONS OF COLONSAY AND JURA. (EXPLANATION OF SHEETS 19 AND 27, with the WESTERN PART OF SHEET 20.) BY S. B. WILKINSON. WITH NOTES BY J. J, H. TEALL, D.SCj, F.R.S., and B. N. PEACH, LL.D., P.R.S. PUBLISHED BY ORDER OF THE L0BD8 COMMISBIOMEKS OP HIS MAJESTY'S TREASURY . GLASGOW : PBINTED FOB HIS MAJESTY'S STATIONERY OFFICE By JAMES HEDDERWICK & SONS Ltd. At ".The Citizen" Press, St. Vincent Place. And to bo purchased from W. & A. K. JOHNSTON, 2 St. Andkbw Square, Edinbuboh ; B. STANFORD, 12, 13, and 14 Long Acre, London ; HODGES, FIGGIS & Co., Ltd., Grafton Stbbet, Dublin; From any Agent for the sale of Ordnance Survey Maps ; or through any Bookseller from the Ordnance Survey Office, Southampton. 1907. Price, Two Shillings cmd Sixpence. LIST OF PUBLICATIONS OF THE GEOLOGICAL SURVEY OF SCOTLAND. -Maps on One-inch Scale. 1. Wigtownshire, South- Western Districts, is. 2. Wigtownshire, South-Eastern Districts. 4?. 3. Wigtownshire, Western Districts. 6s. 4. Wigtownshire, East Part ; Kirkcudbright, portion of S.W. Division, 6«. 5. Kirkcudbrightshire, Southern Districts. 6s. 6. Kirkcudbrightshire, E. margin ; Dumfriesshire, S. margin. 4s. 7. Ayrshire, South- Western Districts. 6s. 8. Kirkcudbrightshire, Ayrshire, and Wigtownshire. 6s. 9. Kirkcudbrightshire, N.E.; Dumfriesshire, S.W. 6s. 10. Dumfriesshire. 6s. 11. Roxburghshire and Duinfriesshire. 4s. 12. Argyllshire (Kintyre, S. half of). 4s. 13. Ayrshire, Turnberry Pointi and S. part of Arran (Solid and Drift editions). 4s. 14. Ayrshire, Central Districts. 6s. 15. Dumfriesshire, N.W. ; Ayrshire, S.E. ; and Lanarkshire, S. 6s. 16. Dumfries, Selkirk, Peebles, Lanark, and Roxburgh shires (parts of). 6s. 17. Roxburghshire, Selkirkshire, and Dumfriesshire (parts of). 6s. 18. Roxburghshire, E. part. 4s. 19. Argyllshire (S. part of Islay). 4s. 20. Argyllshire (Kintyre, Gigha I., part of Islay). 4s. 21. Argyllshire ; Arran, Central and N. part ; Bute, S. part ; Cumbraes, Ayrshire (part of N.W.) (SoUd and Drift editions). 4s. 22. Ayrshire, Renfrewshire, Lanarkshire (parts of). 6s. 23. Lanarkshire, Central Districts ; Ayrshire (part of W. ). 6s. 24. Peeblesshire, Lanarkshire, Edinburghshire, Selkirkshire (parts of). 6s. 25. Berwicksliire; parts of Roxburgh, Selkirk, jind Edinburgh. 6s. 26. Berwickshire and Roxburghshire (parts of). 4s. 27. Argyllshire ; parts of Islay and Jura, Oronsay. 4s. 29. Argyllshire, Ayrshire, Buteshire, Dumbartonshire, and Renfrewshire (parts of). 6s. 30 Renfrewshire ; parts of Dumbarton, Stirling, Lanark, and Ayr. 6s. 31. Lanarkshire, Stirlingshire, Linlithgowshire, Dumbartonshire, Edinburgh- shire (parts of). 6s. 32. Edinburghshire, Linlithgowshire, Fifeshire, Peeblesshire (parts of). 6s. 33. Haddingtonshire and parts of Edinburghshire and Berwickshire. 6s. 34. Eastern Berwickshire. 4s. 37. Argyllshire (part of). 6s. 38. Perthshire, Stirlingshire, Dumbartonshire, Argyllshire (parts of). 6s. 39. Perthshire, Clackmannanshire, Stirlingshire, and Fife (parts of). 68. 40. Fife and Kinross. 6s. 41. Fife, East part ; Haddingtonshire, North part. 6s. 46. Perthshire, Argyllshire (parts of). 6s. 47. Perthshire. 6s. 48. Perthshire, Forfarshire, and Fifeshire (parts of). 6s. 49. Forfarshire and Fifeshire (parts of). 4s. 55. Perthshire (Solid and Drift editions). 6s. 56. Perthshire, Forfarshire (parts of). 6s. 57. Forfarshire and Kincardineshire (parts of). 6s. 57A.Kincardineshire, S.E. comer. 4s. 65. Aberdeenshire, Forfarshire, Perthshire (parts of). 6s. 66. Kincardineshire, Forfarshire, Aberdeenshire (parts of). 6s. 67. Kincardineshire and Aberdeenshire (parts of). 48. 70. Inverness-shire (West-Central Skye, with Soay). 6s. 75. Invemess-shire, Elginshire, Banffshire, Aberdeenshire (parts of). 6s. - ^ MEMOIRS OF THE GEOLOGICAL SURVEY. SCOTLAND. THE GEOLOGY OF ISLAY, INCLUDING ORONSiY AND PORTIONS OF COLONSAY AND JURA. (EXPLANATION OF SHEETS 19 AND 27, with the WESTERN PART OF SHEET 20.) BY S. B. WILKINSON. WITH NOTES BY J. J. H. TEALL, D.Sc, F.R.S., and B. N. PEACH, LL.D., F.R.S. PUBLISHED ]5Y ORDER OP THE LORDS COMMSSIONERS OF HIS JIAJESTy's TREASURY. GLASGOW : PRINTED FOR HIS MAJESTY'S STATIONERY OFFICE By JAMES HBDDERWICK & SONS Ltd. At "The Citizen" Press, St. Vincent Place. Ai-d to bo puroha.=ed from W. & A. K. JOHNSTON, 2 St. Andrew Square, Edinburgh ; E. STANFORD, 12, 13, and 14 LONG Acre, London ; HODGES, FIGGIS & Co., Ltd., Grafton Street, Dublin ; From any Agent for the sale of Ordnance Survey Maps ; or through any Bookseller from the Ordnance Survey Office, Southampton. 1907. Price, Two Shillings and Sixpence. y-' PREFACE. This Memoir describes the geology of the island of Islay, the strip of Jura bordering the Sound of Islay, and Oronsay lying to the south of Colonsay, which are included in Sheets 19, 27, and part of 20 of the one-inch map of Scotland. These areas were surveyed by Mr. S. B. Wilkinson under the supervision of Dr. B. N. Peach. When the survey of Islay was in progress, Mr. Clough visited the island and assisted with the detailed mapping of the tract adjoining Portaskaig on the east coast. The geology of Islay presents features of special interest. The acid and basic gneisses of the Ehinns have been correlated with the Lewisian gneiss of the North-West Highlands, and are there covered unconformably by a succession of sediments resembling some of the subdivisions of the Torridon Sandstone in the counties of Sutherland and Eoss. The character of the metamorphisni presented by the gneisses of the Ehinns is similar to that seen in the masses of Lewisian gneiss which have been driven westwards by the post-Cambrian thrusts in the north-west of Scotland. The rock-groups in the south-eastern part of Islay have been linked with corresponding types in the Eastern Highland sequence. This Memoir has been written by Mr. Wilkinson, with the collaboration of Dr. B. N. Peach. I am responsible for the micro- scopical petrography and for certain paragraphs relating to the Lewisian gneiss. The list of publications referring to the geology and mineralogy of the island has been prepared by Mr. Tait. The photographs reproduced in Plates I. to VIII. were taken by Mr. Eobert Lunn of the Geological Survey. The Memoir has been edited by Mr. L. W. Hinxman under the supervision of Dr. Home. J. J. H. TEALL, Director. Geological Survey Office, 28 Jermyn Street, I,ondon, 3rd May, 1907. CONTENTS. CHAPTER I. Page Area and Physical Features, ...... 1 CHAPTER II. Progress of Geological Eesearch in Islay, .... 6 CHAPTER III. Formations and Rock Groups, ...... 9 General Geological Structure of the Area, . . . 10 Folding and Metamorphism of the Strata, . . . 11 CHAPTER IV. Lewisian Gneiss of the Rhinns of Islay, . . .15 CHAPTER V. Torridouian, . . ... . . 19 Rhinus Conglomerate — Kilchiaran Slates and Grits — Bowmore Grits, . ... .25 CHAPTER VI. Metamorphic Rocks of Southern Series — • Group I. — Order of Succession of the Strata — Laphroaig Quartz-schists ; Port Ellen Phyllites ; Islay Limestone ; Black Slates and Phyllites, . 28 Group II. — Beinn Bhan Quartzite and Conglomerate — Junction of the Quartzite Group with the Eocks of the Port Ellen Belt — Junction of Quartzite with the Eocks of the Central Belt, ... 36 CHAPTER VII. Metamorphic Eocks of Northern Series — Order of succession of the Strata and their Lithological Characters — Eelations of the Portaskaig Conglom- erate to the Eocks of the Central Belt — Structural arrangement of the members of the Northern Series, 42 VI CONTENTS. CHA.PTER VIII. Page Volcanic Eocks of Lower Old Eed Sandstone age (?) . . 55 CHAPTER IX. Trias (?) . ... .56 CHAPTER X. Intrusive Igneous Eocks later than the Gneiss of the Ehinns of Islay, 57 Epidiorite and Hornblende schist, 57 Felsites, Mica Traps, Lamprophyres, . . . . 61 CHAPTER XL Dolerites, ... ,63 CHAPTER XII. Faults, 67 CHAPTER XIII. Glacial Deposits — Marine Alluvium and Eaised Beaches — • Blown Sand — Peat — Fresh-water Alluvium, . . 69 CHAPTER XIV. Economic Eesources, . . .... 76 APPENDIX. List of Publications referring to the Geology and Mineralog}^ of Islay, ■. . . 79 CONTENTS. LIST OF ILLUSTRATIONS. FIGURES IN TEXT. Page Fig. 1. Sections across the Rhinns of Islay through Balchiaran to Port Charlotte 20 „ 2. Plan of shore section, Port nan Gallan . . .40 „ 3. Sectionof strata at CleituaBaintighearn, Port nan Gallan 40 „ 4. Generalised section from Portaskaig to Bonahaven Bay, east coast of Islay ... ... 49 ,, 5. Folded quartzite near Biod nan Sgarbh, on shore four miles north-east of Loch Gruinard . .51 ,, 6. Sketch map showing direction of Ice-flow, Islay . 70 PLATES. Plate I. Kilchiaran slates and grits (Torridonian ?) Frontispiece. Eilean Mor, seen from Tor Lagain, north-west of Islay. TO FACK PAGE ,, II. Acid and basic gneLss traversed by basic dykes, Oi-say, south end of Rhinns, Islay . . . 16 „ III. Basement conglomerate of Kilchiaran slates and grits, Dun Mideir, 2| miles north-west of Port Charlotte, Islay . . . . . . . 21 „ IV. Puckered and overfolded phyllites (Kilchiaran Series), shore between Kilnane and Tayvullin, west side of Loch Gruinard, Islay . . . 25 „ V. Basal conglomerate (Southern Series), Port nan Gallan, Mull of Oa, Islay 39 ,, VI. Foliated and contorted dyke of lamprophyre in Torridonian (?) phyllites, shore, half a mile north of Portnahaven, Islay . . . . . 61 „ VII. Cleaved lamprophyre dyke in grits (Kilchiaran Series), north side of Kilchiaran Bay, four miles west of Port Charlotte, Islay . . . . 62 ,, VIII. Dolerite dykes traversing Kilchiaran slates and grits, cliff, west side of Kilchiai-an Bay, four miles west of Port Charlotte, Islay ..... 65 THE GEOLOGY OF ISLAY. CHAPTER I. The island of Islay, one of the principal islands of the Inner Hebrides, has an area of two hundred and thirty-five square miles, and forms part of the county of Argyll. Its greatest length, measured from north to south, is 25j miles, its greatest breadth about twenty miles. The island is divided into two distinct areas, which at an earlier period, indicated by the 50-feet raised beach, evidently formed two separate islands, but are now connected by the strip of marine and freshwater alluvium that lies between Loch Indaal and the head of the tidal estuary of Loch Gruinart. The western area, known as the Rhinns of Islay, extends in the form of a peninsula from Ardnave Point in the north to Rhinns Point in the soiith, a distance of rather more than seventeen miles ; with a maximum breadth, measured between Coul Point on the western coast and Tynacoille on the shores of Loch Indaal, of about six miles. The land rises gradually inland, the highest point being Beinn Tart a' Mill, 735 feet above sea-level. From the coast inland the lower slopes are under cultivation and yield good crops, while from the margins of the cultivated land extensive tracts of undulating mooi'land extend to the higher ground. The whole of the western coast line from Ardnave Point to Portnahaven is, with the exception of one or two small areas, bold and rugged, particu- larly the clifis in the neighbourhood of Sanaig, and from Kilchiaran Bay to the southern point of the Rhinns of Islay. The northern portion of the peninsula is more undulating and mostly occupied by large tracts of peat moss and marsh, while huge sand-dunes rise on both sides of the entrance to Loch Gruinart. The second and larger portion of the island lies to the east of Loch Gruinart and Loch Indaal, extending from Rhuvaal Point in a south-south-easterly direction to the Mull of Oa, and is bounded on the east by the Sound of Islay and on the south by the Atlantic Ocean. The highest ground is found along an irregular line drawn from a point about midway along the Sound of Islay to Port Ellen, forming the principal watershed of the island. This ridge is con- tinued at a lower elevation from Port Ellen to Beinn Mhor, the southernmost point of the island. In the region to the north of the valley of the Sorn the ground rises gradually towards the centre of the area in a succession of ridges 2 Area and Physical Features. and valleys, the greatest elevation reached being 1192 feet above sea-level. The country to the east of Loch Gruinart is generally barren and uninteresting in appearance, and contrasts with the coast scenery of the eastern and south-eastern seaboard, which is bold and varied in the extreme, particularly in the south-east, where numerous skerries and islets form picturesque groups. The cliffs of the Oa promontory are very fine, and rise from the sea to a height of 656 feet. The chief watershed in this area coincides for the greater part of its course with the quartzite ridge that extends north-east and north from Port Ellen to Portaskaig. It follows a sinuous course along the tops of the principal hills of the island, including Beinn Sholum (1136 feet), Beinn Uraraidh (1490 feet), Beinn Bheigeir (1609 feet, the highest point in Islay), Glas Bheinn (1544 feet), and Sgorr nam Paoileann (1407 feet). The streams on the south-eastern slopes of this water parting run directly to the sea, as do those on the west as far as Sgorr Bhogachain, to the north of which hill the river Laggan receives the western drainage. A second watershed, more or less parallel with the above-mentioned, extends in a south-westerly direction from Ehuvaal Point in the north-east, over Beinn Thrasda (852 feet), Sgarbh Breac (1192 feet), Guir Bheinn (1036 feet), Tais Bheinn (671 feet), and Beinn Oham, to the head of Loch Indaal. The water drains off the south-eastern slopes of these hills into the river Sorn, while on the north-west many of the streams run directly to the sea and others into the river which falls into the head of Loch Indaal. A minor watershed, running south-west from Beinn Bharradail to the east of Bridgend, separates the valley of the Laggan from that of the Sorn. The largest river basin in the island is that of the Laggan, whose head-waters drain the western side of the quartzite ridge — the principal watershed of the island — and the south-eastern slopes of Beinn Bharradail. At Kilennan a large burn of the same name joins the Laggan, and the river from this point follows a winding course westwards towards the sea between high banks of boulder clay, often passing through wide areas of alluvial land. At Bridge House, where the lower road between Bowmore and Port Ellen crosses the river, the latter runs for a short distance between walls of rock, but immediately below the bridge enters a wide plain occupied by marine and freshwater alluvial deposits and blown sand. Just before entering the sea, the Laggan is joined by the Duich river, which flows out of Lochs Beinn Uraraidh and Leathan an Sgorra, near the summit of the quartzite range. The Laggan is a very swift river, and rises and falls with great rapidity, owing in a great measure to the number of sheep- drains which carry off the surface water very quickly. The Sorn comes next in size and in extent of its drainage basin. It is fed by the waters of Loch Finlaggan and a succession of smaller lochs in the neighbourhood of Ballygrant, the various streams which flow out of these lochs combining at Emeraconart to form the river Sorn. About half a mile below this point its volume is increased by the waters of the burn that drains the hill slopes Area and Physical Features. 3 around Ballachlavin and Duisker. The river continues its course through a tract of flattish drift-covered country to a point just below the cloth mill, one mile east by north of Islay House. Here the stream has cut through the boulder clay to the underlying rock and flows through a deep gorge, which at the Stone Cottage expands into a wide tract of alluvial land where the river is joined by a burn from the north-west slopes of Beinn Bharradail. The old course of the river wound through these alluvial flats, but the stream now pursues a straight course westward from Stone Cottage to Bridgend and falls into the sea at the head of Loch Indaal. The area north of Cnoc Donn is watered by a stream which formerly entered the sea at Craigens on Loch Gruinart. Its course was, however, altered many years ago in order to facilitate the reclamation of the Gruinart Flats, and it now falls into Loch Indaal. The principal stream on the eastern side of the quartzite range is the Abhuinn Staoin or Kintour river, which Hows from Beinn Uraraidh and Sgorr Bhagachain, and passes through a beautifully wooded district to the farmstead of Kintour, whence it winds through a tract of alluvium to the sea at Aros Bay. All along the eastern slopes of the range above mentioned there are numerous smaller burns running directly to the sea. Many of these are intercepted and used in working the distilleries, built at intervals along this coast. Extensive tracts of low ground are found in different parts of the island. From Kilennan and Loch Tallant in the Laggan valley there stretches south to Port Ellen a vast extent of gently-sloping ground covered with drift and large areas of peat moss. The glacial deposits consist of boulder clay, which contains a large percentage of quartzose material, and affords a ver)?^ cold and infertile soil. As these slopes along their seaward margin approach the raised beaches and alluvial flats the appearance of the country changes, the latter formations yielding rich crops whose brilliant green colour contrasts in a very marked degree with the barren heather- covered or rushy boulder clay slopes. Another large area, very similar to the last, lies to the east of Loch Gruinart, where the drift-covered slopes fall gently from the base of the quartzite hills to the raised beaches and dunes of blown sand along the coast-line. Loch Gorm, the largest sheet of water in the island, occupies a shallow depression in the superficial deposits between Kilchoman and Ballinaby in the northern part of the Rhinns. It is about one mile and a half long by one mile and a quarter broad, and contains two islands, upon one of which are the ruins of an old castle. Further to the north are Loch Corr, connected with Loch Gorm by the little river Le6ig ; Loch Laingeadail, surrounded by peat bog ; and Ardnave Loch, situated between raised beaches and hills of blown sand. In the area to the east of Loch Gruinart the lochs are very numerous and occur at various elevations. Loch Smigeadail in the north-east lies at an altitude of 700 feet above sea-level. Lochs Ardnahoe, Staoinsha, and Torrabolls form a group of small lakes a short distance north of Portaskaig. The two last-named lochs supply water to the distilleries of Bonahaven and Gaol Isla respectively. 4 Area and Physical Features. Finlaggan Loch, 169 feet above sea level, with its very interesting ruined castle, lies in the narrow valley between Cnoc an Tigha and Eobolls Hill. Three lochs are grouped together to the east of the village of Ballygrant, viz., Loch Cadhan, Loch Chuirn, and Loch Lossit. The two latter are beautifully situated, and contain water of exceptional clearness and brilliancy issuing from springs in the adjacent limestone. Loch 0am occupies a hollow between two quartzite hills about three miles north of Bridgend, and nearer that village is Loch Skerrols, a fine sheet of water partially arti- ficial, a dam having been built to retain a head of water sufiicient to drive the mills at Islay House. Loch Tallant, three miles south of Bridgend, appears on the map as a loch, but is little more than a marsh, and is rapidly becoming filled up with water plants. A small lochan about a quarter of a mile west of Loch nam Breac is known locally as the loch of the " tailless trout," trout having been taken out of it deficient in some of their fins. A number of small lakes of different sizes — Lochs na Breac, Beinn Uraraidh, Sholum, Uigeadail, Leathan an Sgorra, and Leorin — with many other smaller pools, occur along both sides of the quartzite ridge north-east of Port Ellen. Many of the small lochs on the east side of Laggan Bay have been considerably reduced by drainage, and are to a large extent choked with vegetation. On the promontory of the Oa are Upper and Lower Glenastle Lochs, and near the Mull of Oa Lochs Ardachie and Kinnabus. The climate is exceptionally mild ; the annual mean temperatui'e is 47.5°, that of the three winter months being as high as 40.1°. The tenderest plants will thrive if sheltered from the winds, and fuchsias grow with great luxuriance. Snow does not lie even on the hills for any length of time, but the rainfall is large, the average annual precipitation measured at Bridgend being 49.13 inches. The months of April, May, and June are generally dry ; after that period fine weather cannot be depended on. There is, however, generally a fortnight or three weeks in October and November before the fall of the leaf when the climate is all that can be desired, and the colouring of land, sky, and sea superb. The scenery of Islay is very varied, some districts being very bleak and barren, while others have the character of a lowland rather than a highland country. The raised beaches around Bridgend, at the head of Loch Indaal, form very eSective natural features, which have been taken advantage of for planting in the grounds of Islay House. The timber has grown well, and considering the severe nature of the storms that visit the island, many of the trees are above the average, some of the beeches and silver fir in Daill Wood being very fine. On both sides of the road from Bridgend to Port- askaig the scenery again is more like that of the lowlands, with verdant pastures, well-cropped fields, and patches of natural timber. This fertility is due to the underlying limestone, for in the same area, directly the quartzite is reached the growth of timber ceases. A similar change is seen in the southern area, and over the wide tracts of boulder clay which contains a large quantity of siliceous material. Between Port Ellen and Ardtalla, in the south-east of Area and Physical Features. 5 the island, the scenery is very striking. The white gleaming sum- mits and slopes of the quartzite range rise conspicuously in the background, flanked by dark igneous rocks that weather out into picturesque jagged ridges clothed with the indigenous tree growth of the country. These nearer features combine with the brilliant colouring of the sea, the blue line of the distant hills of Knapdale, and the peaks of Arran beyond Cantyre, to invest this portion of the West Highlands with remarkable beauty. S. B. "vV, CHAPTEE 11. Progress of Geological Eesearch in Islay. The geology of Islay has, during the past hundred years, attracted the attention of several eminent observers. As far back as the year 1789, Mr. Abraham Mills, in two letters communicated to the Royal Society,* recognised the intrusive character and basaltic nature of the dolerite dykes of Islay. He also refers to the high-level beaches, and considers them to indicate a recession of the sea. A similar conclusion was reached by Robert Jamieson, writing in ISOO.f The results of the examination of the Western Islands by that far-seeing observer Macculloch, were published in 1819, | and, with regard to the geology of Islay, anticipate in several points the con- clusions of later geologists. Macculloch points out that on the east shore of the Ehinns, clay slates (Kilchiaran slates and grits) alternate with a felspathic hornblendic gneiss which he compares to that of lona and the Outer Hebrides. He also refers to the con- glomerate of Portaskaig and Lossit Hill as a primary con- glomerate, correlating it with a similar rock on the Garvelloch Isles and on Schehallion (boulder-bed of Perthshire); and also pointing out that in addition to pebbles of granite foreign to the island, it contains fragments of the limestone upon which it lies. He was also probably the first to discover the igneous nature of the rocks of Glas Bilean, in the Sound of Islay. He describes the rock as an amygdaloidal trap, and suggests that it may once have covered a large area and have been the source of the trap veins (Tertiary dolerite dykes) of Jura and Islay. Professor J. Nicol, in his description of the metamorphic rocks of Islay,§ 1844, confirms the observations of Macculloch as to the alternation of clay slate with gneiss on the eastern shore of the Rhinns, and the contents and position of the conglomerate of Portaskaig and Lossit Hill. In their paper on the Western Islands of Scotland, communi- cated to the Geological Society in 1861,|| Sir E. Murchison and [Sir] A. Geikie make the following references to the geology of Islay : — The grits and schists (Torridonian) of the north-west part of the island are regarded as the equivalents of the lower quartz-rock (Cambrian) of Sutherland, at that time supposed to be of Lower * Philo. Trans. Roy. Soc, Vol. LXXX., p. 73. 1790. t Mineralogy of the Scotti-sh Isles, Vol. II., pp. 146-166. 1800. t Western Islands of Scotland, Vol. II., p. 223 ; Vol. III., pi. 22. 1819. § Guide to the Geology of Scotland, 1844, p. 225. II Quart. Jour. Geol. Soc, Vol. XVII., p. 206. Progress of Oeologioal Besearch in May. 7 Silurian age. • They describe the whole of the area of the Rhinns as occupied by these grits and schists, into which the granitoid igneous rocks of the southern part of the promontory (Lewisian gneiss) have been intruded. The (Islay) limestones of the central valley are referred by these observers to the horizon of the Assynt lime- stone, and the overlying quartzite to that of the "upper quartz- rock " (over-thrust Cambrian quartzite) of West Sutherland. They further point out that the Islay limetones extend north-east through the Garvelloch Isles to Lismore and Shuna, and that the overlying quartzite and " upper schistose series," after traversing Jura and Scarba, reappear on the north of Oban. Attention is also drawn to the great development of hornblendic igneous rock in the form of intrusive sheets — their parallel arrange- ment, and the contact alteration which they have produced on the surrounding rocks. The results of the recent work of the Survey in Islay have been also in some degree anticipated by the late Mr. James Thomson, F.G.S. His views are embodied in a paper read before the British Association at Edinburgh in 1871,* and are further expanded in a communication to the Glasgow Geological Society in 1875. t Mr. Thomson was the first to pronounce definitely as to the Laurentian (Lewisian) age of the gneiss of the Ehinns of Islay. He lays stress on the difference in age between these rocks and those of the central parts of the island, and points out their close lithological resemblance to the fundamental gneiss of Sutherland and the Outer Hebrides. The rest of the metamorphic rocks of Islay he assigns to three difierent periods. The strata along the west side of the Sound of Islay, between Bonahaven Bay and Lossit (dolomitic shales and flags, quartzites, and Portaskaig con- glomerate of the survey), he groups with the slates and grits of the area between Skerrols and Sanaig, regarding the whole as of Lower Cambrian (Torridonian) age, and further suggests the resemblance of the shales and flags of the Sound of Islay — with suncracks, rain drops, and annelid markings — to similar rocks of Cambrian (Tor- ridonian) age in Sutherlandshire, and on the Longmynd in Wales. To the Upper Cambrian (Torridonian) is assigned the quartzite of the south-eastern belt, together with the underlying slates and limestones of the central region, while the quartzite of the Mull of Oa is separated from that mentioned above and included with the schistose series to the east (Port Ellen phyllites) as being on the horizon of the lower members of the Lower Silurian. Mr. Thomson also notes the occurrence of a breccia-conglomerate on either side of the eastern belt of quartzite, but regards that on the east as the base of an overlying schist series. He further recognises the granite fragments of the Portaskaig conglomerate as foreign to the island, and attributes their presence to the agency of ice. * " On the Stratified Rocks of Islay." Brit. Ass. Rep., 1871, Vol. XLI., p. 110. t " On the Geology of the Island of Islay." Trans. Geol. Soo. of Glas- gow, Vol. v., p. 200. 8 Progress of Geological Research in Istay. The ofiicial survey of Islay was begun in the year 1890 and con- tinued during the next eight years. In the Annual Summary of Progress for 1893* attention was first drawn to the close resemblance of the dolomitic shales and quartzite of the north-eastern part of the island to the " fucoid-beds " and quartzite of West Sutherland and Eoss. The occurrence was noted of a conglomerate at the base of the quartzite, similar to the " boulder-bed " of Central Perthshire, and it was suggested that the Islay and Jura quartzite might be a con- tinuation of that of the Central Highlands. In 1896t a possible Lewisian age was attributed to the gneisses of the south-west pro- montory ; but it was not until the next year that the identification of the rocks of the Ehinns with the Lewisian gneiss and Torridonian of the North-West Highlands was confirmed in the official publica- tions of the Survey. t In the former year Messrs. Peach and Wilkinson had come to the conclusion that the south-eastern or Beinn Bhan quartzite rested unconformably upon the limestone and schists. Further observation in 1897 confirmed their views as to the relations of the schists and quartzite on either side of the Beinn Bhan quartzite belt, and to the overlap of the latter across the limestones on to the phyllite zone. Finally, in 1899,§ a critical examination by the same two officers of the coast sections on the west side of the Sound of Islay estab- lished the order of succession of the strata which has been adopted in the present memoir. L. W. H. * Summary of Progress of the Geological Survey for 1893, p. 265. + lUd for 1896, p. 26. t ,, for 1897, pp. 62-64. § ,, for 1899, p. 66. CHAPTEE III. Formations and Eock Grodps. General Geological Structure OF THE Area. The following formations and groups of rock enter into the com- position of the area under consideration : — Eecent and Post-Ter- tiary. Glacial. Triassic ? (Lower ?) Old Red Sandstone. Northern Series. Sedimentary. {Blown sand. Peat Freshwater alluvium Marine alluvium and raised beaches . 1. Striae ....... 2. Boulder clay. 3. Moraines. 4. Sands and gravels. Breccia and red sandstone of the Mull of Oa. I Basic lavas and red sandstones of Glas j Eilean MetamorpMc. '5. Dolomitic shales and massive cream- coloured dolomites .... 4. False-bedded quartzite with annelid <^ "pipes" ...... 3. Shales, flags, and dolomite . 2. Quartzite with band of conglomerate 1. Portaskaig conglomerate Sig"n on Map. Pool X Xi (This series is everywhere separated from the Torridonian rocks by the Loch Skerrols thrust-plane.) Southern Series, correlated with Central Highland Eocks. C ' TT J Quartzite and conglomerate " ■ 1 (Beinn Bhan quartzite) . ' d. Islay limestone = Lismore and Loch Awe limestone of the mainland . Black slates and phyllites p J I (Esknish slates) = Easdale ^ ■ I slates .... b. Port Ellen 1 phyllites . I Ardrishaig j a. Laphroaig f =phyllites. i^ quartz-schist J A, 9' 10 General Geological Structure of the Area. c. Bowmore grits = Applecross and (in part)^ Aultbae groups of the mainland ilchiaran slate and grit series a. Epidotic grit and basal con- <^ b. Kilchiaran slate and grit series 1 -pi- u • V t ? lan. ; T^ -J 4.- -i J T, 1 I Diabaig ( I a. Epidotic grit and basal con- > p ° \ [_ glomerate . . . . J ^' j Lewisian. Gneiss of the Ehinns of Islay. . . . gl Igneous. Foliated in f Epidiorites B" whole or in^ Lamprophyres and mica traps . . S"' part. (^Felsites F Unfoliated. Basalts and dolerites . . . . B General Geological Structdee of the Area. For convenience of geological description the island of Islay may be divided iato two portions, viz., the Ehinns or western area and the eastern area. The western area comprises the whole of that portion of the island linown as the Rhinns of Islay together with a belt of coimtry lying round the head of Loch Indaal. It is separated geologically from the eastern area by the Skerrols thrust-plane, which follows a sinuous southwards course from Bun-an-uillt on the east shore of Loch Gruinart and crosses Loch Skerrols, where the evidence of great movement is plainly seen. It thence passes to the east of Bridgend by Tallant Farm, where it is shifted eastwards by a normal fault. Beyond that point its course is concealed under superficial deposits, but probably continues to the shores of Laggan Bay. With the exception of numerous intrusive dykes, the western area is entirely occupied by rocks of pre-Cambrian age. Torri- donian grits, slates, and conglomerate form the northern and larger portion of the region, while the acid and basic gneisses of the Lewisian complex occupy that part of the Rhinns which lies south of Kilchiaran Bay and Bruichcladdich. Rocks of Torri- donian age, largely covered by blown sand and marine deposits, also form the island of Oronsay and that portion of Colonsay which falls within the map. The eastern division of Islay is occupied by two series of meta- morphic rocks — Northern and Southern Series — whose respective ages and mutual relations have not yet been determined with absolute certainty. The rocks included in the Northern Series cover the greater part of the north-eastern region between the Loch Skerrol thrust- plane and the Sound of Islay, and consist of shales, dolomites, quartzites, and conglomerates, the last-named occurring both at the base and at higher horizons of the quartzite. The shales and dolomites, repeated by folding and step faults, form several irregular masses in the centre of this area, and fringe the shore of the Sound at Bonahaven Bay and Na Feamindean. The siliceous and conglomeratic rocks that underlie the shales and dolomite General Geological Structure of the Area- H occiipy considerable areas on either side of the irregular masses described above, and form a narrow strip along the shore of the Sound of Islay as far south as Eudha Bhoraraic, where the basal or Portaskaig conglomerate is separated by a line of discordance from the quartzite of the southern area, about to be described. The second or Southern Series is made up of quartz-schists, slates, phyllites, and limestones (Islay limestone), together with an overlying zone of quartzite — the Beinn Bhan quartzite — and conglomerate. The two last of these subdivisions are considered to be on the same horizon as the corresponding rocks of the Northern Series, A^ith which they closely agree in lithological character. The whole series has further been correlated with a similar sequence of rocks in the Central Highlands or Perthshire Series. The rocks of the Northern Series in the north-east part of the island are clearl]^ younger than the phyllites and Islay limestone of the Southern Series upon which they rest unconformably, and fragments of which are included in the conglomerates at their base. Similar relations obtain between the quartzite and conglomer- ate on the one hand and the schists and Islay limestone on the other, along the western margin of the southern belt of siliceous rocks, south of the head of the Kilennan river ; a fact which strengthens the supposition, referred to above, that these quartzites are on the same horizon as those of the Northern Series. The rocks of the Southern Series occupy the whole of the southern and south-eastern parts of the island. The Beinn Bhan quartzite forms a belt gradually widening to the north that extends across the island from the ilull of Oa to the Sound of Islay. On either side of this belt the phyllite and limestone zone is disposed in bands of unequal breadth, the one extending from the Mull of Oa to within a short distance of Portaskaig, the other fringing the south-eastern shore between Port Ellen and Maol Ardtalla. A narrow strip of quartzite, let down by a fault with a down- throw to west, also forms the north-west side of the Oa promontory. Sills of epidiorite and hornblende schist occur in great numbers within the slate and phyllite belt on the south-eastern coast between Port Ellen and Carraig Mhor, and there is also a group of similar intrusions around Beinn Bhan in the centre of the island. Dolerite dykes of Tertiary age with a general north-western and south-eastern trend are abundant in most parts of the island, and visible in greatest abundance along the north-western and south- eastern shores of the Rhinns. L. W. H. Folding and Metamorphism of the Steata. Islay, like the Central Highlands to which it properly belongs, bears evidence of having been subjected to great tangential pressure, whereby its rocks have been thrown into a succession of subparallel overfolds, the longer axes of which trend generally in a north-eastern and south-western direction. The island is a continua- 12 FoldiJig and Metamorphism of the Strata. tion of a tract of countiy wliicli lies to the north-west of the great central belt in which the strata are vertically folded, forming a pseudo-fan structure, the axis of which traverses the West of Scotland from about the south-west end of Loch Awe to the head of Loch Sween. In conformity with the structure of that area, the axial planes of the Islay folds are for the most part inclined towards the south-east and south-south-east, though there is a small belt in the centre of the island where a minor fan-like structure has been partially developed, on the south-east side of which the planes dip in an opposite direction. In addition to this overfolding, there is evidence of over-thrusting along the Loch Skerrols thrust-plane. Up this inclined plane the rocks of the central part of Islay have been thrust bodily westwards over the Torridonian strata of the Ehinns. It is therefore obvious that the direction of movement was from south-east to north-west. In the western area all the rocks, with the exception of some newer dykes, are involved in a series of minor overfolds whose axial planes all dip south-eastwards. The type of folding is well illus- trated in the frontispiece, a view of the well-known Sanaig rocks seen from the south-east. Plate I. is a photograph of folded strata showing intense puckering near the western shore of Loch Gruinart. These puckered rocks of the Ehinns are further arranged in a compound anticline the axis of which coincides roughly with the central line of the promontory. This is well illustrated on sheet 1 9 by the disposition of the Torridonian sedimentary rocks upon the Lewisian gneiss and also in the horizontal section (fig. 1). Unfor- tunately in the ground at the head of Loch Indaal, between the Ehinns and the rest of the island, the rocks are concealed under superficial deposits. At Blackrock, where the Torridonian arkoses are first seen, the evidences of folding and deformation are very slight. There is, however, a constant dip towards the east and an evident upward succession in that direction. The rocks in this district may therefore be looked upon as forming the eastern limb of the Ehinns compound anticline. The structure of the eastern area is that of a central compound anticline or fan-like arrangement of the strata flanked on each side by a compound syncline. The anticline coincides roughly with the central plane along which the members of the underlying phyllite- limestone group come to the surface, though, owing to pitch of the folds and faulting, the overlying conglomerate-quartzite group extends across its north-eastern extension. The axial planes of the minor folds entering into this structure are variously inclined. The northern compound syncline includes all the members of the Northern Series which form the northern range of hills. In this syncline the strata are thrown into a series of comparatively gentle overfolds, somewhat inconstant in the direction of dip of their axial planes. The structure is further somewhat obscured by a succession of large north-east and south-west step faults. The northern margin of the compound syncline is truncated by the sea, so that the phyllites and limestones do not reappear in that direction. Folding and Metamorphism of the Strata. 13 The southern syncline also contains a quartzite and conglom- erate (Grotip II. of the Southern Series) and coincides roughly with the southern range of hills which stretches from the Sound of Islay to the Mull of Oa. The strata involved are all more or less sharply and isoclinally folded, with axial planes dipping south- east and south-south-east. To the south-east of the quartzite belt the phyllites reappear, but unaccompanied by the Islay limestone, pointing to an overlap of the quartzites over the latter in the inter- vening ground. The phyllites, together with the epidiorite sills with which they are abundantly injected, are involved in the same series of easterly inclined overfolds, the whole system forming a second smaller compound anticline which gives rise to the narrow plain which borders the coast to the east of Port Ellen. The crest, however, is marked by the outcrop of the Laphroaig quartz-schists a little to the east of Port Ellen, and its southern boundary by the basement members of the southern quartzite series from near Ardmore Point to the extreme south-east of the island. Over the greater part of the island the evidences of mechanical destructive deformation are more apparent than those of construc- tive metamorphism, and where the latter has taken place it is nowhere of a high grade. In these respects also Islay is in accord with corresponding areas on the western seaboard of the mainland, thoiigh the rocks of Islay are even less altered than those of the mainland. In the western area the effects of dynamic metamorphism are most apparent along the eastern shores of the Rhinns. In this part of the area the Lewisian rocks have been so much crushed that their original structures have been for the most part obliterated, while the overlying Torridonian sediments show an almost equal amount of deformation. The crushing is most highly developed near the junction of the two formations. Towards the south-west side of the Ebinns, however, though the rocks are much puck- ered and folded, the crushing is not so great, so that the igneous structures of some of the pre-Torridonian intrusions are still quite apparent (Plate II.). The Torridonian sediments, together with a series of lamprophyre dykes and sills by which they are invaded, have been folded and cleaved along a set of planes common to them and the underlying Lewisian rocks. The pelitic sediments have been converted into slates, the grits into flaser rocks, and the gneiss pebbles in basement conglomerates have been drawn out and flattened (Plate III.) ; but beyond the production of white mica along the movement planes there has been little or no constructive metamorphism. In the Blackrock and Bowmore parts of the area round the head of Loch Tndaal the Torridon arkoses show but slight signs of deformation or alteration beyond a partial bleach- ing. As the outcrop of the Loch Skerrols thrust-plane is approached, signs of crushing become more and more apparent, till close up to the thrust-plane the grits are rolled out into a platy mylonite. In the eastern area, on the eastern side of the thrust-plane, the quartzite shows similar evidence of intense deformation, which 14 Folding and Metamoiyliism of the Strata. gradually decreases and dies out within a mile of the outcrop of the thrust-plane. Beyond the influence of the thrust the rocks of the central compound anticline are but little shattered and metamorphosed, and that only where the folding is most intense. In the phyllite-limestone group of the central belt the argillaceous rocks are usually cleaved into slates, and towards the south-west end of the belt on the shores of Loch Indaal the members of the lower division become lustrous phyllites, but this part of the area more properly belongs to the southern compound syncline. Towards the north-east end of the central belt the rocks show little or no signs of intimate deformation beyond the cleavage of the slates, the oolitic grains of the Islay limestones often retaining their spheroidal form. Even when there is evidence of consider- able movement of the adjacent rocks, the dolomitic beds, although full of sand grains, do not seem to have set up magnesian and calc- silicates, although the same type of rock elsewhere on the mainland is proved to be particularly favourable to the production of such minerals. In the east of the central belt and in the northern compound syncline, the rocks are in many places so little aifected that the sun-cracks and worm-casts which thej' contain have not been deformed, and many of the surfaces of the sandy dolomitic shales still glisten with derivative flakes of mica. Most of the rocks of this part of the area cannot be regarded as metamorphic in the strict sense of that term. In the southern compound syncline, soiith of the central belt, there is evidence of increasing compression of the folds, accom- panied by a corresponding rise in the grade of metamorphism, until towards the southern edge of the belt the rocks approach the condition of those in the Central Highlands. In the southern compound anticline also, the Port Ellen or Ardrishaig phyllites become more like those of the typical mainland areas than the equivalent rocks of the central belt of Islay. The fact that the beds on each side of the Loch Skerrols thrust- plane, away from the immediate influence of the thrust, show less signs of folding, deformation, and metamorphism than those at a greater distance appears to be of great significance. It seems to point to the fracture having taken place at an early stage in the folding of the country so that these rocks were enabled to move in mass and thus to escape further compression. B. N. P. CHAPTEE IV. Lewisian Gneiss. Nearly the whole of the southern portion of the peninsula of the Rhinns of Islay, to the extent of about nineteen square miles, is occupied by rocks belonging to this formation. A somewhat sinuous line, corresponding to the outcrop of the basement beds of Torridonian age, between Bruichladdich on the east coast and Kilchiaran Bay on the west, bounds this area on the north. In the mapping of the district certain basic masses have been separated out and lettered B°. These are, in part at least, of later date than the rocks of the fundamental complex, but the intense movement which has affected the area and modified the original relations as well as the structure of many of the rocks, often makes it impossible to speak with confidence as to the relative ages of the different varieties. The rocks of the fundamental complex consist of acid and basic gneisses of probable igneous origin. The most striking petro- graphical feature of the specimens from various parts of the area which have been examined under the microscope is the evidence which they afford of changes due to pressure-fluxion. Relics of the original constituents lie in a matrix of secondary granulitic or mylonitic material, mainly composed of quartz and felspar, but containing also minute scales of biotite and chlorite, films of sericite and grains of epidote. The more basic as well as the more acid members of the series show, as a rule, the same evidence of profound modification by this dynamic action, the general result of which has been to crush the original constituents and thus to produce microscopic breccias, not holocrystalline schists. In this respect the character of the metamorphism is similar to that seen in those masses of Lewisian gneiss which have been brought forward by the great post-Cambrian thrusts in the north-west of Scotland, and differs from that which is characteristic of the zones of secondary shearing in the otherwise undisturbed areas of Lewisian gneiss occurring in the same district. It is difficult to speak with confidence as to the original characters of the rocks of the fundamental complex, but they probably consisted of biotite- and hornblende - gneisses, not dissimilar from those occurring in areas of Lewisian gneiss. Microcline and microperthite were certainly present as consti- tuents of the more acid varieties, while a basic felspar, now represented by saussuritic aggregates, occurred in the more basic members of the series. J. J. H. T. A study of the area of the Rhinns, especially along the southern shoro-line, shows that the rocks referable to the "fundamental 16 Lewisiau Gneiss of the Bhinns of Islay. complex " are cut by a series of later basic intrusions (B"). Probably the best locality for observing the relation of the dyke-like masses to the members of the fundamental complex is on Orsay, the island south of Portnahaven upon which the lighthouse is built, where the heavy seas have washed the rocks bare of drift and raised-beach material over a wide area. Here the fundamental complex consists chiefly of a pink por- phyritic gneiss, with eyes and stripes of pink felspar set in a darker granulitic matrix and enclosing knots and elongated masses of the early basic material which are now mostly striped out into horn- blende-schist. These rocks are cut by dykes and elongated masses of basic material, the larger of which are very coarse in the centre and become fine-grained towards the edge (Plate II.). They also give off numerous dykes, which when fairly large behave in the same manner as the main mass ; the smaller dykes are, however, fine- grained throughout and also sheared, while the coarse parts only show a few lines of movement. The felspars are invariably repre- sented by greenish saussuritic material, and hornblende is very conspicuous. The large intrusive mass west of the lighthouse is traversed by a band of dolomitic material, weathering a rusty ochreous colour. The walls of this intrusive mass rise at an angle of about 80° and trend in the north-eastern and south-western direction common to all the similar masses in the Rhinns area. Another good section is exposed along the ridge running from Lossit Bay to Kilchiaran. At Carn the rock is rather massive and coarse-grained. Between Am Glean and the sea the junction of this basic mass with the early complex is clearly shown: the former becomes finer-grained and darker towards its edge, and the line of junction resembles the chilled edge of an intrusion into the rocks of the early complex. The latter consists of a coarse-grained pink acid gneiss with occasional lenticular bands and irregular masses which vary in basicity from ultra-basic hornblendites to rocks con- taining as much felspar as hornblende or mica. None of this more acid material is seen to transgress the boundary of the dioritic rock, and it is inferred from these facts that the latter rock was intruded into the rocks of the original complex. The basic rock at Carn and Cnoc Breac shows little sign of parallel structure, but on each side of the waterfall on the Tormisdale stream a marked folia- tion is seen in places. This rapidly increases in intensity from this point westward to the sea, until near the junction with the acid gneiss of the originajl complex both rocks are found to be traversed by a set of foliation planes which strike north-east and south-west and dip north-west at high angles. North-west from Carn the foliation also increases, the direction of strike and dip being the same. Another interesting section is seen at Port Mias Sgier, north-west of Portnahaven. Here the gneiss represents the acid phase of the original complex sheared and granulitised. This is succeeded by a mass of much-decomposed igneous rock, a large part of which has been converted into epidote prior to the shearing move- ments to which it has been subjected. For more than 100 yards from its edge it is seen to be drawn out in a north-eastern and south- Leiuisian Gneiss of the Rhinns of Islay. 17 western direction, the epidote veins being traversed by a series of cracks at right angles to the direction of stretching. In the cove under Oladdich this rock assumes a massive character, and where not converted into epidote resembles the rock of Cnoc Breac and Kilchiaran referred to below, of which it is probably the southern- most extension. It also resembles the large basic intrusion in Orsay. S. B. W. The basic masses are described on the tablet as consisting of epidiorite and hornblende-schist, but they include also diorites, augite-diorites, and augite-hypersthene-diorites or hyperites. Some are perfectly massive with the original structures well preserved, while others show a well-marked parallel structure. A specimen from Kilchiaran (5593) may be described as a gneissose diorite. It consists essentially of green hornblende and plagioclase with granular aggregates of apatite and iron-ores zoned with sphene as accessories. Epidote also occurs. Another specimen from the same locality (5595) consists of greenish-brown hornblende, uralitic hornblende containing kernels of unaltered augite, and plagioclase which is often crowded with small scales of white mica and grains of epidote. Iron-ores and apatite are also present. Another variety occurs at a point one and a half miles north of Oladville House (6861). It is a dark-green massive rock containing large ii'regular individuals of hornblende which often measure half an inch across and show lustre mottling on their cleavage faces. The constituents are brownish-green hornblende, nearly colourless augite, biotite, a basic plagioclase. and iron-ores. A specimen from Cnoc Breac (6864) is of exceptional interest. It is a coarse-grained massive rock essentially composed of brownish- green hornblende, pale-green augite, hypersthene, biotite, and a basic plagioclase. The original structures are well preserved, and the rock may be described as an augite-hypersthene-diorite. Specimens of augite-diorite from Cultoon (6870-6872) are similar in character but contain no hypersthene. The pale-green augite occurring in the rocks from the two last-mentioned localities is precisely similar to that occurring in the pyroxene-gneisses 'of the north-west of Scotland, where it is also found, not infrequently, to be associated with hypersthene. J. J. H. T. The cliff at the south end of Lossit Bay affords a very fine section of one of the basic masses with some gneiss of the original com- plex. The latter appears to be traversed by a mass of pegmatite. The rocks here have been beautifully polished by the blown sand. On the east coast of the Rhinns, between the road and the coast- line, a lenticular mass of highly acid red pegmatitic rock can be traced in a north-east direction between Easter Ellister and Octofad. Another interesting shore section is that extending from a little north of Bruichladdich by Port Charlotte to Carn Cottage. Here the rock has been intensely sheared, and when examined under the microscope is seen to consist very largely of mylonitic material. In the Gleann Mor stream, about five hundred yards west of Port Charlotte, the gneiss shows signs of intense movement and is drawn out into platy mylonite ; the dip of the planes is generally south- 18 Leivisian Gneiss of the Bhinns of Islay. east at low angles. Further down the burn the gneiss is less deformed, and in the stream behind the distillery both acid and basic material can still be recognised. Along the line of junction between the Lewisian gneiss and the Torridonian sediments at this locality the rocks are traversed by a set of planes of schistosity that are clearly common to both forma- tions. It has further been observed that although crushing and mylonisation of the older rocks is more or less prevalent throughout the region, the amount of shearing and deformation increases towards the margin of the sedimentary rocks, and is most intense along the actual boundary-line between the two. This is in all probability due to the unequal resistance offered by the gneiss and the overlying sediments to the shearing movement. S. B. W. CHAPTER V. TORRIDONIAN. The Torridonian rocks of Islay may be divided into three well- marked groups. A. The Rhinns conglomerate series — a schistose grit with a few thin seams of phyllite. The grit is often highly epidotic and in places forms a local base, highly conglomeratic and containing pebbles of the underlying gneiss. This series is comparatively thin. B. The Kilchiaran slates and grit series — consisting of grey, green, and almost black slates and phyllites alternating with grey schistose grit and thin bands of sandy limestone. Owing to the intense folding that these rocks have undergone, it is impossible to form a true estimate of their thickness, but it must be considerable. 0. The Bowmore grits — a series of red, green, and grey grits or arkoses, whose characteristic feature is the fresh condition of the felspar grains and pebbles of which they are largely composed. In places they are very coarse-grained and contain well-rounded pebbles of quartzite, felsite showing fluxion-structure, granite, and other rocks. Towards their upper limit they become fine-grained and alternate with slaty and phyllitic bands. These more massive grits are less folded than the rocks of Group B, and may be of great thickness. A comparison of the Torridonian rocks of Islay with those of the same formation as developed in the North-west Highlands, and tabulated in the annual report of the Director-General of the Geo- logical Survey for the year 1893, shows that Groups A and B resemble in lithological character the Diabaig group as developed in the Sleat district of Skye, and may occupy the same horizon ; the Rhinns conglomerate corresponding to the basal epidotic grits of Sleat and the Kilchiaran slates and grits to the sub-divisions b, c, d, established by Mr. Clough in Skye. The resemblance between the Bowmore group (C) and the members of the Applecross group (2) as there described is even more striking. The more eastern and probably the upper portion of the Bowmore grit corresponds in character with a portion of the Aultbea group (3) of the northern region* A. Conglomerate and Schistose Epidotic Grit. — Several sections in the Rhinns show that a zone of conglomerate and epidotic grit forms the base of the sedimentary rocks which rest unoonformably * Annual Report of the Geological Survey and Museum of Practical Geology for the year ending December 31, 1893. 20 Torridonian Strata. I i l! a ca .3 M M li -t= c o O t3 >; "-a =« "S is M air< •S go .9 a^ 13 ■"'S M o g^ j3 " ."S .|pq O) to '^ a, a) op 13 '2 I h:i -! "S .^■^ .9 a on the Lewisian gneiss in that area. Following the junction-line between the two formations northwards along the shore from a point on the west coast a short distance south-west of Kilchiaran Bay, the observer finds the Torridonian sediments resting in an inverted position against the gneiss, and dipping south-east at 70° to 80°. The rocks at the junction consist of dark pliyllites with alternations of dark gritty bands, containing masses of gneiss similar to the adjoining coarse acid rock, one of the masses being two feet long. The conglomeratic beds are succeeded by phyllites and epidotic grits, which pass up into a succession of tough grits. The beds here resume their normal position and dip north-westwards out to sea. Two specimens of these grits from this locality have been examined microscopically. One (5605) is a coarse grit composed of sub- angular fragments up to ^ inch in diameter embedded in a fine-grained dark-greenish matrix. The large fragments consist for the most part of quartz rock, one contains biotite in addition to quartz, another is an aggregate of microcline. Amongst the smaller fragments occur microcline, micro- perthite, ordinary striated plagioclase, and quartz. Sericite, biotite, and colourless micro-crystalline material enter into the composition of the matrix. The other (5606) is a grey felspathic grit containing blue quartz and large grains of felspar (mostly microcline). The matrix shows pressure-fluxion and is composed partly of smaller grains of the same two materials and partly of mylonitic material, including a little sericitic mica. The softer phyllites and epidotic rocks weather out more readily than the gneiss on the one side and the grits on the other, producing a hollow along their outcrop. As we follow the line of junction of the gneiss and sediments south-westwards along the east side of this hollow to Eilean Liath, the junction of the dark slates or phyllites with the massive gneiss is found to be of such a nature as to preclude all possibility of any thrust or fault occurring between them. The cleavage-planes can fe ;5u _o 3 The Rhinns Conglomerate Series. 21 be traced from the epidotic grits and phyllites into the gneiss, and at one locality where the gneiss is traversed by a fine- grained dyke, the one set of cleavage-planes is common to the sediments, the gneiss, and the material of the dyke. In several places, however, small fragments of the basal grits are seen adhering to the surface of one of the basic intrusions. At Carn Mor, where a basalt dyke crosses the junction, the latter bends to the north round a knot of sheared basic material. At this point and beyond the alluvium at Am Braid, where the junction-line becomes more irregular, though still running north- east, smaller inliers of gneiss and basic rock occur within the area occupied by the grits, doubtless appearing on the crests of folds which can be seen in section a little to the south at Garn Mor. The junction continues to be of this nature for the next half mile, the grits being in contact successively with the pink acid gneiss and with a sheared basic igneous rock like that seen in the road-cutting east of Kilchiaran. At a point one-quarter of a mile east of Dun Bhar-a-chlaom the outcrop bends almost at right angles and the grits transgress the bands of sheared gneissose rock, while the folding and puckering seem to continue in the same direction as that of the slate and grit already described. A considerable area of rock is here covered with peat, but the evidence favours the continuity of the sediments beneath the peat with those further to the east. An inlier of gneiss and diorite is seen in the stream three hundred yards to the east of Dun Buin Dubha, surrounded on three sides by highly epidotic grit contain- ing pebbles of quartz and gneissose material. Eather more than a mile south of this point the junction-line between the sediments and foliated basic rocks emerges from beneath the peat and can be traced in a south-west direction for half a mile (Plate III.). Immediately south of Dun Mideir several sections show an epidotic grit charged in places with large well-rounded pebbles of gneiss, basic rock, and quartz, which have been elongated parallel to the outcrop, i.e., north-east and south-west, the dip of the strata being to the south-east, at 30° to 40°. Both epidotic grit and conglomerate, and the underlying diorite, are foliated by a common set of planes more or less parallel to the junction-line, and where the pebbles are absent from- the epidotic grit, it is difficult to separate the sediments from the diorite along the boundary, although a few feet from the junction the igneous character of the basic rock can easily be distinguished. About two hundred yards south-south-west of Dun Mideir the junction-line bends back upon itself and runs in an east-north-east direction, the conglomerate for a short distance following the junction, although appearing to dip below the acid gneiss with which it is sheared along the line of contact, the foliation planes in both sediments and gneiss being inclined to the south-east. A tract of peat now obscures the rocks for a short distance, but 1;^ miles east of Dun Mideir they are again exposed, and the conglomerate being gently folded or nearly horizontal, the boundary-line once more bends nearly at right angles to its former course. A large tract of peat again obscures the position of the 22 Torridonian 8l/rata. sediments and gneiss, but to the south-east small knolls of epidotic grit and conglomerate appear at intervals for half a mile to Cnoc Thornasaig, where the junction of the conglomerate with the gneiss is once more visible, both rocks being intensely sheared and drawn out in a direction more or less parallel to the boundary. From Cnoc Thornasaig the outcrop can be followed for about one mile south-south-west towards Loch Gearach, the sheared epidotic grit being in places conglomeratic. For some distance the rocks are drift-covQ.red, but on the roadside five hundred yards due south from the outlet of Loch Gearach the junction of the gneiss with the conglomerate is exposed in a knoll, the conglomerate here containing very large boulders of well-banded gneiss. About one hundred yards south-west of this knoll the boundary bends back upon itself, the conglomerate being once more seen on the road- side in a knoll peering through the drift. From the disposition of the gneiss the epidotic grits probably extend in a north-eastern direction. A number of pebbles from the conglomerate of Dun Mideir, 1;|- miles north-east of Kilchiaran (7006-7008), together with a speci- men (7004) from one of the large boulders mentioned above, have been examined by Dr. Teall, who identified them with specimens taken from the solid rock of the gneiss area. The matrix of the conglomerate is mylonitic, with epidote and sericitic mica. From the last-mentioned locality, near Loch Gearach, the epidotic grits extend in a northerly direction. To the south-west of Cnoc Breac, near the old road, they become very coarse, with pebbles of quartz and gneiss in a schistose matrix which has been intensely puckered and folded ; the pebbles also have been flattened by the movement to which they have been subjected. These grits are repeated by a system of folds which dip in a south-east direction at about 30°. At Sunderland Farm, however, the beds are nearly flat. An outlier of these rocks, much crushed and my Ionised, occurs at Port Charlotte, forming a narrow stripe about a mile in length along the shore on each side of the village. The sediments are first met with between Bruichcladdich and Port Charlotte at a point one hundred and fifty yards north of Loch Indaal Lighthouse, where an olivine dolerite dyke traverses the section and hides their junc- tion with the gneiss. A little way from this dyke a small boss of crushed gneiss appears from beneath the mylonised sediments. The latter here consist of phyllites, slates, and sheared green epidotic grits or greywackes, the last-named being predominant. The dip of all the sediments is towards the south-east at low angles, and that of the foliation planes in much the same direction. These rocks follow the coast-line to the south of Port Charlotte, where, a little beyond the last house, their undoubted sedimentary origin is apparent, the bedding planes being still distinctly seen, while the sand grains stand out upon the weathered surfaces of the green epidotic and chloritic greywackes, which are interbedd«d with dark slates or phyllites. In the little bay of Port an-t-Sruthan, one-third of a mile south of Port Charlotte, sediments are seen resting upon a rounded boss of gneiss. They are green and highly epidotic, and Tlie Kilchiaran Slate and Grit Series. 23 contain boulders of banded gneiss similar to the rock with which they are in contact, masses of epidosite, and some elongated frag- ments of quartz which strongly resemble the pebbles of a conglo- merate. The sediments continue for some little distance to the south of the burn, and similar conglomeratic beds are seen at intervals in conjunction with the gneiss. Inland the junction of the gneiss and sedimentary strata is covered by drift, but the latter do not probably extend very far westwards, as mylonitic gneiss is seen in the stream immediately behind Port Charlotte. At the south-western extremity of the Ehinns, between Portnahaven and Claddich, the coast-line, for a distance of about half a mile, is occupied by rocks which probably belong to Groups (A) and (B) of the Torridonian (Islay) Series. The evidence for this identification is, however, not altogether conclusive, the rocks having been so much altered that microscopic examination throws little light on their origin. The field evidence, however, favours this sedimentary origin. At a point about 200 yards north-west of Portnahaven the igneous gneisses are succeeded by phyllites and quartzo-micaceous rocks that resemble altei-ed greywackes, the junction of the gneiss with the phyllites being quite sharp, and almost vertical. Prom this locality to Port Mias-sgeire the shore section displays phyllites and sheared grey- wackes altered into granulitic mica-schists, and elongated bosses of gneiss surrounded by a peculiar series of rocks consisting of epidotic schists and phyllites that contain what appear to be elongated eyes and knots of gneiss enclosed in a mylonitic matrix. All these rocks show considerable signs of movement, and it is possible that this rock may have been a conglomerate. The larger bosses of gneiss that lie to the west of this little cove are succeeded eastwards by phyllites and rocks resembling the foliated epidotic grits of Kilchiaran. In the neighbourhood of Port Mias-sgeire there are several dykes and sills of igneous rock which show a considerable amount of cleavage and distortion. The foliation planes are also much contorted, while a subsequent cleavage trending north-east and south-west cuts this contorted foliation. Most of these intrusions traverse the sediments and have been cleaved and altered together with them. These schists of presumed sedimentary origin may be the same rocks that occur at Kilchiaran and Port Charlotte. They appear to attain considerable thickness, but this is not really the case, as whenever exposed in a clear section there is seen to be a constant repetition of inverted folds, so that their true thickness is but insignificant. The axis of the folds is north-east and south-west, and the materials are all drawm out in the same direction. B. The Kilchiarcm Slate and Grit Series. — These slates are well exposed in several sections along the little valley between Kilchiaran Bay and Kilchiaran House, and are seen at the roadside in conjunction with the tough greyish grits of Group A in a nearly vertical position. In the north corner of Kilchiaran Bay the rock has been quarried and a rough inferior slate obtained, but the quality of the slates is not improved by the numerous north-east and south-west dykes by which they are traversed, and which were 24 Torridonian Strata. intruded prior to the cleavage of the slates. The slates form the greater part of the coast-line northwards from Kilchiaran Bay to Traigh Machir, but are interfolded in several places with the underlying epidotic grit ; the largest fold of the latter occurring one-quarter of a mile south of An Gro-is-sgeir. At the northern margin of the Traigh Machir a very interesting section of the slates, traversed by igneous rocks of diffei'ent ages, is seen. The area northwards to Coul Point and Coul Farmhouse is occupied by hard, tough, greenish-brown grits, separated towards their northern limit by slaty partings. These grits are not epidotic like those of the basal beds, and probably occupy a higher horizon as intercalations in the slaty group. They are repeated by a series of gentle folds, and it is probable that the grit masses further north at Saligo Bay may belong to this zone, though the evidence that this is the case is not conclusive. At Coul Point intensely crumpled slates succeed the grits, and can be traced for some distance inland over Gnoc Mor, north-east of Coul Farm. From Goul Point north- wards to Saligo Bay the shore section shows bare rock, and both on the coast and on the inland side of the raised beach the slates are well developed. They are intensely puckered and cleaved, the dip of the cleavage planes being south-east, at angles varying from 30° to 75°. They are traversed by igneous intrusions' of several types. On the southern shore of Saligo Bay the grits reappear, interfolded with the slates. Both rocks are cleaved, but the slaty bands have yielded to the movements in a much greater degree than the more resistent grit. Inland, to the north of Ballinaby, the same inter- folding of the grits and slates continues, the cleavage planes in the slates dipping to the south-east at 40°. This alternation of slates and grits continues northwards along the coast as far as Sanaigmore, a distance of more than four miles, the slates being predominant between Saligo Bay and Rudha Lamanais ; the grits to the north of that point, and between An Glachan and T6n Mhor. Where the grits occur in mass the cliff scenery is very bold, and the folding of the beds is beautifully exposed, as at Tdn Lagain, two miles north of Saligo Bay. (See Plate I. — Frontispiece.) The softer slates, on the other hand, are eroded by the sea into gyoes and narrow coves such as Gleann Tuath, which is filled up in a great measure with blown sand. At Goul and Saligo the rocks have been highly polished by the sand which is blown against them with terrific force. The band of grits north of Gleann Tuath forms the celebrated clifis known as " The Sanaig Rocks." These grits are brownish in colour and highly cleaved, and alternate with calcareous flaggy bands which efiervesce slightly with acid ; some slate bands also occur which are also cleaved and folded. The cleavage planes dip south-east at 25° to 50°, the axis of the folding having a north-east and south-west direction. The coast- line from Sanaigmore north-eastwards to Ardnave Point affords a continuous section of slates pierced by innumerable basalt dykes. The whole of the Ardnave promontory is occupied by these slaty rocks, with thin bands of greyish grits and schistose greywackes. The rocks are all intensely puckered and repeated by folding, the > w < OS c OS > c ^ >> ^ (J u w o ■— ' to »-l as ,— I^ . C y: cu u cu o C/] j: ^ u K o J S ^-1 "u o —