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And to be purchased from EK. STANFORD, 12,.13 and 14 Lona Acre; Lonpon; . W. & A. K. JOHNSTON, Lrpv., 2 St. ANDREW Square, EDINBURGH ; ‘ HODGES, FIGGIS & CO., Lrp., 104 Grarron Srrezt, Dustin. From any Agent for the sale of Ordnance Survey Maps ; Or through any Bookseller from T. FISHER UNWIN, 1 Apetrnr Trrracn, Lonpon, W.C., who is the sole Wholesale Agent to the Trade outside the County of London. —— 1916. Price Seven. Shillings and Sixpence. LIST OF PUBLICATIONS OF THE GEOLOGICAL SURVEY OF SCOTLAND. Geological Map (25 miles to the inch) of the British Islands. (Second edition, 1912.) Price, Colour-printed, 2s. ; Uncoloured, 1s. Maps on the Scale of Four Miles to One Inch. (Colour-printed.) Sheet 12. Perthshire, Forfarshire, Kincardineshire, &c. (1910.) 2s. 6d. . 15. Fifeshire, Edinburghshire, Haddingtonshire, &c. (1912,) 2s. 6d. », 16. Galloway and part of Ayrshire, &c. (1907.) 2s. 6d. », 1%. Roxburghshire, Dumfriesshire, &c. (1907.) 28. 6d. 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Lanarkshire, Central Districts ; Ayrshire (part of W.). (1872.) 19s. 8d. 2? eee WON HOO WO~IMUHM Ob et ‘aJep 9IqIssod 4setp1ve oy} 38 pensst oq [TIM dem oy, ‘uojdiiosop e soysturny TIOWMS]L SI} YOIYM Jo dew yYyout-euQ oy} jo Surmutad-snojoo 24} pokelap sey Ie@M ey} 01 Onp HOM Jo oinsserg “IDUOY) SIATN NAIF) UNV STAIN Nagy fae ‘(aoaudsipodg) “T ALWITd 53 MEMOIRS OF THE GEOLOGICAL SURVEY, SCOTLAND. THE GEOLOGY OF BEN NEVIS AND GLEN COE, AND THE SURROUNDING COUNTRY. (EXPLANATION OF SHEET 53.) BY E, B, BAILEY, B.A., anv H. B. MAUFE, M.A, WITH CONTRIBUTIONS BY C. T. CLOUGH, M.A.; J. S. GRANT WILSON ; G. W. GRABHAM, M.A.; H. KYNASTON, B.A. ; W. B. WRIGHT, B.A. PUBLISHED BY ORDER OF THE LORDS COMMISSIONERS OF HIS MAJESTY’S TREASURY. EDINBURGH: PRINTED UNDER THE AUTHORIT STATIONERY OFFICE By MORRISON & GIBB LIMITED, Avr TANFIELD. OF HIS MAJESTY’S And to be purchased from E. STANFORD, 12, 13 and 14 Lone Acre, Lonpon ; W. & A. K. JOHNSTON, Lrp., 2 Sr. ANDREW SQuARE, EDINBURGH ; HODGES, FIGGIS & CO., Lrp., 104 Grarron Srreer, DuBLin. From any Agent for the sale of Ordnance Survey Maps ; Or through any Bookseller from T. FISHER UNWIN, 1 Apr.pni1 TERRACE, Lonpon, W.C., who is the sole Wholesale Agent to the Trade outside the County of London. —_— 1916. Price Seven Shillings and Sixpence. L 42 tel BISB4. Vib A 487 ss4 PREFAGE. etter THE region described in this memoir is situated in one of the most rugged parts of Scotland. Ben Nevis (4406 ft.) is the highest mountain in Great Britain, and several peaks in the eastern part of the area rise to heights of 3000 ft. and over. Between them run glens correspondingly deep and often remarkably picturesque, the most famous being Glen Coe, Glen Nevis, and Glen Etive. The great hollow of Loch Linnhe, forming the continuation of the long line of Loch Lochy and Loch Ness, cuts obliquely across the region from south-west to north-east, and admits the sea far inland. For boldness of relief and for opportunities for studying the origin of geographical features this tract is not surpassed in Britain. The mapping was begun in 1895, and was carried on under the superintendence of Dr. B. N. Peach till 1903, and thereafter under Mr. C. T. Clough as District Geologist. Mr. J. 8. Grant Wilson mapped most of the ground north-west of Loch Linnhe, and on the south-west side of that loch he took in hand considerable areas around Fort William, Onich, Ballachulish, Appin, and Glen Creran. Mr. E. B. Bailey, in addition to a small area in Glen Tarbert, mapped much of the eastern portion of the Sheet as far south as Glen Coe, and subsequently, after an interpretation of the geological structure of the district had been suggested by Mr. Maufe and himself, he re-examined most of the ground between Fort William and Appin. Mr. Maufe mapped Ben Nevis and the greater part of the volcanic rocks of Glen Coe, and a small district south of Glen Tarbert. The southern and south-eastern margins of the Sheet were surveyed by Mr. Kynaston ; Mr. Wright undertook the examination of the area east of Kinloch- leven; Mr. Grabham, the upper part of Glen Etive; Mr. Clough mapped a small area near Dalness; and Dr. Peach collaborated in the survey of Glen Coe in addition to mapping a limited tract near Glen Gour. The memoir has been edited and mainly written by Mr. Bailey. The geology is both varied and complex. Upon a groundwork of crystalline schists an extensive covering of lavas was spread in the time of the Lower Old Red Sandstone, but of this covering parts only have survived denudation in Ben Nevis and in the mountains around Glen Coe. At this time also great intrusions of granite rose, and igneous dykes were injected in vast numbers, more especially in the two “swarms” of Ben Nevis and Glen Etive. Some suggestions offered with regard to their origin may prove to be applicable to dyke-swarms in other regions. The superficial, hypabyssal, and plutonic rocks of Lower Old Red Sandstone age form a suite of unusual perfection. Their description is accompanied by analyses of iii iv representative rock-types. In Tertiary times several dykes were injected along cracks having a general north-westerly direction. No less interesting are the phenomena of contact-alteration induced within the aureoles surrounding the plutonic masses. Schists, lavas, and even earlier members of the intrusions all display in vary- ing degrees the changes due to heating in the neighbourhood of the molten rocks. The geological structure of the area presents some remarkable features, the existence of which was not suspected in this part of Seotland. As the detailed six-inch survey progressed, it became apparent that some of the complicated sequences and repetitions of rock-types in the schists were capable of interpretation if it could be shown that the schists had not only been folded but that the folds had been prostrated and pushed forward horizontally in a recumbent position. The suggestion, first made by Mr. Maufe, was proved by Mr. Bailey to be applicable to a considerable region between Ben Nevis and Appin, and the existence of recumbent folds, accompanied by slides and fold faults, was announced in papers laid before the Geological Society of London. The cauldron-subsidences or sunken voleanic areas of Glen Coe and Ben Nevis, surrounded by ring-dykes, form another feature of unusual interest in the tectonics of this region. In Glen Coe especially the subsidence can be connected with the peripheral upwelling of magma. The character of the cauldron subsidences was described before the Geological Society of London by Messrs. Clough, Maufe, and Bailey. The glaciation at its maximum was effected by an ice-sheet by which the whole district was smothered, and which was advancing towards the Atlantic Ocean from an ice-parting situated just outside, or possibly passing within the north-eastern part of the area included in the map. The course of the ice was determined mainly by the great hollow of Linnhe, and at a later stage almost every glen was occupied by a valley-glacier. Thanks are due to the Councils of the Geological Society of London and of the Geologists’ Association for permission to reproduce several illustrations relating to the district which have appeared in the publications of those Societies. A, STRAHAN, Director. GEOLOGICAL SURVEY OFFICE, 28 JERMYN STREET, LONDON, 16th April 1915. M&GLtd Wt 9959/13 8-16 500 G2 CONTENTS. —— CHAPTER I. INTRODUCTION. PAGB PHYSIOGRAPHY. : . é ‘ ‘ ‘ § 1 CHAPTER ILI. HISTORICAL REVIEW. Tue HIGHLAND ScHISTS P 4 . : . , . 14 THE Ienzous Rocks . . : ‘ 5 : 5 a. ae Ben Nevis - : , : , 2 2 « At Glen Coe . é 3 5 . . Ig GLACIATION AND REcEN? Deposits ‘ g 3 : Z . 20 CHAPTER III. FORMATIONS AND TECHNICAL TERMS. TABULAR STATEMENT OF FoRMATIONS : “i 2 z . 23 Discussion or TERMS 2 : z : . 3 . 25 CHAPTER IV. METAMORPHIC ROCKS. LocHABER AND APPIN—DETAILED DESCRIPTIONS. j : . 29 A. Onich to Fort William i 2 3 a é . 49 B. Appin to Kentallen . : ; : . 384 C. Aonach Beag . : ; ; 5 i - 38 D. Callert ; : : 5 f 5 . 421 E. Ballachulish . : . ‘ : . 45 F. Glen Creran . 5 z : ‘ : . 50 G. Glen Coe to Glen Etive : 4 ‘ . 56 H. Kinlochleven . ; “ 2 s : . 62 1st Hypothesis. : ‘ ‘ . 64 2nd Hypothesis. : 3 ; ; : . 68 Minor Folds . ‘ ‘ . 73 I. Stob Dearg to Allt Coire an Easain (Sheet 54) o. TE CHAPTER V. METAMORPHIC ROCKS—(Continued). LocHaBER AND APpPpin—Résumé j ‘i z j ; . 80 CHAPTER VI. METAMORPHIC ROCKS—(Continued). ARDGOUR DISTRICT. ‘ é : . : ‘ . 84 vi CHAPTER VII. ROCKS OF LOWER OLD RED SANDSTONE AGE. INTRODUCTION . SEDIMENTARY AND ACCOMPANYING Voucanic Rocks oF THE GLEN Cox DistRicr : Sequence from Loch Achtriochtan to the Buachaille Etive Beag Sequence on the North Side of Glen Coe Sequence in Buachaille Etive Mor c Sequence in Cam Ghleann and Sron na Creise . Sequence around Dalness, Glen Etive Outliers. 5 Summary of the Volcanic History CHAPTER VIII. ROCKS OF LOWER OLD RED SANDSTONE AGE—(Continued). Tuer BounpaRy Fautt anp aunt INTRUSIONS OF GLEN COE South of Dalness . 5 3 Dalness to Glen Coe Gleann Charnan , North of Glen Coe South of Glen Coe to the River Ba Conclusions CHAPTER IX. ROCKS OF LOWER OLD RED SANDSTONE AGE—(Continued). Tue Granite Complex or ETIvE The Cruachan Granite The Meall Odhar Granite The Starav Granite Mechanics of Intrusion CHAPTER X. ROCKS OF PROBABLE LOWER OLD RED SANDSTONE AGE. Brn NEvIis , The Volcanic Pile The Inner Granite The Outer Granite General Conclusions CHAPTER XI. ROCKS OF PROBABLE LOWER OLD RED SANDSTONE AGE—(Continued). Puutonic MAssES OTHER THAN THOSE OF GLEN Cor, ETIVE, AND Brn NEvIs ‘ _ ‘ The Moor of Rannoch Grani te The Ballachulish Granite The Mullach nan Coirean and Meall a? Chaoruinn Granites . The Granites of the North-West Coast of Loch Linnhe ‘ Various Small Bosses of More Basie Material SPREE, aad “Diorite, Monzonite, Kentallenite, Cortlandtite) . : PAGE 89 92 93 96 97 100 102 104 105 107 110 110 110 110 117 117 119 119 122 123 125 128 130 131 132 133 135 135 136 137 138 138 vii CHAPTER NII. ROCKS OF PROBABLE LOWER OLD RED SANDSTONE AGE—(Continued). PAGE DYKES AND SHEETS . : : : : : ‘ . 141 Early Lamprophyre Sheets. 5 i ‘ : ‘ 141 Lamprophyres, Unclassed —. ‘ , . 141 Early Felsite and Andesite Intrusions of Glen Coe . i . 142 The Etive Swarm of North-North-East Dykes : . 148 The Ben Nevis Swarm of North-North: ast Dykes . : . 148 Dykes in the District North-West of Loch Linnhe . . 148 CHAPTER XIII. ROCKS OF PROBABLE LOWER OLD RED SANDSTONE AGE—(Continued). PETROLOGY. 3 ‘ ‘ ‘ 3 ‘ . 150 Introduction . ; 150 Discussion of Classification ; : j . 151 Plutonic Rocks . ; : ; 151 Hypabyssal Rocks : 156 The More Acid Plutonic Rocks 4 . 158 Aplogranites ; : . ; . 158 Granitites, Tonalites, Banatites, Adamellites, Aplodiorites, Grano- diorites, and Cale-tonalites . i 4 . 159 The More Basic Plutonic Rocks ‘ , 167 Appinites . , 168 Augite-diorites and Monzonites . ; 169 Kentallenites j z : j 170 Kentallenite-aplites 5 171 Cortlandtites 172 The Hypabyssal Rocks 172 Felsites and Porphyries 173 Porphyrites 174 Malchites < 175 Lamprophyres : 176 Unclassified ae j . 178 The Lavas 3 : 178 Rhyolites . : 179 Hornblende- and Biotite-Andesites “ ‘ . 180 Pyroxene-Andesites and Olivine-Basalts . 182 CHAPTER XIV. CONTACT-ALTERATION DUE TO PLUTONIC ROCKS OF PROBABLE LOWER OLD RED SANDSTONE AGE. METAMORPHISM OF THE SCHISTS BY THE VARIOUS GRANITE MASSES SOUTH- Fast oF Locu LINNHE x 187 Limestones ‘ 2 2 ; ; . 188 Ballachulish Limestone : ‘ . 188 Appin Limestone . 194 Pelitic Schists . é 195 Leven Schists ; 195 Appin Phyllites . : 201 Ballachulish Slates ; 202 Psammitic Schists ‘ 5 . 202 Eilde Flags ; 203 METAMORPHISM OF Lavas, BRECCIAS, FINE SEDIMENTS, AND INTRUSIONS or PROBABLE Lower OLD Rep SANDSTONE AGE 3 » 203 The Lavas of Glen Coe and Ben Nevis : : ; : 203 Hornblende-Andesites . § : 203 Rhyolites . : ; . : . 3 . 204 Lavas in General . . ‘ js : : z . 205 Pack Breccias of Glen Coe . : ‘ i 5 ‘ . on Fine Sediments of Glen Coe . ; : : ae Intrusions 2 ‘ 4 ; : i ; ‘ Ben Nevis Dykes . ‘ ‘ : ; : ; . 20 Early Fault-Intrusion —. 3 . : . 208 Dykes earlier than Fault- Intrusion 2 : ‘ . 208 Fault-Intrusion . ‘ A ‘ : ‘ . 208 Etive Dykes ‘ g ; . 208 Glen Ure Augite- -Diorite , : 3 . . 209 Moor of Rannoch Granite ‘ : $ ‘ ‘ . 209 Effects resembling Contact-Alteration s 3 : "i . 210 CHAPTER XV. RUDHA NA H-EARBA OUTLIER OF SUPPOSED MIDDLE OLD RED SANDSTONE AGE _.. ; . : ‘ . 211 CHAPTER XVI. PRODUCTS OF IGNEOUS ACTIVITY LATER THAN THE OLD RED SANDSTONE AGE. West-North-West Dykes ; ; ‘ ‘ . 212 Breccia and Nowtelias: -Basalt of Coire na Bi Neck . ; - 214 CHAPTER XVII. FAULTS : ‘ é . 215 CHAPTER XVIII. PLEISTOCENE AND RECENT. Ick-SHEET AND VALLEY GLACIERS. 3 : - . 217 Striz of the Maximum Stage . ‘ : : ‘ 3 . 217 North-West of Loch Linnhe Fe é , , . 217 South-East of Loch Linnhe : . ‘ i . 218 Glacial Erosion 220 Boulder Clay, Moraines, and Erratic Blocks, with a description of the Glacial Drainage Phenomena of Meall Cumhann . 221 Fluvio-glacial Gravels . ‘ ‘ 3 a é ; . 223 Ratsep BEACHES ‘ ‘ ‘ 5‘ 224 Hundred-Foot Beach , : . 2 3 . 225 “Twenty-five-Foot Beach ” ‘ ; ‘ - 226 Peat and Alluvium . d r _ . 227 aad CHAPTER XIX, ECONOMICS. RooFING SLATES : ‘ ‘ : 5 . 229 The Slate Belts. a : - 4 . 229 The Slate Rock . ‘ ‘ 5 § - 229 Method of hiaia. - i : % : 230 GRANITE : . : - 231 Ballachulish : : ‘ . 231 Glen Nevis , : 2 : : F . 232 Other Granites. a) (BS Sg . 232 Kentallenite (“‘ Black Granite ) 5 : . 232 QUARTZITE. ; : : ; a g 239 Appin Quartzite . : f : é : ; . 232 Binnein Quartzite is , : : ‘ ‘ . 233 LIMESTONE AND MARBLY , 3 ; 4 : . 233 Limestone ‘i , s . ‘ ’ . 233 Marble. é : é : ‘ : < . 233 1X PHYLLITE Minera Vuins Galena. . Hematite . Barytes LIST OF FIGURES IN TEXT. Fig. 1. Section across Appin Fold, Onich Shore. » 2. Section across Appin Fold, north of Cuil Bay 3. Section across Island of Shuna and Glen Stockdale on the Main- 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21, 22. 23. 24. 25. 26. 27. 28. 29. 30. 31, 32. 33, land . Looking up Ben Nevis from Stob Bin. The Aonach Beag Core of Ballachulish Limestone folded into a syncline well seen in Aonach Beag (4060 feet), and also in Meall Cumhann in thie middle distance . Map showing Outcrops in Callert: District . . Section showing the Relation of the Ballachulish Slide to the Tom Meadhoin Fold . Crag, south-west side of Gleann Chirnan. Cale-silicate-hornfels (Ballachulish Limestone) lying discordantly, through the inter- vention of the Ballachulish Slide, upon the handed eve phyllitic series of the Leven Schists . Map of Stob Ban and Meall a’ Bhuirich, with Serial Sections . Sections north of Loch Leven and across Loch Leven, illustrating oe of the Appin, Aonach Beag, and Ballachulish olds . Sections of Fig. 9 showing alternative interpretation Sections across Loch Leven and south of Loch Leven, illustrating the nature of the Ballachulish Fold Generalised Map and Section, showing the Relation of the Appin, Aonach Beag, and Ballachulish Cores — . Map of the Old Red Sandstone Volcanic J istrict of Argyllshire and Inverness-shire, showing the Distribution of Dykes in Relation to the Etive and Ben Nevis Granite Complexes View up Coire nam Beith from Road, Glen Coe Sections across Cauldron-Subsidence of Glen Coe . Map of Coire nan Lab and Coire Cim Section through Meall Dearg Map of Coire Odhar-mhor and Coire Mhorair Section through ridge west of Coire Odhar-mhor_ . Map of Stob Mhic Mhartuin Diagrammatic Sketch of Glen Coe Boundary- Fault, as expored i in the cliff of Stob Mhic Mhartuiu . % Map of Stob Beinn a’ Chriuaiste Map of Cam Ghleann and Coire an Easain District Diagram illustrating Subaerial and Subterranean Cauldron- Subsidences, accompanied by Volcanic and Plutonic Accumula- tions of Igneous Rock : : x Map and Section of Ben Nevis : Igneous Rocks, supposed to belong to the Lower Old Red Sand- stone Suite Map showing Distribution of early Felsite and Andesite Dykes, Glen Coe . Map of Multiple Dyke i in the bed of Allt Fhaolain, Glen Etive . Porphyrite Dyke traversing Rhyolites at the foot of Buachaille Etive Beag ‘ Porphyrite Dyke traversing “Moor of Rannoch Granite in bed of River Etive, 800 yds. above Kingshouse (Sheet 54) Ratio of Plagioclase to Orthoclase in the Cruachan, Ben Nevis, Moor of Rannoch, and Ballachulish Granites 4 Contact Aureoles in the District south-east of Loch Linnhe Glacial Flow-Lines during the Maximum Stage of Glaciation PAGE 233 234 234 234 234 31 35 38 39 40 42 61 71 74 75 78 81 90 94 105 108 109 111 112 113 114 115 116 126 129 136 142 145 146 147 161 191 218 PLATE oP) I. II. III. x LIST OF PLATES. Ben Nevis and Glen Nevis Gorge Physiographical Map, showing original drainage system (1) Upper hanging portion of River Leven (2) Stob Ban, showing landslip . Geological Map of Lower Glen Creran . ‘ . Interfolded Quartzite and Mica-schist in moun- tains south of Glen Nevis . Sgurr a’ Bhuic, Glen Nevis. Folded Glen Coe Quartzite . Gearr Aonach and Aonach Dubb. Rhyolite Lavas . . Stob Dearg. Rhyolite Lavas on Schists . Glen Coe Fault. Stob Mhic Mhartuin . . Coire Leis, head of Allt a’ Mhuilinn. Ben Nevis Volcanic Rocks and Inner Granite . Photomicrographs of Leven Schists . (1) An Steall, Glen Nevis (2) Roche Moutonnée, Glen Nevis . Frontispiece facing page 1 8 51 63 76 89 100 114 131 196 220 PLATE II. PHYSIOGRAPHICAL MAP SHOWING ORIGINAL DRAINAGE SYSTEM, The lines of dots show the original drainage system, intercepted by shatter-belt-valleys (Loch Linnhe and Loch Leven), and by secondary watersheds (W). Contours every 250 feet, The 1000, 2000, 3000, and 4000-it. contours emphasised by shading. THE GEOLOGY OF THE DISTRICT OF BEN NEVIS AND GLEN COE. CHAPTER L. INTRODUCTION. SHEET 53 of the one-inch map of Scotland is divided into two parts by Loch Linnhe running north-east and south-west. The portion of the district lying south-east of Loch Linnhe, and north of Loch Leven, and of the river Leven, which drains into the latter, belongs to Inverness-shire. All the rest falls to Argyllshire. The main centres of population are Fort William, East and West Laroch, together constituting what is generally spoken of as Balla- chulish, Onich, Kinlochleven, Clovulin, and Portnacroish (“ Appin” on the railway). Cultivation is restricted to the immediate vicinity of the coast-line and, even there, is extremely local. A considerable strip of sheep ground, half a dozen miles wide, flanks Loch Linnhe on the south-east, but, except for this, practically the whole country is given over to deer. The slate quarries of Ballachulish have been worked for genera- tions, and have a great name in Scotland. The rise of Kinlochleven is a notable development of the last few years. An important aluminium factory has been established here at the head of Loch Leven in the heart of the wild, romantic scenery of the district “’twixt Ben Nevis and Glen Coe.” It derives its water-power froma great reservoir which has been constructed in the valley of the river Leven, for the most part to the east of the border of the present map. The progress of this enterprise will be watched with interest by all who are concerned with the future of the Scottish Highlands. PHYSIOGRAPHY.* - The district is a much dissected portion of the main Highland plateau,} with a summit-level east of Loch Linnhe of about 3,000 * Part of the district has been treated in a preliminary account : E. B. Bailey, ‘The Geology of the Neighbourhood of Fort William,’ Proc. Geol. Ass., 1911, vol. xxii. Pp 179. + Cf. A..Geikie, ‘The Scenery of Scotland,’ 1901, chap. vii. I 2 Physiography. ft. Ben Nevis, 4406 ft. high, and several other massifs, such as Bidean nam Bian, 3766 ft. high, south of Glen Coe, overlook the neighbouring ridges and appear to be features inherited from an earlier geographical cycle. West of Loch Linnhe the general level for the dissected plateau is some 500 ft. lower than on the east. In the vicinity of Loch Linnhe the eastern segment of the main high-level plateau merges southwards into the much lower plateau of Lorne, to which the island of Shuna and neighbouring portions of the mainland may reasonably be referred. There is no sharp boundary here between the two plateau regions such as is en- countered along the Pass of Brander at the foot of Ben Cruachan (Sheet 45). Sir Archibald Geikie has repeatedly drawn attention to the fact that the drainage system traversing the high-level plateau is of later date than the Tertiary volcanic outbursts of the Hebrides. The present district furnishes a good illustration of this, for many of its valleys, such as Glen Tarbert, have been cut across Tertiary basalt dykes. Through valleys and hanging valleys are strikingly represented in the drainage system. Some of the through valleys have clearly been determined by the structure or grain of the country; while others, so far as one can judge, are altogether independent of it. Of the first type the best illustration is the valley of Loch Linnhe, which lies along the shatter-belt * of the Great Glen, a continuous feature traversing Scotland from side to side; another shatter-belt valley is that of Loch Leven and Loch Eilde Mor; and yet another holds the upper part of Loch Etive (Sheet 45), and the lower part of Glen Etive, as well as the Lairig Gartain, and, north of Glen Coe, Allt Lagan na Féithe. It must not be imagined, of course, that structurally determined valleys are necessarily through valleys maintaining their individu- ality for long distances at a time. In some cases the influence of structure can only be recognised for a comparatively short interval. Thus the river Nevis, above the entrance of Allt Coire na Gab- halach, has been deflected for about a mile along a shatter-belt, while farther downstream, east of Meall Cumhann, it follows for a rather longer distance a fold which was originally in large part occupied by easily eroded calc-silicate-hornfels. Then again, Glen Etive, in its upper reaches, and several tributary streams of Glen Coe, have been guided by north-north-east_ dykes of porphyrite (and porphyry); and, in like manner, Glen Stockdale, Gleann na h-Iola, and Glen Creran have been locally compelled to accept the direction of the strike of the schists. It is impossible to pass without reference an important book by Prof. Gregory, published since the present chapter, apart from this interpolation, was written. Prof. Gregory’s theme is the origin of * The term—shatter-belt, is adopted from Dr. Marr, who i i Address, dealing with ‘The Influence of the Geological Ute ce ea Lakeland: A study in Physiography,’ Quart. Journ. Geol. Soc., 1906, vol. Ixii, (especially pp. cii.-cxxiii.), materially assisted in the development of the con- ceptions set forth in this chapter. Dr. Marr has, of late years, come to regard glacial erosion as of more importance than when he delivered this address, but many of his conclusions remain unaffected by this change of opinion, Tectonic Valleys, 3 fiords, and he includes Loch Linnhe and Loch Leven as characteristic examples of the class. His discussion is wide, and embraces the main topics dealt with in this chapter. In many instances his arguments and conclusions agree with those set forth below, but they wwe illustrated with a wealth of detail drawn from other lands, which gives them an additional value. In one important particular, Prof. Gregory’s treatment differs from that adopted here, for he attributes a more direct share to earth-movement in the shaping of Scotland, especially the West Highland coast-line. A similar interpretation has been advocated for years in connection with the Norwegian fiords by de Geer and other Scandinavian geologists. The earth-movements which Prof. Gregory appeals to are supposed to have accompanied the foundering of the North Atlantic Ocean in late Miocene or early Pliocene times. Some few de- pressions as the Minch, inside of the Outer Hebrides, and the Central Valley, south-east of the Highlands,* he attributes to trough- faulting. The fiords of the West Highlands he believes originated for the most part as gaping faults and joints which have been worn into troughs by subaerial and glacial erosion. According to Prof. Gregory not only did new faults originate, but old faults moved again. In this connection it is worth quoting from Dr. Horne’s f description of the Inverness earthquake of 1901. “ A long crack or fissure . . . formed in the middle of the towing-path (of the Caledonian Canal, near Dochgarroch Locks), and could be traced at intervals for a distance of 600 yds. In no place was the fissure more than half an inch wide. The position of the Great Glen fracture laid down on the six-inch field map coincides with the trend of this fissure.” Professor Gregory believes that the severance of the original drainage that crossed the line of Loch Linnhe (Plate IT.), was accom- plished by Tertiary subsidence on the north-west side of the Great Glen Fault; it must be admitted that the Highland Plateau north- west of Loch Linnhe is about 500 ft. lower than to the south-east, though the correspondence seems scarcely definite enough to base an argument upon. While it is impossible to do justice to Prof. Gregory’s position in a short notice, the following comments may be offered : (1) Prof. Gregory probably somewhat overrates the proportion of structure-guided valleys in the West Highlands. (2) The main features, apart from straightness, of the structure- guided valleys are reproduced in valleys independent of such guidance. (3) The shatter-helts which can be examined along the line of certain structure-guided valleys, as at the head of Loch Leven, seem quite sufficient in their nature to locate deep, straight valleys without assuming gaping fissures. (4) If such gaping fissures were very commonly developed during movements affecting the West Highlands in Middle Tertiary times, how isit that we never come upon any that have been choked with the gravel and sand of the period ? * Prof. Gregory, by mistake, ascribes this view to the writer: J. W. Gregory, ‘The Nature and Origin of Fiords,’ 1913, p. 175. + J. Horne, in ‘The Geology of the District round Beauly and Inverness,’ Mem. Geol. Survey, 1914, p. 69. 4 Physiography. (5) A similar conflict of opinion formerly existed concerning the origin of the Zambezi Gorge.* Some attributed its zig-zag course to a tectonic rent, but it is now admitted that the gorge is being ex- cavated along intersecting joints which lie latent, so to speak, until the adequate conditions of erosion present themselves. This agrees with the long-accepted interpretation of structure-guided valleys in the West Highlands. Through valleys of the second type, that is independent of structure, are well represented on both sides of Loch Linnhe: on the north-west lie Glen Scaddle, Glen Gour, and Glen Tarbert; on the south-east, Glen Nevis, the Lairigmér and River Leven valley, and Glen Coe. Two distinct interpretations have been adopted in regard to such through valleys in the Scottish Highlands. Cadell,t Mackinder,t and Peach and Horne§ regard them as representative of a drainage system which once led continuously across Scotland, more or less from west to east, but is now in large measure broken up into segments owing to the development of subsequent valleys along special lines of weakness. The late Professor Tarr,|| on the other hand, regarded them as a result of glacial erosion. The last-named author summarised his position as follows :— “The through valley condition, a great aid to travel in glaciated lands, is found in the Alps, in Alaska, in Central New York, and in the Scottish High- lands. It is as characteristic of glaciated lands as are the hanging valley, the steepened slope, and the ‘canal’ valley, and, so far as I am aware, is all but unknown in regions which glaciers have never occupied.” This appeal from glaciated to unglaciated topography, to ascertain such features as have resulted from glacial erosion, is a most encouraging sign of the times. A like comparison was employed many years ago by Ramsay J in relation to rock basins; the extension of the principle, and its detailed application is, however, mainly due to American geographers such as Davis** and Tarr. But when attention is paid to the through valleys of Scotland as a whole, their distribution seems too systematic to admit of a semi-accidental origin connected with glaciation, The evidence points strongly to the alternative conclusion, namely that the majority of them are remnants of a once continuous drainage system. In Plate II. ex- pression is given to this alternative interpretation. It may be urged by some that the correspondence of the through valleys on the north-west with those on the south-east of Loch Linnhe, as * GW. Lamplugh, ‘The Geology of the Zambezi Basin around the Botoka Gorge (Rhodesia),’ Quart. Jowrn. Geol. Soc., 1907, vol, lxiii. p. 165. H. a ‘The Dumbartonshire Highlands,’ Scot. Geog. Mag., 1886, vol. ii, p. 337. } H. J. Mackinder, ‘ Britain and the British Seas,’ 1907, p. 126. § B. N. Peach and J. Horne, in ‘Report on the Scientific Results of the Bathymetrical Survey of the Scottish Fresh-Water Lochs’ (Murray and Pullar) 1910, vol. i. p. 457. | R. S. Tarr, ‘Glacial Erosion in the Scottish Highlands, Scot. @ 1908, vol. xxiv. p. 580, Be ect eens q A.C. Ramsay, ‘On the Glacial Origin of certain Lakes in Switzerland, the Black Forest, Great Britain, Sweden, North America, and elsewhere,’ Quart. Journ. Geol. Soc., 1862, vol. xviii. p. 201. : ** W, M. Davis, ‘Glacial Erosion in North Wales,’ Quart. Journ. Geol. Soc. 1909, vol. Ixv.p. 281, ’ Corroms. 5 shown in this figure, is fanciful; but the same principle is involved in regard to the former connection of the Lairigmor and River Leven valleys, now separated by the valley of Loch Leven, and is for this particular case self-evident. Once a river system has been broken up, following the develop- ment of subsequent streams along such lines of weakness as the Loch Linnhe and Loch Leven shatter-belts, secondary watersheds must originate as a matter of course in the isolated segments. According to the view adopted here, the low cols of the through valleys of Scotland are, for the most part, such secondary watersheds in the process of development, rather than primary watersheds which have been all but obliterated by glacial erosion. That glacial erosion has, however, modified these cols is in some cases almost certain. Thus a powerful lateral stream which issues from the north directly upon the col of Glen Tarbert hangs distinctly above this col, while at the same time the main valley where it crosses the col is broad and open in its cross section. This combination of characters seems to necessitate a belief in powerful glacial erosion on the site of the col; in fact, vigorous erosion is exactly what one might expect in such a situation, and Mr. Wright * has emphasised its importance in relation to certain flat-bottomed, high-level passes trenching spurs of the Binnein Mor massif north of Kinlochleven. The through valleys near their cols have, as a rule, very low gradients. This is admirably illustrated in Glen Coe and in the long upper reaches of the river Leven—now occupied by the Black- water Reservoir supplying power to the Kinlochleven Aluminium Works (Plate III. p. 8). It would, therefore, appear that the rivers of the original drainage system had approached base-level before they were beheaded. Such a condition clearly favours the formation of long obsequent streams in valleys subsequently broken up into isolated segments. A well-graded trunk valley, once it has lost a considerable proportion of its water through beheading, cannot possibly keep its course clear. One deltaic cone after another, built by tributary streams, obstructs the channel, and serves in succession as its corrom or delta-watershed. Supposing the tributary streams all of equal importance, the first stream beneath the breach, by which the main valley has been beheaded, builds the first corrom. The resultant self-imposed diversion, although intermittent, probably gives this stream increased erosive power, by putting it into com- munication with the beheading river system, and thus enables it to keep in check the further growth of its deltaic cone. But the second stream, sheltered, as it were, behind the first-formed corrom, now carries on its work of deposit with increased vigour until eventually it establishes a second corrom to supersede the first. And so the position of the corrom is shifted farther and farther along, in what was originally the downstream course of the main valley, until a position of equilibrium is reached—that is, until the remnant of the consequent drainage is truly balanced by the successive additions made to the obsequent stream. The word corrom, which has been used above to signify a delta- watershed, is based on the Gaelic cothrom, a balance. The stream * W. B. Wright, in ‘Summary of Progress for 1907,’ Mem. Geol. Survey, 1908, p. 63. See also p, 220 of this Memoir, 6 Physiography. already mentioned, which issues upon the col of Glen Tarbert, is known as Allt a’ Chothruim (the stream of the balance) since, at its point of entry, it has built up a great cone, upon which it is balanced, as it were, with the possibility of flowing either east or west. The name was borrowed from this Glen Tarbert example to describe delta-watersheds in general, once the physiographical importance of these structures was recognised by Professor Kendall.* The divide at the head of Glen Nevis isacorrom. Asa matter of fact the present-day corroms of Glen Tarbert, Glen Nevis, and other valleys of this district have only a trifling importance in fixing the positions of the various watersheds, but they may not unreasonably be taken as symbolical of pre-glacial corroms of much greater significance. In many through valleys of the Highlands the modern water- shed is not a corrom. In some cases this may have resulted from the shifting of the position of the watershed by glacial erosion, but it is not unlikely that in other instances landslips helped, in pre- glacial times, to determine the divides between the consequent and obsequent drainage of the segmented rivers. The watershed of the Lairigmor valley is situated at the foot of Stob Ban, the bulging flanks of which are constituted of landslip material (Plate IIL, 2, p. 8). The slipped debris does not in this mstance reach down to the valley floor, but it is easy to realise that a similar great fall of rock in another instance might well turn the drainage of a beheaded river. A striking example of a landslip reaching right across a stream, and more or less blocking its course, is afforded in the valley of Allt Coire Gabhail, on the south side of Glen Coe. A flat strath has been built up behind the landslip, but there has not been any chance of reversal of drainage in this case, since the valley here is steeply bounded and has not been beheaded. Segmentation of a through valley system, such as has been out- lined above, may well rejuvenate a mature river system by a process of short-circuiting. This conception leads us to inquire into the origin of the hanging valleys of the district. First, it may be pointed out, that the hanging-valley system shows evident trace of glacial action. A fine illustration of this is afforded by a hanging valley, or corrie, draining into Glen Nevis, 14 mile east of Polldubh (Plate I.). A powerful stream issues from this corrie and races to the bottom of Glen Nevis, 1000 ft. below. From the road we see the tributary stream standing out as a long ribbon of foaming water, for it flows down the untrenched slope of the main valley. The latter at this point is of U-shaped cross section, unhampered by projecting spurs; it is the “canal” type of valley, as defined by the late Professor Tarr. Aqueous and subaerial erosion could not have been so selective as to carve this gigantic hollow for the main river, without furnishing a gorge, however small, for the independent accommodation of the powerful tributary stream. Another some- what similar example of a hanging valley is afforded by the Allt Coire a’ Mhail which cascades down an unnotched rock face at Steall farther up Glen Nevis (Plate XIL, 1, p. 220). , Turning to a side issue we may mention that this Steall water- * P. F. Kendall and E. B. Bailey, ‘The Glaciation of East Lothian south of the Garleton Hills,’ Trans, Roy. Soc, Hdin., 1908, vol. xlvi. p. 1. Glen Nevis. 7 fall affords an interesting illustration of how erosion is sometimes actually hindered by steepness of grade. At the foot of the fill one can pick up rounded pebbles smashed in two. They have been brought to the edge of the fall and hurled down without an oppor- tunity of doing effective work. Above the waterfall the gradient is much less pronounced, and the pebbles, retained in pot-holes, have succeeded in lowering the bed of the stream, some 30 ft. in places, since the glaciers retired.* The fall, it is true, is over quartzite, and the pot-holes above are cut in hard mica-schist, so that the comparison which has just been made must not be emphasised too strongly. The important point, to return to the question of glacial erosion, is that the Steall waterfall, where it passes over the quartzite, has no bounding walls, although the rock on either side is a continuation of the same quartzite as is washed by the descending waters. It seems incredible that this condition could have been fully developed with- out glacial intervention. But inquiry cannot cease here with the mere recognition of ice- work in the development of the present-day hanging-valley system. Scottish geologists are well nigh unanimous in regarding glacial erosion as responsible for the majority of the rock basins of the Highlands.f It is unnecessary, therefore, to point out that on this assumption many tributary valleys in one part or another of the country must have been left hanging through glacial over-deepening of corresponding trunk valleys. It is well, however, to consider each case on its merits, and, accordingly, it is advisable to inquire whether glacial over-deepening can be called in to explain the hanging tributaries of Glen Nevis. This glen affords an imposing example of a L-shaped canal valley in the four miles of its course in which it drains north-westwards towards the open country near Fort William. Several minor hanging tributaries rush swiftly down its slopes from Ben Nevis. At Polldubh the glen turns so as to run east and west, and at the same time it is narrowed by a projecting spur of very hard contact-altered mica-schist. The valley thus con- stricted exhibits the V-shaped cross section of well-advanced river and subaerial erosion; it is therefore very difficult to believe that ice can have had any effective part in the erosion of the glen at this point. This inference is much strengthened by the V-shaped cross section of two powerful tributary valleys, Allt a’ Choire Dheirg and Allt Coire a’ Mhusgain, which enter Glen Nevis at Polldubh in normal adjustment with the level of the main river. Thus it appears that at Polldubh a pre-glacial section of Glen Nevis is preserved, and, as this does not hang above the canal portion of the valley farther downstream, it is a fair inference that the latter has not been over- deepened by glacial erosion, however much it may have been widened. Above Polldubh, Glen Nevis is again for a couple of miles * It must not be supposed that the gorge seen in the photograph above the fall is post-glacial. On the contrary, it is glaciated almost to the bottom, where pot-holes appear. / + No more convincing example can be given than Loch Coruisk, described by Dr. A. Harker, ‘Ice-Erosion in the Cuillin Hills, Skye,’ Trans. Roy. Soc. Hdin., 1901, vol. xl. p. 238. Many other authors might be cited, and the subject has been most completely dealt with by Dr. Peach and Dr. Horne in their report already referred to. 8 Physiography. of the “canal” type, and receives from the north the waters of the particularly fine hanging valley to which special attention has been drawn already. Then beyond a right-angled bend as we ascend the glen, the river for half a mile flows hurriedly through a* deep and narrow gorge cut in the base of a high-level gap which shows intense glacial erosion (Plate I.). The gorge itself cannot be of post-glacial origin, for it shows ice-moulding almost to the very bottom. It is at the same time far too narrow to be the direct work of glacial erosion. Thus here, as at Polldubh, it would seem that a pre-glacial section of Glen Nevis has escaped, with little modification—although, on the other hand, it must be admitted that no proof can be given that the gorge is not the work of a sub-glacial torrent.* : Altogether Glen Nevis affords fairly satisfactory evidence of the existence in this district of a hanging-valley system, which has not been determined by glacial over-deepening. This inference is cor- roborated by an inquiry into the drainage system of the river Leven. The upper reaches of the Leven valley are thoroughly mature, and belong to a widespread high-level valley topography, which includes in the clearest possible manner the hanging portions of the down- stream tributaries of the Leven itself. A little below the reservoir, which now occupies the site of the string of lochans shown in Plate IIL, 1, the river plunges from its high-level to its low-level valley in a series of waterfalls. The form of the entire low-level valley from the falls to the sea does not suggest glacial but aqueous and subaerial erosion. Allt a’ Choire Odhair-bhig and Allt a’ Choire Odhair- mbhoir, tributary streams which join the Leven less than a mile below the falls, have done very little to adjust themselves to the low-level valley; Allt na h-Eilde and Allt Coire Mhorair, which enter more than a mile farther downstream, have,’ however, eaten their way back, so that their hanging portions are well removed from the valley edge. The case of Allt na h-Kilde is particularly important. The lip of the hanging valley in this instance is cut back for a mile from the Leven. The stream tumbles over the lip in the form of a waterfall and reaches a low-level valley. The latter is graded with the Leven, and yet, so far as one can speak with confidence in such matters, it is obviously not the work of glacial erosion—although, at the same time, it has suffered a certain amount of glacial modification. Since’ then we may take it that this lower adjusted portion of the Allt na h-Eilde is pre-glacial, it follows that the deepening of the Leven valley is pre-glacial too—an inference which many would regard as justified on a consideration of the form of this valley taken by itself. Other instances might be added, all pointing in the same direction. The conclusion which the evidence favours is that the original drainage system of the district had reached a stage of maturity, only to be rejuvenated as a result of widespread beheading and resultant short-circuiting: the main valleys were cut back by waterfall and cataract action ; the tributary valleys made use of their opportunities as soon as presented; a hanging-valley system was thus developed, * Throughout this discussion the possibility of very extensive inter-glacial erosion is not dealt with; it is well to realise, however, that Prof. E, J. Garwood regards such erosion as of prime importance. See ‘Features of Alpine Scenery due to Glacial Protection,’ Geographical Journal, 1910, p. 310. PLATE III. 1, Upper hanging portion of River Leven, now submerged beneath Blackwater Reservoir, 2. Stop BAN, near watershed of Lairigmor Valley, showing comparatively recent landslip, Corries and Landslips. 9 such as occurs in the Zambezi district at the present tine;* then followed intense glaciation; in many cases trunk valleys were opened out, side spurs were truncated, and the walls of certain minor hanging tributaries totally obliterated. It appears, then, that the glaciation of this district has accentu- ated a hanging-valley system previously in existence. Concurrently it has often produced very obvious results in the modelling of the walls and bottoms of the various glens in addition to the removal of lateral spurs. Allt Coire an Eoin, flowing north-east from Aonach Beag of the Ben Nevis group, and Amhainn Coir’ an Iubhar, entering Glen Tarbert near Loch Linnhe, occupy valleys which are particularly well worthy of a visit in this connection. Their moutonné surfaces convey to even a casual observer a more vivid impression of the immense grinding power of ice than would reams of description. In such cases grinding has sometimes been associated to a quite obvious extent with plucking, the importance of which has been emphasised by American geologists. Plate XII., 2 (p. 220), shows a roche moutonnée by the roadside in Glen Nevis partially destroyed by plucking, which has removed a great block, thus interrupting the beautiful curving outline imparted to the surface by previous grinding; morainic drift is banked some little way up against the broken surface. At the head of Allt Coire an Eoin and Amhainn Coir’ an Iubhair, one enters upon truly magnificent corries, as fine as any in Scotland. Corries, indeed, are a characteristic feature of the physiography of this district in general, and accordingly we shall now pass on to consider how such great cliff-bound amphitheatres can have originated. There is a general consensus of opinion that corries are restricted in their typical development to glaciated lands, and are therefore of glacial origin. From the preceding discussion of through valleys and hanging valleys, for which, as we have seen, a similar claim is also advanced, it will be evident that this line of argument must be followed with great caution; still it seems to the writer that the case for the glacial origin of corries has been established. The only agency at present at work in the district which might be regarded as responsible for the production of corries is landslipping. Reference has already been made to big landslips on Stob Ban above the watershed of the Lairigmér valley (Plate IIL, 2, p. 8). There are many other important landslips, especially in the Coire na Ba glen, in the valley east of Binnein Mor, in the Allt Coire Rath glen, and on the two sides of the long ridge which separates Allt Coire Rath from Allt Coire an Koin. Behind the slipped material a hollow is left, which is often of semi-circular form and backed by steep crags. Mr. Clough, + in his description of the Cowal district of Argyllshire, has pointed out that hollows thus left by landslips often resemble corries; in fact, some of the larger landslip-hollows are actually small corries. He has further suggested that many of the crag features of Highland valleys may be due to pre-glacial landslipping followed by glacial removal of the fallen material. Such may well be the case. Where slipping occurs along the two sides of a straight ridge, *@. W. Lamplugh, ‘The Geology of the Zambezi Basin around the Batoka Gorge (Rhodesia),’ Quart. Journ. (eol. Soc., 1907, vol. lxili. p. 165. + C. T. Clough, ‘The Geology of Cowal,’ Mem. Geol. Survey, 1897, p. 276. 10 Physiography. the cracks limiting the slips are sometimes straight themselves, and determine the production of a more or less continuous “knife-edge” aréte, The best locality to convince any observer of this peculiarity, and of the considerable importance of landslips generally in shaping the physiography of some parts of this district, is the ridge summit separating Allt Coire Rath from Allt Coire an Eoin. Here one may examine not only innumerable slips but also vertical cracks which extend deep down into the solid rock in preparation for the fall of other great masses. In almost every instance the knife-edge character of the ridge has been maintained, but a little south of Stob Coire an Easain the divide has been breached; in this case a high-level col has been formed, and the slipped material, which has travelled to the west, has built a prominent little hill (“Meall Tionail ”), easily recognised even from a distance of several miles. Many of the landslips referred to above, have probably resulted from the instability of valley sides over-steepened by glacial erosion. Slipping in such cases is a step towards the re-establishment of gentler gradients, although its immediate result is often to produce cliffs and crags. Special conditions, tending to repeated landslipping, and the rapid transport of slipped material, are required for really successful corrie formation. Such conditions are apparently afforded at the bergschrund of a glacier—the crevasse, that is, which forms every summer between a glacier-head and the containing valley. The bergschrund is in many respects analogous to the landslip cracks which occur so abundantly in some parts of the present district. The tug of the glacier before the bergschrund forms will be a powerful incentive to landslipping on a large and small scale, while the glacier itself is admittedly an efficient carrier of slipped material. The importance of the bergschrund in connection with corrie formation was first suggested by Johnson.* In 1883 he descended a bergschrund at the back of a miniature glacier of the Sierra Nevada. The crevasse was about 150 ft. deep and for the most part was walled on both sides by ice. About 20 or 30 ft. from the bottom, the fissure reached the valley side, and from this point down the parting had taken place, not between ice and ice, but between ice and rock. Johnson found that the zone of bare rock thus exposed bore evidence of intense and repeated frost action. In fact, he regards this localised frost action as the main element in the problem of corrie formation, although he states that plucking likely supplements the initial rupturing due to the frost. We are, however, inclined to lay greater stress on the plucking, but this is a point which can only be settled by further investigation of present- day glaciers. According to another view elaborated by Dr. Harker + the bergschrund is not taken into consideration, and corrie formation is attributed to direct glacial grinding. “ Erosion,” he points out, “near the sources of a river is very feeble; a glacier, on the other hand, springs into being, like Athene, fully armed.” It may be noted that the corries in this district seem to favour a north-easterly aspect. On erealises this on looking north from Bidean nain Bian across country towards Ben Nevis. The coinci- *W. D. Johnson, ‘The Profile of Maturity in Alpine Glacial Erosion,’ Journ. Geol., 1904, vol. xii. p. 569. + ‘Notes on Subaerial Erosion in the Isle of Skye, Geol. Mag., 1899, p. 485, Rock Basins and Moraines. 11 dence is not sutticiently striking, however, to need special explanation were it not that many observers in various northern lands have noticed a similar orientation: The effects have been very reasonably ascribed to the conjoint influence of sun and wind upon the course of glaciation, for even now the snow lies long in shady hollows whither it has been drifted before the prevailing westerly gales; thus, in the great corrie of Ben Nevis, there are patches of snow which are seldom, if ever, entirely dissipated by the summer's heat. We may now pass on to notice a few instances of glacial over- deepening recorded in rock basins within the limits of the present district. Loch Eilde Mor and Loch Eilde Beag are probably, both of them, rock basins. They are situated on the shatter-belt which has determined the position of Loch Leven. Loch Eilde Mor is 100 ft. deep.* The shallow lochans in the flat upper reaches of the river Leven are, some of them, almost certainly rock basins. Loch Achtriochtan in Glen Coe is another example. Mr. Maufe has pointed out that this loch lies in soft phyllites behind a barrier of relatively harder rocks which here cross the glen on‘the down- stream side of a powerful fault; differential erosion of the softer material has given rise to the loch basin. Loch Coire na Creiche among the mountains on the other side of Loch Linnhe may also be cited as a clear example of a rock basin. Other rock basins might be mentioned, but the only ones of any size are those which probably exist submerged beneath the salt waters of Loch Linnhe and Loch Leven. Closed basins undoubtedly occur along the beds of these two lochs, as is clearly indicated by the soundings. Some of the complications of these submarine valleys are due to accumu- lations of gravel, as at the narrows of Corran, North Ballachulish, and Caolasnacon, but it is reasonably certain from analogy that others are due to rock barriers.f Enough has now been said of glacial erosion. Glacial accumu- lation also deserves attention as a formative element in the topo- graphy. The main glacial accumulations are of the nature of morainic debris, more or less restricted to the valleys. Well-shaped moraine ridges are rare, but a few occur, including a fine terminal crescent on the lip of the Allt Coire Giubhsachan valley, where the latter overhangs Glen Nevis. Most of the morainic drift has a hummocky surface. There is no more characteristic type of scenery than that afforded by hummocky drift where developed to perfection, as in the upper reaches of the river Leven valley and on the Moor of Rannoch at the head of Glen Coe. A few words may now be added in regard to certain post-glacial and late glacial changes in the physiography of the district. The summits of the high ridges were certainly exposed to frost action during late glacial times when the valley bottoms were still occupied by glaciers. This has contributed to render the results of frost conspicuous on many of the mountain tops. It is common, where the slope is gentle, to find the whole surface littered with slabs and fragments which have been prized loose by water freezing in joint fissures, but which have not travelled appreciably from their **Bathymetrical Survey of the Fresh-Water Lochs of Scotland,’ under direction of J. Murray and L. Pullar, vol. ii., 1910, p. 70. + J. Geikie, ‘The Great Ice Age,’ 1894, chap, xix. 12 Physiography. original source. The great domed surface of Ben Nevis affords a capital example of such an accumulation merging insensibly into genuine scree on the steeper slopes. On other mountain tops the debris covering, especially where it is composed of comparatively small fragments, has crept forward in a succession of steep-fronted, flat-topped waves or terraces. The movement appears to take place in times of storm, when rain is driving, and the whole surface is laden with water. In exposed situations the direction of movement, as indicated by the steep fronts of the terraces, is clearly determined rather by the direction of the prevalent gales than by that of the hill slope, so long as the inclination of the latter is not pronounced. In such cases any heather that manages to survive has assumed a prostrate habit and turned its head away from the blast. Patches of the surface are swept bare, and Lilliputian lochs are excavated with miniature storm-beaches to match. The turf in front of the advancing terraces of debris is turned up on end, broken and over- whelmed. Frosts and torrents have been busy, too, among the cliffs and crags, picking out every little shatter-belt, and thus dissecting the relatively smooth surface left by glaciation. Great gullies have been cleared, and debris sent hurtling down to build widespreading cones of scree on the lower slopes. This type of frost action is illustrated very finely in Glen Coe, where the scree actually encroaches upon the road in times of flood. One can see that this scree formation, like the large-scale slipping so pronounced in the district north of the Leven valley, is busily undoing the work of the glaciers, and tending to produce a topography more in harmony with existing conditions. While the ridges show signs of the severe treatment to which they have been exposed since they emerged from the glaciers, the stream-courses have also suffered to some slight extent. Many of the rivers have cut gorges a few feet deep, and it is interesting to see how their mode of attack varies with the nature of the rock. If, for instance, the stream is crossing quartzite, it quarries out fragment by fragment, making use of joint and bedding planes; if, on the other hand, mica-schist has to be removed, pot-holes are drilled, or a continuous channel is ground out with smooth flowing contours. Beautiful examples of breached pot-holes, forming arches through which the water plunges, may be seen from the road where Allt Nathrach runs down to join Loch Leven. But of all post-glacial changes, the most important from the human point of view is one that is limited to the coast-line. A more or less continuous terrace, often backed by low, rocky cliffs, fringes the shores of Loch Linnhe and Loch Leven, and carries most of the roads and railways of the district. It is a record of the activities of the sea at a time when the land stood some 30 ft. lower than at present. Fertile beach deposits, dating from this period, have determined the sites of the crofting villages of Onich North Ballachulish, and Clovulin. : We may bring this discussion of the physiography of the district to a close by considering the influence of rock structure, which is responsible for many conspicuous minor features. Most of the country is composed of crystalline schists of various kinds, each with a more or less characteristic type of weathering. Thus white quart- Glen Coe and Glen Etive. 13 zite, interfolded among darker mica-schist, is strongly developed in the district between Ben Nevis and Glen Coe, and gives rise to remarkable scenic effects (Plates V., VI., XII.). In Glen Coe itself, lavas of Old Red Sandstone age occur in striking contrast to the surrounding schists. Here the abrupt face of Aonach Dubh, built up of tiers of andesite flows, capped by rhyolite and agglomerate, at once attracts attention. The ridges which follow to the east, the two first of which, together with Aonach Dubh, constitute the Three Sisters of Glen Coe, are largely composed of rhyolite, with a peculiarly grand and massive type of scenery (Plate VII.), reaching its culmination in Stob Dearg (Plate VIII.). There are also several plutonic intrusions in the district with a typical amorphous outline well illustrated in the hills, formed of the Ballachulish Granite and Glen Scaddle Epidiorite, on the two sides of Loch Linnhe. In the southern portion of the map a small part of the Etive granitic complex is included. This complex is divided into an outer rim and an inner core. Mountains constituted of the granite of the inner core, the Ben Starav Granite, are altogether remarkable for the extreme bareness of their slopes, which recalls the nakedness of the Norwegian Highlands. E. B. B, CHAPTER II. HISTORICAL REVIEW. Tue HIGHLAND SCHISTS. In 1810 MacKnight* traversed a considerable portion of the Western and Central Highlands of Scotland, and visited many points of interest in the district dealt with in this Memoir. He viewed all geological phenomena from the standpoint of Wernerian doctrine, and found no evidence in favour of Hutton’s theories regarding the metamorphism of the schists or the igneous origin of granite. Thus he thought that the schistose strata had been precipitated from a primitive ocean, and that their quartz veins, high dips, crinkling, and contortion were all developed during the process of accumulation. He expresses keen satisfaction in having been able to trace a succession from the clay-state of Ballachulish, through the mica-slate of the Fort William shore of Loch Linnhe and the gneisses of Ardgour, to the granite of the western end of Glen Tarbert. This he evidently regarded as a normal downward sequence of deposit ; in Glen Coe, however, and Ben Nevis, he found a development of granite, and other rocks now classed as igneous, which he assigned to an overlying formation of later date than the surrounding schists. MacKnight also recorded the association of limestone with the slates at Ballachulish, and the occurrence of both granular quartz and limestone at Onich. Macculloch’s writings at once strike a different note. He showed that the quartzites + of Jura, Ballachulish, and Assynt are, in large measure at any rate, true mechanical deposits, since they contain worn and blunted pebbles of quartz and felspar. He further drew attention to various sections in which the Jura and Ballachulish quartzites are overlain by, or interbedded with, clay-slates and mica- slates; and on the basis of this association he argued for the sedimentary nature of the slates. Previously, of course, both slates and quartzite (granular quartz) had often been regarded as crystalline precipitates from Werner’s primitive ocean. Incidentally, in the presentation of his evidence, Macculloch has given an excellent account of the transition zone linking the quartzite of Ballachulish, that is the Appin Quartzite, with the _ eT ite Late ed ‘On the Mineralogy and Local Scenery of Certain Districts in le s of Scotland, Mem. Wernerian Nat. Hist. Soc., 1811., vol. i. pp. 307-357. + J. Macculloch, ‘ Remarks on Several Parts of Scotland which exhibit Quartz Rock, and on the Nature and Connection of this Rock in general,’ Trans. Geol. Soc,, 1814, vol. i1, pp. 450-487, 4 Schists. 15 well-known Ballachulish black slates (op. cit., p. 483). We quote the following :— _. 33. Fine sandstones, not to be distinguished from the floetz sandstones, and, like many of them, striped in endless alternations by black clay. From the series at Ballachulish. These belong to the quartz rock, which alternates with clay slate, and show the transitions between these two substances, The term “ floetz sandstones” used in this description merely signifies normal sandstones of such formations as the Carboniferous System. Later, Macculloch* drew attention to the fact that the Balla- chulish granite metamorphoses the schists in its vicinity, and sends veins into them, and, further, that it contains innumerable fragments of these schists as inclusions. But Wernerian theories still dominated MacKnight,+} who, returning to the district, described the contact-altered rocks round the Ballachulish granite as gneisses occupying their normal position in his imaginary granite, gneiss, mica-slate, clay-slate succession. MacKnight now regarded the Ben Nevis granite as part of a great underlying mass, forming the foundation of the Scottish Highlands ; the rocks of the upper portion of the mountain (the lavas) he correlated as before with the Glen Coe complex, and interpreted them as an overlying formation. Both Macculloch t and MacKnight refer to the limestone of Lismore, which extends into the southern edge of the present map, and is continued northwards into Shuna. Macculloch nightly points out the resemblance of this limestone to that of Islay. Five years after his death, in 1835, Macculloch’s geological map of Scotland § was published. In this he indicated in a generalised manner the positions of many of the limestone, quartzite, and clay-slate out- crops of the district, especially in the neighbourhood of Loch Leven and Portnacroish (Appin Station). The igneous origin of the Ben Nevis complex had _ been confidently denied by MacKnight. Von Oeynhausen and von Dechen,|| however, pointed out that the granite has forced its way through the surrounding gneiss and schists, aud that it extends laterally into these rocks in the form of veins. It is interesting to find them referring in their descriptions of Glen Nevis to “a rock composed of alternate lamin of white felspar and green mica” which occurs, for a considerable distance, bordering the vranite margin. This curious rock has since been shown by Mr. Grant Wilson and Dr. Teall to be a cale-silicate-hornfels produced by thermal metamorphism from a thick mass of schistose limestone; but * J. Macculloch, ‘ Observations on the Mountain Cruachan in Argyllshire, with some Remarks on the Surrounding Country,’ Trans. Geol. Soc., 1817, vol. iv. p. 126. + T. MacKnight, ‘Mineralogical Notices and Observations,’ Mem. IVerneriun Soc., 1821, vol. iii. p. 113. t J. Macculloch, ‘A Description of the Western Isles of Scotland, including the Isle of Man,’ 1819, vol. i. p. 265. § J. Macculloch, ‘A Geological Map of Scotland, 1840. | C. von Oeynhausen and H. von Dechen, ‘Der Bein Nevis am Loch Eil,’ Karsten’s Archiv fiir Mineralogie, Geognosie, Bergbau und fHuttenkunde, Band IL, 1830, p. 38; and ‘ Observations on the Mountain Ben Nevis, and on some other places in Scotland, Proc. Geol. Soc., 1834, vol, i. pp. 94-96. 16 History of Research, for many years its nature was not understood, and in the earlier maps of the district it is always classed as “ gneiss.” Von Oeynhausen and von Dechen also drew attention to the grey, granular limestone of the Spean section, which is situated not far north of the limit of the district at present under discussion. r Nicol* recognised, probably from the summit of Ben Nevis, that quartzite plays an important réle in the geology of the country lying between Glen Nevis and Glen Coe. He expressed the opinion that the quartzite here forms the tops of many of the hills, and we find this theory of an overlying quartzite constantly recurring in his later writings. Thus he groups together the quartzites of Islay, Jura, and Loch Linnhe, and maintains that they “cannot be older than the Lower Silurian Period ” t and that they are associated with mica-slate upon which they appear to rest. On reading paragraphs f and g of Nicol’s note, there can be no doubt that he correlated the quartzites mentioned above with those of the North-West Highlands. Murchison { adopted similar views in that he provisionally classed the quartzites and limestones of the Loch Linnhe district with the fossiliferous deposits (now known to be Cambrian) of north-west Sutherlandshire. Later he elaborated this interpretation in conjunc- tion with Sir Archibald Geikie.§ Finding opposing dips on the two sides of Loch Linnhe near Fort William, they postulated an anticline along the line of the loch, bringing up the Sutherlandshire series from beneath the general gneiss and schists of the Highlands. It is strange to find that Nicol’s hypothesis of an overlying quartzite is here turned upside down without involving any change in his correlations. It is now acknowledged, however, that it was quite impossible, either for Nicol or for his successors, to arrive at any true conception of the structure of this district as a result of brief visits. This latter-day caution is, in fact, foreshadowed in Murchison and Geikie’s own paper, for in it they insist on the presence of isoclinal folds as an important factor in the structure of the Highlands (op. ctt., pp. 201, 203, Figs. 17, 18). It must be added that the correlation put forward both by Nicol and Murchison of the Loch Linnhe quartzites with those of Sutherlandshire carries very little weight at the present day. In this matter also the need for caution has been increasingly recognised. Among other points of interest we find the Ballachulish slates correlated with those of Kasdale—a correlation which is still regarded as probable, while “the general interlacing of the quartzose and the schistose series” in the Loch Leven district is noted as important (op. cit., p. 214). Harkness j| followed with a paper inspired by Murchison’s generalization regarding the age and position of the Highland * J. Nicol, ‘Guide to the Geology of Scotland,’ 1844, p. 165. + Note explanatory of Nicol’s Geological Map of Scotland, 1858, p. 5. 1 R. I. Murchison, ‘First Sketch of a New Geological Map of the North of Scotland, Quart. Journ. Geol. Soc., 1859, vol. xv. p. 420.* § R. I. Murchison and A. Geikie, ‘On the Altered Rocks of the Western Islands of Scotland, and the North-Western and Central Highlands,’ Quart. Jowrn. Geol. Soc., 1861, vol. xvii, pp. 205-215. || R. Harkness, ‘On the Rocks of Portions of the Highlands of Scotland south of the Caledonian Canal ; and their Equivalents in the North of Ireland,’ Quart, Jowrn, Geol. Soc., 1861, vol. xvii. pp. 265-268. Schists. 17 schists. He draws a section across the Ardsheal peninsula, north of Cuil Bay, showing schist (Cuil Bay Slates and Appin Phyllites) lying in a syncline above quartz-rock (Appin Quartzite) with locally a band of limestone (Appin Limestone) intervening (op. cit., Fig. 8, p. 266). It is probably no more than a coincidence that this interpretation agrees with the views at present in vogue, for it is doubtful whether there is sufficient local evidence to indicate the synclinal structure of this peninsula. The last paper to be noticed is by Nicol.* He regarded the clay- slate, limestone, and quartzite of Appin and Ballachulish as uncon- formable to the mica-slate, and as probably belonging to an overlying formation. In bad weather he visited sections in Glen Nevis, and these again led him to suspect the presence of an unconformable overlying quartzite (op. cit., pp. 205-206). It may be mentioned here that long afterwards Dr. Peach and the late Mr, Grant Wilson considered the pebbly quartzite of Appin and Ballachulish as unconformable even to the Ballachulish Slates accompanying it. This view has, however, been relinquished owing in no small measure to the presence of the passage zone already described by Macculloch. The quartzite which Nicol saw in Glen Nevis is on quite a different horizon from the Appin Quartzite, but is equally conformable in its relations. Apart from this question, Nicol gives an interesting account of the schistose beds—partly “clay-slate,” partly “ mica-slate ” —which intervene between Loch Leven and the southern edge of the map (op. cit., p. 202). He draws special attention to the frequent cross-foliation of these rocks, and, led astray by definite segregation of quartzose material along cleavage planes in the more micaceous layers, he concludes that the foliation must have originated under the same conditions as the bedding. He points also to the con- vergence of dips between Appin and Glen Creran as evidence of a syncline, For an outline of the history of research since the Geological Survey began mapping the district, the reader is referred to the various “Annual Reports” and “Summaries of Progress” published by the Survey in the interval (see Bibliography). A condensed account of this history has also been given in a recent paper dealing with the structure of the district.t Since the appearance of this paper various modifications have been introduced, mainly as a result of criticisms advanced by Mr. R. G. Carruthers—who has continued the mapping of the complex district lying north-east of Kinlochleven. The nature and extent of these modifications will be found set forth in Chapters IV. and V. of this Memoir. Detailed references to the work done on the contact alteration of the schists in the neighbourhood of the various plutonic rocks of the district will be found on pp. 187-203, E, B. B, THE IGNEous Rocks. Ben Nevis——As will have appeared already from various state- ments in the preceding paragraphs, Ben Nevis early attracted the * J. Nicol, ‘On the Geological Structure of the Southern Grampians,’ Quart. Journ. Geol. Soc., 1863, vol. xix. p. 180. ; + E. B. Bailey, ‘Recumbent Folds in the Schists of the Scottish Highlands,’ Quart. Journ. Geol. Soc., 1910, vol. Ixvi. pp. 586-620, 2 18 History of Research. attention of geologists; this was partly because of its outstanding height, but more especially on account of its unique structure. Williams * in 1810, and MacKnight f in 1811, described the main features in its geology. The flat country at its foot is underlain by gneisses and schists; the lower half of the mountain itself is formed of granite, whilst the upper portion consists of a mass of “ felspar- porphyry,” which is now known to represent a succession of lavas and agglomerates. Both the above-mentioned writers believed that the “porphyry” overlay the granite, and that the latter in turn overlay the schists. Ami Boué{ and Thomson § made the same deductions from their observations. On the other hand von Oeynhausen and von Dechen,|| writing in 1830, held that the granite was intruded into the gneisses and schists, and the “ porphyry” of the summit injected into the granite. These conclusions were endorsed by Macculloch,4 Nicol,** Murchison, and Geikie,t + and received general acceptance for a number of years. Von Oeynhausen and von Dechen observed the main mass of the granite rising up through and truncating the edges of the Highland schists, They described the variations in composition and texture within the granite itself, noting especially the grey basic facies on the northern margin, which they called “syenite,” and the fine-grained porphyritic. modification found in contact with the summit porphyry. With respect to the last-named rock mass they commented on the wide occurrence of the brecciated structure so well displayed on weathered surfaces, but erroneously concluded that the whole constituted a single intrusion. It was always recognised that the junction of the granite and “porphyry ” of the summit was an important one. Williams thought that the “porphyry” was a distinct stratification overlying the granite, and so, too, apparently did Rhind.t}{ MacKnight thought that the junction was horizontal, but considered that there was a transition between the two rocks. Von Oeynhausen and von Dechen, by follow- ing the complete circuit of the “ porphyry ” showed that the junction was everywhere vertical. This is clearly the case, and is demon- strable by a study of the course of the junction-line on a contoured map, or of actual sections in the field, in particular the sections exposed on the north-west shoulder of the hill, and on the ridge which connects the summit of Ben Nevis with Carn Mor Dearg. * J. Williams, ‘The Natural History of the Mineral Kingdom,’ 1810, vol. i. (2nd ed.) pp. 409 and 469. + T. MacKnight, ‘On the Mineralogy and Local Scenery of Certain Districts in the Highlands of Scotland,” Mem. Wernerian Soc. Nat. Hist. 1811, vol. i. . 322-357. BE t Ami Boué, ‘ Essai géologique sur ’Reosse’ (circ. 1820), p. 66. § T. ga ‘Outlines of Mineralogy, Geology, and Mineral Analysis,’ 1836, p. 208. ; || Von Oeynhausen and von Dechen, ‘Der Bein Nevis am Loch Eil,’ Karsten’s Archiv fiir Mineralogie, etc., Band II., 1880, pp. 38-54. { J. Macculloch, ‘Observations on the Mountain Cruachan in Argyllshire with a cae on the Surrounding Country,’ Trans. Geol. Soc., 1817, vol. iv. pp. 117-138. Ee J. Nicol, ‘Guide to the Geology of Scotland,’ 1844, pp. 161-165. +t R. I. Murchison and A. Geikie, ‘First Sketch of a New Geological Map of Scotland, with Explanatory Notes,’ 1862. tt W. Rhind, ‘The Geology of Scotland and its Islands,’ 1842, p. 82. Ben Nevis and Glen Coe. 19: Bryce * also followed closely the junction-line between lavas (“ greenstone”) and granite, and, although he did not rightly appreciate the relations of the two rocks, he “noticed in places portions of siliceous slate, sometimes of conglomerate structure entangled in the greenstone, or dividing it from the granite.” He probably refers here. to the strip of basal breccia and underlying schist exposed between the lavas and the granite in the corrie north-east of the summit. In 1874 Professor Judd+ made an important advance, and embodied a description of the geological structure of Ben Nevis in his account of the Newer Paleozoic Volcanoes of the Highlands. He saw that the central “porphyry” is composed of a succession of lava sheets, frequently separated by masses of agglomerate, and that it is veined by the granite; the structural relations of the latter he did not clearly understand in that he thought it underlay the voleanic rocks. He realised, however, its steep outer junction against the schists. Dr. Teall,t in 1888, described specimens of the lavas, and showed that they are hornblende-andesites; at the time no other good example of hornblende-andesite was known in Britain, though now, of course, the type has been widely recognised. Much more recently Dr. Mackie § has given an account of a specimen of fine-grained agglomerate from the hill. The results obtained during the course of the Geological Survey of Ben Nevis have, for the most part, been published already in an Appendix to the Summary of Progress of the Geological Survey for 1909. || Glen Coe.—Most of the geologists who have examined Ben Nevis have also visited Glen Coe. Macculloch,] MacKnight,** Boud,tt and Nicol {{ have all given brief accounts of their observations. They recognised the existence of crystalline schists and gneisses, granite or syenite, dykes of “felspar-porphyry,” and masses of “compact felspar” and “hornstone,” but the complex relations of these rock masses were scarcely, if at all, understood. At a considerably later date Stevenson §§ gave a fairly accurate account of the phenomena of permeation which may be observed in * J. Bryce, ‘On the Geological Structure of the Ben Nevis Group of Mountains,’ Proc. Phil. Soc. Glasgow, 1864, vol. v. p. 105. tJ. W. Judd, ‘The Secondary Rocks of Scotland. Second Paper, On the Ancient Volcanoes of the Highlands and the Relations of their Products to the Mesozoic Strata,’ Quart. Journ. Geol. Soc., 1874, vol. xxx. pp. 291-294, tJ. J. H. Teall, ‘ British Petrography,’ 1888, p. 287. § W. Mackie, ‘The Occurrence of Volcanic Tuffs on Ben Nevis,’ Trans. Edin. Geol. Soc., 1907, vol. ix. pp. 69-72. . || H. B. Maufe, ‘The Geological Structure of Ben Nevis,’ Summary of Pro- gress for 1909 (1910), Mem. Geol. Survey, pp. 80-88. . i { J. Macculloch, ‘Observations on the Mountain Cruachan in Argyllshire, with some Remarks on the Surrounding Country,’ Trans. Geol. Soc., 1817, vol. iv. pp. 132-135. : a T. MacKnight, ‘On the Mineralogy and Local Scenery of Certain Districts in the Highlands of Scotland, Mem. Wernerian Nat. Hist. Soc., 1811, vol. i. pp. 311-318, +t A. Boué, ‘ Essai géologique sur 1ficosse’ (circ. 1820), p. 67. tt J. Nicol, ‘Guide to the Geology of Scotland,’ 1844, pp. 159-161. §§ W. Stevenson, ‘On the Origin of Granite,’ Proc, Roy. Phys. Soc, Hdin., 1867, yol, iii. p. 167, , % 20 . History of Research. Glen Coe. “Near the granite and porphyry,” he writes, “ where the metamorphism becomes extreme, flesh-coloured felspar is added, chiefly between the slaty and quartzose laminz.” The value of this description is much reduced by the extravagance of some of the accompanying speculations. In 1874 Prof. Judd* included a short description of Glen Coe in his account of the Newer Paleozoic Volcanoes of the Western Highlands. He recognised a great series of “ felstone ” lavas, under- lain partly by schists and partly by granite, and remarked on the close similarity in petrological characters between the Glen Coe lavas and those which make up larger areas in Lorne and the Ochil and Cheviot Hills. He also called attention to the multitude of dykes traversing schists, granites, and lavas alike. Murchison and Geikie in their 1861 map of Scotland had coloured the Lorne “traps” as of Old Red Sandstone age, thus endorsing an opinion expressed long previously by Thomson.t Judd, as we have seen, was more cautious, and, of course, Sir Archibald Geikie was not able for many years to take up a positive position. “No certain proof,” he writes, in 1897, in his ‘‘ Ancient Volcanoes,” “ has been noted that they belong to the Lower Old Red Sandstone.” For Geikie,t the lavas of the Glen Coe district were but “picturesque outliers” of the Lorne series. The finding of fossils of Lower Old Red Sandstone age § in Lorne by Mr. Macconochie, and in Glen Coe by Mr. Tait and Dr. Peach,|| has now settled the point in a most satisfactory manner. The account just given, bringing us down to the work of the Geological Survey, is taken from the historical portion of a recently published paper dealing with the geology of the Glen Coe volcanic rocks and their associated intrusions.§]/ For further details the reader is referred to the original paper. H. B. M. ‘ GLACIATION AND RECENT DEposIts. Various other subjects of research have been pursued from time to time within the limits of the district. Reference to the biblio- graphy of this Memoir will show that the Boulder Committee of the Royal Society of Edinburgh from 1878 to 1884, under the convenor- ship of David Milne Home, issued several reports, which touched upon its glaciology. It is interesting to find that kentallenite and augite-diorite boulders in Glen Creran, and along the shore south from Kentallen, early attracted attention. At the time there was a * J. W. Judd, ‘On the Secondary Rocks of Scotland. Second Paper, On the Ancient Volcanoes of the Highlands, and the Relations of their Products to ae ae Strata,’ oe Journ. Geol. Soc., 1874, vol. xxx. PP. 294-295. - Thomson, ‘Outlines of Mineralogy, Geol d Mi is,’ 1836, vol. it p 184 gy, Geology, an ineral Analysis, : t Sir = Geikie, ‘Presidential Address,’ Quart. Journ. Geol. Soc., 1892, vol. xlvili, p. 95. ase aoe ‘The Old Red Sandstone of Lorne,’ Nature, 1897, vol. lvi. pp. 10%, lov. || B. N. Peach and H. Kynaston in ‘Summary of P ? for 1902 Geol. Survey, 1903, pp. 79, 130. ONE ee 4 C. T. Clough, H. B. Maufe, and E. B. Bailey, ‘The Cauldron-Subsidence of Glen Coe, and the Associated Igneous Phenomena,’ Quart. Journ. Geol, Soc. 1909, vol. lxv. pp. 611-676, ‘ Kentallenite Erratics. 21 great tendency to look for a westerly or north-westerly source for all erratics, and accordingly specimens of the “black granite” boulders were dispatched to Prof. Judd for identification with the gabbros of Mull; it is not surprising considering the date, 1877 or 1878, that Prof. Judd considered it “certain they were derived from the Western Isles.” On general grounds this conclusion was, however, immediately disputed by Prof. J. Geikie,* who stated that he had never seen Hebridean erratics upon the mainland of Scotland. In fact, local evidence might have been adduced to throw doubt upon the sugges- tion, for, thirty years before, Maclaren } had illustrated a masterly account of the glaciation of Scotland by a detailed description of certain conspicuous roches moutonnées at Ballachulish, and had shown, it now seems conclusively, that these rocks had been moulded by land ice moving from east to west, Heddle { strengthened James Geikie’s position immensely since, in the following year, 1879, he found kentallenite or, as he described it, a rock “identical in composition with the boulders,” occurring in situ above the path which leads between Glen Creran and Balla- chulish, a couple of miles north of Salachail. In spite of this discovery, however, the idea that the “black granite” boulders came from Mull was not finally relinquished for years. It is now known, of course, thanks to the mapping of the late Mr. Grant Wilson and the petrographical descriptions of Teall, Hill, and Kynaston,§ that “black granite” (kentallenite and augite-diorite) is represented by several outcrops in the district, and that it has an individuality of composition which entirely distinguishes it from the Tertiary gabbros. This later work is dealt with in Chapter XIII. ; it is scarcely necessary to add that through it kentallenite has become a familiar rock type to petrographers in all parts of the world. It may be stated that although ice-rafts are frequently invoked by the reporters of the Boulder Committee, many of the phenomena of transport were correctly: attributed to local glaciers, especially in the later reports. Colin Livingston of Fort William and the late Prof. Heddle were both particularly clear on this point. While dealing with the subject of erratics, it is convenient to direct attention to a pebble found by Mr. Currie|| in far-away Jona, since he claims that it has been transported by ice from Glen Coe. One of the andesite lavas of Glen Coe contains a beautiful red epidote first found by Witham in 1824 and described, under the name “ withamite,” by Brewster J in the following year. The Iona erratic is said to be identical with this Glen Coe lava, both in its general * J. Geikie, ‘On the Glacial Phenomena of the Long Island or Outer Hebrides,’ Second Paper, Quart. Journ. Geol. Soc., 1878, vol. xxxiv. ve 867. + ©. Maclaren, ‘On Grooved and Striated Rocks in the Middle Region of Scotland,’ Edin. New. Phil. Journ., 1849, vol. xlvii. pp. 169-172. tM. F. Heddle, in ‘Sixth Report of the Boulder Committee,’ Proc. Roy. Svc. Edin., 1880, vol. x. p. 630. ; § J. B. Hill and H. Kynaston, ‘On Kentallenite, and its Relations to other Igneous Rocks in Argyllshire, Quart. Journ. Ceol. Soc., 1900, vol. lvi. pp. 581-540, | J. Currie, jun., ‘On an Iona Erratic containing Withamite,’ Trans. Hd. Geol. Soc., 1899, vol. vii. pp. 115-118. ; ; { D. Brewster, ‘Description of Withamite, a New Mineral Species found in Glen Coe,’ Edin. Journ. Sct., 1825, vol. ii. pp. 218-221. 22 History of Research. characters and in its containing ‘a considerable quantity of withamite. It is quite probable from the direction of ice flow that Mr. Currie is right in deriving this pebble from Glen Coe, sixty miles distant, but it also seems possible that it may have come from some unknown source in Mull; this possibility is suggested owing to Mr. Harker’s * description of rosy epidotes which “may be termed withamite” occurring in some of the lavas of Skye. ; An analysis of the Glen Coe withamite has been given by Thomson,f and another, and much more complete one, by Heddle.f The raised beaches of the district have not attracted very much notice. Chambers§ has referred to them, and has figured the problematical terraces of Corran (wrongly called Connel in his description). .. Gwynn Jeffreys || has also published an exceedingly valuable list of fifty-nine marine species of Scandinavian facies from a bed found by Captain Bedford at Fort William. The significance of these shells, and of certain mosses found by Messrs. Wright and Maufe beneath the 25-ft. beach of the same locality, and described by Mr. Dixon,{] will be discussed in Chapter XVIII. Although not rich in lakes the district has shared in the advantages of the great Scottish fresh-water loch survey, carried through under the direction of Sir John Murray and Mr. Laurence Pullar during the years 1897 to 1909. The results have been published in collected form (1910), and a reference to the bibliography of this Memoir will enable. the reader to find details regarding the four small lochs which have been sounded in Sheet 53. E. B. B. * A. Harker, ‘Tertiary Igneous Rocks of Skye,’ Mem. Geol. Survey, 1904, p. 52. A T. Thomson, ‘ Outlines of Mineralogy, Geology, and Mineral Analysis,’ 1836, vol. i. p. 377. t M. F. Heddle, ‘ Chapters on the Mineralogy of Scotland,’ (chap. viii. Silicates), Trans. Roy. Soc, Ed., 1900, vol. xxxix. pp. 355, 356. § R. Chambers, ‘Ancient Sea-Margins as Memorials of Changes in the Relative Level of Sea and Land,’ 1848, p. 92. | J. G. Jeffreys, ‘Notice of an Ancient Sea-Bed and Beach near Fort William, Inverness-shire,’ Rep. Brit. Assoc., Cambridge Meeting, 1862, pp. 73-76. 4 H. N. Dixon, ‘Some Neolithic Moss Remains from Fort William,’ Ann. Scot. Nat. Hist., 1910, pp. 108-111. CHAPTER III. FORMATIONS AND TERMS. TABULAR STATEMENT OF FORMATIONS. THE following classification has been adopted in the index of Sheet 53 POST-GLACIAL, Peat. GLACIAL AND POST-GLACIAL. Fresh-Water Alluvium :— Lowest Terrace. High Terrace. Higher Terrace. Alluvial Cone. Fluvio-glacial Gravel (only separately mapped at Corran). POST-GLACIAL. Marine Alluvium :— Present Beach. Low Raised Beach. “95-Ft.” Raised Beach, about 35 ft. above O.D. Raised Sea Margins are algo shown, GLACIAL. Morainic Drift. Boulder Clay. TERTIARY AND DOUBTFUL. Dolerite Dykes, mostly N.W. or W.N.W. Monchiquite W.N.W. Dyke (Glen Coe). Breccia Vent with Nepheline Basalt (Allt Coire na Ba). DOUBTFUL MIDDLE OLD RED SANDSTONE. Rudha na h-Earba Conglomerate. LOWER OLD RED SANDSTONE AND DOUBTFUL. Minor Intrusions :— Porphyrite Dykes, mostly N.N.E. (some with slight: foliation near Starav Granite are distinguished by a symbol). Fine-grained Diorite N.N.E. Dykes, sometimes porphyritic (Glen Etive District). 23 24 Formations. Andesite W.N.W. Dykes (Glen Coe District). Lamprophyre Dykes and Sheets. Quartz-Porphyry, Felsite, and Rhyolite Dykes. Plutonic Intrusions :— Granite, including much of the “ Fault-Intrusion” of Glen Coe. Porphyrite Facies of “ Fault-Intrusion.” Porphyrite Facies of “Early Fault-Intrusion.” Augite, and other Diorites, including two outcrops of “ Early Fault Intrusion.” * Kentallenite and Cortlandtite. Contemporaneous Igneous Rocks :— Agglomerates, including some Sedimentary Breccias. Rhyolite Lavas. Andesite Lavas separated locally into Hornblende- and Augite- Andesites. Sediments :— Conglomerates, Sandstones, and Shales. METAMORPHIC ROCKS OF UNCERTAIN AGE NORTH-WEST OF LOCH LINNHE. Stratigraphical Relations of the Sediments are doubtful. Intrusions :-— Epidiorite and Hornblende-Schist. Augen Gneiss (Sgurr Dhomhnuill). Of mixed Origin :— Felspathised Sedimentary Gneiss. Sediments of “ Moine Series ” :— Limestone. Quartzite. Psammitic Gneiss. METAMORPHIC ROCKS OF UNCERTAIN AGE SOUTH-EAST OF LOCH LINNHE. Intrusions :— Hornhlende-Schist (S.E. of Glen Creran). Sediments of Western Part of District (excluding a small tract south of Fort William), in Stratigraphical Sequence (perhaps inverted) :— Cuil Bay Slates. Ajpin Phyllites and Limestones. Appin Quartzite. Ballachulish Slates. Ballachulish Limestone. Leven Schists. Glen Coc Quartzite. Discussion of Terms. 25 Sediments forming a sequence of bands between the Pap of Glen Coe and the Blackwater reservoir, and reappearing in part, it is thought, south of Fort William. It is uncertain whether the repetition of rock types in this sequence is due to folding or inter- stratification :— Glen Coe Quartzite. Binnein Schists.: Binnein Quartzite. Eilde Schists. Eilde Quartzite. Eilde Flags. Stob Quartzite. Reservoir Schists. Reservoir Quartzite. Reservoir Flags. MINERAL VEINS. Hematite (E.N.E. of Glenure House). In addition to giving the position of the various rock groups, the map carries signs to show how the inclination of the bedding and cleavage, or foliation, of the sedimentary and schistose rocks varies from place to place. Faults and slides (fold-faults) are distinguished from unbroken junctions, while a device is employed in certain cases to indicate which of two adjacent igneous intrusions is the later. DISCUSSION OF TERMS. The folding of the schists within Sheet 53 is extremely complex, and it may be helpful to explain the terminology employed in its description. The idea of an isoclinal fold is sutticiently familiar to form a starting-point. In an isoclinal fold the two limbs of the fold have assumed a rough parallelism, for a lesser or greater distance ; the inclination of the parallel limbs we take as defining the inclina- tion of the fold itself; and experience teaches us that it may be at any angle. Now it has been found that the isoclinal folds of some of the better known mountain chains often lie at low angles, and such prostrate folds are said to be recumbent. Recumbent folds, where well-developed, frequently attain to surprising dimensions, and can be traced for miles as more or less flat sheets extending in a direction at right angles to the direction of strike of the folding. These large-scale folds generally occur in groups, in which the apices of the recumbent anticlines all point in one direction, and the apices of the synclines in exactly the opposite direction. The direction in which the apex of a fold, whether anti- cline or syncline, points, defines the direction in which that particular fold closes. In the other direction the fold is said to open or gape (cf. fold of Glen Coe Quartzite, Fig. 6, p. 42, which closes to the N.W. and gapes to the S.E.). In a group of recumbent folds, the successive anticlines generally overlap, sometimes for miles. Thus a whole district may be carved 26 Formations and Terms. by erosion out of an assemblage of. superimposed recumbent anti- clines, separated of course by recumbent synclines. A group of anticlines, one need scarcely point out, is always accompanied by a group of synclines. The two kinds of fold are complementary to one another in the sense that the billow of a wave is complementary to the hollow. On the other hand anticlines are said to be homologous, one to another, and, likewise, synclines. The recognition of the anticlinal, or synclinal, nature of a small- scale fold, whether normal or isoclinal, is often comparatively easy. In the great majority of cases an anticline, considered as a whole, closes upwards and gapes downwards, while a syncline does exactly the reverse. With a large-scale recumbent fold this apparently simple criterion often ceases to be applicable. The available ex- posures are, time and again, too limited to present a picture of the fold as a whole, and a partial view is quite insufficient. Recumbent folds are never really flat; they often undulate quite sharply (Fig. 12, p. 81), and this introduces a difficulty in the matter of nomencla- ture: wherever an undulating recumbent fold dips down towards the direction of its close, it is at that place synclinal in form, because it is closing downwards; vice versa it is anticlinal in form wherever it rises in the direction of its close. Thus a recumbent anticline is almost certain, at some point or other, to be of synclinal (or as many prefer to say, pseudo-synclinal) form, and vice versa. Further, it is easy to imagine a recumbent anticline drooping consistently from its origin to its close, in which case its form is synclinal (or pseudo- synclinal) throughout. Where a sufficient basis of well-established fact exists, the words pseudo-synclinal and pseudo-anticlinal are preferable to the words synclinal and anticlinal. It is unwise, however, to insist upon the use of these terms in the initial stages of inquiry. They pre-suppose knowledge which may be very difficult of attainment. A concrete example will illustrate this point quite clearly. One of the major recumbent folds of Sheet 53 is known as the Appin Fold. It was realised some years ago that the Appin Fold is of synclinal form in its outcrop, running south-west from Glen Nevis, through Onich, (Sections C and D, Figs. 9 and 10, pp. 74,75). We can at once record this fact of observation by simply saying that the fold, at this part of its course, is of synclinal form; but we are not in a position to elaborate, and to say whether or no this synclinal structure is pseudo-synclinal. The use of the terms synclinal and anticlinal in an observational sense is in strict accord with the established British custom in regard to dips. For instance the dip of an overturned bed which might be stated as 170° to the west is, in almost all cases, recorded as 10° to the east. The advantage of this apparently slip- shod procedure is the possibility of recording a fact of observation, without waiting for a theoretical discussion involving, perhaps, a large element of uncertainty. It may be asked: How are recumbent anticlines and synclines to be distinguished? The answer is best conveyed by the consideration of a simple case, of frequent application in the Alps, though not as yet in Sheet 53. Let us take the case of recumbent folds developed in previously unfolded strata, which, owing to the presence of fossils or otherwise, can be divided into a stratigraphical sequence of which Folds and Slides. 27 the older portions can be distinguished from the younger. In such a case the recumbent folds which carry older rocks into the heart of younger are anticlines, and vice versa. The rocks in the interior of such folds are said to constitute fold-cores. Under the conditions postulated a recumbent anticline has a core of older rocks, and a recumbent syncline of younger rocks. In Sheet 53 there is good reason to believe that the recumbent folds, which have been detected, were developed in previously un- folded rocks. The conditions outlined above are so far complied with; but though a stratigraphical sequence has been determined in the strata affected by the folds, it is impossible, as yet, to be certain which end of the sequence is the older. What is possible is to divide the folds of the district into two complementary suites according as their cores belong to the one end or the other of the stratigraphical sequence. As might have been anticipated from the experience won by workers in other mountain chains, the homo- logous folds of the one suite close in one direction and their complements in the other. Now, as may readily be imagined, the rocks included in recumbent folds have very commonly broken. The immense extent of many of these folds is due, in large measure, to the rupture of the folding rocks, and their displacement along fold-faults or slides, which as a rule lie approximately parallel to the folds themselves. Slides do not belong all to one category. Some are developed in, and more or less completely replace, the lower limbs of recumbent anticlines ; these are known as thrusts. Others are developed in, or replace, the lower limbs of synclines, and are called /ags. Thrusts and lags are complementary to one another in the sense that anticlines and syn- clines are said to be complementary. In many regions thrusts predominate to the more or less total exclusion of lags. In Sheet 53, however, this is not the case. Complementary slides have been recognised (Section G, Fig. 11, p. 78), and, until a means has been devised of distinguishing between the recumbent anticlines and synclines of the district, it is impossible to say which of these complementary slides are thrusts and which are lags. As a result of all this folding and sliding one frequently finds that the original stratigraphical sequence has been profoundly altered by inversion, repetition, and deletion. Under these circumstances one must be careful not to accept a structural sequence, even though it hold throughout a considerable area, as necessarily agreeing with an original stratigraphical sequence. Convincing evidence that a structural sequence has not been determined by sliding is sometimes afforded by the presence of passage zones linking the various members of the sequence together. But even such a definitely united sequence may have been determined, in part, by folding, leading to repetitions of one and the same stratigraphical group on several distinct structural levels; and it may also have suffered complete inversion. In Sheet 53 it has been found possible to arrange many of the schists in a stratigraphical sequence, but not to determine which member of the sequence was originally at the bottom, and which at the top. The major recumbent folds of Sheet 53 have been in certain cases sharply bent into later or secondary folds. These secondary folds are of considerable magnitude, some thousands of feet, and are of isoclinal 28 Formations and Terms. character. As they affect rocks already rearranged by previous folding, they are not included in the definitions of anticlines and synclines given above. More generalised definitions are required, and may be stated as follows :— An anticline is a fold which carries rocks belonging to the lower part of a structural sequence (whether this sequence be strati- graphical or not) into the heart of rocks belonging to the upper part. A syneline behaves in complementary fashion. E. B. B. CHAPTER IV. METAMORPHIC ROCKS. No definite correlations can be established between the schists on the two sides of Loch Linnhe owing to the effect of the Great Glen Fault which runs along the bottom of the loch. Thus it is necessary to describe the schists of Lochaber and Appin, south-east of Loch Linnhe, separately from those of Ardgour, north-west of the same. In addition to this, the Lochaber and Appin country is of such geological complexity that it has been found convenient to split it up into nine local subdivisions as follows :— A. Onich to Fort William p. 29 B. Appin to Kentallen . p. 34 C. Aonach Beag p. 38 D. Callert . p- 41 E, Ballachulish . p. 45 F. Glen Creran . é p. 50 GQ. Glen Coe to Glen Etive p. 56 H. Kinlochleven ; : ; ‘ p. 62 I. Stob Dearg to Coire an Easain (Sheet 54) . p. 77 These various districts will be taken in the present chapter in the order given above. No definite geographical boundaries will be assigned to any one of them, for our object is to make the local descriptions cover the whole of the country south-east of Loch Linnhe, avoiding at the same time both undue overlapping and arbitrary isolation. The general results obtained from this survey of the evidence will then be discussed in Chapter V. The Ardgour district, north-west of Loch Linnhe, will be considered by itself in Chapter VI. Some of the technical terms employed in describing the structures of the schists have already been discussed in Chapter IIT. It has already been pointed out that most of the schists included in the map are of sedimentary origin, but that schistose igneous rocks also occur, both in Glen Creran and in Ardgour. These igneous rocks will be treated along with the sediments with which they are associated. LOCHABER AND APPIN—-DETAILED DESCRIPTIONS. A, Onich to Fort William.—The country lying between Onich and Fort William affords a capital introduction to the schists of the whole district south-east of Loch Linnhe. An important and complex isoclinal fold, already alluded to in Chapter IIT. as the Appin Fold, runs north-east from Onich to Ben Nevis. Its existence is clearly indicated by the mapping of the outcrops of five easily 2g 30 Schists S.E. of Loch Linnhe. distinguished rock groups. Beginning with the Appin Phyllites, which occupy the heart of the fold at Onich, these five groups are as follows :— Appin Phyllites (2) * Appin Limestone (3) Appin Quartzite (4) Ballachulish Slates (5) Ballachulish Limestone (6). A brief description may now be given of these various groups. The Appin Phyllites and Appin Limestones (2 and 3) may be taken together, since on the Onich shore, where both groups are well- exposed, the limestone is in two beds—one at the margin of the Appin Quartzite (4); the other at some little distance from this margin and within the phyllitic series. The Appin Phyllite Group consists of grey pelitic sediments, with which, in many outcrops, bands of flaggy quartzite are very abundantly intercalated. On the Onich shore the quartzite beds appear to be restricted to that portion which intervenes between the two beds of Appin Limestone already referred to. Mr. Clough has found pebbles in these quartzite beds; for the most part the pebbles are merely grains of quartz (and perhaps quartzite), but at about 150 yds. west of the “ Nether Lochaber” Monument, which stands on the outcrop of the Appin Quartzite to the east, Mr. Clough found a 1}-in. pebble of porphyry (slide number 12,909) in a bed of phyllite intercalated with quartzite and certainly belonging to the phyllite-limestone succession and not to the Appin Quartzite. The occurrence of such igneous pebbles immediately suggests comparisons with the Portaskaig Conglomerate of Islay (one-inch map, Sheet 27) and the Garvellachs (Sheet 36), and the Loch na Cille Conglomerate of the Tayvallich peninsula (Sheet 28). The rest of the group consists of homogenous massive grey mica-slates, well exposed on the foreshore of the small promontory east of Onich Pier. The Appin Limestone is a sandy magnesian limestone or dolomite. Both beds in the Onich shore section weather a pale cream or pink colour with dark stripes, and have thus won for themselves the descriptive designation “tiger rock.” Many of the dark ribs are pale in their interior, but others again are dark throughout. The cream- coloured portions are frequently arranged in the form of elongated nodules. The phyllites and limestones are constantly repeated in the shore section by isoclinal folds (Fig. 1). The presence of these folds has only been detected by mapping the country to the north; the beds exposed on the shore always dip towards the south-east. The results attained by mapping inland cannot, it is true, be relied upon in detail, since the country is only partially exposed, but one can very easily satisfy oneself that the phyllites and limestones do not extend far north, and that their outcrop is surrounded in this direction by that of the succeeding group, the Appin Quartzite (4). "3 The Appin Quartzite (4) is a massive, white, false-bedded, pebbly quartzite containing pebbles of quartz and felspar. It flanks the outermost crop of Appin Limestone both east and west of Onich, and _* The numbers here attached to the various rock groups have been selected with reference to the needs of the district as a whole, e Onich to Fort William. 31 makes a great show in the hills a mile or two farther north. The eastern outcrop of the quartzite continues to the shore section by the roadside at the “Nether Lochaber” Monument. The exposure here is capital, but unfortunately the junction of the quartzite with the limestone to the west, laid bare on the foreshore, is a broken one owing to the presence of an unimportant fault not shown on the map. The Appin Quartzite is, in turn, flanked by a striped transition series (4') which links it with the Ballachulish Slates (5). This Striped Series consists of alternations of thin beds of quartzite with seams of black slate. The series is represented in the cliff of the raised beach behind the road east of the “Nether Lochaber” Monument, but is rather poorly exposed. It can be better studied in inland sections round the northern extremity of the quartzite outcrop. The Ballachulish Slates (5) are well-exhibited in a large quarry by the roadside east of the outcrop of the Striped Series just mentioned. The group consists of black roofing slates with large undeformed cubes of pyrites. The slates are well-exposed again on W N.W. “0 x t 2 MILES E.S.E. Fic. 1.—Section across Appin Fold, Onich Shore. Appin Phyllites (2), Appin Limestone (3), Appin Quartzite (4), Ballachulish Slates (5), Ballachulish Limestone (6), Leven Schists (7), Eilde Flags (9). the shore west of Onich, and inland they build large grassy hills stretching north to Glen Nevis. Their outcrop is easily followed and completely surrounds that of the quartzite except where the structure is complicated, for a short space on the western side of the fold, by the presence of a normal fault. Near the Ben Nevis Granite the slates become hard and brittle and spotted with cordierite, but retain their black hue. The Ballachulish Limestone (6) succeeds the Ballachulish Slates in a very clear shore section in the promontory west of Onich. The portion in association with these slates is a dark grey, sandy, though not very impure, banded limestone. The western part of the outcrop is very much more impure, and some of it is of a cream colour. The limestone is also well-exposed east of Onich, where it forms conspicuous little crags at the hill-top above North Balla- chulish which are readily located from the road. Here again the Ballachulish Slates are followed by a dark, banded, fairly pure limestone which is succeeded by much more impure pale grey lime- 32 Schists S.E. of Loch Linnhe. stone with a prominent cream-coloured band near the junction with the next succeeding group, the Leven Phyllites (7). It is easy to follow the eastern and western outcrops of the Ballachulish Limestone, bounding the Ballachulish Slates, until on the southern slopes of Glen Nevis these two outcrops unite. Before this happens, however, at about a mile from the margin of the Ben Nevis Granite, the more impure portions of the Ballachulish Lime- stone assume the character of a flaggy, greenish-white, calc-silicate- hornfels—totally unlike the original limestone in appearance. The group is exposed in this condition along the roadside in Glen Nevis, where it appears to be immensely thick, largely as a result of packing by folding. On the hill-slopes south of the glen, a considerable proportion of that part of the limestone which borders the Balla- chulish Slates has been too pure to alter completely to calc-silicate-horn- fels; these purer beds have recrystallised as black limestone with numerous long prisms of tremolite darkened with included carbon. Reference must now be made to a structural feature of prime importance which can be recognised on the southern slopes of Glen Nevis. The Ballachulish Slates on the top of the hill overlooking the glen are clearly resting upon the Ballachulish Limestone cropping out below (Section CO, Figs. 9 and 10, pp. 74,75). This relation can be readily made out even from the road as soon as one has passed Glen Nevis House on the way up the valley. The Ballachulish Slates make a single little black eminence against the sky-line, and on either side, and below, the contact-altered Ballachulish Limestone can be seen cropping out with regular bedded structure in summits and slopes which are characteristically grassy and green. On climb- ing the side of the valley, so as to get into touch with the exposures, we find the evidence correspondingly more convincing, It is in fact obvious that the Appin Fold in this section is of synclinal form. In agreement with this observation the filling up of the fold-core towards the south-west is associated with a constant appearance of a south-westerly pitch, indicated by the featuring of the hills every- where between Glen Nevis and Onich. The Appin Fold, in the district now considered, loses its regularity of development as soon as we pass beyond the outcrop of the Ballachulish Limestone (6). On the south-east the limestone is succeeded by a great thickness of Leven Schists (7), while on the north- west it is flanked by no less a thickness of flags, which may be correlated with the Hilde Flags (9) met with again to the east of Kinlochleven. The Leven Schists (7) are excellently exposed in ice-moulded crags at the mouth of Loch Leven. They consist of a thick homo- geneous mass of pale sandy greenish-grey phyllites, with a regular alternation of more or less sandy layers, resulting in a persistent colour stripe; conspicuous little flakes of biotite are very common. These phyllites are connected through a banded transition zone with the Glen Coe Quartzite (8), which is well exposed in an anticlinal fold at Tom Meadhoin near the eastern limit of their outcrop. The Leven Schists of this outcrop may also conveniently be studied in Glen Nevis. They have been strongly contact-altered here by the Ben Nevis granite, and so are harder and darker than usual, and are often conspicuously spotted owing to the develop- ment of cordierite, Onich to Fort- William. 33 The banded transition zone linking the Leven Schists (7) to the Glen Coe Quartzite (8) is of very limited thickness in the present district. It may be seen in many sections, for instance at the Steall waterfall, Glen Nevis, where grey phyllites are intercalated with fine- grained quartzite bands, and, what is more peculiar, with carbon- aceous leaden-grey, or black, pelitic seams; and sometimes too with very sandy calcareous beds, a foot or so in thickness. The Glen Coe Quartzite (8) which follows this banded series, is a thick, fine-grained, well-bedded quartzite; it is markedly non- felspathic as it appears in Tom Meadhoin, and also very free from mica, East of the Glen Coe ()uartzite (8) of the Tom Meadhoin and Mam na Gualainn outcrops are several broad bands of mica-schist and quartzite followed by the type outcrop of the Eilde Flags (9) surrounding Loch Eilde Mor. It is an open question at present whether these bands are mere repetitions of the Leven Schists and Glen Coe Quartzite, as the writer is inclined to believe, or whether, as Mr. R. G. Carruthers thinks, they include two additional strati- graphical horizons of mica-schist and quartzite. The point scarcely affects the description of the present district, and its discussion will be reserved for Section H, dealing with Kinlochleven (p. 62).- Eilde Flags, recognised on lithological grounds, do, however, enter largely into the present district near the coast between Onich and Fort William. Their occurrence here throws very little light upon the stratigraphical question touched upon above, for in this particular outcrop the flags either come into direct anomalous contact with the Ballachulish Limestone, and even for a short distance with the Ballachulish Slates, or are separated from these rocks merely by a single band of quartzite and mica-schist, too thin to show on the one-inch map. The Eilde Flags (9) running past Fort William on the north-west side of the Ballachulish Limestone, are quartzo-felspathic flagstones, rich in biotite and muscovite. Their great characteristic is an even- bedded structure shown by the constant alternation of more and less micaceous layers. A belt of flags, containing more pelitic material than usual, borders the Ballachulish Limestone from Glen Nevis to the promontory west of Onich; in type this part of the group is intermediate between the more typical flags and the pelitic rocks characteristic of the Leven Schists ; they le north-west of the thin band of quartzite and mica-schist which has already been referred to as locally separating the Ballachulish Limestone from the Eilde Flags of this district. In Fort William itself there are some beds of hard black schist which appear to be an integral portion of the group, although no similar occurrence is known elsewhere. A few strips of massive white fine-grained quartzite have been mapped out from the flags in the vicinity of the coast; they probably belong to the Glen Coe Quartzite (8), or, at least, to one of the quartzites, if more than one exists, stratigraphically intervening between the Leven Schists and Hilde Flags. It is clearly necessary to account in some way for the striking difference between the two sides of the Appin Fold beyond the outcrop of the Ballachulish Limestone. In the Onich district, where the fold-core is deep enough to include all groups from 2 to 6, it 3 34 Schists SF. of Loch Linnhe. might be argued that the flags on the north-west are merely a local facies of the Leven Schists, but this explanation breaks down farther north, for the contrast between the two groups is as marked as ever on the southern slopes of Glen Nevis, although the fold-core here must be comparatively shallow, since it contains only the Ballachulish Limestone. We are thus driven to postulate a slide along the one side or the other of the Ballachulish Limestone to account for the discrepancy. The crags above Glen Nevis show a fine interlaminated passage zone between the Ballachulish Limestone (here represented by calc-silicate-hornfels) and the Leven Schists on the south-east, and accordingly the slide must lie on the other side of the limestone. This slide may conveniently be spoken of as the Fort William Slide. From Glen Nevis to Onich it cuts out more or less completely the Leven Schists (7) and the Glen Coe Quartzite (8), and thus brings the Hilde Flags (9) into conjunction with the Ballachulish Limestone (6). (Sections A-D, Figs. 9 and 10, pp. 74, 75, and Fig. 1, p. 31). The existence of the Fort William Slide is further indicated by a detailed examination of the junction line between the limestone and the flags. In the first place, the Leven Schists and Glen Coe Quartzite are locally represented in a very attenuated form; in the second place, the Ballachulish Limestone, about three miles north of Onich, suffers a similar attenuation (Section D, Figs. 9 and 10), and for a space is entirely absent. The type section of the Fort William Slide is afforded by the river Kiachnish, where the slide is repeated owing to a normal fault shown on the one-inch map. Greatly attenuated Leven Schists and Glen Coe Quartzite intervene in both exposures between the lime- stone and the flags. The quartzite has a more flaggy structure than usual, and minute examination reveals three or four instances of very gradual transgression of one divisional plane across another. But, though the whole of the attenuated quartzite, from its banded edge against the Leven Schists to its contact with the Hilde Flags, is laid bare in the river, there is no more evidence of disruption than one is accustomed to in ordinary exposures of schistose rocks. A mere inspection of the Fort William and other slides of the district as a whole, would often fail to suggest the importance attaching to these structures. Probably the sliding has seldom, if ever, been confined to a single isolated plane, while it has doubtless been associated, to an important extent, with plastic deformation. Only on some such hypothesis of distributed differential movement is it possible to account fer the simultaneous thinning of neighbour- ing groups of rocks so often observed in connection with these slides, The Fort William Slide runs out to sea in the promontory west of Onich. The Leven Schists and Glen Coe Quartzite are both absent in this section, but unfortunately a thin porphyrite or lamprophyre dyke has been intruded along the plane of the slide, obscuring to some extent the junction relations of the Ballachulish Limestone and the Hilde Flags; the latter, however, are evidently intensely sheared at their margin. B. Appin to Kentailen—This district includes the south-westerly continuation of the Appin Fold, The heart of the fold is here Appin to Kentallen, 35 occupied by the Cuil Bay Slates (1), a group which is unrepresented in the Onich section. The Cuil Bay Slates (1) are black slates, connected by a transition zone of grey and black pelitic sediments with the Appin Phyllites (2). This transition zone is quite unlike any portion of the Ballachulish Slates (5); otherwise the two slate groups are indistinguishable. The Cuil Bay Slates make an important outcrop at Cuil Bay; on the north-east this outcrop is surrounded by Appin Phyllites, but to the south-west it continues beneath the waters of Loch Linnhe, and appears again on the coast at Lurignich. Mapping renders it ‘apparent that the Cuil Bay Slates occupy the heart of a minor isoclinal fold—the Cuil Bay Fold (Fig, 2)—included within the limits of the Appin Fold. The Appin Phyllites (2), following the Cuil Bay Slates, are some- what more sandy than at Onich, so that much of the group consists of grey quartzose mica-schist. Towards the Appin Limestone, flaggy quartzite intercalations are common, as is the case with the portion Se Ly 7 oat. SSNS ey Tees pea as WA NONSEA LEVEL ’ WANT N.W. 0 1 2 MILES Fic. 2,—Section across Appin Fold, north of Cuil Bay. of the group which falls between the two beds of Appin Limestone on the Onich shore; but here, in the Cuil Bay Fold, only one bed of Appin Limestone has been definitely proved, and this lies at the margin of the Appin Quartzite. Near Kentallen the Appin Phyllites have been greatly altered by the Ballachulish Granite. or a distance of a quarter of a mile the more pelitic beds are converted into massive, spotted cordierite- hornfels, and the portions containing much flagey quartzite are scarcely distinguishable from the Striped Series (4!) where similarly metamorphosed. Two isolated outcrops of the Appin Phyllites occur in a broken minor fold to the south-east of the Cuil Bay Fold. One of these, immediately west of the Ballachulish Granite in Glen Duror, consists of interbanded quartzite and pelitic sediment in a baked condition ; the other, six miles farther south on the west side of Glen Stockdale, consists of grey phyllites. For convenience the broken fold including these two outcrops may be termed the Glen Stockdale Fold; its tectonics will be considered later. The Appin Limestone (3) is well exposed in the Cuil Bay Fold, 36 Schists SL. of Loch Linnhe. especially between the mouth of Glen Duror and Appin Station. At the latter place it may be seen passing through a transitional pebbly calcareous bed into the Appin Quartzite alongside. The Appin Limestone is well-developed again in the Glen Stockdale Fold, where, for some distance south of Salachan glen, it is clearly in two beds as at Onich. The bed next to the Appin Quartzite is massive and white; the other bed is flaggy and of the “tiger rock type” so well displayed on the Onich shore, The. Appin Quartzite (4) has exactly the same massive pebbly character as it has farther north. It flanks the Cuil Bay Fold on both sides in perfectly regular fashion. Its south-eastern outcrop is found on mapping to be disposed in a minor fold, the Beinn Sgluich Fold, separating the parallel Cuil Bay and Glen Stockdale Folds already referred to. The Striped Series (41) and Ballachulish Slates (5) are intermittently exposed along the axis of this Beinn Sgluich Fold, and may be conveniently studied near the roadside leading east from Appin Station. The Ballachulish Slates (5) also occur in a long strip in the Glen Stockdale Fold, where they are associated with the Ballachulish Limestone (6). The latter forms a continuous outcrop from the southern limit of the map north-east into the hornfels country of Glen Duror. Glen Stockdale affords excellent sections of this important group; the passage of the grey portion of the limestone into the cream-coloured portion and of the latter into the Leven Schists lying to the south-east is capitally shown in the tributary burns of the valley south of Glenstockdale House. Both the Ballachulish Slates and the Ballachulish Limestone are met with again in the island of Shuna; only the grey portion of the limestone is represented. The Leven Schists (7) oceupy a broad strip of country between Glen Stockdale and Glen Creran. For half a mile from the edge of the Glen Stockdale outcrop of Ballachulish Limestone the group is represented by pale greenish-grey phyllites of the type predominant in northern exposures. Beyond this, thin dark seams begin to put in an appearance, just as they do farther north in the banded passage zone bordering the Glen Coe Quartzite. In fact, the next three miles measured across the strike consist in the main of an exaggerated Banded Series with an enormous development of quartzose intercala- tions and black pelitic seams finely interlaminated. The south-eastern boundary of this banded outcrop is the Ballachulish Slide, to be described later on. Here we may note that the effect of the slide is to bring the Banded Series into direct contact on the south-east with an outcrop of Ballachulish Limestone; there is no intervening zone of grey phyllite. A third of a mile within the north-western limit of the Banded Series narrow outcrops of quartzite are met with intermittently along a line of strike passing east of Salachan. In the north-east this quartzite is a massive, white, non-pebbly rock; in the south-west, on the borders of the map, it is equally massive and white, but highly pebbly with large grains of quartz and felspar. The south-westerly exposures are very clearly linked by transition types with the surrounding Banded Series. They therefore represent one of two things; on the one hand they may be a mechanical intercalation of Appin to Kentallen. M7 Glen Coe Quartzite of abnormal pebbly facies—such as will fall to be described on the south-east slopes of Glen Creran; on the other they may be a local stratigraphical intercalation of pebbly quartzite belonging to the Banded Series. The prominence of the Banded Series gives the Leven Schists of the Appin district a distinctly peculiar facies. The change naturally does not come in abruptly: the road and railway cuttings along the shores of Loch Leven, north-east of the Ballachulish Granite, already show an unusual number of black seams; farther south, on the other side of the granite, the new characters are more strongly marked, and, gathering strength, they gradually become very pronounced in Sheet 45—the one-inch map to the south of Sheet 53. On this account the Banded Series in Sheet 45 has been mapped and described as belonging to a black slate group, wrongly correlated with the Ballachulish Slates. Mr. Clough first suggested, in conversation, the correlation, at present accepted, with the Leven Schists. Excellent exposures of this great Banded Series may be examined by the road- side near Creagan Station (Sheet 45). We shall now turn to a consideration of the tectonics of the Appin district. It is apparent from the mapping, when allowance is made for local complications near the margin of the Ballachulish Granite, that the Kentallen and Appin district continues, with certain modifications, the structure already elucidated between Fort William and Onich. Thus, in the light of the Glen Nevis evidence, which shows that the Appin Fold as a whole is of synclinal form, one may at once interpret the minor folds of Cuil Bay and Glen Stockdale as synclines, and the intervening Beinn Sgluich Fold as an anticline (see Fig. 2). The broken nature of the Glen Stockdale Fold has already been referred to. Definite groups are persistently missing in the south- east limb of this syncline as the result of a powerful slide which may be called the Glen Stockdale Slide. The existence of this slide is most easily appreciated on entering the syncline from the south-east, somewhere in the vicinity of Glenstockdale House. After leaving the Leven Schists (7) we first meet with the Ballachulish Limestone (6), and beyond this with a narrow belt of the Ballachulish Slates (5). So far all is as regular as it is along the corresponding outcrops between Onich and Glen Nevis; but on continuing our traverse we fail to encounter the striped Transition Series (4'), and instead step directly on to the massive pebbly quartzite (4), or, according to the locality, on to the Appin Limestone (3) or the Appin Phyllites (2). A comparison of Figs. 1 and 2 (pp. 31, 35) will show that the Fort William and the Glen Stockdale Slides are complementary to each other—both of them deepen the Appin Syncline, although they occur on opposite sides of the same. Another slide locally replaces part of the limb common to the Beinn Sgluich and Glen Stockdale Folds. Its presence is clearly indicated by the mapping west of Glenstockdale House. For a space this slide cuts out both the Appin pebbly Quartzite (4) and its striped margin (4!) and brings the Ballachulish Slates (5), which form the core of the Beinn Sgluich Fold, into contact with the Appin Lime- stone (3) on the south-east. The structural relations of Shuna and the mainland are inferred 38 Schists S.E .of Loch Linnhe. from unpublished results obtained by Mr. Maufe* in Sheet 44. It is fairly certain that, to the south of the Creag Islands in the Firth of Lorne (Sheet 44), the undersea outcrop of the Ballachulish Lime- stone of the upper limb of the Appin Fold is carried by a south- westerly pitch into Lismore and Shuna. On the way it touches at the southern extremities of Eilean na Cloiche and Eilean nan Gamhna of the Creag Island archipelago. Expression is given to this interpretation in Fig. 3, where it is suggested that the Glen Stockdale Slide is bent down into the Cuil Bay Syncline, and thus comes to underlie Shuna. On this hypo- thesis there is no difficulty in accounting for the proximity of the Ballachulish Limestone, in Shuna, to the Appin Phyllites of the eastern limb of the Cuil Bay Fold in the promontory of the mainland opposite. Another point remains :—it does not seem impossible that some of the black slates in the heart of the Cuil Bay Fold at Cuil Bay itself may belong to the Ballachulish Slate Group above the Glen Stockdale Slide. All that can be said with certainty in this connection is that some portion of the black slates here exposed must belong to a NW. ' 2 MILES S.E. eo L i Fic. 3.—Section across Island of Shuna and Glen Stockdale on the Mainland. Cuil Bay Slates (1), Appin Phyllites (2), Appin Limestone (3), Appin Quartzite (4 Ballachulish Slates (5), Ballachulish Limestone (6), foe (7). distinct group, the Cuil Bay Slates, because of the passage zone linking them with the surrounding Appin Phyllites. C. Aonach Beag.—Reference has already been made in describing the district between Onich and Fort William to the Leven Schists of Glen Nevis intervening between the crop of the Ballachulish Lime- stone near the entrance of the glen and that of the Glen Coe Quartzite at Steall. In the hills on either side a thick limestone is preserved which has been completely eroded away in the glen itself. This limestone occurs as a layer in the Leven Schists and has been folded into a pro- minent syncline (Fig. 4). It catches on to the northern extremity of the Stob Ban ridge, south of Glen Nevis, then on to Meall Cumhann, north of the glen, then on to Aonach Beag, and, finally, in accordance with a north-easterly pitch, it descends into the valley of Allt Coire an Eoin. The limestone is generally in the condition of a flaggy, greenish- white calc-silicate-hornfels; it has however escaped contact-alteration * H. B. Maufe, in ‘The Ceology of the Country near Oba d : Explanation Sheet 45, Mem. Geol, Survey, 1908, p. 37, Et Sema Aonach Beag. 39 in some of the sections east of Allt Coire an Edin, and is there found to be pale grey or cream colour, and very sandy and impure. It is regarded as a highly compressed fold of the Ballachulish Limestone in the heart of the Leven Schists, and not as an independent strati- ee intercalation in the latter; the reasons for this view are as ollows :— 1. The limestone is linked with the Leven Schists through transition beds, and is of the same type as that portion of the Ballachulish Limestone which is similarly connected. 2. It is not structurally interposed between Ballachulish Limestone and Glen Coe Quartzite. Thus we have already seen that in Glen Nevis the Leven Schists, which follow to the east of the Ballachulish Limestone of the Appin Fold, pass beneath the syncline holding the Aonach Beag Limestone, and connect directly with the Glen Coe Quartzite beyond (cf. Section A, Figs. 9 and 10, pp. 74, 75). 3. The type of folding required by this interpretation is independently exemplified close at hand in Stob Ban, where the Ballachulish Limestone and Slates occupy the core of the Ballachulish Fold (Section C, Figs. 9 and 10). Accepting this interpretation of the Aonach Beag Limestone as a fold-core, one is struck with the extent of the secondary disturbance to which it has been subjected. The steep-sided syncline into which the core has been bent has a depth of probably not less Fic. 4.—Looking up Ben Nevis from Stob Ban. The Aonach Beag Core of Ballachulish Limestone folded into a syncline well seen in Aonach Beag (4060 ft.), and also in Meall Cumhann in the middle distance. than 2000 ft. (Fig. 4, and Sections A-C, Figs. 9 and 10). Once a fold-core has been buckled in this fashion it is obvious that its de- velopment, as a fold-core, must have come to an end. The secondary folding of the Aonach Beag Fold has been accompanied by the production of a very pronounced vertical strain- slip cleavage, which affects a belt of country about a mile in width, including the Glen Nevis gorge. The strain-slip cleavage cuts and displaces quartz-veins, as in analogous instances described by Mr. Clough * in Cowal and Mr. Wright} in Colonsay. In agreement with the views expressed by these authors it would seem that the quartz-veins in the present instance were formed in connection with the development of the Aonach Beag fold-core, and were buckled and broken when the latter suffered its subsequent corrugation. * «The Geology of Cowal,’ Mem. Geol. Survey, 1897, p. 9, etc. + ‘The Two Earth-Movements of Colonsay,’ Quart. Jowrn. Geol. Soc., 1908, vol. lxiv. p. 302, Fig. 3. 40 Schists SE. of Loch Linnhe. Mati ha: fig latin. : BEC wt Dip, amount in degrees. wt Dip of Cleavage. DB Steep Dip. B Steep Dip of Cleavage. x Vertical Beds. X Vertical Cleavage. —— Ballachulish Slide. ++ Exposures of attenuated Ballachulish Limestone. suit Boundary of aureole within which the Ballachulish Limestone is represented by Calc-silicate-hornfels. The succession is Appin Quartzite (4), Ballachulish Slates (5), Ballachulish Limestone (6), Leven Schists (7), Glencoe Quartzite (8), G=Granite. Fig. 5.—Map showing Outerops in Callert District. Callert. Al D. Callert.—The district which now falls to be described lies between Allt Coire nam Feusgan, north of Stob Ban, and Callert House, on the shores of Loch Leven. It is specially important, as its interpretation provides a key to the tectonics of a large tract of country on the south side of Loch Leven. As already pointed out, the Aonach Beag Syncline is exposed at the northern extremity of the Stob Ban ridge. About half a mile to the south-east is another syncline. In this second syncline, as in the first, a folded limestone (calc-silicate-hornfels) is encountered, with Leven Schists above and below; in this case, however, the limestone itself is in two layers, separated by a parting of hornfelsed black slates (Section C, Figs. 9 and 10, pp. 74, 75). The section of the syncline on the hillside reads as follows :— Thick Leven Schists in the heart of the syncline. Upper layer of Cale-Silicate-Hornfels, 100 ft. Black Slate Hornfels, 100 ft. Lower layer of Cale-Silicate-Hornfels, 100 ft. Basement of thick Leven Schists in contact at both sides with Glen Coe Quartzite. The black slates in the middle of this section belong to the out- crop worked in the Ballachulish Quarries, so that here is a fold-core of undoubted Ballachulish Slates, separated by Ballachulish Limestone from Leven Schists above and below, and bent into a sharp secondary syncline. The fold which has suffered this secondary deformation will henceforward be spoken of as the Ballachulish Fold. It is distinct from the Aonach Beag Fold (Section C, Figs. 9 and 10) since its core, with its included Ballachulish Slates, is fuller than that of the latter, although it is exposed farther to the south-east, in which direction, as will presently appear, it unquestionably closes. The secondary syncline of Stob Ban, affecting the Ballachulish Fold, continues south-west of the outcrop of the Mullach nan Coirean Granite. Here the black slates are thicker than in Stob Ban, and occupy the heart of the syncline to the exclusion of the overlying layer of Ballachulish Limestone and Leven Schists. On either side of the slates the underlying layer of Ballachulish Limestone (calc-silicate-hornfels) is strongly developed on the hill-top near the granite margin, and the Leven Schists beyond are also thick. Exposures are intermittent, but on crossing the Lairigmor valley (Fig. 5) one realizes a great change in the development of the Ballachulish Fold: the lower limb of the fold shows marked attenuation, leading in many cases to the total disappearance of one or more recognisable subdivisions. This attenuation is connected with the most interesting slide in the whole district, to wit—the Ballachulish Slide. A brief examination demonstrates that the Ballachulish Slide must have developed before the Ballachulish Fold was bent into the secondary syncline, to which reference has so often been made; it is found, in fact, that the eastern and the western outcrops of the lower limb of the Ballachulish Fold show precisely similar phenomena of attenuation (Figs. 5 and 6). This important feature is particularly striking perhaps in the hollow west of Main na Gualainn, since here 49 Schists S.E. of Loch Linnhe. the syncline is of very considerable complexity (Section D, Figs. 9 and 10), as is demonstrated by the mapping. Three groups are affected by attenuation in connection with the Ballachulish Slide in this district. These three are the Leven Schists (7), the Ballachulish Limestone (6), and the Ballachulish Slates (5). Naturally one can appreciate the attenuation of the last-named group, the Ballachulish Slates, only where Appin Quartzite is present in the heart of the Ballachulish Fold—elsewhere one cannot distinguish the slates in the upper limb from those in the lower. This requisite condition is fulfilled for a mile and a half along the western side of the secondary syncline, and for a short space along the eastern side, near Loch Leven. On the western side of the syncline the Ballachulish Slates (5) and Limestone (6), which might be expected to intervene between the Appin Quartzite (4) and the Leven Schists (7), are, in some exposures, reduced to a few feet in thickness, and, in others, cut out altogether. In like manner on the TOM MEADHOIN H SLIDE A LacHutls = FY SEA RES, LEVEL SM KE ASS SS o Ya ! 2 MILES L a 1 a N.W. S.E. Fic. 6.—Section showing the Relation of the Ballachulish Slide to the Tom Meadhoin Fold. The succession is Appin Quartzite (4), Ballachulish Slates (5), Ballachulish Limestone (6), Leven Schists (7), Glen Coe Quartzite (8). eastern side of the syncline, in the promontory east of Callert House, both groups are entirely missing. The attenuation, and frequent complete disappearance, of the Ballachulish Limestone in the lower limb of the Ballachulish Fold can be proved in section after section in addition to those mentioned in the previous paragraph. In fact it is a phenomenon of perfectly general occurrence between the Lairigmor valley and Loch Leven; the recognition of this feature depends upon the constant presence here of Ballachulish Slates in the core of the Ballachulish Fold (whether accompanied by Appin Quartzite or no). This core of Ballachulish Slates separates a thin development of Ballachulish Limestone (where present) in the lower limb of the fold from a thick development in the upper limb. The thick development is only preserved from erosion in the heart of the secondary syncline, and includes a central outcrop of slightly calcareous silvery-grey quartzose schists.* * These slightly calcareous rocks were originally classed with the L Schists. E. B. Bailey, ‘Recumbent Folds in the Schiste of the Scottish Wieblenda,! Quart. Journ Geol. Sov. 1910, vol. xvi. p. 587, Pl, XLIL. , Callert. 43 The phenomena of attenuation of the Leven Schists in connection with the Ballachulish Slide are particularly instructive. On the north-west of the complex syncline, west of Mam na Gualainn, lies a thoroughly well-exposed anticline of Glen Coe Quartzite, forming Tom Meadhoin (Fig 6). The quartzite here is separated from the outcrops of limestone, black slate, and pebbly quartzite, lying to the south- east, by a narrow belt of Leven Schists, in one extreme case (Section D, Figs. 9 and 10) reduced to 30 ft. in thickness. And yet, the Leven Schists, when followed round the nose of the Tom Meadhoin Anticline, are found as a band a mile wide, which presents an uninterrupted outcrop in spite of its dissection into hills and valleys. Clearly then a slide exists along the south-east margin of the Tom Meadhoin Anticline. Since the Tom Meadhoin (Doire Ban) Fold of Glen Coe Quartzite closes towards the north-west, beneath the Ballachulish Slide (Section D, Figs. 9 and 10), the complementary Ballachulish Fold of Limestone, Slate, etc., above the slide must close towards the south-east. In the Ballachulish district to the south of Loch Linnhe this deduction becomes almost self-evident (Sections F-H, Fig. 11, p. 78); even in the Callert district it is supported by the much greater prominence of the Appin Quartzite on the western than on the eastern side of the secondary syncline into which the Ballachulish Core is thrown (Figs. 5 and 6). The exposures of the Callert district, though disconnected, are fully sufficient to establish the mapping upon which the foregoing conclusions are based. The accompanying map will enable any geologist to verify the evidence for himself in the course of one or two days. The section near the road is itself quite representative, and may be examined in the course of an hour or so (cf. Section E, Fig. 11, p. 78). The reduced lower limb of the Ballachulish Fold is exposed in the shore section close by the road on both sides of the secondary syncline. In the promontory east of Callert House the Appin Quartzite (4) marks the centre of the fold, though in so sheared a condition that its pebbly nature cannot be definitely recognised in the shore section, and has to be verified in an exposure at the north end of the little wood which here borders the road. East of the quartzite the Ballachulish Slates (5) and Limestone (6) are entirely missing, as well as the phyllitic portion of the Leven Schists (7). The result is that, in this direction, the pebbly Appin Quartzite (4) approaches very close to the non-pebbly Glen Coe Quartzite (8); the banded portion of the Leven Schists (7) is all that remains to separate the two. On the other side of the syncline, in the promontory opposite Eilean Choinneich, the centre of the Ballachulish Fold is marked by the Striped Series (4!). West of this the Ballachulish Slates (5) are nearly or quite absent, and the Ballachulish Limestone (6) is repre- sented by not more than six feet of dark grey limestone. West of the limestone again comes a narrow outcrop of grey phyllites of the Leven Schist Group (7), then four yards of black slates, which perhaps belong to the Leven Schist Group, or perhaps to a little syncline bringing down Ballachulish Slates. West of this black slate outcrop lies the Banded Series of the Leven Schists (7) with its grey phyllites, quartzite ribs, calcareous beds, and thin black seams. 44 Schists SE. of Loch Linnhe. Beyond this, at the roadside, the Glen Coe Quartzite (8) is itself exposed, In the upper limb of the Ballachulish Fold in the same neighbour- hood, the Ballachulish Slates (5) are best developed on the west side of the syncline. They are well exposed close by the road and in the burns west of Callert House. On the east side of the syncline the slates are much sheared and greatly reduced in thickness, so that, although they make a well defined outcrop, it seems rash at first sight to correlate them with the slates on the west side of the syncline. To dispel one’s doubts, however, one need only follow the crumpled black schist inland, along what is virtually a continuous exposure, till at about a mile, or a mile and a half, from the coast, one finds it in a much less disturbed condition, and unmistakably of Ballachulish type. In this part of its course the black slate outcrop is, moreover, separated from the Leven Schists outside by a thin representative of the Ballachulish Limestone. The map, Fig. 5, shows how convincing the evidence is, especially when taken in conjunction with the synclinal structure so beautifully revealed in the ridge north of Stob Ban. The marginal portion of the overlying thick development of the Ballachulish Limestone (6) is well exposed in a little promontory on the western side of the syncline, and again in the fields near Callert House on the eastern side. The limestone is grey in both these outcrops; it is distinctly paler and also more impure and phyllitic in the eastern than in the western, and it is not at all certain that the eastern outcrop belongs to the normal marginal portion of the lime- stone at all. It is quite possible, for instance, that there is a con- tinuous stratigraphical sequence in the limestone from the western pure outcrop—certainly in normal contact with the Ballachulish slates—through the slightly calcareous beds of the central outcrop— to be described in the next paragraph—into the fairly impure phyllitic limestone of the eastern outcrop. If so, the Ballachulish Slates beyond are introduced by a slide not shown in the map or sections. The writer inclines to this view, but it is, after all, a detail of not much importance. The central outcrop, consisting of slightly calcareous schists, may be seen in a little stream north of Callert House; but to obtain a satisfactory exposure of it one must go to the burn which crosses the outcrop a mile from the road. The Tom Meadhoin Anticline should certainly be visited if time permits. As will appear from the description already given, this is the most compact section, north of Loch Leven, for demonstrating the existence of the Ballachulish Slide. In this connection an excursion should be made, if possible, to compare the very different developments of the Ballachulish Limestone and the Leven Schists on the two sides of the Lairigmor valley. The best localities for this purpose occur :-— (1) North of the valley on the hill-top near the granite margin. (2) South of the valley where the outcrop of the Ballachulish Slide makes a horse-shoe bend on the west side of the syncline (Section D). (3) Also south of the valley, but on the east side of the syncline where three exposures of the Ballachulish Limestone are shown in Ballachulish. 45 Fig. 5 intervening between the Ballachulish Slates and the Leven Schists. In all except one of these localities the Ballachulish Lime- stone is in the condition of calc-silicate-hornfels; it is important to bear this in mind or else the group will not be recognised, especially where it is reduced to a few feet in thickness. There is only one exposure in the Callert district which can be cited as showing very pronounced evidence of special movement where beds are missing in connection with the Ballachulish Slide. This section is in Allt a’ Choire Dhuibh. At the head of the burn there is a small gorge which exhibits a sequence from Appin Quartzite, through much diminished representatives of Striped Series and Ballachulish Slates to Leven Schists—the Ballachulish Limestone is omitted. There is evident smashing of the beds here, but whether this is connected with the formation of the slide or whether it is of late and independent origin is quite uncertain. In enumerating exposures of interest in the Callert district, one must refer again to the little syncline on the Stob Ban ridge. This affords the most important individual section in the whole one-inch map, owing to the clear light it throws upon the synclinal disposition of the Ballachulish Fold in all the country lying ,within the limits of the Sheet. E. B. B. £. Ballachulish—On the southern shores of Loch Leven the Ballachulish Limestone (6) of the upper limb of the Ballachulish Fold has a widely extended outcrop in direct continuation of that which is exposed on the north side of the loch. This outcrop accordingly marks the centre of the secondary syncline into which the Ballachulish Fold is thrown (cf. Section F with section E, Fig. 11, p. 78). We propose for the present to concentrate our attention upon the various rocks occupying the gape of the Ballachulish Fold upon the west side of this secondary syncline. Here we encounter not only Ballachulish Slates (5) and Appin Quartzite (4), already met with in like position north of Loch Leven, but also Appin Limestone (3) and Appin Phyllites (2). In fact, one may study these various groups at Ballachulish just as conveniently as anywhere in the Appin Fold itself. There is no change in facies: the descriptions already given of the deposits in the Appin Core hold at Ballachulish without alteration. At the same time, for convenience, a brief recapitulation of the characters of the groups is given below with special reference to type sections included in the Ballachulish Fold. The Appin Phyllites (2) are grey phyllites with, in many ex- posures, a very large proportion of flaggy fine-grained quartzite intercalations. Their character is well displayed on the western slopes of Gleann an Fhiodh. In the valley west of Beinn Bhin, owing to the proximity of the Ballachulish Granite, the phyllites of the group are in the condition of massive spotted cordierite-hornfels, and the portion with quartzite intercalations comes to resemble the Striped Series (41); it will be remembered that similar characters are developed in the rocks of this group on the other side of the granite at Kentallen in the Appin Fold. ‘The Appin Limestone or Dolomite (3) presents both the cream or pink dark-banded “ tiger-rock ” type met with at Onich, and the whiter 46 Schists S.E. of Loch Linnhe. more massive type found in one of the beds south of Glen Salachan. The tiger-rock type is exhibited in the “marble quarry” near the entrance of Gleann an Fhiodh, on the west side of the river. The massive type is exposed on the top of Sgorr a’ Choise in close conjunc- tion with the pebbly quartzite: it weathers with the faintest of blue- white tints. In the Allt Gitbhsachain valley, and again south-west of the path which crosses the Sgorr a’ Choise ridge, the group is greatly altered by the Ballachulish Granite; in the last-named exposure dedolomitisation has produced a grey limestone studded with innumerable little crystals of forsterite. The Appin Quartzite (4) is a massive pebbly quartzite, which is well exposed, for instance, in Beinn Bhin. Its pebbles have suffered remarkably little deformation, and in Sgorr Dhearg, to the south, it weathers down in many places to a coarse grit. The Ballachulish exposures of the Striped Series (4") connecting the Appin Quartzite with the Ballachulish Slates (5) are specially good. The rocks of this group may be conveniently studied at the mouth of Gleann an Fhiodh in the stream above the bridge. In Sgorr Dhearg they build the greater part of a cliff, which makes a very conspicuous object as seen from the village; they must be fully 500 ft. thick here. As the name implies, the Striped Series is characterised by an incessant alternation of white and grey quartzite and quartzitic material, with black pelitic seams. At the one end quartzite predominates and pebbly seams are common ; at the other end black slate predominates and pebbly seams are rare orabsent. A few calcareous layers are found locally in the heart of the Striped Series. Owing to abundance of pyrites in the black beds, the whole group has a pronounced rusty weathering. The Black Ballachulish Slates (5) are opened up in huge quarries at the roadside. The Ballachulish Limestone (6) is well seen along the road east of the slate quarries; it is grey here, and much of it is tolerably pure, though sandy and banded. At Bridge of Coe the limestone is less pure and more phyllitic, but still grey. Intermediate exposures on the hill slopes include a large proportion of very slightly calcareous material. As in the case of the Callert exposure (p. 44), it is likely that a minor slide—not shown on the map, and quite distinct from the Ballachulish Slide—bounds the limestone outcrop along Glen Coe, cutting out its purer portions in this position. The best section of the Ballachulish Limestone in the Ballachulish district is in Allt Socaich, a mile south of the village. There is here a very typical exposure of the darkest type of the limestone; this is followed by interbanded limestones and slates, and these by the strongly developed cream-coloured edge of the limestone group, in obviously normal contact with the Leven Schists. The Leven Schists (7) on the west of the outerop of the Ballachulish Fold, between the loch and the granite, have been referred to already in the description of the Appin and Kentallen district ; they are grey phyllites, with numerous small porphyroblasts of biotite, and sometimes garnet; they contain many interbedded thin black pelitic seams and, in the eastern part of the section, numerous bands of fine-grained quartzite also. A great mass of Leven Schists of perfectly normal type follows above the Ballachulish Ballachulish. 47 Limestone of the upper limb of the Ballachulish Fold in Meall Mor, ete. ; this overlying mass will be dealt with in connection with the Glen Coe and Glen Etive district. EB. B We may turn from this brief stratigraphical review of the rocks of the Ballachulish district to a consideration of their tectonic relations. The gape of the Ballachulish Fold is highly complex, as indicated by the mapping, and includes within itself three minor folds—the Beinn Bhan, the Gleann an Fhiodh and Sgorr a’ Choise Synelines, The synclinal structure of these three folds shown in sections F and G (Fig. 11, p. 78) has not been determined directly, since such pitches as have been noted in connection with them are too steep to be relied upon in deciding such a matter; however, the anticlinal nature of the complementary fold, separating the Beinn Bhan and Gleann an Fhiodh Synclines, is very clearly shown in Sgorr Dhearg. The beds in which the arch is shown to such good advantage belong to the Striped Series, and are crossed by a cleavage parallel to the axial plane of the fold. This critical section is laid bare in an easily accessible cliff due east of the summit of Sgorr Dhearg, and south of the precipice which is so conspicuous from the village ; it should be visited by any one who wishes to check the interpretation of the structure of the district given in Sections F and G (Fig. 11, p. 78). The regularity of the anticline, as exposed in the cliff, is broken somewhat by a slide which makes a conspicuous hollow with marked discordance of the beds on its two sides; this slide is shown on the map, but is of such trivial importance that it does not interfere appreciably with the reading of the section. A more important slide is indicated by the mapping as running along the base of the cliff on the south-east side of the anticline; it does not, however, in any way impair the regular development of the anticlinal arch exposed in the cliff face. H. BBM, EB. B We may now give more particular attention to the Beinn Bhan Syncline. The mapping of the Appin Phyllites (2), Appin Limestone (3), Appin Quartzite (4), and the Striped Series (41) of Beinn Bhin and Allt Gitbhsachain shows that these rocks have a perfectly regular arrangement, except in the two following instances :— (1) It has not been found possible to trace the outcrop of the Appin Lime- stone completely round that of the Appin Phyllites in the valley of Allt Giibhsachain ; exposures are bad here, but the limestone is certainly absent at some pointe, probably as a result of sliding. (2) The junction beween the Appin Quartzite and the Striped Series on the east face of Beinn Bhan is a minor slide. There is a marked discordance of dip and strike of the rocks on the two sides of this slide, so that its presence can be realised even from a distance ; in fact, one can quite well recognise this minor slide in the instructive view that one gets looking south-west from Ballachulish Station. The regularity of the Beinn Bhin Syncline breaks down altogether when we pass outwards from the Striped Series (4°). One need scarcely leave the roadside to realise this important feature. At the west end of the village there is a large quarry in Ballachulish Slates (5). This quarry is opened up on the centre of 48 Schists SH. of Loch Linnhe. the anticline separating the Beinn Bhan from the Gleann an Fhiodh Synclines; with neighbouring exposures it indicates beyond doubt that the Ballachulish Slates are represented in full force on the east side of the Beinn Bhan Fold (Sections F and G, Fig. 11, p. 78). Westwards, the Striped Series (4), dipping off the Ballachulish Slates, crosses the road and occupies the foreshore. The quartzite of Beinn Bhan follows, but before reaching the road it swings round the nose of the Beinn Bhan Fold and runs up the hill-face on the far side of Allt Gitbhsachain ; the change of strike as the quartzite thus bends round the end of the fold is perfectly clearly exposed. Map in hand, one can appreciate its significance even from a distance; to verify it more fully one need only ascend Allt Gitbhsachain behind St. John’s Church to the point where the quartzite crosses the stream. The Striped Series (4!) follows the quartzite in its altered course, but the Ballachulish Slates (5), which have gone out to sea, never reappear: instead, one meets with a thin representative of the grey part of the Ballachulish Limestone (6), bordered on the west by the banded transition series, which links the Leven Schists (7) with the Glen Coe Quartzite (8). The best section, showing the absence of the Ballachulish Slates, is afforded in the small promontory west of St. John’s Church ; the rusty-weathering Striped Series is evidently much disturbed both here and in the adjacent railway cutting. Immediately to the west of the Striped Series, which on the coast itself includes a thin bed of sandy limestone, there lies an outcrop of typical grey Ballachulish Limestone, measuring 12 yards across; west of this follows a banded quartzo-phyllitic series, obviously the Banded Series connecting the Leven Schists with the Glen Coe Quartzite, the latter not exposed in this particular neighbourhood. The absence of Ballachulish Slates between the Striped Series and the Ballachulish Limestone, and also of the greater part of the Leven Schists between this limestone and the banded edge of the Glen Coe Quartzite indicates a double slide along the west side of the Beinn Bhan Syneline (Sections F and G, Fig. 11, p. 78). A glance at the map will estabish the fact that this double slide is the con- tinuation of the Ballachulish Slide which has already been recognised on the opposite side of Loch Leven. E. B. B. A very instructive excursion can be undertaken from Ballachulish up Gleann an Fhiodh and Allt Socaich, finishing with an ascent of Sgorr a’ Choise. Such a traverse will enable any one who has the map of the district in his hand to form a clear conception of the minor folds of Gleann an Fhiodh and Sgdrr a’ Choise, which are illustrated in Sections F and G (Fig. 11, p. 78). For completeness one may start with a visit to the grey, banded portion of the Ballachulish Limestone (6) of the upper limb of the Ballachulish Fold, exposed by the roadside east of the village, This is followed to the west by the Ballachulish Slates (5) opened up in an enormous quarry, Still keeping to the road one next meets with the Striped Series (4!) in its natural position, bordering the slates ; this transition zone is more fully exposed in the river Laroch above the bridge at the mouth of Gleann an Fhiodh ; it oceupies the bed of the burn as far as the Established Church, where the pebbly Appin Quartzite (4) puts in an appearance. The Appin Limestone (3) is wanting in the river section at the eastern margin of this Ballachulish. 49 quartzite outcrop, for a slide of sinall importance brings the latter into direct contact with grey-green phyllites (Appin Phyllites, 2) interbanded with thin ribs of cream-coloured limestone: unfortun- ately the slide itself is not exposed. The missing outcrop of Appin Limestone is well seen in the “Marble Quarry” on the west side of the river ; the limestone here is of the “tiger rock” variety, cream and pale pink in colour, with dark stripes. The phyllites mentioned in the previous paragraph mark the centre of the Gleann an Fhiodh Fold in this line of traverse. Beyond them the return outcrop of Appin Limestone makes a con- spicuous feature in the bed of the river; then above a waterfall the Appin Quartzite and Striped Series follow, both greatly attenuated. This attenuation, which is maintained along the strike to the south, is the evidence which has led to the recognition of the slide S” of Sections F and G (Fig. 11, p. 78). The attenuated zones are well exposed in the bed of the river and show the following signs of disruption :— (1) The face of quartzite over which the water tumbles at the fall is slickensided ; (2) The quartzite a little farther up stream is in a crushed condition and is crossed by well-defined planes of movement ; (3) The Striped Series beyond is of greatly diminished thickness and is crossed by similar planes of movement which cut the bedding structures at so small an angle that their preseuce is only revealed on careful scrutiny. Beyond the Striped Series there is, especially in Allt Socaich, a fine exposure of the Ballachulish Slates followed by an outcrop of folded dark-grey banded limestone, obviously part of the Ballachulish Limestone. On the east of this latter there is found a succession of slates, both black and grey, with numerous bands of limestone, mostly dark, some cream-coloured. It was previously considered that this succession represented a reappearance of the Ballachulish Slates,* but it seems much more probable after re-examination that it belongs to the Ballachulish Limestone. The last outcrop of cream-coloured limestone is several feet thick and separates thin black slates on the north-west from a great uninterrupted outcrop of Leven Schists on the south-east. Mr. Clough detected in the black slates obvious signs of movement, probably due to the continuation across the stream of a slide which has been established on independent grounds on the summit of Sgorr a’ Choise, immediately to the south. (8”’, Section G, Fig. 11, p. 78). H. B. M., E. B. B. At this point it is well to leave the stream section and follow the outcrop of the cream-coloured limestone to the top of Sgorr a’ Choise. At the summit a steeply pitching minor fold, to which reference has already been made, introduces the Striped Series, Appin Quartzite and Appin Limestone—the latter of the massive white type. The effect of the slide S’” now becomes exceedingly obvious, for whereas on the north-west side of the fold the Ballachulish Slates and Appin Quartzite (including the Striped Series) are developed in force, on the south-east side these two groups are reduced to 3 ft. each in thickness; at the same time * Cf, for instance, E. B. Bailey, ‘Recumbent Folds in the Schists of the Scottish Highlands,’ Quart, Journ, Geol, Soe., 1910, vol. Ixvi. plate xii. p. 612, 4 50 Schists SE. of Loch Linnhe. the grey portion of the Ballachulish Limestone so strongly developed in Allt Socaich is missing, so that the cream-coloured edge of this limestone is only separated from the white Appin Limestone, in the heart of the Sgérr a’ Choise Fold, by some 6 ft. of strata. The attenuation can be followed in its various stages of development at the steeply pitching nose of the fold; here, as a result of the movement, the bands of the Striped Series are broken up into a series of lenticles. If the traverse be continued along the ridge of Sgérr a’ Choise so as to reach the path that leads down to Gleann an Fhiodh once more, it will be found that the quartzite of the north-west limb of the Sgorr a’ Choise Fold is presently cut out by yet another slide. The effect of this slide is further indicated in a severely “scrubbed” face of the quartzite on the hillside south-east of the point at which the path crosses the stream; the quartzite here is in the condition of the Durness Quartzite of the North-West Highlands where cut across by one or other of the well-known thrusts of that region. C.T. C, E. BB. In the upper part of Gleann an Fhiodh the fold of Appin Phyllites, which figures so prominently in the western slopes of this glen farther north, terminates. The structures involved are obviously very complex, and the exposures are insufficient to elucidate them in detail. Before turning to the next district an important feature of Sections F and G demands attention. In these sections the general synclinal disposition of the Ballachulish Fold and Slide, already recognised in the Callert district on the north side of Loch Leven, is indicated as continuing in the Ballachulish district on the south side of the same. This feature will be justified in the sequel when we come to consider Glen Coe and Glen Etive, where it is an easy matter to recognise the re-emergence of the Ballachulish Fold and Slide. It has not been found possible, however, to follow the accompanying minor structures in like detail, and accordingly the bending of the minor folds of Beinn Bhan, Gleann an Fhiodh, and Sgorr a’ Choise, and of the associated minor slides as drawn in Sections F and G is not based upon field observation. The long tongue of Ballachulish Limestone, west of Allt na Muidhe, is now attributed to a secondary anticline, and not to a primary anticline, as in previously published sections. It seems fairly certain that a secondary anticline continues from Sgdr nam Fiannaidh, Glen Coe, south-west to the outcrop of the Ballachulish Limestone in Glen Ure. E. B. B F. Glen Creran.*—The development of the gape of the Ballachulish Fold between Fraochaidh and Salachail (Section H, Fig. 11, p- 78) is so similar to that just described in the Ballachulish district, that a brief description of it will suffice. The Ballachulish Slide is in two branches between which is a thick band consisting of the pure dark- grey portion of the Ballachulish Limestone, associated in places with outcrops of the Ballachulish Slates. North-west of the double slide, one steps on to the banded portion of the Leven Schists; south-east of * E. B. Bailey, ‘The Ballachulish Fold near the Head of Loch @ (Argyllshire),’ Quart. Journ, Geol, Soc., 1914, vol, lxx. p. 321, protane PLATE IV. BALLACHULISH ORE le IL FPP P APL OOP EE. Explanation SP PAUL 6a! flo LET Es Y PP ote FB oS & Ls Jf. 4A - ee Fp SE ee 5 Sy SDNY vor iF oy }) ee RAMS Granite, etc Li ae Appin Phylites (2) and Limestones (3) i YF / on Aes @ < ji “®) eee, ve “p /; f asnactloith GY fo ! “Spock atte“ oe or oN LP Mhic/Chailein “7 p-G 7 Fr ott ARES A “ Its oooa0o°o o 0 o 0 = 3 S oe acs Pebbly Quartzite S: oo 0 0 > ch N & AN Ve Peete s LESS LK Striped Series (where separately mapped ) r—o o— o — 0 Ne Ballachulish Sheepfold \ % \ SS, \ aR rn AT Banded. Series SISTYIG UPAOT oQBANDED LEVEN _scHisTs “f 1 Glen Coe Sak eta =. 737A ( ee Quartzite J BALLACHULISH ~ CORE™. [N\\ e Minor tgneous intrusions, both schistose’& non-schistose, omitted. , : < Dips m degrees. .A°C.S. Sgorr a Choise Slide || #~ Undulating Dips. B.S. Ballachulish, Sude + Vertical Beds. €¢ Dipping Beds ee contorted h I MILE |] “~~ Vertical Beds strike | —— Faults —_ Slides. GroLtogican Map or LowER GLEN CRERAN. Glen Creran. 51 the same, one meets a folded complex of Appin Quartzite, Limestone, and Phyllites. The Appin Phyllites in many exposures contain thin black seams suggesting a passage to the Cuil Bay Slates (1). The gape of the Ballachulish Fold is narrower in Glen Creran than it is at Ballachulish. At first the continuations of the Beinn Bhan, Gleann an Fhiodh, and Sgorr a’ Choise Synclines, or equiva- lent structures, can be recognised, but the first two disappear south- wards ; capital exposures are afforded by Allt Coire Mulrooney and its tributary streams. The Sgorr a’ Choise Slide (S’”, Sections G and H, Fig. 11, p. 78), is well developed in this district, and we may note a few localities where its presence can readily be detected. First let us pay attention to two streams which run into Allt Eilidh across the path that descends the Sgorr a’ Choise ridge making for Salachail. Mapping indicates that the effect of the slide hereabouts is greatly to reduce the thickness of the Ballachulish Slates (5) and Limestone (6), intervening between the Appin Quartzite (4), of the Sgorr a’ Choise Fold, and the Leven Schists (7), outcropping to the south-east. In the more northerly stream, the attenuated groups are represented by a very few feet of black slate and calc-silicate-hornfels. In the more southerly, the conditions are probably the same, but there is a small gap in the exposures where the limestone might be expected. In another burn, running down the hill-slope behind Salachail, the Sgorr a’ Choise Slide is laid bare in a small waterfall. Its effect here has been to cut out the dark-grey part of the Ballachulish Lime- stone, and thus leave the cream-coloured portion by itself with an extensive outcrop of Ballachulish Slates on the one side and Leven Schists on the other; the cream-coloured limestone is traversed by obliquely convergent planes of movement; the section is a particularly clear one; its main features are repeated in the river Creran, but the exposures here are much disturbed by subsequent faulting. C1. 0, BE BB. From the river Creran the Sgérr a’ Choise Slide can be followed by mapping to Allt Coire Mulrooney, since along this part of its course it brings Ballachulish Slates, and eventually Appin Quartzite, into direct contact with Leven Schists (Plate IV.). A small outcrop of the highly impure portion of the Ballachulish Limestone is met with in natural association with these Leven Schists some distance down- stream from the position of the slide. Three-quarters of a mile to the south-west, in Easan Diblidh, the following succession is well-displayed, starting from the road :— Grey Leven Schists (7) Highly impure pale Ballachulish Limestone (6) Sgorr w Choise Slide Thin White ae Quartzite (4) Typical, though thin, White Appin Limestone (3) Grey Phyllites, Quartzite Bands, and Calcareous Beds(Appin Phyllites) (2) Grey Phyllites with Black Seams (also Appin Phyllites) (2) Eastern Branch of Ballachulish Slide Black Ballachulish Slates (5), and Black Ballachulish Limestone (6) Western Branch of Ballachulish Slide Quartzose and Micaceous Schists of Banded Series of Leven Schists (7%). This succession continues for another three-quarters of a mile south-west along the strike, to the little stream that on crossing the 52 Schists SE. of Loch Linnhe, road enters the Fasnacloich grounds. Here both Appin Quartzite and Limestone have been detected, though in a very attenuated form. South-westwards no more is seen of these two groups, but the grey Appin Phyllites can be followed for another half-mile until they die out in the heart of Ballachulish Slates. The inference is that the Sgorr a’ Choise Slide has converged upon, and finally met, the eastern branch of the Ballachulish Slide. The Ballachulish Slates from this point on have a simple outcrop, but it is probable that their internal structure is highly complex; it is thought indeed that, during the folding movements, masses of Appin Quartzite, Limestone, and Phyllites, now exposed on the east side of Glen Creran, were actually squeezed right through these Ballachulish Slates. The Ballachulish Slates may be followed to the roadside at Loch Creran, where they have been quarried for wall-building material. They are separated at the roadside from the banded Leven Schists to the west, by a thin outcrop of black Ballachulish Limestone. A thicker mass of pure black limestone belonging to same group lies immediately east of the slates and has been burnt for lime. In the road section the eastern black limestone is succeeded by a thin strip, not separated on the map, of banded non-caleareous schist and white limestone, and this by a broad belt of grey highly impure calcareous schist, which undoubtedly belongs to the Ballachulish Limestone. Calcareous schists continue to occupy the slopes above the road as far as the Post Office, except for an interruption of phyllite, the position of which is shown on the map. It is doubtful whether this phyllite band should be regarded as a stratigraphical or a mechanical intercalation in the Ballachulish Limestone; it is grey with black streaks, recalling parts of the Appin Phyllites, but it is difficult to believe that it can belong to that division. On crossing Loch Creran the outcrops of limestone, black slate, pebbly quartzite, ete, which mark the central portion of the Ballachulish Fold, turn abruptly in their course and run north- eastwards towards Glen Ure. In so doing they enter into the metamorphic aureole of the Cruachan Granite wherein the impure limestones are converted into cale-silicate-hornfels, and the argil- aceous rocks are correspondingly disguised. The first step in tracing the outcrops of the various groups east of Glen Creran is easy. The broad belt of Ballachulish Limestone in the upper limb of the Ballachulish Fold, which west of the glen is represented almost wholly by impure calcareous schists, is readily recognised on the east side near the mouth of the river Creran. The calcareous rocks on this side of the glen, in wide-spread exposures reaching north-eastwards to the river Ure, are practi- cally always in the condition of cale-silicate-hornfels. The almost complete absence from this outcrop of limestone too pure to yield calc-silicate-hornfels, suggests that a slide (not shown on the map, but possibly a branch of the Sgorr a’ Choise Slide) may everywhere form the south-east boundary of this outcrop of the Ballachulish Limestone. South-east of the broad expanse of cale-silicate-rock an almost continuous band of Ballachulish Slates can be recognised. It is very thin near the alluvium at the head of the loch, and about half a mile farther north-east it disappears altogether, evidently as a result of Glen Creran. 53 sliding. When, a little farther east, the group reappears, it is in the condition of black hornfels, spotted with cordierite. East of the fault that passes by the sheepfold (Plate IV.), the black horn- fels, representing the Ballachulish Slates, is extremely prominent and gives rise to a sombre row of crags, conspicuous at a distance. The Striped Series of the Appin Quartzite which follows is much less distinctive in its characters, and one cannot, in the contact-aureole, distinguish it with certainty from parts of the Appin Phyllites or of the Banded Series of the Leven Schists. Its outcrop in most cases, however, is limited by hornfelsed black slate on the one side and pebbly quartzite on the other. In the burn flowing north- west to the sheepfold the Striped Series is shown in Plate IV., terminating somewhat abruptly against the Banded Series of the Leven Schists. The mapping naturally becomes rather difficult in this case, but fortunately the Banded Series here no longer closely approaches the Striped Series in character, as nearer Loch Creran, for it contains a large proportion of grey pelitic mica-schist- hornfels. The Pebbly Quartzite division of the Appin Quartzite is of great assistance to the mapper, for it retains its normal characters even close up to the granite. It is a massive, pure-white quartzite with pebbles of quartz and felspar. The value of the quartzite is particu- larly evident in the Allt Buidhe glen, where its outcrops, taken in conjunction with that of the hornfelsed Ballachulish Slates, very clearly define the essential features of an unusually intricate bit. of structure. It is noteworthy that almost all the dips seen in the Allt Buidhe district are extremely steep, and that the form of the outcrops is not appreciably influenced by topographical features of the magnitude of the Allt Buidhe valley. The Appin Limestones and Phyllites come next in order. Narrow outcrops of calc-silicate-hornfels have been noted at the eastern margin of the Balliveolan outcrop of Appin Quartzite, and are referred with some hesitation to the Appin Limestone horizon. Along the southern margin of the next outcrop of Appin Quartzite, about half a mile south of Taravocan, a band of marble, or a corre- sponding hollow, is constantly seen. In colour this marble may be white, pinkish, or grey. A grey specimen, taken from close to the fault which leads down to the sheepfold, was found to contain forsterite (17310) indicating its original dolomitic character. Another specimen, however, from the same band farther west showed no forsterite (17309). Across the fault, the pebbly Appin Quartzite is again frequently bounded on the side remote from the Striped Series by a land of marble, as shown on the map (Plate IV.). A grey marble, from the edge of the quartzite at the top of the declivity leading down to the stream that flows north-east to the sheepfold, has proved on examination to be particularly rich in forsterite and green spinel (17311). There can be no doubt whatever as to the correctness of the correlation of all these marginal outcrops of marble with the Appin Limestone. The Appin Phyllites in their outcrops east of Glen Creran, that is within the aureole of the Cruachan Granite, are often practically indistinguishable from the Striped Series (just as at Kentallen and in 54 Schists SE. of Loch Lnnnhe. Allt Gitbhsachain on the two sides of the Ballachulish Granite). There is a further difficulty in that the Banded Leven Schists near Loch Creran are also of very similar appearance. In fact, in mapping certain hornfelses with quartzite ribs as Appin Phyllites one has to be guided to a large extent by their relations to the more recognisable divisions on either side. There is, however, good in- ternal evidence for assigning the banded hornfelses half a mile south of Taravocan to the Appin Phyllites, for various exposures show alternating beds of impure calc-silicate-hornfels, phyllite-hornfels, and quartzite, an assemblage very reminiscent of parts of the Appin Phyllite Group. Near the southern margin of the Appin Phyllite outcrop just mentioned there is a thick bed of irregularly weathering white marble (sometimes grey), with dark streaks. It is first seen north- west of a wail half a mile due south of Taravocan, and can readily be followed thence to the fault about a mile farther east. Two specimens (17307-8) examined contain no forsterite, but there can be little doubt that the marble belongs to the Appin Limestone position, probably to that part of the group which is interbedded with the Appin Phyllites. One main reason for this belief is the occurrence, close at hand, of black marbles, associated with black- slate-hornfels which obviously belong to the purer part of the Balla- chulish Limestone, and at the same time differ markedly from the pale marble now under consideration. East of the fault the pale marble (Appin Limestone) is strongly developed in Allt Buidhe near a sheepfold not shown on the map. But here, owing partly to a poverty of exposures in the valley bottom away from the stream-bed itself, no attempt has been made to separate the Appin Limestone from the closely adjacent Balla- chulish Limestone—visible in a hollow immediately south of the river. The two limestones have been mapped together as Ballachulish Limestone, and the dentate outcrop shown for the latter can only be regarded as approximately correct. The Appin Limestone, of this composite outcrop, does not extend far east of the fault, for the broad belt of limestone (mainly cale-silicate-hornfels), which turns north- wards from Allt Buidhe near the margin of the Cruachan Granite, belongs wholly to the Ballachulish horizon. All the rock groups east of Glen Creran, from the pelitic Leven Schists to the Appin Limestones and Phyllites just described, belong to a continuous stratigraphical sequence, comparatively little com- plicated by mechanical disruption or repetition. The Callert and Ballachulish districts, taken in conjunction with that of Glen Coe and Glen Etive, yet to be considered, teach us plainly that this stratigraphical sequence is referable to the upper limb of the Ballachulish Fold. Beyond the Appin Limestone and Phyllites, we step on to the lower limb of the Ballachulish Fold, greatly curtailed and com- plicated by the Ballachulish Slide. The latter, as on the opposite side of the glen, is sometimes single and sometimes double, including Ballachulish Slates and Limestone between its two branches. The Ballachulish Slates and Limestone are very well defined in character in their outcrops between a right-angle bend in a wall half a mile south of Taravocan and the fault that runs north-east to the Glen Creran. 55 sheepfold shown in Plate IV.: the slates are represented by black pelitic hornfelses spotted with cordierite; the limestone by black marble of very much the same appearance as the black pure part of the Ballachulish Limestone outside contact aureoles. East of the fault, Ballachulish Slates have nowhere been recognised in the lower limb of the Ballachulish Fold, but the Ballachulish Limestone is often quite thick. In Allt Buidhe, as already men- tioned, the outcrop shown for the Ballachulish Limestone (Plate IV.) includes Appin Limestone as well. Where it turns northwards, how- ever, leaving the stream and converging slowly upon the Cruachan Granite, it consists of a great thickness of calc-silicate-hornfels, with minor outcrops of dark-grey marble, and doubtless belongs wholly to the Ballachulish horizon. After a considerable interruption, due to the operation of the Ballachulish Slide, the limestone reappears once more in streams draining north-west towards Glenure House. It is here in the condition of calc-silicate-hornfels, and is about 40 ft. thick. The Banded Series of the Leven Schists is very largely composed of impure quartzite bands in the neighbourhood of Loch Creran, and its characters are considerably blurred by contact-alteration. Farther north-east, near the margin of the Cruachan Granite, the Banded Series contains a large proportion of grey mica-schist- hornfels, thus showing a divergence from the Loch Creran, or Creagan type. The Glen Coe Quartzite outcrops, shown in Plate IV., truncated by the Cruachan Granite, are particularly interesting on account of their coarsely pebbly nature, for they contain large and abundant grains of both quartz and felspar. They thus approach the Appin Quartzite in character, but are very clearly distinguished by the existence of a banded passage zone of quartzite and grey pelitic schist into the Banded Series of the Leven Schists alongside. There can be no question of mere mechanical interrhixture here, for many of the semipelitic layers are thickly strewn with pebbles. It is worth while recalling that Mr. Macgregor and the writer have already assigned a similar pebbly quartzite, in Glen Strae, east of the Cruacban Granite, to the Glen Coe horizon.* It is believed that the Glen Creran and Glen Strae outcrops are in the same structural position in regard to the Ballachulish Slide. The complex course of the Ballachulish Slide, east and west of the lower part of Glen Creran (late IV.), is a very fine illustration of the extent to which the Ballachulish Fold and Slide have suffered from secondary folding. The Ballachulish Slide has been thrown into a deep isoclinal fold, running north-east and south- west, and steeply overturned towards the south-east; in the upper part of Allt Buidhe, a subsidiary branch-fold runs south-east, and here again the folding is of steep isoclinal character. Moreover, such dips as may be taken to give an indication of the pitch of the secondary folding are for the most part extremely steep, so that it is only by reference to districts lying to the north and east that one can realise which is the upper, and which the lower, limb of the Ballachulish Fold in the vicinity of Glen Creran. *E, B. Bailey and M. Macgregor, ‘The Glen Orchy Anticline (Argyll- shire),’ Quart. Journ. Geol. Suc., 1912, vol. lxviii. p. 172. 56 Schists SE. of Loch Linnhe. One point remains. East of Glen Creran there are very numerous small sheets of compact hornblende-schist, only a small proportion of which have been mapped. Scattered examples occur as far east as Beinn Fhionnlaidh, where a narrow porphyritic band, belonging to the group, may be seen traversing the Glen Coe Quartzite near a small lochan, close to the margin of the Cruachan Granite. Most of them, however, occur nearer the glen, where they are associated indifferently with the schists above and below the Ballachulish Slide. This feature in their distribution shows that they must have been intruded after movement had ceased along the Ballachulish Slide. Their foliation, which is often very pronounced, is most probably connected with the secondary folding which has affected this great slide to such a notable degree. Certain straight dykes, cutting the cale-silicate-hornfels south of Glen Ure, and lettered P? in Sheet 53, were regarded by Mr. Kynaston as related to the rocks discussed above. It is now thought that they are early members of the Old Red Sandstone suite of intrusions which have suffered contact-alteration. E. B, B. G. Glen Coe to Glen Etive.—The section on the south shore of Loch Leven, north of the mouth of the River Coe, agrees closely with the Callert section on the opposite side. A thick development of Ballachulish Limestone (6), pale grey, and very micaceous, is followed eastwards by an obscure hollow which, to judge from the Glen Coe exposures to be described immediately, must mark the position of an outcrop, of black slates. Beyond the hollow comes quartzite, striped with black slate, succeeded by very definitely pebbly quartzite referable to the Appin Quartzite (4). Beyond again lies an extensive outcrop of the Banded Series of the Leven Schists (7) followed by Glen Coe Quartzite (8) (Section E, Fig. 11, p. 78). E. BB The pebbly Appin Quartzite does not continue far inland, and, in fact, is not seen again south of the coast section. The black slate band is, however, much more persistent, and gives rise to a knoll by the roadside, 200 yds. east of the entrance to Glen Coe House. The black pelitic sediments are in this knoll puckered and sheared just as in the exposures east of Callert House. The rocks imme- diately to the north-east belong to the Banded Series of the Leven Schists. For half a mile the black schist outcrop is hidden below alluvium. It then reappears, and can readily be followed for a mile a short distance back from the road. The schist, although clearly distinet from any of the phyllitic seams of the Banded Series beyond, does not strikingly recall the Ballachulish Slates (5), as represented in the gape of the Ballachulish Fold, because it is always relatively thin, and its cleavage planes intensely crumpled. It is, however, un- questionably the same band as occurs to the east of Callert House, and, as has been shown (Fig. 5, p. 40), the correlation of this band with the Ballachulish Slates appears certain. Opposite Leacantuim the limestone occurring south-west of the black schists is much darker and purer than elsewhere in Glen Coe, aud, in fact, is much more of the character one expects of the Balla- Glen Coe. 57 chulish Limestone near its contact with the Ballachulish Slates. Beyond this point it comes within the range of the An t-Sron and other vranites of Glen Coe, and is converted to pale greenish calc- silicate-hornfels ; extremely beautiful specimens of this hornfels can be obtained from the little crags south of the road a few hundred yards west of Clachaig Hotel. Tremolite occurs here in conspicuous clusters of large, pale green crystals, and zoisite may also be found, though less abundantly, in stout, white prisms. Just at the roadside, immediately west of the wood at Clachaig, the black schist band fails, and one may hammer over a section in which banded quartzo-felspathic rocks, representing the transition zone between the Leven Schists and Glen Coe Quartzite, are directly overlain by fine-grained cale-silicate-hornfels. The interest of this exposure is that it reproduces precisely the conditions to be described presently in the sections which have come to be known as the Windows of Etive, three miles farther south. On the north side of Glen Coe, overlooking Loch Achtriochtan, there is much confusion owing to igneous intrusions, but it is easy to separate large outcrops of highly impure calc-silicate-hornfels, resting in places upon a thin development of black schist, and this in turn upon the Banded Series of the Leven Schists dipping steeply south- wards off (len Coe Quartzite. Up to this point the Glen Coe section has been continuous. It is now interrupted by a great fault which crosses the valley at the mouth of Loch Achtriochtan. This fault is dealt with very fully in Chapter VIIL, where it is shown to be the boundary of a circum- scribed region of subsidence, for the most part occupied by down- thrown lavas of Old Red Sandstone Age. As a rule, these lavas are strongly tilted away from the bounding fault, and so erosion has been enabled in several instances to lay bare the underlying schists for some little distance within the region of subsidence. Such is the case at Loch Achtriochtan where a wide V-shaped outcrop of greenish-grey phyllites is exposed between the escarpment of the lavas and the boundary fault. It is obvious that these phyllites, thrown down by the fault against the limestone outside, must structurally overlie it (Section E, Fig. 11, p. 78); in fact, it is clear that they are the faulted continuation of the great mass of Leven Schists which follows to the south of the limestone, in accord- ance with a general southerly dip, in its outcrop west of the fault. It is interesting to note that these down-thrown phyllites have escaped the contact-alteration so marked outside; this, as will appear later, is due to the fact that the uprise of granite in this locality was contemporaneous with the subsidence of the schists and lavas included within the circuit of the great Glen Coe Fault. H. B, M, Though lost to sight for the time being, the limestone reappears within the region of subsidence in the form of a minute outcrop, too small to show on the one-inch map, in the corrie north of Aonach Eagach at the west foot of Meall Dearg. It here under- lies the basement conglomerate at the bottom of the Old Red Sandstone volcanic pile (Fig. 16, p. 108). It is so near the granitoid porphyrite of Meall Dearg that it is in the condition of calc-silicate- hornfels. 58 Schists S.E, of Loch Linnhe. The next exposure of the Ballachulish Limestone (6) is a mile and a half farther east, at the head of Allt Coire Mhorair (Fig. 18, p. 111). The limestone is thick, pale grey, very micaceous, and not contact- altered; it recalls at once the characters of the limestone as seen at the entrance to Glen Coe. It dips steeply off 10 ft. of black slates, referable to the Ballachulish horizon (5), and these in turn are separated by some 50 ft. of grey Leven Schists (7) from a thick development of Glen Coe Quartzite (8) with banded edge. The section is at once so special in its characters, and so precisely similar to that which has been traced up Glen Coe to the boundary fault of the region of subsidence, that there can be no doubt in recognising the equivalence of the two (Section E, Fig. 11, . 78). Z Looking at the one-inch map the significance of tracing the Glen Coe outcrop of the Ballachulish Limestone so far to the east becomes at once apparent. The little syncline on the northern ridge of Stob Ban, described in connection with the Callert district, has increased enormously in scope when traced across Loch Leven. It has taken into its heart a great mass of Leven Schists, spreading eastward beyond Glen Creran, and forming a covering to the Ballachulish Limestone and all the other rocks which occupy a more central position in the recumbent Ballachulish Fold. It will be convenient henceforward to speak of this syncline, into which the Ballachulish Fold has been bent as the “Glen Creran Syncline.” One other point stands out. In all the sections from the mouth of Glen Coe to Allt Coire Mhorair there is good evidence for the persistence of the Ballachulish Slide. In all of them there are dis- crepancies of the same kind as led to the detection of the slide in the Callert district; of these the most conspicuous is the persistent absence of any thickness of grey phyllites beneath the Ballachulish Limestone (with its associated Ballachulish Slates and Appin Quart- zite, where these occur). E. B. B We shall now turn to consider the thick covering of Leven Schists in the heart of the Glen Creran Syncline. There is some variety from point to point in this great mass, but it has not been found practicable to subdivide it into groups, and such local differences as do occur are largely the effects of metamorphism. The prevailing type is a homogeneous greenish-grey quartzose phyllite or mica-schist, with alternating layers of somewhat more and somewhat less sandy nature. Bright little flakes of biotite are highly charac- teristic. Additional minerals are found in the eastern portion of the district. Small blades of actinolite occur in Gleann Leac na Muidhe, and towards Glen Etive large crystals of the same mineral are frequently met with. Garnets also are found in Gleann Leac na Muidhe, particularly along the edges of quartzose bands. In the district south of the An t-Sron Granite and east of Beinn Fhionn- laidh garnets come to be so abundant that much of the deposit assumes the character of a garnetiferous mica-schist. Contact-alteration is very marked in the neighbourhood of the An t-Sron Granite, and on the south-west side of Fionn Ghleann; the baked phyllites standing out as a cragey ridge well seen from Glen Coe. The development of the actinolites Glen Etive. 59 and garnets does not appear to be in any way connected with this contact-metamorphism, and seems in fact to be of much older date. I. BM, The garnetiferous mica-schist is strongly hornfelsed in the neigh- bourhood of the Cruachan Granite also. On the slopes of Beinn Ceitlein, south-east of Glen Etive, the garnets are usually surrounded by dark rims, partly composed of biotite, and elongated in a definite direction, thus showing that the rock has been considerably sheared since the garnets were developed. As the neighbouring granites are very rarely sheared, this in itself renders it highly probable that the garnets are of earlier date than the granites—a view supported by microscopic evidence given in Chapter NIV., where it is shown that the garnets have suffered marked contact-alteration in the aureoles of the various granites. On the same north-western slope of Beinn Ceitlein the garnetiferous mica-schist sometimes includes thin seams containing dark green actinolite, occasionally arranged in radiate forms, in a matrix of mica-schist, with abundant flakes of biotite. C. T. C. The great layer of folded Leven Schists which has just been described is often one or two thousand feet thick, and out of ita whole series of hills and valleys has been carved. In Glen Etive and its tributaries, Gleann Fhaolain and Gleann Chirnan, erosion has cut sufficiently deep, however, to lay bare the foundation upon which the Leven Schists rest (Sections G and H, Fig. 11, p. 78). The openings thus pierced by erosion have come to be known as the “ Windows of Etive,” and in them we find the following sequence of rocks revealed beneath the great covering of garnetiferous mica- schist :— Calc-silicate-hornfels. Alternating mica-schist and quartzite. Non-pebbly quartzite, finely banded near its top. On the slopes of Beinn Ceitlein the calc-silicate-hornfels appears to be not less than 15 or 20 ft. thick. In the valleys north of Glen Etive it is often considerably thicker, and may range up to about 100 ft. In like manner the alternating series of mica-schist and quartzite is sometimes extremely thin and hardly represented at all on Beinn Ceitlein, while it is over 100 ft. thick in the valleys to the north. The calc-silicate-hornfels is a fine-grained greenish- white rock in which sinall prisms of tremolite can often be recognised. The other important constituents are granular malacolite and felspar ; calcite is rare. © T. CO, H. BM. It is instructive to compare this succession, revealed by erosion in the heart of the great Glen Creran Syncline, with the successions which have been described already around its rim. One is struck at once with practical identity of the Etive sections with those displayed by the roadside west of Clachaig Hotel, Glen Coe. It seems necessary, therefore, to correlate the cale-silicate-horn- fels of the Windows of Etive with the Ballachulish Limestone (6), the alternating series below with the Banded Series of the Leven Schists (7), and the fine-grained quartzite with the Glen Coe Quartzite (8). While the more northerly sections exposed in the Windows of 60 Schists S.E. of Loch Linnhe. Etive agree so closely with the Clachaig section, the most easterly of them, on the slopes of Beinn Ceitlein, recalls even more closely the Allt Coire an Easain sequence to be described presently (p. 77). In both Beinn Ceitlein and Allt Coire an Easain the calc-silicate- hornfels is thinner than elsewhere, while the Banded Series may vanish altogether. A feature which the Allt Coire an Easain section does not share with any exposure in the Windows of Etive, however, is its diminished quartzite and the occurrence of Hilde Flags beyond. c.T. C,H. BM, EB. B. Just as certain valleys cut down through the thick mass of Leven Schists included in the heart of the Glen Creran Syncline, so a few of the more prominent summits reach up above it. Small outliers of Glen Coe Quartzite, margined by Banded Series, have thus been preserved on the top of the Leven Schists in Beinn Maol Chaluim, Stob Dubh, and Beinn Fhionnlaidh (Sections G and H, Fig. 11, p. 78). There is thus evidence of a great recumbent fold in the Glen Etive district,* with Glen Coe Quartzite forming the caps of the hills and the floors of the valleys, and a thick layer of Leven Schists sandwiched in between. The calc-silicate band presents an obvious difficulty, however, since while it occurs above the Banded Series in the lower limb of the fold, no definite representative of it has been located in what would appear to be the corresponding position in the upper limb. At first the simple interpretation was adopted that the calc-silicate- hornfels represents an impersistent limestone locally developed between the banded portion and the more pelitic portion of the Leven Schists. When the districts to the north came to be studied in detail, however, and a connection was established between Callert, Glen Coe, and Glen Ktive, this view had to be abandoned, for, as already stated, it seems certain that the Glen Etive calc-silicate-horn- fels represents the Ballachulish Limestone.t Accordingly, the problem is no longer, why are there not two limestone bands in Glen Etive, but why are there not two great masses of the pelitic portion of the Leven Schists? The answer, as in Glen Coe, is that the pelitic portion of the Leven Schists, which one would expect to find below the Ballachulish Limestone in Glen Etive, has been cut out by the Ballachulish Slide (Sections E-H, Fig. 11, p. 78). H. B. M. Upon minute investigation of the Gleann Chirnan exposures it was found that the calc-silicate-hornfels is separated by a discernible plane of discordance from the Banded Series below. This plane is less marked by any appearance of disruption than the well-known thrust planes of the North-West Highlands. It only occasionally transgresses the bedding and foliation of the schists above and below. Sometimes, in addition to such transgressions, one finds small, flat, isoclinal folds affecting the schists on the one side or the other of the plane without involving the plane itself (Fig. 7). In other cases, however, the plane seems to be folded alone with the schists it separates. CT. C., EB. B, B. . H. B, Maufe, in ‘Summary of Progress for 1905,’ Mem. Geol. Survey, 1906. pesl. age oi Ve B. Bailey, in ‘Summary of Progress for 1908,’ Mem. Geol. Survey, 1909, p. 51. ’ Glen Etive, 61 Attention has already been drawn to the outliers of Glen Coe Quartzite on Beinn Fhionnlaidh. The one lying farthest west is interesting as affording yet another example of recumbent folding, as illustrated in Section H, Fig. 11, p. 78. E, B. B. Fie. 7.—Crag, 8.W. side of Gleann Charnan. Cale-silicate-hornfels (Ballachulish Limestone) lying discordantly, through the inter- vention of the Ballachulish Slide, upon the banded quartzo-phyllitic series of the Leven Schists. The upper portion of Leinn Ceitlein is composed of a very considerable outcrop of Glen Coe Quartzite, separated by a gently inclined fault from the Leven Schists of the lower slopes. The fault is a branch of the Glen Coe boundary fault, and probably throws down to the east, although it is itself inclined towards the west. Accepting this direction of downthrow it does not necessarily follow, however, that the quartzite below the fault is the down-thrown 62 Schists SE. of Loch Linnhe. continuation of the layer which caps Stob Dubh outside.* The difficulty is that the inclination of the quartzite under the fault is rather steeper than the inclination of the fault itself, so that it is certainly quite possible that the Beinn Ceitlein quartzite is the con- tinuation of the lower quartzite of Glen Etive lying a little beneath the cale-silicate-hornfels. That such may be the case is indeed suggested by the presence of a very restricted outcrop of green schist, apparently calc-silicate-hornfels, in close association with the Beinn Ceitlein Quartzite, about 100 yds. north-east of the lochan on the north-east side of the mountain, This green schist only forms a narrow strip running east and west for 100 yds. _ It lies 50 yds. outside the main branch of the Glen Coe boundary-fault, and is bounded on the north by a vertical discordance, of ex- tremely inconspicuous type, which is probably of much the same age as the main fault. On the south side the green schist, although its boundary is not clear, seems to come steeply over a thin alternating series of semi-pelitic beds mixed with quartzite. c. T. C, H. Kinlochleven.—The district included under this title is an extensive one reaching from Invercoe, on Loch Leven, up to the north-eastern corner of the map; its southern limit is in the main the boundary fault of the cauldron-subsidence of Glen Coe. The geology of the district is of more than usual difficulty. In 1910, when the writer published his paper on “ Recumbent Folds in the Schists of the Scottish Highlands,” + it was thought that (apart from the Appin horizons) all the mica-schist bands in the neigh- bourhood of Loch Leven could be referred with certainty to the Leven Schist position, and all the quartzite bands to the Glen Coe Quartzite. These mica-schist and quartzite outcrops extend as far east as Loch Eilde Mor, the type locality of the Eilde Flags. The last-named group of rocks has an outcrop varying from 14 to 2 miles in breadth, and is succeeded in turn, from west to east, by a narrow belt of quartzite (broad in the river Leven and in Stob Coire Easain, Sheet 54), a broad belt of mica-schist or pelitic gneiss, a narrow belt of quartzite, and, finally, a belt of flags terminating against the Moor of Rannoch Granite. Largely on account of evidence derived from the neighbourhood of Loch Dochard, south of Sheet 53, this eastern mica-schist was referred to the Leven Schists, the bounding quartzites to the Glen Coe Quartzite, and the flags bordering the Moor of Rannoch Granite to the Eilde Flags. Before arriving at these conclusions the writer had very carefully considered the alternative view that there might be several horizons of quartzites and mica-schists in the Kinlochleven district, and two of flags; but, as stated above, the evidence, so far at least as the rocks outcropping west of Loch Eilde Mor were concerned, seemed at the time conclusively in favour of the restricted sequence :— Leven Schists. Glen Coe Quartzite. Eilde Flags. * At first this interpretation was regarded as probable. Cf. CO. T. g H. B. Maufe, and E. B. Bailey, ‘The Cnl@ron. Sh bold enien of Glen oe oe the Associated Igneous Phenomena,’ Quart. Journ. Geol. Soc., 1909 vol. lxv, p. 628, and Fig. 2, p. 627. 1 E. B, Bailey, Quart, Journ. Geol, Soc., 1910, vol, lxvi. p. 586, *suozIIOY, TeorydesZyeIy4s yOUTsIP UO a1v soqizjzeNh pue systyos-vorur asoyy [[e Woryeyardraqut JayjouR 0} BuUIpdovwr fsystyoy ulrouUg ay} TYIA ISIS warsy oyy puv ‘opZqcenh UUUIG oy) ITA payefastoa st aytZqawnty ao— ualyy oy} uoTyeJo1dsajUL aUO 0} Surp0Oy SLAON UOT) JO YIMOS SUTLITMOW UL YSTIG-BoI]E PUR ayIZ}luNe) pappoywazUy “SLATN NAT) : ( , ‘ T t ; * 7 t VW FLEE “VW GEES ueoyorusenay uy Ay OOLE Seog ureuUutg ud qo, oRpod Ww ATT Ulsuul = 7 “VF TO9€ os es a LOFL Op[ly LNsy WILY vans youuryqrey uy ‘A ALV1d Kinlochleven. 63 During the last few years, Mr. Carruthers has continued the mapping of this complicated region towards the north-east, and, finding it difficult to accept the restricted stratigraphical sequence, has re-examined most of the sections relied upon by the writer, with the exception of that at Loch Dochard. As a result of this extended inquiry, which occupied about two-and-a-half months, Mr. Carruthers * believes that the following stratigraphical sequence should be adopted :— Leven Schists. Glen Coe Quartzite. Binnein Schists. Binnein Quartzite. Eilde Schists. Eilde Quartzite. Eilde Flags. Stob Quartzite. Reservoir Schists. Reservoir Quartzite. Reservoir Flags. The names employed in this list are all, except one, founded upon localities within Sheet 53 (the Reservoir is the Blackwater Reservoir on the east edge of the map). This plan is adopted purely for the convenience of the reader of this memoir. It should be understood that the localities do not necessarily coincide with the most characteristic development of the various strata. The name Stob Quartzite is based upon Stob Coire Easain, Sheet 54, where the quartzite is much more conspicuously developed than anywhere in Sheet 53. The concrete application of these names is indicated by an appropriate system of lettering on the one-inch map, and in pictorial fashion in Plate V. To avoid repetition it may be stated here that pebbles can very rarely be detected in any of these rock groups except in the quartzose portion of the Eilde Flags bordering the Eilde Quartzite. The writer has returned on various occasions to the district, twice accompanying Mr. Carruthers, and has found that he had previously failed to appreciate the full strength of the evidence in favour of an extended stratigraphical sequence; and, what is more important, that he had been led astray in regarding the restricted sequence as demonstrable on the evidence as yet considered. As a result the writer accepts Mr. Carruthers’ interpretation as an alternative hypothesis on an equal footing with that of the restricted sequence. The more complex stratigraphical hypothesis admits of a corres- ponding simplicity in regard to structure. The sections in Figs. 9 and 10, pp. 74, 75, illustrate the two alternative views. Com- parison of the sections in Fig. 9 with those in the original account (Plate XLITL., Quart. Journ. Geol. Soc., vol. xvi.), shows two important modifications: in the first place Mr. Carruthers, on evidence obtained in the northern continuation of the district, has established the probability that the flags of the Loch Kilde Mor outcrop connect underground with the flags of the Loch Linnhe outcrop; in the * See especially R. G. Carruthers in ‘Summary of Progress for 1912,’ Mem. Geol. Survey, 1913, p. 52. The following account of Mr. Carruthers’ position is based upon a manuscript report drawn up for the Director of the Geological Survey in 1912, 64 Schists SB. of Loch Linnhe. second place the writer has found reason to believe that the quartzite of the Binnein outcrop is anticlinal in structure, unless, as Mr. Carruthers holds, it is an independent stratigraphical intercalation. Tt will probably assist the reader if the two hypotheses be con- sidered separately, taking Mr. Carruthers’ first. KINLOCHLEVEN: First HYPOTHESIS. A. Reference to the table on p. 63 will show that Mr. Carruthers holds that three distinct quartzites are met with in the Loch Leven district before the Eilde Flags are reached. In so far as Sheet 53 is concerned, much the most telling fact adduced in support of this view is that the broad belt of mica-schist, west of the quartzite running through the Pap of Glen Coe, is distinguishable in its characters from the mica-schist outcrops farther east. To use Mr. Carruthers’ names, which, as already mentioned, can be followed on the one-inch map by paying attention to the lettering of the out- crops, the Leven Schists west of the Glen Coe Quartzite are dis- tinguishable from the Binnein Schists and Hilde Schists east of the same. The Leven Schists are in the main a great mass of pale greenish-grey, pelitic mica-schist, with porphyroblasts and patches of biotite, which have a distinct greenish colour in hand-specimens, and innumerable fine white or yellowish stripes (}; to } in. thick) of quartzose material. Apart from its persistent lamination, the group is extremely homogeneous; ribs of quartz-granulite and quartzite are restricted to a narrow transitional zone bordering the Glen Coe Quartzite. In this Banded Series, black carbonaceous seams are commonly developed, often interlaminated with yellowish quartzose material, thus yielding what Mr. Carruthers aptly terms “wasp rock.” The Binnein Schists, and Eilde Schists, admittedly very similar to one another, are distinguished from the Leven Schists by the almost universal occurrence of quartzose ribs, up to about 2 ft. in thickness, intercalated with the mica-schist. Even where a con- siderable outcrop is seen free from such quartzose ribs, the mica- schist is distinctly darker and more definitely grey than the pelitic portion of the Leven Schists, and the biotite is more evenly dis- seminated. The Binnein and Eilde Schists agree fairly well in character with the Banded Series of the Leven Schists, especially where they include good representatives of “wasp rock” near the quartzite junctions; but they are much thicker than the Banded Series anywhere north of Loch Leven. Mr. Carruthers supports the view that the Glen Coe Quartzite is a stratigraphical intercalation between the Leven Schists, on the one side, and the Eilde Schists on the other, by pointing to differences distinguishing the two margins of the quartzite in the Glen Nevis district. The section which he regards as typical in this respect is afforded by Sgurr a’ Bhuic, 14 mile north-east of Steall. According to his notes :— “No folding can be seen here, and the strata dip regularly north-west at 20° or so, At the top of the cliff are thin-banded flaggy qua: taiten, signer and grey-weathering, in all perhaps 150 ft, thick, These shade down into massive Kinlochleven: First Hypothesis, 65 pure-white or pinkish-weathering quartzites, which continue to the foot of the cliff, where they pass suddenly down into a zone of mica-schist with irregular bands of quartz-biotite-granulite and quartzite.” Other sections, 1, 2, and 3,in which 1 contrasts with 2 and 3, are cited by Mr. Carruthers to illustrate the same point, as follows :— (1) On the Leven Schist side of the quartzite : the cliff in the north bank of Glen Nevis near Steall, at the foot of Meall Cumhann “where about 60 ft. of thin- and parallel-banded sugary quartzites can be seen passing gradually down into massive pure quartzite, and upwards into mica- schist with quartzose lines half an inch or so thick.” (2) On the Binnein Schist side of the quartzite: near the junction of Allt Coire na Gabhalach with the Water of Nevis, where “very massive pure quartzites are seen, in beds 6 ft. thick or more, abruptly passing into a banded edge with somewhat irregular bands of quartzite and quartz-biotite-granulite.” (3) Also on the Binnein Schist side of the quartzite: on the hilltop, An Garbhanach, and on the ridge connecting Am Bodach and Sgurr a’ Mhaim, where “the abrupt transition from massive quartzite into irregularly banded quartz- biotite-granulite and mica-schist is again well seen in excellent sections round the Am Bodach Syncline.” B. If we accept for the sake of argument that these observations prove that the Glen Coe Quartzite is a stratigraphical intercalation between the Leven Schists and Binnein schists, then we may readily admit that the Glen Coe (uartzite is distinct from the Eilde Quartzite, for the latter seems to be linked quite naturally with the Ejilde Flags. The next point in Mr. Carruthers’ thesis is the strati- graphical independence of the Binnein Quartzite. This he believes can be established on three grounds :— 1. The lithological distinctness of the Binnein Quartzite as a whole from the Glen Coe Quartzite on the one side, and the Eilde Quartzite on the other. 2. The distinctness of the two sides of the Binnein Schists. 3. The distinctness of the two sides of the Binnein Quartzite. The evidence adduced under these three headings will now be considered more in detail. (1) “The Binnein Quartzite,” Mr. Carruthers writes, “is emphatically the quartzite of the Loch Leven district. Even at a distance its brilliant white-weathering on the hillsides contrasts strongly with the dull grey of the Glen Coe and Kilde Quartzites,” There is no doubt of the truth of this statement, although occasion- ally, as by the roadside, near the island, a mile from the head of the loch, and also on the northern extension of Sgor an Fhuarain, the Binnein Quartzite adopts a grey-weathering and sugary aspect. Taking the district as a whole, the characters of the Quartzite Bands may be summarised as follows :— GLEN COE QUARTZITE. The Glen Coe Quartzite, from the northern edge of the map to Glen Nevis, is a very pure non-felspathic quartzite, with—in the northern outcrops at any 5 66 Schists SE. of Loch Linnhe. rate—occasional small ferriferous spots. Between Ben Nevis and the Lairig- mor valley the quartzite presents two contrasted facies: the Stob Ban outcrop is pure, and white-weathering ; the Am Bodach outcrop is relatively impure, sugary, and egrey- or pink-weathering. Between the Lairigmor valley and Loch Leven, the two facies are less contrasted: the Tom Meadhoin outcrop is pure and white, while both the outcrops bordering Beinn na Caillich, though relatively felspathic, often weather fairly white. South of Loch Leven the felspathic facies prevails through the Pap of Glencoe and to the summit of Garbh Bheinn ; the non-felspathic facies is strongly developed, however, on either side of the upper reaches of Allt Gleann a’? Chaolais. The small outcrop within the Glen Coe Fault at the head of Allt Coire Mhorair has not been re-examined of late years. BINNEIN QUARTZITE, The Binnein Quartzite is always remarkably pure, and almost always white- weathering. In the neighbourhood of Kinlochleven ferriferous spots are a marked feature, but they are local and do not extend far into Garbh Bheinn on the south. The Binnein Quartzite can readily be distinguished from the fels- pathic facies of the Glen Coe Quartzite developed in the Loch Leven district : the detached anticlinal outcrop of very pure, and to a trifling extent spotted, quartzite at Caolasnacon is, I believe, undoubtedly an inlier of the Binnein Quartzite. EILDE QUARTZITE, In the Hilde outcrop, quartzite, of the purity common in the Binnein out- crop, is only met with in the comparatively insignificant exposures north of Glen Nevis. From Meall Doire na h-Achlais southwards to Allt Coire Mhorair, south of the Leven, the quartzite is uniformly felspathic. Towards its junction with the Eilde Flags there is a markedly impure belt, probably indicating a passage into the flags ; and in the broader part of the outcrop, near Coire an Lochain, the more impure beds are extensively repeated by folding. It is the purer part of the Hilde Quartzite which concerns us chiefly. Mr. Carruthers expresses his opinion of it as follows :—“ The quartzite of the Sgurr Eilde Mor ridge is very different from that of Binnein Mor. Most of the rock is very poor and flaggy, and might be called quartz-biotite-granulite rather than quartzite. The remarkable contrast between these two quartzites is very well seen on crossing from Binnein Mor along the ridge to the south-east. After leaving the massive pure-white quartzite of the Binnein range, a narrow belt of grey mica-schist with abundant quartz-biotite-granulite bands is passed, and then there appear many hundred feet of platy ‘quartzite,’ of poor quality. grey- weathering, and very felspathic. It is only in the heart of the Sgurr Eilde Mir outcrop that good quartzites predominate. Even here they are (relatively to the Binnein rock) felspathic, and they degenerate still farther to the east, where they merge into the pinkish and grey, banded, siliceous granulites of the Kilde Flags... . The continuation of the Eilde Quartzite south of the Leven is also very poor in quality, and has the same banded felspathic appearance as to the north, much of it being very close in type to the Eilde Flags to the east.” The writer cannot refrain from criticising the description just quoted on the ground that it seems to him to convey a very exaggerated impression of the nee! of the Eilde Quartzite. This is one of the few matters of observation and record upon which agreement has not been attained. Below are given the writer’s own impressions regarding the Eilde Quartzite. The Kilde Quartzite everywhere south of Glen Nevis is readily distinguish- able from the Binnein Quartzite, since it is definitely more felspathic tan the latter. All the same, it is, in its thick purer portions, neighbouring the Hilde Schists, a thoroughly good quartzite—a statement based upon the recent re- examination of a whole series of sections: south of Meall Doire na h-Achlais: the southern ridge of Na Gruagaichean (with Carruthers) ; the paths going u to Loch Eilde Mor; the dry alternative channel of the River Leven - od oa north of the main tributary entering Allt Coire Mhorair from the west. In al these exposures the quartzite seems to the writer practically indistinguishable from the felspathic facies of the Glen Coe Quartzite, characteristic of the shores Kinlochleven: First Hypothesis. 67 of Loch Leven. There seems, however, to be a greater tendency for the felspar to be concentrated along the planes of bedding and current-hedding ; in the Glen Coe Quartzite it is more evenly disseminated. The detached outcrop of quartzite, showing from beneath the Glen Coe lavas at Stob Dearg, is of Eilde type. (2) To illustrate the distinctness of the two margins of the Binnein Schists, Mr. Carruthers selects the Allt Nathrach section, 14 mile W.N.W. of Kinlochleven. A narrow belt of Binnein Schists here separates Glen Coe Quartzite, in the Am Bodach Syncline, from Binnein Quartzite lying farther south-east. The western third of this outcrop of Binnein Schists is “grey mica-schist with occasional quartz-biotite-granulite ribs”; the eastern two-thirds are “ mica- schist, with a remarkably regular and rapid alternation of grey quartz-biotite-granulite ribs, each about 2 in. thick.” (3) The distinctness of the two margins of the Binnein Quart- zite is illustrated by Mr. Carruthers in the continuation of the same Allt Nathrach section. The mica-schist with regular ribs, just referred to as occurring on the west side of the Binnein Quartzite, is not met with on the east, but instead, “ grey mica-schist with many quartz-biotite-granulite ribs, rather lenticular and contorted.” Mr. Carruthers lays more stress, however, upon sections § of a mile N.E. of the summit of Binnein Mor. “The western edge of the quartzite, shows 100-200 ft. of regular parallel-banded quartzites in beds 6 in. to 1 ft. thick, associated with afew feet of ‘wasp rock’ before they merge into the Binnein Schists. The eastern edge, on the other hand, is of very massive and pure quartzites, in beds 6 ft. or more thick with only 30 ft. or so of irregularly banded quartz- biotite-granulite and quartzite (without ‘wasp rock’) between them and the Hilde Schists. These features are particularly well seen in the stream which flows east from the lochan between Binnein Mor and Binnein Beag.” C. The Eilde Flags, stratigraphically intercalated, according to Mr. Carruthers, between the Hilde Quartzite and the Stob Quartzite, are evenly-bedded, quartzo-felspathic, micaceous, gneissose flagstones, considerably more crystalline than their accepted equivalents in the Fort-William district. -Pebbles are very rare in the group as a whole, but Mr. Wright recognised a quartzose pebbly zone, with large pebbles of quartz and felspar, near the western margin of the flags, on both sides of the river Leven east of Allt na h-Eilde. In the original sections drawn across the district the writer showed the Eilde Flags of the Loch Kilde Mor outcrop as occupying a rather shallow syncline. It now seems extremely probable from Mr, Carruthers’ work to the north that they connect underground with the flags of the Fort-William outcrop. Mr. Carruthers regards this probability as strong support of the view that the Eilde and Stob Quartzites are stratigraphically distinct. The question of the relations of the Stob Quartzite, the Reservoir Schists, and the Reservoir Flags cannot be discussed conveniently in relation to the outcrops included in Sheet 53, where difficulties are introduced ‘owing to the proximity of the Moor of Rannoch Granite. Mr. Carruthers bases his interpretation upon northern exposures, especially those of Lairig Leacach, Sheet 54. In this section the various bands are met with in the same order from 68 Schists S.E. of Loch Linnhe. west to east as in Sheet 53, Their characters may be summarised as follows :— STOB QUARTZITE. White, massive, and pure in centre, some of it like the Binnein Quartzite, some more felspathic ; flaggy at the edges, especially the eastern edge against the Reservoir Schists. The Stob Quartzite is thicker in Lairig Leacach than anywhere in Sheet 53 except on the river Leven. RESERVOIR SCHISTS, Western division : reddened mica-schist or gneiss, without garnets except in a central band which is studded with large garnets. Central division ; “ Ermine Rock,” a non-garnetiferous mica-schist or gneiss, with large biotite pseudomorphs after actinolite, giving the rock a very character- istic appearance—hence the name “‘ Ermine Rock” proposed by Mr. Carruthers. The Ermine Rock is thick and persistent, and has already been followed by Mr. Carruthers from Loch Treig in Sheet 63, across the corner of Sheet 54, con- taining Lairig Leacach, to the river Leven, a distance of 12 miles. Eastern division : red mica schist or gneiss without garnets. RESERVOIR QUARTZITE, Thin yellowish quartzite, not so well developed in the Lairig Leacach section as in Sheet 53, where it is well exposed near the foot of the Reservoir and on Beinn a’ Chrilaiste. RESERVOIR FLAGS. Pinkish and grey granulites like Eilde Flags, very thin in the Lairig Leacach but thicker southwards towards the Moor of Rannoch Granite, which latter greatly curtails their outcrop in Sheet 53. The Lairig Leacach section continues eastwards to Loch Treig across a broad outcrop of garnetiferous mica-gneiss, followed by miles of country composed of flags. According to Mr. Carruthers both the mica-gneiss and the flags are additional members of his stratigraphical sequence, but this is a matter which in the writer's opinion does not immediately affect the interpretation of Sheet 53. KINLOCHLEVEN: SECOND Hyporussis. In setting out this hypothesis the same group-names will be employed as in the previous section. Two suggestions are offered :-— A. The Binnein and Eilde Schists should perhaps be correlated with the Leven Schists, and the Binnein and Hilde Quartzites with the Glen Coe Quartzite. B. The Reservoir Schists should perhaps be correlated with the Leven Schists, the Stob and Reservoir Quartzites with the Glen Coe Quartzite, and the Reservoir Flags with the Hilde Flags. : These two propositions are independent, and will be considered separately. A, THE SCHISTS AND QUARTZITES WEST OF LOCH EILDE MOR.— The main difficulty in the way of the hypothesis of one quartzite and one mica-schist in the Loch Leven district (apart from the Appin horizons) is the admitted distinctness of character of the Leven Schists as compared with the Binnein and Eilde Schists. Two considerations make the writer hesitate to draw a confident conclusion from this important observation of Mr. Carruthers :— It is possible that the Banded Series of the Leven Schists, : : T of Loch Leven, has been persistently reduced by sliding, he eee while it is easy to Kinlochleven: Second Hypothesis. 69 suppose that in the Binnein and Hilde outcrops the same series has Leen redupli- cated by folding. It is equally possible to imagine that lateral variation plays an important part. The differences between the Leven and Binnein Schists seem to the writer less marked than the differences of the Leven Schists in the two parts of the type outcrop separated by the Ballachulish Granite. The two causes just cited certainly co-operate in the Glen Creran district to introduce a strong contrast between the Leven Schists of the upper and lower limbs respectively of the Ballachulish Fold. But it is much more difficult to invoke their assistance in the present instance; for, supposing the Glen Coe Quartzite outcrop through the Pap of Glencoe and Stob Ban is a fold, one has no reason to suspect that it continues far north-west underground before it closes—in fact one naturally considers the Tom Meadhoin Anticline (Fig. 6, p. 42) as marking the local limit of the extension of the quartzite fold in this direction. The other difficulties raised by Mr. Carruthers seem to the writer to be of less weight. The variations of facies of the Glen Coe Quartzite along its out- crop are as great, in the writer’s eyes, as the variations which dis- tinguish the Binnein Quartzite from the Eilde Quartzite. North of the Lairigmor valley, for instance, the Glen Coe Quartzite in the Stob Ban outcrop is as pure as the Binnein Quartzite, and in the parallel Am Bodach outcrop is as impure as the Hilde Quartzite— and the contrast persists for 4 miles. The lack of detailed agreement between the two sides of bands, which according to Mr. Carruthers are stratigraphical intercalations, and according to the alternative hypothesis are folds, can scarcely be relied upon in such a district as the present. The contrast, just cited, between the Stob Ban and Am Bodach outcrops of the Glen Coe Quartzite is a sufficient indication of this. Another good example is afforded by differences on the two sides of the band of quartzite which, crossing Allt Coire na Ba, terminates on the north side of Na Gruagaichean (Plate V., p. 63). Although the quartzite here is admittedly a fold, there is no detailed agreement in regard to the characters of its two edges, or of the mica-schists following immediately beyond. Garbh Bheinn.—In Garbh Bheinn south of Loch Leven the Binnein Quartzite is in two main outcrops, separated by a fold of Eilde Schists which terminates, north of the loch, on the slopes of Sgor an Fhuarain. The more easterly of these two quartzite outcrops ends in the valley bottom south of Garbh Bheinn, and allows the Eilde Schists just mentioned to unite with the main outcrop of Eilde Schists lying farther east—the exposures are scattered, but Mr. Carruthers and the writer are agreed that they are sufficient to establish this particular point. The other belt of Binnein Quartzite on entering the ill-exposed region (further complicated by the pres- ence of ramifying intrusions) is lost sight of. But a banded series of mica-schist and quartzite, which the writer has no hesitation in regarding as a continuation of the Binnein Schist outcrop, can be traced along the sides of a felsite dyke across Meall Dearg into Coire nan Lab (cf. Fig. 16, p. 108, where the Binnein Schists are shown as “Phyllite”), bounded to the south by well exposed Glen 70 Sehists SE. of Loch Linnhe. Coe Quartzite. Near the east margin of Fig. 16 it will be seen that the “ Phyllites” (Binnein Schists) terminate against a fault which introduces quartzite. The quartzite east of the fault is very like the Glen Coe Quartzite west of the same—all the quartzite in Coire nan Lab is slightly more felspathic than one expects the Binnein Quartzite to be. If now the quartzite east of the fault is Glen Coe Quartzite, there is certainly very little room in the low ground occupied by igneous intrusions (or obscured by moraines) for out- crops of Binnein Schists and Binnein Quartzite before exposures of Eilde Schists are reached. This difficulty is met if we suppose that the western outcrop of the Binnein Quartzite terminates in the same manner as the eastern, allowing the Binnein Schists and Eilde Schists to join hands across. Such a relation, if established, would suggest very strongly that the Binnein and Eilde Schists are one group. Coire Mhorair.—tThe schists of Coire Mhorair, within the Glen Coe boundary-fault, have already been described, p. 57. The “Limestone and Calcareous Schist” of Fig. 18 (p. 111) undoubtedly belong to the Ballachulish Limestone; the “Phyllite” to the Leven Schists ; the “ Quartzite ”—so far as it is included within the southern boundary fault—to the Glen Coe Quartzite. Between the limestone and phyllite there is, as already stated, a thin band of black slate (Ballachulish Slate), but it is too thin to show on the map. The same black slate makes a small exposure beneath the conglomerate shown in Fig. 18 at the head of Coire Odhar-mhor, a little farther east. Beyond this the Glen Coe Lavas hide the schists for a space. When the latter re-appear in the streams south-west of Stob Mhic Mhartuin, they belong to the Hilde Flag Group. Assuming there is no fault hidden beneath the lavas, there is only } mile measured across the strike between the Ballachulish Slates of Coire Odhar-mhor and the Hilde Flags. The outcrop of steeply dipping Glen Coe Quartzite in Coire Mhorair is itself ~, mile wide, which leaves } mile for the Binnein Schists, Binnein Quartzite, Eilde Schists, and Hilde Quartzite, whereas the last named alone never has an outcrop less than 4 mile broad at any point for a distance of 6 miles north of the Glen Coe Fault. This difficulty is at once met by a natural development of the explanation already suggested for the behaviour of the schists at the foot of Garbh Bheinn. If first the Binnein Quartzite passes beneath the coalescing Binnein and Hilde Schists (supposed to be one group on this hypothesis), and then these schists ‘ass beneath the coalescing Glen Coe and Eilde Quartzites, this would leave only one band of quartzite to separate the limestone, black slate, and phyllite, of Coire Mhorair, from the Hilde Flags farther east. Sections south of Sheet 53—It will be shown, p. 77, that the Coire Mhorair sequence can with great probability be recognised in Coire an Easain (Sheet 54); but the various groups separating the Eilde Flags and Ballachulish Limestone are so exceptionally thin in this burn (probably as a result of sliding), that they afford no safe basis for determining the number of quartzites and mica-schists originally present in the stratigraphical sequence. The conditions are otherwise in the Glen Orchy district to the south where Mr. Kinlochleven : Second Hypothesis. 71 3S ose Se 4 N NARS RRO DNAX SR Ss Highly Inclined ~ Vertical ee SV O'S L.- Limestone Sch. = Mica Schise, Dip, amount in degrees iy es? WX Contorted with Pitch \eF EZ _Q= Guarézite: Fl.= Flags STE ee oa C Y N x \ \ x ? NS; ay LER = IW SENS NS Sea Level Fic. 8.—Map of Stob Ban and Meall a’ Bhiurich, with Serial Sections a, b, and c. L. = Limestones, The sections illustrate the Quartzite, Eilde Flags. Sch.=Mica Schist. Q.=Quartzite. Fl. =Flags. hypothesis that the succession is Leven Schists, Glen Coe 72 Schists SE. of Loch Linnhe. Macgregor and the writer * believe that they have recognised the Ballachulish Slide, with the Ballachulish Limestone above, and with a single thick quartzite a little distance below, resting directly upon Eilde Flags. There is in the miles of outcrop examined no sugges- tion of a multiple succession of quartzites and mica-schists. Meall a’ Bhuirich and Stob Ban (Sheet 54).—Meall a’ Bhuirich exhibits an anticlinal structure. The broad belt of Binnein Quartzite, which constitutes the bulk of the mountain, tapers to a point, 700 yds. north-east of the summit, and pitches at moderate angles beneath the Binnein Schists. Owing to quartzite scree, it is uncertain how far the Binnein Schists extend south-west along the south-eastern margin of this anticline. On the map (Fig. 8, p. 71) they are shown terminating as soon as the obscure scree-covered ground is reached, but it is quite possible that they continue beneath the scree and join with the Hilde Schist outcrop beyond. If no theoretical point of importance were involved, one would scarcely question the equivalent of the Binnein Schist outcrop, east of the anticlinal nose just described, and the Eilde Schist outcrop, south of the scree. Both lie structurally above a thick mass of Binnein Quartzite occurring to the west; and both are separated from the Eilde Flags to the east by relatively thin bands of quartzite. If the two outcrops of mica-schist are equivalent, it follows that the Binnein Quartzite outcrop, as a whole, is anticlinal in structure— a conclusion already suggested by its behaviour in the neighbourhood of Garbh Bheinn and Coire Mhorair. Further, the Binnein Schist outcrop, east of the anticlinal nose of quartzite on Meall a’ Bhuirich, is obviously the structural continuation of the Binnein Schist outcrop that runs east of the Stob Ban anticline of quartzite. Five miles north-east of Stob Ban, Mr. Carruthers finds that the eastern Binnein Schist outcrop terminates synclinally. On the correlation suggested above, it follows that the Hilde Schist outcrop is also synclinal in structure, and that the Hilde Quartzite is continuous underground with the Binnein Quartzite. This view is in accord with the local evidence in other respects : (1) The quartzite separating the known Eilde Schists from the Eilde Flags in Meall a’ Bhuirich, especially the more northerly outcrop shown in Fig. 8, is of the degree of purity characteristic of the Binnein rather than the Eilde outcrop. (2) The quartzite of Stob Ban is of very high purity, and rises as a complex anticline from beneath the Binnein Schists, thus agreeing in composition and structural position with the Binnein Quartzite of Meall a’ Bhuirich, and yet it includes an extensive core of Hilde Flags without the intervention of zones corresponding to the Eilde Schists and Eilde Quartzite. It is clear that alternative interpretations may be offered. For instance, the Hilde Schists and Hilde Quartzite outcrops in Meall a’ Bhuirich may both terminate in the scree-covered ground, against the Meall a’ Bhuirich Slide, the existence of which is fairly obvious on other grounds ; and their apparent reappearance to the north may be entirely deceptive. And, similarly, the non-appearance of Eilde Schists and Hilde (uartzite may be due to a hiatus, either mechanical or stratigraphical, in the sequence at this point. * E. B. Bailey and M. Macgregor, ‘The Glen Orchy Anticline (Argyllshire),’ Quart. Journ, Geol. Soc., 1912, vol. Ixviii. p. 164, . aes Kinlochleven: Minor Folds. 73 (B) THE SCHISTS AND QUARTZITES FAST OF LOCI EILDE MOR.— The evidence for correlating the Reservoir Schists with the Leven Schists lies wholly in the ground to the south of Sheet 53. Beinn Udlaidh and Loch Dochard.*—Rocks, which it appears justifiable to regard as the continuation of the Eilde Flags, occupy a large tract of country about Beinn Udlaidh and Loch Dochard. In Beinn Udlaidh the following downward structural sequence can be established : Flags. White fine-grained pure Quartzite. Grey pelitic Mica-Schist, often garnetiferous. Between the quartzite and mica-schist a banded series is locally well developed, sometimes with black carbonaceous seams giving rise to “ wasp rock.” The quartzite and mica-schist of Beinn Udlaidh are disposed in a very simple, and very perfectly exposed, recumbent anticline of a minimum horizontal extent of 2 miles. At Loch Dochard there is another important outcrop of mica-schist, which, as it terminates towards the north-east, and includes massive cores of tremolite-schist (probably altered limestone), is regarded as marking the position of another fold. From the general structure of the ground, if the Loch Dochard mica-schist is in a fold at all, as appears tolerably certain, it must be in an anticline repeating the Beinn Udlaidh mica-schist. A few feet of white quartzite locally occur at the margin of the Loch Dochard mica-schist, and strengthen this correlation. A glance at a map, showing the geographical position of the Loch Dochard Fold in relation to the outcrop of the Reservoir Schists,f makes it appear highly probable that the mica-schists in the two cases are the same; while the non-appearance of the Loch Dochard Limestone in the Reservoir Schists suggests that the latter are in a fold. On this reading, the quartzites bounding the Reservoir Schists are to be correlated at once with the quartzite of Beinn Udlaidh, and the Reservoir Flags with the Eilde Flags. The further correlation of the Loch Dochard Limestone with the Ballachulish Limestone, the Reservoir-Dochard-Udlaidh Schists with the Leven Schists, and the Stob-Reservoir-Udlaidh Quartzite with the Glen Coe Quartzite may be suggested as extremely probable. The abundance of actinolite in the “ Ermine Rock” of the Reservoir Schists reminds one very definitely of the same feature in the Leven Schists of the Glen Etive region, and (in smaller crystals) in the Leven Schists of the type outcrop near the river Spean (Sheet 62). The quartzites appear to be indistinguishable. KINLOCHLEVEN: Minor Fo.ps. The Kinlochleven district, quite apart from its uncertainties of interpretation, is worthy of special attention on account of the many beautiful examples of folds which it displays (Figs. 9 and 10, pp. 74,75). The well-bedded quartzites, and the steep, bare hill-slopes, are conjointly responsible for this attractive peculiarity. * E. B. Bailey and M. Macgregor, ‘The Glen Orchy Anticline (Argyllshire),’ Quart. Journ. Geol. Soc., 1912, vol. Ixviii. p. 164. + Lettered h, Plate XLIJ., Quart. Journ. Geol, Soc., 1910, vol. Ixvi. p. 587, Schists S.E. of Loch Linnhe. ‘ ‘OL ‘SI ‘SWOTJOOS GAIZBUID}[C 09g “(g) ey1zq1UnG 0079 woyH ‘(L) syst MeAoTy “(9) ouoysoUTT YsrNYorTIvA “(G) seqe1g ystngourieg ‘(p) epzqaend ulddy s} worsseoons oy} vq) stsoqjodAy ot} UO UMBIP ole SUOT}OaB OTL, *(S][NVJ-P]OJ) SOPI[S oWOIpUl SoU] YOBlq OL ‘Sp[O YsipNoryieg pure ‘Svog yoruoy ‘aiddy ey} Jo suoryeper oyy Styeaysny]E aaa] Yoo] ssoroe ‘q pue ‘UeAey Yo] Jo WON ‘O-V suouovIg—'6 “OL Saw tt uw al 6 ~ 8 “a 9 s - £ 2 : 0 ct i See 75 “NeOlM pe en Se S f{LFWNEEG AS : Wie RE RR ES Ne RAF Wy Ey Z Se RS ES Lg So i i = LL SS CO iN oS WW EETEN if WZ fe _ BR ATEN Wh : OU GALAMIT Se - o7 3 BIOT e7 NLLAT HIOT J @ AG (nv auioa a aaarTyopay) 40 3 8) FO1TS WEN DRIYTTVE \ YINOSGIEL MT WIYOD HSITHHIVITWE — _ zy¥09V NIddY a eee 29 zi S ~~ 91VOND VN NNIZE NIEM ONS ee Ne, fis nos | WSS SS : GAUEES ES Cig GPE ¢ STS SS Se ew NESS WpE SLE, mS NN SS 3 4 Nas @ oe Mh ) YS, oe Nldd¥ YIM 1Y0s a NHALT UDALL pans 9 0 ; Waar [YIOPULY fO AHNDYIA-H VN 3 2 YOY 270f [ff 3A4I0I A01s Nv@ 901s 4 263g yoynov —3AIWIOl H WN 9 SOE cw, bey Ly SS Ly ‘ TIAII VIS GLLEELLLLIIIE OLY. * SERED OA ee a L Le Gro CG. : ee = @ 6 WOM FATT ‘HIOT t eee op: SUOON MNvHWnd 99 HOvNOY 19 snvaw OW NIANNIG TIAII VIS : =_— : / —— *S0Ob'3 y x ; \ ANY SSESS Ne FANS WSS wv Tivaw Nivsv3 bz 3u103 8O01S P2enoy 7 Minor Folds. ‘Kinlochleven *(LH'E) SBeLq Toaresoyy (Org) aneqend Mosresoy {(Pog'T) systqog Hoarsay (H's) ayzszend ois (6) sBerg epua ‘CO'a) aqyzqrend opp “(qoS'H) SISOS OPT (‘Oug) spzrend ursuurg (WoS Ug) sysiyog Wouamlg—* sMo]]oy Sv st ‘(g) oqIzjzend 90) Ueyx) 1oqjv ‘UOTssedoNS oY} 4VT} sisoqjodéy aq} Uo UMBICT ‘6 ‘B1q 0} oatqeuseye ‘Q-V SMoyIegS—'OT “PLT ——— SIA ZH Mb ol (Ss) JAITS HSITAHIVITVS NO INILSTH FHOD HSTINHIVIIVE ONG 78 ay su09 Niddy YUM LyOs9. JIARI VIS a CW SsssQ“sw HP, wazrvas 6 OS 7% _ Z 76 Schists SH. of Loch Linnhe. The more noteworthy folds will be considered in connection with the various quartzite outcrops. THE GLEN COE QUARTZITE—Three minor recumbent folds, super- imposed, are seen in spectacular fashion (Section A, Figs. 9 and 10, pp. 74, 75) by an observer stationed at the north-east corner of the map and looking south-west along the Stob Coire Easain ridge (reached from Spean Bridge Station, Sheet 62). Farther south, Sgurr a’ Bhuic, immediately north of Glen Nevis (Plate VI.) shows a very fine upright arch of Glen Coe Quartzite with a core of Binnein Schists (partly obscured by a landslip). The south face of Glen Nevis (Section B, Figs. 9 and 10, pp. 74, 75) exhibits the very fine small-scale recumbent folds illustrated in Plate V., p. 63, with the steeper folds of Sgurr a’ Mhaim in the background. The lower part of Stob Ban, cut into by Allt Coire a’ Mhusgain, gives another good section of a flat fold (Section C, Figs. 9 and 10, pp. 74, 75). The Am Bodach Syncline of Glen Coe Quartzite is less obvious, but more important, than the folds just mentioned. The confluence of the quartzite of this outcrop with that of the main outcrop to the north-west, can be followed through the difficult ground south of Garbh Bheinn. On the northern slopes of Garbh Bheinn itself, the synclinal structure of the outcrop is demonstrable, although it takes time and careful searching to establish it owing to the frost-riven nature of many of the exposures. In Stob Coire na h-Eirghe, north of Loch Leven, the centre of the outcrop shows its synclinal structure much more clearly than in Garbh Bheinn, but the margins of the outcrop consist of thick masses of steeply inclined quartzite without obvious folding (Section C, Figs. 9 and 10, pp. 74, 75). Farther north, the quartzite terminates in an obscure grassy slope beyond An Garbhanach (Plate V., p. 63). In An Garbhanach the quartzite is very highly inclined with its normal strike, and it is fairly obvious that its non-continuance to the foot of the slope is due to erosion having laid bare the underlying Binnein Schists. The latter rest flatly upon an inlier of quartzite exposed in the valley bottom (Plate V.). It is uncertain whether this quartzite inlier is in underground continuation with the main Glen Coe outcrop to the west or with the Binnein outcrop to the east. Not only has the Glen Coe Quartzite been bent down into the Am Bodach Syncline, but the two layers of quartzite thus formed, with the intervening layer of Binnein Schists, have been sharply re-folded in Beinn na Caillich (east of Mam na Gualainn, Section D, Figs. 9 and 10). THE BINNEIN QUARTZITE.—The isolated outcrop of Binnein Quart- zite in the heart of the Binnein Schists at Caolasnacon, on Loch Leven, is anticlinal in structure, as may be deduced fairly certainly from the exposures north of Loch Leven, and, independently, from its situation between the main outcrop of Glen Coe Quartzite and the Am Bodach Syncline. Similarly, at Meall a’ Bhuirich, the Binnein Quartzite passes beneath Binnein Schists. E. B.B. Elsewhere, however, within Sheet 53, the Binnein Quartzite in many conspicuous folds always overlies the Binnein Schists. The steep, compressed anticlines and synclines shown in Binnein Mor ‘dyspury fq parnosqo aurpayue jo WRey UL syst ulauuig APIS PURTY-IYSLU UO JUTPOUAS AG POMOT[OF 9}IZ]IUNGH BOM Uapry JO oul UP “SIAMN Nady) OAH .V wuADg IA ALY 1d Kinlochleven: Minor Folds. 77 and Binnein Beag are among the most striking and picturesque features of the whole district (Plate V., p. 63, and Section B, Figs. 9 and 10, pp. 74, 75). The southernmost anticline of Binnein Schists extends south into Sgor an Fhuarain. As a rule the pitch of these folds is quite moderate, but at the head of Allt Coire na Ba a quartz- ite fold terminates with vertical pitch, and the section in the cliff above the stream has the appearance of a sheet of corrugated iron set on end. W. B. W. The Binnein Quartzite, where it rests in folds upon Binnein Schists, passes in folds eastwards beneath Hilde Schists. This feature can be easily recognised on a large scale in the hill-slopes north of Kinlochleven (Section C, Figs. 9 and 10, pp. 74, 75). South of Kin- lochleven, where the Hilde Schists wrap round the main outcrop of Binnein Quartzite, at the foot of Garbh Bheinn, the pitch of the folding is too steep to give reliable information as to which group is uppermost. But from the way in which the outcrop of the Balla- chulish Slide is sweeping across country from west to east, along the course of Glen Coe, it is obvious that the general pitch in this neighbourhood must be steeply towards the south. and so it is justifiable to assume that the main outcrop of Binnein Quartzite in Garbh Bheinn, at its termination, pitches southwards under the Eilde Schists. From the explanation given in the previous paragraph it follows that the fold of Hilde Schists, reaching north to near the summit of Sgor an Fhuarain, must be a syncline. It encloses a definitely synclinal outcrop of quartzite on the ridge of Garbh Bheinn, a fact that somewhat strengthens this interpretation ; it would be interest- ing, however, to find out to which quartzite this outlier should be assigned, before basing too much upon its occurrence—in character it certainly approaches the Binnein type. The northern termination of the Eilde Schist Fold shows very steep pitches on Sgor an Fhuarain. The appearances definitely suggest a steeply pitching anticline, and were interpreted in this sense originally ; but there is no difficulty in accepting the fold as a syncline, with steeply inverted pitch, in accordance with the evidence outlined above, In the slopes south of Kinlochleven, an isolated anticline of Eilde Schists rises high into the heart of the main outcrop of Binnein Quartzite. The synclinal structure of the quartzite outcrop on the west is well displayed, and it is clear that this anticline of Eilde Schists is in underground connection with the Kilde Schist Syncline described in the previous paragraph (Section D, Figs. 9 and 10, pp. 74,75). West of the Kinlochleven Anticline of Eilde Schists, the Binnein Quartzite is disposed in a conspicuous syncline, well seen in the ruck cutting within the private grounds of the Kinlochleven Aluminium Works. This is followed by an anticline visible on the hillside beyond. THE EILDE QUARTZITE—The Eilde Quartzite is involved in the flat belt of folding which reaches from Meall an Doire Dharaich to Allt Coire na Bu. Here it obviously overlies the Eilde Schists, just as the latter overlie the Binnein Quartzite. , I. Stob Dearg to Coire an Easain (Sheet 54).—The Coire Mhorair section north of Glen Coe has already been discussed (p. 57). From SGOR NAM FIANNAIDH Coire Mhorair 78 Schists S.E. of Loch Linnhe. the constant south-easterly strike of the Hilde Flags, from Stob Mhic Mhartuin, through Stob Dearg and Sron na Créise, to Meall a’ Bhuiridh (Fig. 23, p. 116), it appears probable that the schists of Coire Mhorair are buried beneath the lavas only a short way behind the escarpment, This suspicion is strengthened by the emergence of o Oost z 2 bes sd “nO iS Eo Nad z Sy wi = SI = [. o a n eee a i 3 3 =f BP oh > wi PFD, Mls gy oo B 5 ES a S on fits: g 2 gS og. o In a a 7) = ao Bsn ° ©. “a & ui tal wo geo 3 o oO = a Bo ek 8 z 2 e 2 Bly = 8 LY : 2 a} 8 sos NW fo, uy 3 Ss & €2s Sz a “5 Fan MS See is qo ol 2 o HS S yoe x obs zaé gs & aa Z yl Wy oa i o 3 a ges ee VS % ae £6 Ss « ee + Sy pe 7 = oO Hod % S td o, Pog a Se ~ ¢ 808 en g w q 3 eae \ GS & FEA q \ bs es! a Kt SB 8 3ia a \ e ep Ra % oS WN ny a 3 oo u WN” 2 ~ s ° N 3 = fiey x ‘ : Wernoe 5 © cM Ba 8°88 SE eh. “ain Oe A enn 3 oye 5 4 sons <3 yy = =x o Be 6,892 e a a 2 sete on 3 = 5 So. MEA s < z S- ne 2g 48 I ag a a ic = Ef we EE@SE a 4s ig EF oot = go z\ mS Beg w wn 4 Taeg il a tz os Sepes 9 w > a Egos v a ia ay SoHE. 3° © as 2 S44 o eu BF aed wl tT BS we@a a vo a BS s,s S 3 NSM | sa RE GR es = 5 Baars an oO 3 a gan ‘< a 2a wo » a a S S88gc o oe 7 w Zea nod R Zoerge . oo. ey S Re OS eNO! zi Oy aA. Ee SRR eK -3 g ee 3° yg S990 s Go. 3 S 33 esee a =o 2s s Ssangse & 53 aa. zs Wo aS* esa x ie S ee “a g3g2ee c = S a a o = gO R808 SS “ts “ 3 Bae “s00 S = 2 Ao ,fao SS a mn 2Qarger a z rs | Sa gaered z 2 meosag o = A =o 0 2 43 BO mi 2 rt of OSE Cs he if Ss OyNes = ro sAlVes 0 A 2359 es Z i wa? Dos < : z : 0 ZLenda ut = < z= Saatia a + N © aa o oO . 7 + » BsghaaJg uw | 3 o = x = & B= Oo ana Eilde Quartzite at the base of the lavas in Stob Dearg (p. 67). And it is virtually demonstrated by the appearance in Allt Coire an Easain (Fig. 23, p. 116) of the following sequence, reckoned outwards from the edge of the voleanic rocks : Thick hornfelsed garnetiferous mica-schist (probably Leven Schists). Thin calc-silicate-hornfels (probably Ballachulish Limestone). Coire an Easain. 79 Probable position of the Ballachulish Slide. Thin Quartzite. Thick Eilde Flags. The calc-silicate-hornfels is very considerably thinner than the Ballachulish Limestone in Coire Mhorair, but can be paralleled in this respect in some of the exposures south of Dalness, Glen Etive. The only feature peculiar to the section is the thinness of the quartzite underlying the limestone, but one is very likely dealing here with a diminution due to sliding. E. B. B, CHAPTER V. METAMORPHIC ROCKS (Continued). LOCHABER AND APPIN (Léswmé). A BRIEF summary * will now be given of the results, which have been set forth in detail in the previous chapter, concerning the schists on the south-east side of Loch Linnhe. ; The following stratigraphical sequence has been established, but whether it should be read upwards or downwards is a matter for future inquiries to decide :— 9. Hilde Flags (commonly classed with the Moine Gneisses of the Central Highlands). 8. Glen Coe Quartzite (fine-grained).} 7. Leven Schists (grey phyllites, with “Banded Series” against Glen Coe Quartzite). 6. Ballachulish Limestone (dark-grey; with thick cream-coloured margin against Leven Schists). 5. Ballachulish Slates (black). 4. Striped Transition Series of Appin Quartzite (classed with 4 in Figs. 9-11, except in Sections F and G). 4. Massive Appin Quartzite (pebbly). 3. Appin Limestone (cream-coloured, pink, etc.). 2. Appin Phyllites (with large proportions of flaggy quartzite). 1. Cuil Bay Slates (black). In forming a mental picture of the original features of this succession, the thickness of the individual groups may be taken in general as some hundreds of feet. The Appin Limestone is the thinnest group, and is perhaps not much more than 100 ft. thick; the Leven Schists, Glen Coe Quartzite, and Hilde Flags must more nearly approach 1000 ft. All the groups, with the possible exception of 8 and 9, form a continuous stratigraphical sequence linked by passage zones. The original order of superposition is uncertain. Regional metamorphism increases in a south-east direction. Thus the metamorphism of the Eilde Flags about Loch Eilde Mor is of a considerably higher grade than near Fort William. The local effects of contact-metamorphism are dealt with in Chapter XIV. * A fuller statement has already been published. E. B. Bailey, ‘Recumbent Folds in the Schists of the Scottish Highlands,’ Quart. Journ. Geol. Soc., 1910, vol. lxvi. p. 586. + According to an alternative interpretation, 8 and 9 are separated by four groups, two of mica-schist and two of quartzite, see p. 63. The account given in the present chapter 1s not dependent, wpan this part of the sequence. Résumé, 81 Within the limits of the district the facies of individual groups remains wonderfully constant—save that the Leven Schists, in their Lae AT, Te least LRANAW’ PRALINE! CLEN LOCH CRERAN LEVEN synCLine 3 P GLEN ETIVE Groups 1-6 of the table m the text, included tithe. Appin, Aonach Beag, cS and LBullachulish Cores. **|Groups 7-9 where they structurally * ‘loverlie the Ballachulish Core. "| Groups 7-9 where they structurally ev lne: bebweew Whe Lallachulisiy and Appi Cures. Groups 7-9 where they structural dndirtic the Appi Core.” Fia. 12.—Generalised Map and Section showing the relation of the Appin, Aonach Beag, and Ballachulish Cores. outcrop south of the Ballachulish Granite, include what is for them a quite unusual proportion of banded quartzose rocks. 6 82 Schists SL. of Loch Linnhe. The main structural features of the district are presented in outline in Fig. 12. For the sake of clearness the rocks of the district are divided in this figure into two main series, comprising groups 1-6 and 7-9 respectively. These two series are interfolded in great recumbent folds.* The folds enclosing groups 1-6 gape towards the south-east, and have been given definite names, The lowest great fold of this suite is known as the Appin Fold; above it lies the Ballachulish Fold. The Aonach Beag Fold at an intermediate level is of minor importance, and may, in fact, be regarded as an apophysis of the Ballachulish Fold. For convenience, the rocks of groups 1-6, where they occur in the hearts of these three superimposed folds, are defined as constituting the Appin, Aonach Beag, and Ballachulish Cores respectively. The complementary fold-cores closing towards the north-west, and con- stituted of Groups 7-9, have not been given special names. Their outcrops lie in the stippled areas-of Fig. 12, where three main folds are distinguished by the density of the stippling employed. The Ballachulish Core is very fully exposed, for it has been bent into a great basin-shaped structure, right round the rim of which its outcrop has been traced, except where interrupted by the Glen Coe lavas and KEtive Granite. Reckoning from the gape of the Ballachulish Core, on the north side of Loch Leven, to the exposure ofits taper end in Coire an Easain at the extreme eastern edge of the cauldron-subsidence of Glen Coe (Fig. 23, p. 116), we find that the Ballachulish Fold extends at least 12 miles across the strike. If we exclude the Coire an Easain section on the ground of its isolation, and base instead upon Coire Mhorair, north of Glen Coe, the minimum extent of the Ballachulish Fold is still 6 miles—and from this estimate there is apparently no escape. Glen Etive, near Dalness, affords us with good examples of what are known as ‘windows.’ The Windows of Etive have been opened by erosion midway between Ballachulish and Coire an Easain, and allow us a glimpse of the Ballachulish Core in its underground course between these two places. At Stob Ban, between Ben Nevis and Loch Leven, an even more important section is exposed. Here the great basin structure, holding the Ballachulish Core, has narrowed down to a single syncline, and finally rises into the air. When attention is given to the map (Sheet 53), and the sections (Figs. 9-11, pp. 74, 75, 78), it is seen that the limbs of the various recumbent folds are, in many instances, replaced by slides. Thus the lower limb of the Appin Fold is, to a large extent, cut out by the Fort William Slide (Sections C and D, Figs. 9 and 10), and that of the Ballachulish Fold by the Ballachulish Slide (Sections D-H, Figs. 9-11). The two slides just mentioned are strictly homologous. The Ballachulish Slide is the more interesting on account of its more extensive exposure. In all sections from Ballachulish to Coire an asain, with the trivial exception of those north of Allt Lairige Moire,f it cuts out a great thickness of Leven Schists; the distance * A discussion of structural terms is given in Chapter IIT. + The Ballachulish Slide is not shown on the map in these northern ex- posures, owing to the fact that its presence is not sufficiently clearly indicated by the loca] evidence ; but there can be no real doubt of its continuity. Résumé, 83 over which this condition of affairs holds is 14 miles,* measured across strike, and this distance must be taken as a minimum for the dis- placement effected along the Ballachulish Slide. Only some of the slides of the district belong to the Fort William and Ballachulish suite; others are complementary. Thus, for instance, while the slide 8’ in Fig. 11, p. 78, is homologous to the Ballachulish Slide (B.S.), the slides 8’ and 8” are of complementary nature. Another good example of the complementary suite is the Glen Stockdale Slide (Fig. 3, p. 38). An obvious feature of the sections illustrating the structure of the ground is that the earlier formed cores and slides have suffered extensive corrugation of isoclinal type. It seems not improbable that the metamorphism of the many small intrusions of the Glen Creran district was connected with movement of a comparatively late date. These schistose igneous rocks, judging from their field distribution (p. 56), must have been intruded after differential movement along the Ballachulish Slide had come to an end. E. B, B * Or9 miles if the Coire an Easain section is neglected, and the estimate based on Coire Mhorair and Dalness. CHAPTER VI. METAMORPHIC ROCKS (Continued). ArpGour District. Tue foliated rocks lying to the north-west of Loch Linnhe were mapped for the most part by the late Mr. Grant Wilson. In the main these rocks are altered sediments, but they also include the Glen Scaddle complex of epidiorite and other igneous rocks, the Sgurr Dhomhnuill mass of augen-gneiss farther to the west, and numerous small lenticles of hornblende-schist. The metamorphic sediments of this district are of the following main types :— (a) Siliceous gneisses of “ Moine” type, with a. considerable proportion of felspar and mica accompanying the quartz; these represent banded, impure, arenaceous sediments. (b) Highly micaceous gneisses ; these represent argillaceous sediments. (c) Quartz-granulites ; these represent exceptionally pure sandstones. (d) Marble ; this is of restricted occurrence and represents a thick, pure limestone or dolomite. The structural and stratigraphical relations of these various types are very imperfectly known. Their variations of dip and strike are sufficiently clearly indicated on the map. (a) The siliceous gneisses form the bulk of the country. As far south as Glen Gour “ the chief type is a grey flaggy rock, with bands of varying thickness, and lighter and darker in tint, according to the presence of more or less black mica. Felspar and white mica are always present.” * South of Glen Gour the flaggy character of what appears to be the continuation of these siliceous gneisses is lost, owing to an intricate puckering of the bedding planes. In this southern district the crystallisation of the gneisses is exceedingly coarse. (b) The highly micaceous, or pelitic, gneisses occur near the western border of the map, their outcrop, so far as it is included in the present district, terminates in a tongue extending southwards from Glen Gour across Glen Tarbert. A common type is muscovite- biotite-gneiss—“a coarsely crystalline rock with muscovite and biotite in large flakes, and with quartz and felspar, which may be scattered either more or less regularly throughout, or may be arranged in layers, knots, and lenticles of varying size. The rock in places is flaggy, and alternates with layers of granulitic quartz- * J.S. Grant Wilson, in ‘Summary of Progress for 1897,’ Mem. Geol. Survey, 1898, p. 65. 84 Ardgour District. 85 schist of Moine type, while in other places it becomes massive and coarse, and assumes the appearance of a foliated igneous rock. In certain parts of the present area it is much traversed with knots, strings, and veins of quartzo-felspathic material, sometimes carrying muscovite or biotite, or even both micas, and the crystalline texture is often coarse.” In places muscovite is quite inconspicuous, and the rock is then a biotite-gneiss. The material of the tongue which reaches south from Glen Gour affords a good example of this biotite- gneiss. E. B. B. (c) The quartz-granulites are only shown separately on the map ina belt reaching south across Glen Tarbert to Cilmalieu, and in small isolated patches, apparently outliers, on Meall a’ Chuilinn and Maol Odhar farther east, The main outcrop is flanked on both sides by banded, impure, siliceous gneisses of the type (a), while the detached outcrops are completely surrounded by rocks of this character. At the head of Glen Galmadale the main outcrop swings round to the east and thus comes across the line of the minor outcrops already mentioned. This renders it probable that the tongue of micaceous gneiss (6), which terminates on the southern slopes of Glen Tarbert, marks the core of a fold, since it occupies a symmetrical position midway between the main and minor outcrops of quartz-granulite (c). It is quite uncertain whether the siliceous gneisses of type (a) lying on the west and south of the main outcrop of quartz-granulite are on the same horizon as those on the east and north which come into contact with the micaceous gneiss (b). A good example of the interfolding of the western siliceous gneisses and the quartz-granulite is afforded in the mountain, west of Maol Odhar. The crystallisation of the quartz-granulite is often exceedingly coarse, reminding one of the structure of glacier ice. i. B. M., E, B. B. (d@) The Ardgour Marble was discovered by Mr. Wilson in 1902. It consists of a thick bed or beds of crystalline marble weathering pale grey. The outcrops lie between Glen Gour and Glen Scaddle. Specimens examined by Dr. Flett (11176-9 and 11181) have been found to contain calcite, forsterite, colourless pyroxene, felspar, garnet, sphene, and graphite in very variable proportions. The marble is everywhere closely associated with portions of the Glen Scaddle igneous complex. It is accompanied in some of its outcrops by a thinly bedded series of slightly calcareous grey pelitic sediments containing quartzose bands. These sediments have quartz veins developed along their bedding planes, but their degree of regional metamorphism is certainly not very high. Dr. Peach * has accordingly suggested that they have been protected through having been contact-altered by the Glen Scaddle intrusions prior to the period of movement.t As will appear immediately, it is certain that the introduction of the Glen Scaddle Complex antedated a large part of the schist-making movement ; it is doubtful, however, whether it * ‘Summary of Progress for 1903,’ Mem. Geol. Survey, 1904, p. 66. + Cf. The protection of hornfels round the augen-gneiss of Inchbae, Ross- shire, first interpreted by Dr. Peach ; C. T. Clough, C. B. Crampton, J. 5S. Flett, ‘The Augen-Gneiss and Moine Sediments of Ross-shire,’ Geol. Mag., 1910, p. 337, and in ‘The Geology of Ben Wyvis, Carn Chuinneag, Inchhae, and the Surround- ing Country,’ Mem. Geol. Survey, 1912, chaps. v. and vi. 86 Schists N.W. of Loch Linnhe. is older than the whole of this movement. In one locality the dioritic rock is crowded with inclusions of quartzite, gneiss, etc., including some of pelitic character containing much kyanite. Here the foliation of the containing rock is indistinct, and it seems as if some of the inclusions may have been foliated before their immersion in the igneous magma. The rocks entering into the Glen Scaddle Complex have been classified by Mr. Wilson * as follows :— (1) Epidiorite. (2) Granulitic gneiss. ; (8) Diorite dykes, cutting granulitic gneiss and also the surrounding gneissose flagstones. 9 ; (4) Grey basic dykes, now represented by hornblende-biotite-schists ; these cut (1), (2), (3), and the gneissose flagstones. (5) Granulitic gneiss and Aplite (Porphyrite) dykes ; these cut (1), (2), (3), (4) and the gneissose flagstones. 1, The main epidiorite mass is of somewhat irregular shape. It stretches north-westwards from Loch Linnhe, with one unimportant interruption, for about half a dozen miles, and it has a maximum breadth of rather less than 3 miles. In texture the epidiorite varies from coarse to medium, and it is generally of a dark grey colour, which, at several points not far from the margin, gives place to red. It appears to have been originally a diorite and, to a large extent, it has escaped conspicuous alteration. It is often massive or imperfectly foliated (8233), but in many instances it is ina thoroughly gneissose condition. Mr. Wilson t found that the foliation commonly strikes between 15° and 25° west of north, and that it is parallel to the foliation of the surrounding sedimentary gneisses. There are many isolated masses of epidiorite in connection with the Glen Scaddle Complex, and perhaps the numberless lenticles of hornblende-schist found intruded among the sedimentary gneisses of the Glen Tarbert district belong to the same suite. The small mass of epidiorite by the roadside at Gearradh, between Glen Gour and Glen Tarbert, consists of dark foliated rock resembling that of which the: lenticles is composed. The Inversanda outcrop at the mouth of Glen Tarbert is a grey dioritic rock, much shattered owing to the proximity of the Loch Linnhe fault, but with very little foliation. The detached outcrops between Glen Gour and Glen Scaddle are in every way similar to the main mass of Glen Scaddle. 2. The more important epidiorite outcrops are surrounded by a felspathised zone of granulitic gneiss (8235) from 150 to 300 yds. in breadth.{ A typical exposure of this felspathised zone is afforded by the river Scaddle, near a small waterfall, half a mile above the junction of this river with the Cona. It is a fine-grained, banded rock in which close examination reveals a. most unusual proportion of fairly idiomorphic felspar. This felspathic material appears to * J. S. Grant Wilson, in ‘Summary of Progress for 1900,’ Mem. Geol. Survey, 1901, p. 45. } J. 8. Grant Wilson, in ‘Summary of Progress for 1898,’ Mem. Geol. Survey, 1899, pp. 40-42. a J.8.G. Wilson, in ‘Summary of Progress for 1898,’ Mem. Geol. Survey, 1899, p. 40. Ardgour District. 87 have been superimposed upon a thinly bedded, fairly pelitic sediment, considerably more pelitic, for instance, than that which has given rise to the siliceous gneisses occurring farther downstream. The fels- pathised rock has suffered very extensive shearing after the consolida- tion of the introduced felspathic material, in fact, it is quite possible that all the movement chronicled in the rocks of the district followed the epoch of intrusion, but this has not been proved for certain. “3. In Glen Cona the river-section for a distance of half a mile, below the keeper’s house, exposes numerous foliated diorite dykes from 2 to 30 ft. broad which traverse the flagstones. Some of these in turn are cut by small dykes of granulitic gneiss and veins of aplite. To the south-east of Glen Scaddle several broad diorite dykes cut both the acid gneiss and the flagstones. “4. Eleven foliated grey basic dykes, now represented by horn- blende-biotite-schists, were mapped last year on the southern slopes of Glen Scaddle (8236). They intersect both the epidiorite and the granulitic gneiss, while two in Glen Cona traverse the flagstones. “5. On the south side of Glen Scaddle the epidiorite is cut by a large number of dykes and veins of foliated granulitic gneiss and aplite (porphyrite) from 1 inch up to 20 yds. in breadth (8234). Similar dykes have also been mapped to the south-east of Corrlarach in Glen Cona, intersecting the flagstones.” * These aplite veins are often more foliated than the surrounding epidiorite.t Like the Scourie dykes of the North-West Highlands these veins have been specially picked out by shearing movement. More than this, the epidiorite was evidently foliated, in some cases, at least, before the introduction of the aplite veins. The latter are frequently sheared more or less parallel with their length in marked discordance with the general direction of the older and frequently much ruder foliation developed in the surrounding epidiorite. The shearing of individual veins has extended for a short distance into the epidiorite, and the older foliation of the latter has sometimes been dragged sharply round into parallelism with the newer structure of the adjacent vein. These phenomena of intermittent intrusion and foliation recall the sequence which has been worked out in the Lizard Complex in Cornwall.t Just as there are abundant lenticles and sills of hornblende-schist, the connection of which with the Glen Scaddle Complex is uncertain, so, too, there are abundant pegmatite veins cutting the surrounding sedimentary gneisses, which can only be assigned to this complex with a certain amount of hesitation. These veins are, some of them, massive, some foliated, and the same vein may be massive in one part of its course and foliated in another. One pegmatite may often be seen cutting another. Small sills of hornblende-schist intruded into quartz-granulite and cut by pegmatite may be seen in the crags north of Loch nan Gabhar. In this case the igneous rocks may be referred with high probability to the Glen Scaddle Complex which is repre- sented in force a little farther north-east. *J.S. G. Wilson, in ‘Summary of Progress for 1900,’ Mem. Geol. Survey, o 56. Wilson, in ‘Summary of Progress for 1898,’ Mem. Geol. Survey, 1 . 41. a ett, in ‘The Geology of the Lizard and Meneage,’ Mem. Geol. Survey, 1912, pp. 22-25. 88 Schists N.W. of Loch Linnhe. The Sgurr Dhomhnuill mass of augen-gneiss is much more uniform in composition than the Glen Scaddle Complex. It is a large intrusion, and only a portion of its outcrop is included in the present map. “The rock is a foliated granite or augen-gneiss with a granulitic matrix, showing flaser structure, in which are set rounded porphyritic crystals of pink felspar with Carlsbad twinning. In places, however, the porphyritic felspars are absent. The rock closely resembles the well-known augen-gneiss of Inchbae in Easter Ross. “The boundary line between the augen-gneiss and the surround- ing schists is very irregular, due partly to injections, but chiefly to subsequent folding. The augen-gneiss, while sharing in the folding of the region, is foliated in the same direction as the schists. The rock is seen to cut some small masses of epidiorite and hornblende- schist, but in turn is pierced by later dykes of that material. It is also traversed by aplite and pegmatite veins, some of which have shared in the foliation, and are granulitic, while others are still massive and probably belong to the later granites.” * The contact- alteration of the sediments round about this intrusion appears to have been completely obliterated by subsequent shearing and re- crystallisation. E. BB, * Ben. N. Peach and J. S. G. Wilson, in ‘Summary of Progress for 1903,’ Mem. Geol, Survey, 1904, p. 68. SVAWT O1TOSYYL SUIISIG,, MOQ NID “daACE HOVNOY ANV HovNOy wWIvay) TIA “LV Td CHAPTER VII. ROCKS OF LOWER OLD RED SANDSTONE AGE. INTRODUCTION. Rocks which accumulated at the surface during the Lower Old Red Sandstone Period are preserved in the Glen Coe district and, almost certainly, also on Ben Nevis. They are represented in both places mainly by lavas and agglomerates, the stratified rocks forming but a very small proportion of the whole succession. In Glen Coe fragmentary plant-remains, including Psilophyton and Pachytheca devonica, determined by Dr. Kidston, have been found in the basal sediments, and furnish satisfactory proof that the rocks are of Lower Old Red Sandstone age. Fossils have not yet been obtained on Ben Nevis, but both lavas and sediments in this latter locality are exactly like types developed in Glen Coe and in other Lower Old Red Sandstone areas, and also bear the same relation to the High- land Schists and to the accompanying intrusive rocks, so that their Lower Old Red Sandstone age may be assumed with some confidence. The Lower Old Red Sandstone volcanic and sedimentary rocks rest with a violent discordance upon a sculptured surface of the Highland Schists, which is in all probability an old land surface. The topography of the period was, in fact, buried beneath the voleanic outpourings, and is now partially brought to light again by denudation. Unlike the schists the deformation to which these rocks have been subjected is mainly due to faulting. The folding, though sometimes sufficiently intense to produce inversion, is a local pheno- menon, a phase of the faulting, not part of an extended system of crustal buckling. In fact, since Old Red Sandstone times, whatever faulting and denudation have taken place in the region, deformation has not resulted in any apparent system of folds. The sedimentary rocks, consisting of red, grey, and black shales and compact grits are greatly indurated, whilst the volcanic rocks show various changes, due to weathering and the percolation of underground waters, but neither igneous nor sedimentary rocks are cleaved. Amongst the volcanic rocks hornblende- and biotite-andesites are the only types of lava met with on Ben Nevis, whilst in Glen Coe pyroxene-andesite, hornblende-andesite, and rhyolite, together with some transitional varieties are represented. The Glen Coe rocks are identical with types developed in the extensive plateau of Lorne, which lies to the south. The Lorne volcanic series is known to be of Lower Old Red Sandstone age on account of the occurrence of characteristic fish- and plant-remains in the intercalated sediments. Not only are the Glen Coe and hone rocks identical in composition 90) Lower Old Red Sandstone. and similar in habit, but, according to Mr. Kynaston, the succession of rock types is, in a broad way, also comparable. It is not to be assumed, however, that we are here dealing merely with outliers of “| Granite HOF J Lava Dee Wade Yj Lanpiyry’ =? Y Sheets (Scale : ro miles to 1 inch), Fic. 13.—Map of the Old Red Sandstone volcanic district of Argyll- shire and Inverness-shire, showing the distribution of dykes in relation to the Etive and Ben Nevis Granite Complexes. Schists left white ; Dykes shown by light, and Faults by heavy black lines. the Lorne volcanic plateau. The internal structure of the volcanic pile in Glen Coe (Fig. 15, p. 105) leads us to believe that the district was supplied from more than one source, situated in the immediate vicinity, though it is impossible now to point to the site of the volcanic vents. H. BM, Introduction. 91 This local independence of the Glen Coe lavas does not, however, negative the suggestion that they may have been confluent at one time with those of Lorne on the one hand and of Ben Nevis on the other. A small crag of conglomerate made up of well-rounded boulders of quartzite occurs on the hilltop north of Gleann Seileach, near Onich. It is probably im situ and of Lower Old Red Sandstone age, and if these assumptions are correct, its position about equi- distant from Glen Coe and Ben Nevis serves as a connecting link between the two. Fragments of similar conglomerate have also been found just beyond the eastern border of the map involved in a land- slip at the southern foot of Stob Choire Claurigh (Sheet 54) at the head of Allt Coire Rath. E. B. B. In this connection it should be noted that the Old Red Sandstone rocks of Glen Coe, and probably of Ben Nevis also, owe their pre- servation in large measure to faulting, by which they were let down relatively many hundreds of feet. It will be shown that the sub- sidence of the fault-block was accompanied by the uprise of igneous magma, which consolidated in part about the down-thrown mass. Denudation has revealed the consolidated .magma as an almost continuous ring surrounding each “ cauldron-subsidence.” The major portion of this chapter deals with the volcanic and sedimentary rocks of Glen Coe from the point of view of their local variations and their relations to the floor of crystalline schists upon which they accumulated. Chapter VIII., which follows, also deals with Glen Coe, but concentrates attention upon the cauldron- subsidence and the associated “fault-intrusions.” Chapter IX. treats of the great granitic complex of Etive, of which the Glen Coe Fault-Intrusions may be regarded as early northern offshoots. Chapter X. describes the lavas, sediments, and plutonic rocks of Ben Nevis. Chapter XI. follows with an account of several plutonic masses which are tentatively ascribed to the same general period as those of Glen Coe, Glen Etive, and Ben Nevis. Chapter XII. treats of minor intrusions of probable Lower Old Red Sandstone age, the great majority of which are N.N.E. dykes definitely associated with either the Etive or the Ben Nevis plutonic foci. Petrological details are throughout reserved for Chapters XIII. and XIV. In reading the chapters dealing with field-relations it is well to bear in mind that certain terms, such as “ granite,” are used in an extremely broad sense. All the intrusive igneous rocks dealt with in the chapters enumerated above are referred to the Lower Old Red Sandstone period, along with the lavas and agglomerates of Glen Coe and Ben Nevis. The evidence that they are not of later date than the Lower Old Red Sandstone period rests upon the fact that similar dykes and plutonic masses, in the East Highlands, are always earlier than the widespread Orcadian or Middle Old Red Sandstone of that district. The evidence that some of them at any rate are not earlier than the Lower Old Red Sandstone is quite clear :— (1) The Fault-Intrusion of Glen Coe, the Granite Complex of Etive, and the accompanying swarm of dykes can be demonstrated to be later than the lavas of Glen Coe. 92 Lower Old Red Sandstone. (2) The Inner Granite of Ben Nevis and three individuals of the dyke-swarm of that mountain are equally clearly later than the lavas of Ben Nevis. (3) The age-relation of the Outer Granite of Ben Nevis, and of the majority of the dykes, to the lavas cannot be settled; but the analogy of the Outer Granite of Ben Nevis to the Outer or Cruachan Granite of the Etive Complex, and of the dyke-swarm of Ben Nevis to the dyke-swarm of Etive (Fig. 13, p. 90), leaves little doubt as to the Lower Old Red Sandstone age of the entire Ben Nevis Com- plex and its attendant dykes. (4) A small diorite (appinite), east of Sgor na h-Ulaidh, between Glen Coe and Glen Etive, pierces a patch of breccia which perhaps may be an outlier of the Lower Old Red Sandstone Series of Glen Coe (see p. 139). (5) For the rest no direct evidence is available, and we rely upon the close petrographical affinity, which seems to link them together into a single great suite. There is one cause for hesitation, however, in pressing this conclusion: in Lorne and also in Glen Coe, granite. boulders of types similar to those of proved Lower Old Red Sandstone age, are locally abundant in the basement conglomerates of the great volcanic series. Some geologists, notably Mr. Barrow, attach con- siderable weight to these granite boulders, regarding them as evidence in favour of a period of granitic intrusion much earlier than that of Lower Old Red Sandstone times. To the writers, however, this argument does not seem to carry very much weight, since, in Skye, Mull, and Arran, Tertiary granophyres and gabbros are abundantly represented in Tertiary breccias and agglomerates. In Mull the early Tertiary intrusions and later Tertiary breccias are actually exposed in contact.* Dr. Harker + has interpreted this feature of Hebridean geology as due to explosions, bringing deep- seated early-consolidated Tertiary intrusions to the surface in the form of fragments. Now, in Lorne and Glen Coe, to return to the district under consideration, no natural base of the Lower Old Red Sandstone is known, and the conglomerates which contain the granite boulders, contain also an overwhelming proportion of volcanic rocks, which, like the granites, are of types common in the Old Red Sandstone suite. It seems reasonable then to suppose that such basement conglomerates may have formed at a comparatively late stage in the vulcanicity of the district, when granites, of the same general age as the lavas, had already become available, as a source for boulders, through the agency either of explosion or of erosion, or perhaps of both these processes acting in partnership. H. K., HB. M., EB. B SEDIMENTARY AND ACCOMPANYING VOLCANIC Rocks OF THE GLEN CoE DISTRICT. The volcanic rocks occupy a roughly oval area, situated in the south-east corner of the map and measuring nine miles in length by * E. B, Bailey, in ‘Summary of Progress, for 1912,’ Mem. Geol. Survey, 1913, p. 46. + A. Harker, ‘The Tertiary Igneous Rocks of Skye, Mem. Geol. Survey, 1904, p. 24, Glen Coe. 93 five in breadth. They form the centre of the mountainous district about Glen Coe and the upper part of Glen Etive, giving rise to about a dozen peaks over 3000 feet in altitude. On the north side of Glen Coe they constitute the jagged ridge of Aonach Eagach, whence a south-easterly dip brings them down into the central portion of the glen. On the south side they reach a height of 3766 feet in Bidean nam Bian, aud form the northerly spurs of this hill, which are known as the “Three Sisters of Glen Coe” (Aonach Dubh, Gedirr Aonach, and Beinn Fhada, Plate VII, p. 89). Thence they extend eastwards into the two “ Shepherds of Etive ” (Buachaille Etive Beag and Buach- aille Etive Mor), and south of the latter they cross Glen Etive, to build the northern and eastern ridges of Clach Leathad and the shoulder of Beinn Ceitlein above Dalness., The area thus described is surrounded for four-fifths of its cir- cumference by a fault which throws down the volcanic rocks and the underlying schists some thousands of feet. On the south side, where the fault is wanting for a distance of 4 to 5 miles, the volcanic rocks have been invaded by a lobe of the Cruachan Granite which has penetrated into the heart of the volcanic region. Sequence from Loch Achtriochtan to the Buachaille Etive Beag.— A traverse up Coire nam Beith to the top of Bidean nam Bian, and thence to the southern summit of Beinn Fhada, furnishes the best introduction to the study of the volcanic series (Fig. 14, p. 94, and Sections II. IV., Fig. 15, p. 105). A condensed account will suffice, as this portion of the district has been dealt with in detail elsewhere.* The full sequence encountered is as follows, in descending order :— Ft. 7. Andesites and Rhyolites . ‘ . about 300 6. Shales and Grits. . a ” 50 5. Rhyolite . . ‘i : : » 250 4, Hornblende-Andesites. . ‘ : » 900 3. Agglomerate and Shale’. : 5 » 250 2. Rhyolites . ‘ : : » 450 1. Augite-Andesites . _ : 5 » 1500 Total 3700 1, Augite-Andesites—About 17 flows occur in this group, and are well exhibited in the precipitous lower crags of Aonach Dubh. In the stream (Fig. 14) the andesites are separated from the under- lying phyllites by a couple of feet of purple sandy shale containing flakes of phyllite. The lavas closely resemble the basic flows which constitute the greater portion of the Lorne Series. They are dark grey, almost black when fresh, and become purple, reddish, and greenish on decomposition. Small black phenocrysts of augite and red pseudo- morphs after olivine can often be detected. In Chapter XIII. many of these lavas are classed as basalts. The lavas are only sparingly amydaloidal, and their vesicles are filled with calcite, agate, red jasper, and sometimes epidote. They * (©. T. Clough, H. B. Maufe, E. B, Bailey, ‘The Cauldron-Subsidence of Glen Coe, and the Associated Igneous Phenomena,’ Quart, Journ. Geol. Soc., 1909, vol, Ixv. p. 615. 94 Lower Old Red Sandstone, are often much brecciated, with red shale bedded into the interstices between the blocks. Breccias of this type must not be confounded with agglomerates. Good examples are exposed near the summit of the group in Coire nam Beith and on the roadside near Achtriochtan, Fra. 14.—View of Coire nam Beith from Road, Glen Coe. Ly / 2 ®. ape eee z % 3 re ae a" E Nos : 2 By pe ea y ‘ = Fe <3 \ Aonach Dubh > Se 3 Occasional beds of shale and grit intervene among the lavas, as may Se seen to advantage 200 yards east of the upper fall in Coire nam eith. The upper surfaces of some of the flows are reddened in a Manner suggesting contemporaneous weathering. 2. Lhyolites—There are three rhyolite lavas in the type section Buachaille Etive Beag. 95 (Fig 14), each about 150 feet thick. They all show fluxion structure, and the uppermost flow-brecciation in addition. 3. Agglomerates—The next group is a mass of agglomerate made up of blocks of augite-andesite and rhyolite and underlain by about 20 feet of greenish sandy shale. 4, Hornblende-Andesites—The hornblende-andesites of Bidean nam Bian weather with a pale crust, though dark greenish-grey to black on a fractured surface. They carry small but fairly conspicu- ous phenocrysts of plagioclase felspar and hornblende in a compact base, which often exhibits flow-banding. 5, Rhyolite—A black vitreous rhyolite, rich in phenocrysts of felspar, succeeds the hornblende-andesites. Though a thick mass it may be a single flow. It encloses numerous xenoliths of rhyolite and hornblende-andesite, and occasional pieces of quartzose schist. 6. Shales and Grits—Well stratified greenish-grey shales and grits now interrupt the volcanic sequence. They vary considerably in thickness and probably collected on a lake bottom. 7. Andesites and Rhyolites—The topmost group preserved consists of rhyolites and hornblende-andesites accumulated in very irregular fashion (Section IV., Fig. 15, p. 105), H. K., H. BM. Groups 6 and 7 are only preserved on the southern shoulder of Beinn Fhada. Rhyolites in the position of Group 5 occupy much of Beinn Fhada, and also form an outlier on the Buachaille Etive Beag, and probably enter into the contact-altered mass form- ing the southern end of the Buachaille Etive Mor. Their base dips at moderately high angles towards the south. The hornblende-andesites of Group 4 extend continuously from Stob Coire nan Lochan to the Buachaille Etive Beag. The agglomerate of Group 3 marks an important period during which few if any lavas were érupted. The accumulation is of variable character, and often gives place to green shales and grits. The latter are well bedded and show that the structure of the central portion of the Glen Coe area is characterised by a prevalent southerly dip of varying amount. The fine-grained sediments are 100 feet thick in Allt Coire Gabhail. In the western slope of Beinn Fhada they are much thinner, and in the eastern slopes are lost sight of. They reappear, however, at the head of Allt Lairig Eilde, and connect north-eastwards with agglomerate, just as in the type section of Stob Coire nan Lochan. The local variations of Group 2, represented by rhyolites alone in the type-section, are extremely interesting. Traced into the western peak of Bidean nam Bian, where the rocks stand on end near the boundary fault, the rhyolites die out entirely. Followed to the east, on the other hand, through the “Three Sisters of Glencoe” (Aonach Dubh, Gearr Aonach, and Beinn Fhada), the group expands, and in the last of the three, Beinn Fhada, and also in the opposite slopes of the Buachaille Etive Beag it includes a very large pro- portion of andesite flows (Section IL, Fig. 15, p. 105). Among these latter both augite- and hornblende-andesites occur, and their inter- calation with the rhyolites, which they more or less replace in the succession, can clearly be traced in Beinn Fhada. The thickness of the group in this locality must be about 2000 feet. 96 Lower Old Red Sandstone. Farther east again, the andesite-lavas of Group 2 disappear even more suddenly than they came. Not one of them extends round the northern face of the Buachaille Etive Beag, for this consists entirely of rhyolite. The topmost flows of the basic andesites of Group (1) are seen on the roadside below the upstanding mass known as the “Stiddy.” H. B. M, The Sequence on the North Side of Glen Coe.—Owing to the sharp upward tilt of the volcanic rocks in the neighbourhood of the boundary-fault (Section L., Fig. 15, p. 105), only Groups 1 and 2 are found on Aonach Eagach forming the steep northern flanks of the glen. The basement beds are exposed in a number of places, one of the most interesting being between the 1000-ft. and 1250-ft. levels half a mile north-west of Achtriochtan. At the foot of the precipi- tous crags the following succession may be made out in beds dipping at 30° to the south-east. Ft. Andesites e : 2 : - — _ Fine grit j 7 F ‘ : _ 24-3 Green shales . . ‘ a , ‘ 1-3 Coarse conglomerate . ; ; about 30 Green micaceous shales with plant remains : . not more than 20 These sediments are of a local character and are not traceable far in either direction. A short distance further up the hill towards the left the section in the basement beds is :— Ft. Compact banded green shale passing down into green sand- stone : : : “ . 12 Coarse pebbly ¢ grit ‘ : . 10 Red and green ‘shales with sandstone bands . ‘ : ~ 8 Fine grit : . ‘ - A : ‘ . 5 Conglomerate. a 2 : a ‘ - 20 Silvery greenish phyllites ; : . : : ee A thin felsite sill is here intrusive along the plane of uncon- formity between the conglomerate and phyllites. The conglomerates consist of well-rounded bounders and pebbles of quartzite, phyllite, andesite, and granite, and an occasional pebble of quartz-porphyry, in a gritty matrix, which is often very felspathic. The shales are mostly purple and sandy, but occasionally contain bands of darker and more argillaceous material. Sections from the granitic boulders in the conglomerate below the andesites, were also examined and were found to belong to the quartz-diorites rather than to granites. One section shows (10285) much idiomorphic plagioclase, altered biotite, calcareous patches after another ferromagnesian mineral, and some interstitial quartz. In some there is altered green hornblende, and augite may also have been originally present (10286, 10287). Another section is from a biotite-granite (10288). These rocks are of types common among the unfoliated granites of the Highlands, and would appear to have been derived from one or more closely associated plutonic masses. The boulders are often large and always well rounded. They do not resemble any of the varieties of granitic rocks found in the im- mediate neighbourhood, nor could they possibly have come from any portion of the Ben Cruachan or Ben Starav masses of granite, sigce North Side of Glen Coe. 97 these, as will be shown in the sequel, are of later date than the volcanic rocks overlying the conglomerates. The most abundant boulders are those consisting of white quartzite; they reach a length of 3 ft. The larger ones are well rounded, the smaller more sub- angular, and many of them have their surfaces stained red. Two boulders of a basic plutonie rock much decomposed but resembling the kentallenite of Aonach Dubh a’ Ghlinne were noticed. Boulders of pyroxene-andesite are fairly plentiful, and a few pebbles of hornblende-andesite were found, but none of rhyolite or porphyrite. The matrix of the conglomerate is generally a coarse grit, but in some places a sandy shale. Altogether the conglomerate re- sembles the boulder-gravel in the bed of a torrential stream. H. K., HB. M. The basement conglomerate is seen again with like characters in Coire Cam at the foot of Meall Dearg, where it is overturned at an angle of 70° in close proximity to the boundary fault (Figs. 16, 17, pp. 108, 109). There is a further exposure of the base of the group in the stream-bed of Coire nan Lab, but the conglomerate is absent here and the sedimentary beds consist mostly of shale. The base is also seen in Coire Mhorair and Coire Odhar-mhor situated further to the east (Figs. 18, 19, pp. 111, 112, and Section IIT. Fig. 15, p. 105). In the immediate vicinity of the boundary-fault the beds are locally overturned; elsewhere they dip steeply away from the boundary fault towards Glen Coe. From the southern flank of Sron a’ Choire Odhair-bhig a band of ashy grits, shale, and conglomerates, resting unconformably on the schists and succeeded by coarse agglomerate and lavas, may be followed fairly continuously down to the Glen Coe road half a mile west of Altnafeadh. Along this line the dip, in conformity with the change of trend of the boundary fault, is directed towards the south-west at various angles. E. B. B, The andesites (Group 1) overlying the conglomerate in the slopes of Aonach Eagach are the continuation of those already described in Coire nam Beith. The upper limit of the group is, however, very ill-defined on this side of Glen Coe, owing to the incoming of andesites into the succeeding division. The group, moreover, changes its character as it is followed eastwards. Hornblende-andesites appear near its base in Coire Mhorair, and agglomerate, hitherto confined to thin bands intercalated between successive flows, forms an important bed near its bottom farther east in Glen Coe. At the same time the flows of basic andesite gradually die out, and the whole group is extremely attenuated in the neighbourhood of Lochan na Fola. As already remarked, Group 2 on the north side of Glen Coe is represented by rhyolites and andesites in much the same manner as in some parts of Beinn Fhada to the south. Rhyolite forms the conspicuous red cap of A’Chailleach, and its dip to the south may’ be observed even from the road. BM, EB. B. Sequence in Buachaille Etive Mor.—The volcanic series in ascend- ing order consists of basement breccia, with intercalated flows of andesite (Group 1), rhyolites, and rhyolitic flow-breccias, with inter- calated and overlying agglomerate and sediment (Groups 2 and 3), and hornblende-andesites (Group 4). These together attain a thickness of at least 2500 ft, (Sections I. and IL, Fig. 15, p. 105). 7 98 Lower Old Red Sandstone: Glen Coe. The breccias at the base of the series are usually coarse and made up largely of angular fragments of quartzite and quartzose schists, which may have been derived from the underlying schists, and rarely attain 6 in. in diameter. Here and there throughout these breccias thin beds of finer material are met with, and as they are well bedded it appears they must have been laid down under water. The general characters of these deposits are well seen on the north face of Stob Dearg, where they form the smooth rock-slopes towards the base. The best section occurs beneath a conspicuous over- hanging slab which can be easily recognised in the photograph (Plate VIITL, p. 100), at the lower limit of the rocky portion of the hill. The sequence, where the beds are thickest, is as follows, in descending order :-— Ft. Bedded breccia often resembling conglomerate, with fragments of quartzite, micaceous schist, and some of felsite in a gritty matrix ‘ _ ‘ . : 7 ‘i 3 : . so Red shales with cornstones . . .. Be 8 oa cas 4 Purple shales... : é ; ‘ . : : ‘ 4 Greenish and black shales, showing alternations of coarser and more sandy layers with finer bands : , : : . 10 Conglomerate, with angular and subangular boulders of quartzite and quartzose schists (Hilde Flags) in a green sandy matrix . 20 Green shales, some red, and irregular bands of conglomerate > 48 Fine greenish basement breccia containing quartzite fragments . 1-2 Quartzite, much shattered at the surface ‘i é Above the breccia come the rhyolitic lavas, which form the greater part of the east face of Stob Dearg. A band of dark shales was also seen at the base of the same overhanging crag a little farther to the north. Mr. D. Tait searched the dark shales for fossils, and found good specimens of plant remains which were recognised by Dr. B. N. Peach as belonging to Psilophyton, and this identification was con- firmed by Dr. Kidston. Specimens of Pachytheca devonica were also found. The discovery of such characteristic Lower Old Red Sand- stone fossils at once settles the geological age of the shales, and of the volcanic series with which they are associated. Thus it is demon- strated that the Glen Coe volcanic rocks are of the same date as those of Lorne to the south-west. The lava-flows of andesite intercalated with the basement breccia, and with them representing Group 1, are seen on the banks of the stream flowing down past Lagangarbh and, near the river Etive, on the south-east slopes of Stob Dearg; but, though not well exposed, they cannot exceed a thickness of more than a few feet. These flows appear to represent the north-eastward thinning of the andesites so well developed in Glen Coe and about Dalness, The rocks are some- times vesicular, show specks of pyrites but no conspicuous pheno- erysts, and may be dark grey or light grey. Under the microscope (12490, 12772) they are seen to be very much decomposed: pseudo- morphs after some ferromagnesian mineral are set in a groundmass of cloudy felspar laths, with interstitial epidote and iron-ores. Vesicles are frequent, and are filled with calcite and pyrites. In the area about Dalness there is a greater development of andesite lavas and breccias, The latter consist almost entirely of Buachaille Htive Mor. 99 andesite, so that it was found impossible to separate them from the lavas in mapping. Intercalated near the top of the series is a bed of quartzite breccia which runs round the south-west spur of the Buachaille Etive Mor and dips gently to the north-east. Resting on the conglomerates and andesites are the rhyolites and rhyolitic flow-breccias of Group 2 which attain a thickness of about 1500 ft. on the north face of Stob Dearg (Plate VIIL.). The precipitous character of this peak is entirely due to these rocks. The rhyolites tower up in huge vertical walls and precipices, while the numerous joint planes often form a series of step-like ledges, and sometimes give the rock a rudely columnar appearance. The scenery formed by these flinty textured masses cannot fail to impress anyone who has seen the imposing and precipitous eastern face of Stob Dearg, the highest point (3345 ft.) of the Buachaille Etive Mor. At Stob Dearg the lower rhyolites are free from fragments, but throughout the rest of this area they are of the nature of flow-breccias containing abundant fragments of other rhyolites, hornblende-andesites, and frequently of the schists as well. On Stob na Broige, opposite Alltchaorunn, these flow-breccias, although their basal portion has been cut off by the Cruachan Granite, attain a thickness of over 1500 feet without reaching the hornblende-andesites above. Marked metamorphism has been induced by the Cruachan Granite throughout much of the district. The altered rhyolites assume a red colour with partial obliteration of their characteristic flow structure. It is considered probable that a red felsitic rock which forms the top of Stob Dearg may really be intrusive although it is grouped with the rhyolitic lavas in the published map. It has a well-defined fluxion edge without any accompaniment of fragmental material. Intercalated with the rhyolites are beds of agglomerate, flows of andesite, and an occasional parting of shale. The most important bed of agglomerate in this sequence forms a large part of the floor of the corrie west of Stob Dearg. Local beds of red sandstone and shale occur along with it, and one of them, situated near its base, is well seen on the ridge forming the western margin of the corrie. To the east the agglomerate thins out until represented by only a few feet of dark purplish and greenish shales, which, running diagonally down the eastern face of Stob Dearg, have given rise to the feature known to mountaineers as the “Crowberry Traverse.” The shales here are less steeply inclined than in the corrie west of Stob Dearg, on the west side of which they are sometimes vertical. A thin parting of shaly beds, though much shattered by lines of fault and crush, is well seen in a gully immediately to the south-east of the “Crowberry Traverse,” and is accompanied by a bed of ash about 2 ft. thick. Other bands of ash occur among the lavas south of the peak, and are seen in thin slices under the microscope to consist chiefly of com- minuted glass fragments, showing typical ash-structure, with a few recognisable pieces of rhyolite and hornblende- and pyroxene- andesites. Slices of the agglomerate are seen under the microscope to consist of fragments of igneous and metamorphic rocks in varying proportions, set in a matrix of quartz grains and comminuted igneous rocks (12477, 12487). In cases where there is only a small pro- portion of schist, and the agglomerate consists almost entirely of igneous matter, the distinction between it and rhyolitic flow-breccia 100 Lower Old Red Sandstone: Glen Coe. becomes very difficult, and, in fact, impossible once the rocks have been even slightly altered (12487, 12470, 12784). The andesite intercalations, met with occasionally throughout the series, are most numerous near its base in the Dalness area. Where the junctions are well seen, it sometimes appears as though the andesite was of the nature of an inclusion in the rhyolite, since there is no vesicular margin between the two, while the general outline of the junction does not always agree with the strike of the surrounding beds. The andesite which occurs along the granite margin, south of Stob Dearg, is regarded as one of these intercalations in the rhyolitic series of Group 2. The slices prepared from these intercalations show the results of contact-alteration by the neighbouring granite. They were originally hornblende-andesite flow-breccias, but the horn- blende phenocrysts are now only represented by magnetite skeletons and matted aggregates of biotite or, more rarely, of green hornblende in ragged prisms. Phenocrysts of felspar do not occur. The ground- mass consisted originally largely of felspar and granules of some ferromagnesian mineral, but is now a mosaic of felspar, some quartz, and scales of biotite or prisms of green hornblende (12511, 12762, 12764). Group 3 is represented by a bedded breccia full of fragments of the schists. It 1s exposed on the summit ridge of the Buachaille Etive Mor, where it thins out in a southerly direction, just as in the Buachaille Etive Beag and Stob Coire nan Lochan. A small outlier of hornblende-andesites (Group 4) forms the mid peak of the Buachaille Etive Mor, rising to a height of 3325 ft. It includes thin intercalations of rhyolite and of more basic andesite (12464, 12465, 12467). The predominating rock is a dark horn- blende-andesite, containing needles of hornblende, visible to the eye. Under the microscope the effects of contact-alteration are very marked. The original rock is seen to have contained phenocrysts of hornblende, plagioclase, and sometimes mica. H. K., G. W. G. Sequence in the Cam Ghleann and Srin na Créise.—In this, the most easterly part of the volcanic district, the sequence is similar in many respects to that described on the northern end of the Buachaille Etive Mor. In ascending the Cam Ghleann from the position of the boundary fault we encounter several detached outcrops of rhyolite and andesite, and also patches of breccia consisting of angular frag- ments of schist and rhyolite. A short distance farther up we come to a coarse conglomerate underlying the main mass of the volcanic rocks, The section exposed in the stream contains, in addition to schist fragments, large masses of various igneous rocks, conspicuous among which are boulders of a red granite, which attain a diameter of 2 or 3 ft. This granite is particularly interesting as it shows some of the characters which distinguish the Moor of Rannoch rock from those later granites which are intrusive in the volcanic series. In the hand specimen the quartz is seen to occur in large blebs, and a foliation can sometimes be made out. Under the micro- scope (12788) the large composite areas of quartz so characteristic of the Moor of Rannoch rock are seen; the plagioclase has a refractive index differing but little from that of the balsam, indicating a com- "+ 4e STISsOT “(padorod ssva8) sjsiqoy wo (sBvso) sea] oppodyy ‘puvaq, aozg ‘WIA WLW Td Cam Ghleann and Sron na Creéise. 101 position near that of oligoclase. This is another point of agreement with the Moor of Rannoch rock. The fact that in slices these boulders show biotite as their only ferromagnesian mineral (12788, 12789) indicates that if they have been derived from the Moor of Rannoch mass, they must have come from its marginal portions as its interior contains hornblende in addition. G. W. G. Overlying the conglomerate comes a dark basic andesite which is all that remains to represent the thick basic andesites (Group 1) of the Coire nam Beith section. It is succeeded by several feet of shale overlain in turn by thick rhyolitic lavas belonging to Group 2. Except in the floor of the glen, where the andesite is present, the rhyolites form the local base of the series, the junction with the older rocks being well seen on the northern slopes of Sron na Creise. A thin layer of breccia usually intervenes between the rhyolites and the schists. On Srdn na Créise, as on Buachaille Etive Mor, the voleanic series dips at high angles to the south-east, but in the Cam Ghleann and farther east it is lying approximately flat and abuts against a steeply sloping surface of schist. The agglomerates and breccias (Group 3) cap the rhyolites in the large corrie at the head of the Cam Ghleann, whence they sweep round to the north-west and cross the Sron na Créise ridge, but are cut off farther to the west by the intrusion of the Cruachan Granite (Section I. Fig. 15, p. 105). Eastwards they extend round the head of Cum Ghleann into Meall a’ Bhuiridh (Sheet 54, and Fig. 23, p. 116), where they come into direct unfaulted contact with the schists, thus indicating a remarkable overlap. This also is the condition of affairs in Coire an Easain (Fig. 23, p. 116), where remarkably clear exposures illustrate the uneven nature of the floor upon which the breccia accumulated. The breccias often consist almost entirely of fragments of rocks derived from the quartzose and micaceous schist of the neigh- bourhood. It is impossible to draw any hard and fast line between this breccia and the coarse agglomerate with which it is associated, consisting mainly of fragments of rhyolite and andesite. In some places the breccia is comparatively fine grained, and full of small angular fragments of quartzose schist and rhyolite, averaging half an inch or less in diameter; but more usually the main mass of the rock is a confused agglomeratic mixture of angular fragments of all sizes, consisting of quartzose and micaceous schist, rhyolite, and occasional blocks of andesite. The fragments of quartzose schist are sometimes 2 yds. or more in length. As the andesite is approached the agglomerate becomes more and more andesitic, and sometimes con- sists mainly of andesite fragments. The finer-grained portions of the breccia are well bedded, and good indications of bedding are seen on both the east and west slopes of the head of the Cam Ghleann. These finer-grained portions are very rich in quartz grains, and some- times appear to pass into zones of grit. In two or three places on the west side of the Cam Ghleann angular fragments of a rather acid biotite-granite were observed in the breccias. These fragments are not unlike the granite which is exposed farther west in Glen Etive, but they cannot possibly have been derived from any portion of the Cruachan or Starav Granites since the Cruachan Granite, the earlier of the two, is in the immediate neighbourhood clearly intruded into 102 Lower Old Red Sandstone: Glen Coe. the breccias. They must have been derived either from some older mass bared by erosion, or from some quasi-contemporaneous intrusion, fragments of which have been ejected during explosions. The occurrence of granite blocks in the conglomerates and breccias of this part of the district may be paralleled with that of granite and gabbro in the agglomerates of the Cuillin Hills of Skye.* The hornblende-andesites of Group 4 in their normal types succeed the agglomerates, and are remarkably well exposed along the sharp east and west ridge of Meall a’ Bhuiridh, and again along the north and south ridge between Sron na Créise and the Clach Leathad. The rocks weather with a pale greyish or whitish crust. On fracture they are dark grey or almost black in colour, and small plagioclase phenocrysts and prisms of dark hornblende may usually be detected with the naked eye. Flow-structure is also well seen, especially on the Meall a’ Bhuiridh ridge. The Cruachan Granite has invaded these andesites and sends many veins into them. The junctions with the granite in the field are clear and well defined, and decided contact-alteration has been produced. This will be described in Chapter XIV. On the north and east sides of the highest point on the ridge north of the Clach Leathad, 20 to 30 ft. of dark grey laminated shales and grits occur intercalated in the lower portion of the horn- blende-andesite series. These beds are considerably indurated by the granite, and no signs of fossils were observed in them. H. K. Sequence around Dalness, Glen Htive—The schistose floor on which the rocks of Old Red Sandstone Age rest is seen in several places both near the river Etive and some distance above it, and is evidently very uneven. On the north bank of the Etive a breccia about 2 ft. thick, inclining steeply south-west, separates the andesites from the schists, which here are less siliceous than usual in the adjoining localities. Further up on the south side of the river various inliers of quartzite are exposed at different levels within the andesitic rocks. The surface of the quartzite is generally a good deal broken and frequently partially covered with a skin of breccia. c. T. C. In a bed of breccia cropping out round the S.W. end of Buchaille Etive Mor large masses of quartzite appear, and are well seen in Allt Gartain, half a mile up from Dalness, and also on the slope about half a mile E.N.E. of the house. The smaller of these masses, associated with a bed composed of quartzite fragments, appears at first sight to be merely particularly large boulders: but one very large mass in Allt Gartain is evidently in place. It is probable then that they are all stacks surrounded by the volcanic rocks, and that the breccia-bed is scree-material showered down upon and finally covered by the advancing lavas. The basic andesites of Group 1 may be traced continuously from the type-section of Coire nam Beith along the south-western flanks of Bidean nam Bian to the neighbourhood of Dalness. Though not so thick as in the type-section, they present a greater development here than in the district to the north-east. Accompanying them are breccias, consisting almost entirely of andesite, and so similar in ph Harker, ‘The Sequence of the Tertiary Rocks of Skye, Geol. Mag., 1901, p. 507, , Dalness, Glen Etive. 103 appearance to the lavas that it was found impossible to separate the two members in mapping. G. W. G. The andesites exposed in the river Etive between 200 yds. and three-quarters of a mile east of Dalness have suffered considerable contact-alteration. Weathered surfaces are usually very knobby, and, the parts which project the most being usually greener and more epidotic than the rest, it is perhaps possible that they represent the parts which were the most decomposed at the time of the intrusion of the neighbouring granite. Exposures sometimes show conspicuous oval forms rather like the pillows of pillow-lavas, the outer rims of which for the breadth of half an inch or so project somewhat, but the vesicles show no arrangement parallel to these rims. Many parts of the rock have a brecciated aspect, the angular pieces frequently fitting into one another very exactly, with hardly any matrix between. In numerous cases cracks penetrate pieces of andesite for a certain distance and then come to an end as if they had been developed during violent rolling or on falling. It is not clear that any true ash is present. The amygdales often project considerably on the weathered face. They are usually much elongated and consist of epidote, or epidote and a pale green hornblende. Other vesicles are filled with quartz, apparently in a granular or granulitic condition, and to a less extent with plagioclase (11488). In one locality a number of thin veins which cross the local trend of the vesicles almost at right angles were also seen to contain a pale hornblende in considerable abundance. The development of hornblende and felspar in the vesicles, and of hornblende in the veins is doubtless due to contact metamorphism, probably exerted by the neighbouring granite, and it may be concluded that the lavas were considerably de- composed and crossed by strings of carbonate before the meta- morphism was effected.* On the hillside three-quarters of a mile W.S.W. of Dalness an important band of clastic material, sometimes perhaps 300 ft. thick, intervenes between the andesites and the overlying rhyolitic rocks. The character of the clastic beds varies rapidly from place to place. In the burn rather more than three-quarters of a mile ES.E. of Dalness the group is almost entirely composed of coarse breccias of quartzite and andesite, and it is noticeable that the pieces of andesite are often more porphyritic than the underlying andesite lavas: at the very top of these beds there is a little fine-grained highly-altered sandstone containing pieces of quartzite. In the adjacent crags the following succession in descending order can be made out, but the middle group appears to die out rapidly in a westerly direction :— Thickness. in ft. Breccias composed of quartzite and andesite, with occasional bands of sandstone, sometimes ashy . ; ; 50 Grey and pink sandstone with fragments of quartzite . _ : 60 Breccia composed of angular pieces of quartzite from 5 ins. to 12 ins, in length: very little fine matrix . : 50 * Cf, A. Harker and J. 8. Marr, in their account of ‘The Shap Granite and the Associated Igneous and Metamorphic Rocks,’ Quart. Journ. Geol. Soc., 1891, vol, xlvii. p. 297. | 104 Lower Old Red Sandstone: Glen Coe. The above beds are insome places almost horizontal, but in others inclined at gentle angles to the S. or S.S.W. They are often ina greatly altered condition, being close to the granite. The highest members of the volcanic series preserved on the slopes of Beinn Ceitlein are composed of 1000 ft. or so of rhyolitic rocks belonging to Group 2. In certain weathered exposures many of them show included fragments; but no vesicles or distinct flow- structures are seen. Ashes as well as lavas may be present, but a separation has not been attempted owing to the masking of original differences by contact-alteration. : All the beds above referred to lie on the inner side of the line of flinty crush-rock which marks the main inner boundary-fault. On the other side of this line small outliers of sandstone breccia of Old Red Sandstone age occur in several places on the quartzite crags nearly half a mile S.S.W. of Dalness, and again near the northern end of Beinn Ceitlein. The outliers in the first locality appear to be quite outside all the boundary-faults which have been recognised in the district, but most of those in the second locality seem to lie between the inner fault and the outermost. They differ also from those in the first locality in being closely associated with rhyolitic bands, some of which are of intrusive character, while others may possibly represent lava flows. C. T. C, Outliers,—Reference has been made in the previous paragraph to certain small outliers of breccia in the Dalness district. There are a few others elsewhere which deserve attention. The largest of them rests against a steep slope of quartzite, at a height of 2500 to 3000 ft. on Sgor nam Fiannaidh, north of Glen Coe, and consists of quart- zite fragments, together with small pieces of red felsite and porphy- ritic andesite, set in a siliceous matrix. The breccia is pierced by plutonic and dyke rocks belonging to the Old Red Sandstone suite. Its preservation outside of the fault bounding the volcanic rocks points to its having accumulated in a deep hollow, probably due to ordinary erosion.* H. BM. There are other patches of breccia, and associated rhyolite, which present more than ordinary difficulty in their interpretation, They lie far beneath what appears to have been the normal erosion surface of the time, and it is hence suggested that they may have gathered in landslip-cracks or in rents due to volcanic activity, such as are in Iceland termed “gjas.” The main examples are given below. From half to three-quarters of a mile in front of the highly tilted lavas of Stob Dearg there is a belt extending from the Devil’s Staircase to Glen Etive (perhaps even including some of the isolated outcrops in the Cam Ghleann) along which exposures of rhyolite and breccia are constantly making an appearance. The belt for the most part passes through ground much obscured by superficial deposits, and often its existence can only be recognised in isolated stream sections. Careful examination of that part which crosses the face of Stob Beinn a’ Chrilaiste, above Altnafeadh (Fig. 22, p. 115) shows various irregular outcrops of rhyolite, often separated from the adjacent schists by thin breccias, and therefore allied in their * Mr. Manfe takes the same view in regard to the Gleann Charnan breccia described later, p. 139. ‘105 In ine, pieces of rhyolite are also gular fragments of schist. Summary of Volcanic History. Another small patch of breccia, the presence of which is difficult behaviour to the lavas of the district rather than the intrusions. The breccias here consist of small an the Coupal River, along the same 1 common. “4X93 94} UT passnosip sdnois 0} Jeyar J-T sreqtunu aqy, ‘20D Ud[p) JO dUEplsyNg-UOIP[NL,) SsOLIe SUOTJIBI—"G [ “OT S271W oO} 6 e Z 3 t cs ¢ z SUSTYDS: E4 OJTUD AS) TE YOOUTY £0 S00] GES NINLYVHW : .o LOULE YY gy JIHW g04S 9vaea 3A UUM DY py. an 311IvHONa 7 oS sucisngqUy- yung [~* **] PITUDAL) UY IVIL STUMUTPIS! PUD FB z Spdeaoneol Fees syospg -§ sepsopup & FIAIV VIS ae ‘i ¢ ara OYPO 2.207 NVH907 NWN ’ aN 34109 G01S MS: FIAT? WIS JO LOOK" PANT A oF: Sv3e 3AIL3 avTitvwovne aNivHovns te re PON YOQW B3AIL3) =OVAE BAIL HOYVS ayy a's UUDITYD) UD) aTivHOWNa = =43FIWHOVAS Taw JO PDA PAU” a -bhig, E. B, B, SUMMARY OF VOLCANIC HISTORY. (1) The Glen Coe volcanic rocks are of Lower Old Red Sand- stone age. to account for, is found near the foot of Srén a’ Choire Odhair between two branches of the Glen Coe Fault (Fig. 18, p. 111). 106 Summary of Volcanic History. (2) They accumulated on an uneven floor carved by erosion in the Highland Schists. (3) In a few cases patches of breccia and rhyolite seem to mark the sites of volcanic fissures. (4) The lavas are basic andesites (including basalts), hornblende- andesites, and rhyolites. Their extremely irregular arrangement inter so indicates that they were supplied from neighbouring inde- pendent vents. The basic flows frequently show reddened tops as if affected by contemporaneous weathering. (5) The agglomerates of the period cannot be distinguished with certainty from scree-breccias, (6) The sediments accompanying the volcanic rocks are of quite restricted occurrence. They include screes and torrential conglomer- ates, along with shales and grits which appear to have gathered in temporary lakes. Shale is often found filling the crevices of the lavas of the bottom group consisting of basic andesites. (7) Away from the faulted margin of the area the voleanic rocks dip at varying angles towards the south. H. B. M. CHAPTER VIIL ROCKS OF LOWER OLD RED SANDSTONE AGE (Continued). THE BouNDARY FAULT AND FAULT-INTRUSIONS OF GLEN COE. THE volcanic series of Glen Coe occupies a cauldron-subsidence, circumscribed by a fault of some thousands of feet downthrow. The cauldron, which measures 9 miles by 5, dates from the Lower Old Red Sandstone period, and does not, we need scarcely add, find expression as a hollow in the topography of to-day. At its initiation the fault was probably continuous, but its outcrop is now interrupted for about 4 miles by a northward extension of the Cruachan Granite. The sunken area is girdled by a discontinuous igneous complex in the form of an irregular ring-dyke (p. 126) of porphyrite merging into granite. This is the Fault-Intrusion of Glen Coe—so named from its intimate connection with the boundary-fault of the Cauldron Subsidence; some portions of it, generally more or less crushed, have been separated under the designation of Early Fault-Intrusion (Figs. 18-20). The Fault-Intrusion rose up around the subsiding block as the latter settled down, but before we can substantiate this statement it is necessary to give a résumé of the evidence upon which it is based. A full account has already been published to which the reader is referred for details.* The existence of a cauldron-subsidence is suggested by the com- pact area occupied by the volcanic series in the heart of the schists. In many places the lavas abut against the steep even plane of a bounding fault, and there stop abruptly. Such a relation is especially well seen between An t-Sron (Fig. 14, p. 94) and Dalness. The lavas also almost always show a marginal inward tilt, sometimes amounting to actual inversion (Figs. 14-19). Very few outcrops referable to the Glen Coe volcanic series are known outside the outermost recognised branch of the boundary-fault of the cauldron-subsidence. These are patches of breccia, on the slopes of Sgor nan Fiannaidh, at the head of Gleann Charnan and S.S.W. of Dalness. Certain other small outcrops of breccia occur between the main boundary-fault and some outer branch (cf. Figs. 18 and 20). In all these cases the breccias have accumulated in specially deep hollows, often of volcanic origin (p. 104). The importance of the subsidence which has preserved the Glen Coe volcanic rocks from erosion is strikingly illustrated by the distribution of the schists within the downthrown area. The effect * ©. T. Clough, H. B. Maufe, E. B. Bailey, ‘The Cauldron-Subsidence of Glen Coe, and fe ‘Associated Igneous Phenomena,’ Quart. Jour. Geol. Soc., 1909, 1. vol. Ixv. p. 61 7 aTIN i 7 140008 "140002 "14 0001 0 LIN Yd — *AOHDLNO SO SINIT \\9NIGCIE TILYFIA + £ ‘ON/IG@IE AO did <— SSTHAG FLIVAHAYOS ATHVI=d FF ISPHAO FLISTII ATUVG =f ue = x ty Tapp IUD, 2 ONL fee ees s 2 posta 3 poy a Le ae iE S s $ S 3S S =, : ORT HERI SWS ot EL RD PSG Or iat 75N/ ise ae Pepin mai UA ae ADIN EAN CY SINDS 3°9NII | " PE see <7 a Ne aS NII SYS z ie oo ; . rh é eminent 0 aN Ulin c wii iene AST Ate i ee on | NOTSREINT STILT NOR (OULLINO SIHAT INN) Neg Bo | WYD FHI0I 8 KEENE = GV7NYN FY/09 |-i\4 84 ee: 1S0 dVN “xy } Fault and Fault-Intrusion. 109 of the Gleann Charnan branch of the boundary-fault is particularly obvious (Sections G and H, Fig. 11, p. 78). Farther north in Glen Coe the boundary-fault (Fig. 14, p. 94, and sections E and F, Fig. 11) throws the outcrop of various schist eroups forward to the north- east to an extent which indicates a drop of two thousand feet or more. Thus it is that the Ballachulish Limestone is exposed so far to the north as Coire Cim and Coire Mhorair. In Coire Cam the outcrop is very small and is indicated by a note “Schistose Lst.” in Fig. 16. In Coire Mhorair the outcrop is large and easily identified (Fig. 18). The evidence so far advanced shows that the volcanic rocks occupy a region of subsidence. It is now necessary to give in outline the proof that the sunken region is bounded by a curving dislocation, and not by straight faults accidentally intersecting. The demonstra- tion depends upon the possibility of actually tracing the boundary- fault, and this has been effected with great precision, thanks to excellent exposures. The task is made easier by the fact that the SW. OP ethetin,, N.E. “\ =f olla [rm So \ St x Sey eae es PIED S| SERRE early felsite. F dyke o 1000 2000 s000 vonoFt 1 J Fie. 17.—Section through Meall Dearg. F=Fault, accompanied by Fault-Intrusion. fault serves as the inner boundary of the ring-complex known as the Fault-Intrusion. Only three or four minute crops of the Fault- Intrusion are known within the circuit of the boundary-fault, whereas outside, for a distance of about a mile, there are numberless masses of the intrusion; not only so, but wherever an individual mass is in contact with the fault it presents to it a smooth well-defined margin, though elsewhere its boundaries may be highly irregular. This feature is abundantly clear in the one-inch map, and in Figs, 14-23, The contact-metamorphism induced by the Fault-Intrusion is similarly restricted, so that the lavas and schists within the cauldron-subsidence have escaped almost untouched, with the exception of the rocks in the neighbourhood of the large offshoot of the Cruachan Granite. In correlation with this phenomenon the Fault-Intrusion invariably shows chilled margins to the fault where- ever the two are seen in contact from Beinn Ceitlein, south of Dalness, right round its course to Meall a’ Bhuiridh (Fig. 23). Along its external boundaries, on the other hand, while chilling is not un- common, it is anything but universal. 110 Glen Coe Cauldron. A very brief statement of the phenomena encountered along the course of the fault is given below. C. 1. C,H. BM, BE BB. South of Dalness.—The fault is here in two branches. An inner fault is well seen on the hillside between 1100 and 1200 yds, south-east of Dalness. It inclines steeply towards the region of subsidence, and separates quartzite on the south-west from rocks of the volcanic series on the north-east. A highly sheared band intervenes; a thin streak of nearly vertical black flinty crush-rock traversing the rhyolite lavas close to the dividing plane shows incipient crystallisation (11464), A little farther up the hill a grey porphyrite, belonging to the Fault-Intrusion complex, extends along the fault for a distance of about a third of a mile. Its north-eastern margin is chilled and nearly straight, while its south-western margin is distinctly less regular, On the west flank of Beinn Ceitlein the outer branch of the fault can be recognised by evident discordance of the schists on its two sides. It is inclined outwards often at comparatively low angles— about 50° to the north-west. Like the inner branch it is locally accompanied by grey porphyrite acting the part of Fault-Intrusion. The two branches converge, and probably coalesce south-east of Dalness. In Chapter XIV. it is shown that the Fault-Intrusion south of Dalness is contact-altered. There can be no doubt that the effect is due to the Cruachan Granite. The two intrusions are seen together a third of a mile 8.S.E. of Dalness, where abundant red granite strings project from the surface of the porphyrite. Datness to Glen Coe.—The fault is clearly marked in this region as the boundary of the volcanic rocks. It is further distinguished by the presence of a loose fault-breccia which weathers out, yielding a more or less decided hollow (Fig. 14, p. 94). The fault inclines normally at about 70°. Almost along its whole course one finds Fault-Intrusion—pink and grey granite and porphyrite—swelling to a large mass in An t-Sron. It presents the usual smooth chilled edge to the fault. The loose fault-breccia involves this chilled edge, and is thus evidently due to a recurrence of movement along the old line of weakness. This is a quite usual phenomenon with the Glen Coe Fault, and one commonly finds the N.N.E. porphyrite dykes, of considerably later date than the Fault-Intrusion, thoroughly broken where they cross the line of the fault. Where the crush-rock in the fault is of a compact nature, as south of Dalness, at Stob Mhic Mhartuin, and in the Cim Ghleann, the N.N.E. dykes cross without suffering brecciation. The crushing north-west of Dalness affects even a basalt dyke, presumably of Tertiary age. Gleann Charnan—An outer branch of the fault runs down Gleann Chirnan. It is recognisable by its effect upon the schists, and its position is further marked by a line of loose crush-rock. A tongue of Fault-Intrusion extends along its outer side with the usual relationship. H. K., H. B. M, North of Glen Coe—The evidence for the boundary-fault in its sinuous course north of Glen Coe is presented in Figs. 16-22, From 111 North of Glen Coe, “TRIOY ETON puv sipYyW-1eqYPO oat0H Jo dey{—'g] ‘org aul 140008 14 0002 id 0601 6 SLINVI —— “dOYPLNO SO SINIT/ FONIGAIT TVIILYIA + !9NIGOIG SO dd <— "MIOU-HSNUD ALNITS FAISQULNI=9 ‘STIHILVA WIIDTUG~G SIMA FLISTIA ATWYF=4 moe AL YE ie gest pe ee To SFR Ghee ae tN eae eT Te py RE =a - ore WIAA Ct We ist wae PLT TOAHE EE Tare — a ss eee uae SON = { = WO CNEL Re 1 “An en ‘ SS te 4 Xl oe, WA ° Se + = Re & sm ee a; aoe 9 (906 ae 3 ACAR7 | RGR ONE sa\ * at d Yee 4s ce 3 a Ll iz MHDS. = AX. iF Grane TC ag ON Rin (wor ay Lo ak - Ho STS — = mK S x> oy the LAPT | S | SRL ee wizy Ae ite 7 : te YTV May iS : B.D. va » Higa Zt Se eet Fe Wer fi SSE (SOUL Nir Haeg PV RE eo x ofe ete eo Ve Nee ie ag SEAN 9 Or Ve Wie” fe NETL: Cae At ’ ve 1 —— — *- ~ ‘he ts Ds RA ae (CILLIWO SI4Ad aN) CTL) 1 [o ‘UIVHOHN FHI0I 9} arate YOHW-HPHOO F109 ee JO dvw i ca \ A 112 Glen Coe Cauldron, its turning point in Coire Cam, the fault is always inclined outwards at angles varying from 70°-50°, and the same feature has been recognised in the Cam Ghleann (Fig. 23). From Loch Achtriochtan to Coire Odhar-mhor the Fault-Intrusion is mainly represented by pink porphyrite. The “Granitite Fault- Intrusion” distinguished in Fig. 18 is easily separable from the main Fault-Intrusion. Its junction is exposed at the base of Sron Gharbh and is not quite sharp, being marked by a foot or so of hybrid rock. Probably the coarser rock is somewhat the later of the two; ‘its uprise may well have been contemporaneous with that of the main mass of the Cruachan Granite. The numberless intrusions of pink porphyrite between Garbh Bheinn and Glen Coe are particularly interesting for they frequently present unchilled margins. The district. has probably been subjected to extensive pneumatolitic action. On the one hand, the porphyrite NE, oO 1000 2000 3000 +000 Ft 1 J Fic. 19.—Section through ridge west of Coire Odhar-Mhor F, and F,=Early Faults, accompanied by Early Fault-Intrusions. F,—Main Fault with Fault-Intrusion. is often not only richly charged with xenoliths of baked mica-schist and quartzite, but also loaded with quartz grains separated from the quartzite; on the other hand, the quartzite, where it retains its individuality, is frequently saturated with pink felspar from the porphyrite (p. 164). It has already been pointed out that the alteration of the schists due to the intrusion of the Fault-Porphyrite is limited by the boundary-fault. This is particularly well illustrated in Coire Mhorair and the ridge to the east. Inside the fault, pure white quartzites dip steeply beneath thin phyllites and black slates, and these under calcareous schists and limestones showing no trace of contact-alteration. Outside even the inner of the two branches of the fault traced in Fig. 18, quartzite and quartz-schists are en- countered with obscure bedding and local well-marked reddening. In addition, these outer rocks are pierced by many irregular intru- sions of pink felsite which are quite unknown on the downthrow side of the fault, North of Glen Coe. 113 The most interesting single exposure of the Glen Coe Fault is exhibited in Stob Mhic Mhartuin (Plate IX. and Fig. 21). The map (Fig. 20) shows how readily two branches of the fault can be recog- nised by paying attention to the nature and strike of the schists which they traverse. What is not shown on the map is the amount of disturbance and reddening characteristic of the schists between the two branches of the fault, and for about a third of a mile to the north-east. The two branches of the fault are accompanied outside by grey Fault-Porphyrites, but the porphyrites in the two cases are not identical. The inner one can easily be recognised as the main Fault- Intrusion of Glen Coe; the outer is a rather different rock, distin- guished alike by original characters, and by its moved, broken, and baked condition. Traced westwards into Sron a’ Choire Odhair-bhig (Fig. 18) the inner intrusion occupies the whole distance between the two branches of the fault. To the north it exhibits a chilled undis- turbed edge against a banded flinty crush-rock, which on its other KRiAL x ay WA FAULT-INTRUSION EARLY FAULT- A INTRUSION [ B |eneccsa PATCH <—- D/P OF BEDDING —+— VERTICAL BEDDING EILDE FLAGS FAULTS QUARTZITE 1000 FT 2000 fT AMILE [=a 1 1 , Fic, 20.—Map of Stob Mhic Mhartuin. North-north-east dykes omitted. side passes imperceptibly into the strongly sheared margin of the outer porphyrite. No more convincing evidence could be desired: the outer Fault-Intrusion is of comparatively early date, and has suffered from a recrudescence of shearing posterior to its consolida- tion. E. B. B, Before quitting the Stob Mhic Mhartuin section, a few words must be said about the shearing of the rocks along the main fault, illustrated in Plate IX. and Fig. 21. The rocks of the low frontal cliff of the Stob are well bedded quartzites lying within the inner fault. South-west of this dislocation minor planes of movement are encountered crossing the bedding of the quartzite and occupied by a few inches of hard white rock composed of ground-up quartzite, Along the main line of movement the bedded quartzites are truncated by a zone of similar white crush-rock (12329) two feet thick. Beyond this, long tongues of the white crush-rock begin to be isolated in a darker matrix with obvious flow-structure; then these tongues disintegrate, and the proportion of fluxional matrix rapidly increases, The fluxion-breccia which results is a foot 8 114 Glen Coe Cauldron. wide, and next to it is a layer, an inch thick, of black flinty crush- rock (12332). Much has been written on the subject of flinty crush-rocks, and we refer the reader to the paper already mentioned for further in- formation on the subject. The view which has been adopted in explaining the flinty or glassy appearance of these crush-rocks is that they have suffered fusion as a result of frictional heating. In the Stob Mhic Mhartuin example it is probable that the heat was in part derived from the Fault-Intrusion, for the flinty band near its junc- tion with the intrusion carries easily recognisable tiny felspar crystals undoubtedly picked up from the latter. It is extremely EAULT-INTRUSION Fic, 21,—Diagrammatic Sketch of Glen Coe Boundary-Fault as exposed in the cliff of Stob Mhic Mhartuin (Plate IX.). interesting to find that some of these introduced crystals are broken, showing that viscous flow continued in the crush-rock after some part of the Fault-Intrusion had risen into conjunction with it. Thus the subsidence of the rocks within the cauldron and the uprise of the Fault-Intrusion were contemporaneous events. Although the flinty crush-rock of the face of Stob Mhic Mhartuin incorporated a little material from the Fault-Intrusion, it undoubtedly became stiff and rigid before the consolidation of the latter, for at one place a small angular fragment of the crush-rock is embedded, with trifling displacement, in the margin of the igneous rock. It may seem strange at first sight that the Fault-Intrusion presents a chilled margin to the flinty crush-rock, considering the high tempera- ture to which the latter had been raised mechanically before the advent of the intrusion. The apparent anomaly disappears, how- ever, when once we remember that the zone of crush-rock, owing to its very small thickness, can only have carried a negligible supply of PLATE IX. GLEN Cor FauLt. Srov Matic Muarruly. North of Glen Coe. 115 heat in spite of its high initial temperature. Crush-rock and intrusion would cool together, yielding, as it were, a composite chilled edge, G, T. C., E, BB. of ; N\A ia {-) ; ngsheouse he cto is i Beige el sy ww) SY Spy Aer] Spel die’ po oth 4. >7~™ joarcenia acme UMN FAULT-INTRUSION. |—"-——-|E/LDE_ FLAGS VZVAD —— -- —] LINING PARALLEL TO STRINE -—R— .RAYOLITE PATCHES.|.°\-\* | QUARTZITE. eZ B+ | BRECCIA ” Zz). MICA SCHIST. Ns DIP OF BEDDING. A VERTICAL BEDDING. TAs id FOLIATION. SA z FOLIATION. SS UINES OF STRIKE Ge 7 IN MICA SCHIST. FAULT. o 1000 FT. 2000 FT. EMILE Fig. 22.—Map of Stob Beinn a’ Chrilaiste. North-north-east dykes omitted. The older fault plane, north ofthe summit of Stob Mhic Mhartuin, is marked by exactly similar crush-effects. These culminate in the production of a flinty crush-rock against which the early Fault-Intru- sion is clearly chilled. E, B. B 116 Glen Coe Cauldron. jo Oo ° o RSBRECCIA ° Ze.” ° SEX * - fekthoites ENS ae bcs“ \\SINMORNBLENDES - Afi == a 207 | OE ANDESITE . QC —————"« APA IS te Sas to , —_S_ TOMA =r ick Ward VES VY NA ess VIAN Mead BhuindhS, HO eee eS —> A. 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