: 5* ^ ■iLa^^ -■^/.vl - lavs r)^m^,,i^^js ti. .i;\ Q|arn$U HmoetBttg Ethtarjj Strata. Sitm lath BOUGHT WITH THE INCOME OF THE SAGE ENDOWMENT FUND THE GIFT OF HENRY W. SAGE 1891 Cornell University Library 3E 264.H64M6 1905 The geology of Mid-Argyll.(Explanation o 3 1924 004 061 796 Cornell University Library The original of tiiis bool< is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924004061796 MEMOIRS OF THE GEOLOGICAL SURVEY. SCOTLAND. THE GEOLOGY OF MID-ARGYLL. (EXPLANATION OF SHEET 37.) BY J. B. HILL, E.N., WITH THE COLLABORATION OP B. N. PEACH, LL.D., P.E.S. ; 0. T. OLOUGH, M.A. ; AND H. KYNASTON, B.A. WITH PETEOLOGICAL NOTES BY J. J. H. TEALL, D.Sc, P.R.S., and J. S. FLETT, M.B., CM., D.So. PUBMSHBD BY ORDER OP THE LORDS COMMISSIONERS OF HIS MAJESTY'S TBIiASURY. GLASGOW: PKiNTED FOR HIS MAJESTY'S STATIONERY OFPIOE By JAMES HEDDERWICK & SONS Ltd., At " The Citizen " Peess, St. Vincent Place. And to be purchased from JOHN MENZmS & Co., Rose Street, Edinburgh; B. STANFORD, 12. 13, and 14 Long Acre, Lond'ojt ; . HODGES, FIGGIS & Co., Ltd., Grafton Street, Dublin ; From any Agent for the sale of Ordnance Survey 'Maps ; or through any ' Bookseller from the Ordnance Survey OfHoe, Southampton. 1905. Price, Three Shillings. LIST OF PUBLICATIONS OF THE GEOLOGICAL SURVEY OF SCOTLAND. I.— Maps on One-inch Scale- 1. Wigtownshire, South-Western Districts. 4s. 2. Wigtownshire, South-Eastern Districts. 48. 3. Wigtownshire, Western Districts. 6s. 4. Wigtownshire, East Part ; Kirkcudbright, portion of S. W. Division, 6s. 5. Kirkcudbrightshire, Southern Districts. 6s. 6. Kirkcudbrightshire, E. margin ; Dumfriesshire, S. margin. 4s. ,7. Ayrshire, South- Western Districts. 6s. 8. Kirkcudbrightshire, Ayrshire, and Wigtownshire. 6s. 9. 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PREFACE. The region described in this Memoir embraces the districts of Northern Oowal, Argyll, and part of Lome, which are included in sheet 37 of the one-inch map of Scotland. It presents a remark- able development of the various subdivisions of the metamorphic series of the Eastern Highlands and their associated igneous rocks, while in the north-west it contains part of the southern margin of the Old Eed Sandstone volcanic plateau of Lome. The central portion, bounded on the west by Loch Awe, and on the east by upper Loch Fyne, Gaol Ghleann, and Glendaruel, was surveyed by Mr. Hill ; and the south-eastern district by Mr. Clough, except two small tracts in tlie extreme north-east and south-east parts of the sheet, which were respectively mapped by Mr. Dakyns and Mr. H. M. Cadell. In the north-west portion, Mr. Hill surveyed the area extending from Loch na Sreinge across Loch Avich to Barr Phort on Loch Awe, and also the tract south- west of Lochan a' Bhruic from Loch Awe to its western watershed. The remainder of the north-west district was mapped by Dr. Peach, Mr. Symes, Mr. Kynaston, and Mr. Pocock. This Memoir has been written mainly by Mr. Hill, with contri- butions from Mr. Clough, Dr. Peach, and Mr. Kynaston. More detailed descriptions of the Oowal district by Mr. Olough and Mr. Hill have already appeared in the Memoir on the Geology of Oowal. The petrographical examination of the rocks has been done partly by Dr. Teall and Dr. Flett, who supplied descriptive notes of the microscopic sections, and partly by Mr. Kynaston and Mr. Hill. Mr. E. T. Newton, P.E.S., determined the fossils from the shelly marine clays on the south-east shore of Loch Pyne, and furnished the list given in the Appendix. The list of publications referring to the geology and mineralogy of the district has been prepared by Mr. Tait. Special thanks are due to Sir John Murray and Mr. Laurence Pullar for permission to publish their chart of soundings of Loch Eck, and to Dr. H. E. Mill for his chart of Loch Dubh. The photographs reproduced in Plates I. to IV. inclusive have been taken by Messrs. Valentine & Son, Dundee ; that in Plate V. by Mr. John Fleming, Glasgow ; in Plate VI. by Mr. Eobert Lunn ; and the photomicrographs in Plate VII. by Mr. T. 0. Hall, of the Geological Survey. The Memoir has been edited by Dr. Home. J. J. H. TEALL, Director. Geological Survey Ofl&ce, 28 Jermyn Street, London, 7th November 1905. CONTENTS. r PAQB Beepace iii CHAPTER I. Area and Physical Features, ....... 1 CHAPTER II. Formations and Groups of. Rock and their General Distribution, 9 CHAPTER III. Metamorphic Rocks — i. Beinn Bheula Grits and Schists, ... 12 CHAPTER IV. Metamorphic Rocks {continued) — ii. The Green Beds, .18 iii. The Glensluan Schists, 22 iv. Loch Tay Limestone, . . . . • . 23 CHAPTER V. Metamorphic Rocks (continued) — ': V. Garnetiferous Mica-schists overlying Loch Tay Lime- stone, . . . . . . . . . •■ 27 vi. Graphite Schists, ....... 31 CHAPTER VI. Metamorphic Rocks (continued) — vii. Ardrishaig Group, .32 CHAPTER VII. Metamorphic Rocks (continued) — ■ viii. Loch Awe Group : Summary of the Stratigraphy, . 40 CHAPTER VIII. Older Igneous Rocks — Epidiorite and Serpentine, 62 CHAPTER IX. Folding and Progressive Regional Metamorphism, . . . 72 ■ CHAPTER X. Lower Old Red Sandstone — . . Volcanic Series, ., . • ,. . . . . 85 Vi CONTENTS. Page CHAPTER XI. Later Intrusive Igneous Rocks, 92 CHAPTER XII. Faults 123 CHAPTER XIII. Tlhe Glaciation and Glacial Deposits — Recent Deposits, 127 CHAPTER XIV. Economic Resources, . . . . . . • .150 APPENDIX. Part I. Palseontological, 159 Part II. Bibliographical, 160 LIST OF ILLUSTRATIONS. Plate I. Inveraray and Raised Beach platform fringing Loch Fyne, Frontispiece To FACE Pagk II. Miller's Linn, Glen Aray. A band of quartz- porphyry forming waterfall, . . . .111 III. Upper Loch Fyne and the entrance to the Pass of Glencroe. The castle of Dundarave is built on a promontoiy formed of epidiorite, ... 62 IV. The summit of the Pass of Glencroe with Loch Restil. The rugged hill scenery is formed by the Beinn Bheula schists, . . . . . 12 V. Quartz- porphyry cut by basalt dykes in a quarry north of Crarae, Loch Fyne, . . . .117 VI. Contorted mica schist in cliff by the roadside, south of Coylett Inn, Loch Eck. (Plate V., Oowal Memoir), 81 VII. Micro-photographs. The first five illustrate a series showing progressive metamorphism, from an andesitic basalt through intermediate stages to hornblende-schist, or epidiorite. The last slide illustrates a Dalradian grit from the Loch Awe series with little or no metamorphism, At end of volume. FIGURES IN TEXT. Page Fig. 1. Map of Loch Eck, showing soundings in feet, . , 128 „ 2. Map of Dubh Loch, showing soundings in feet, . . 129 DESCRIPTION OF PHOTO-MICROGRAPHS. Fig. 1. 9274. Near Inverliver, Loch Awe. „ 2. 9270. ,, ,, ,, ,, „ 3. ' 4367. Ford, Loch Awe. „ 4. 5525. I mile west of Loch Ederline. See descriptions on pages 65-66. These are examples of the andesitic basalts of the Loch Awe group, in which the original igneous structures are well preserved, and have suffered little modifica- tion by shearing. They are all porphyritic, with large plagioclase felspars in a groundmass of felspar, iron ores, chlorite, and epidote. Slide 9274 contains vesicles filled with calcite and lined by fine-grained opaque films, which probably were originally glassy. Slide 5525 con- tains acicular actinolite between the groundmass felspars. Fig. 5. 4366. Ford, Loch Awe. See description, p. 66. This shows a similar rock with phenocrysts of plagioclase felspar, but the groundmass has been converted by shearing into a schistose aggregate of hornblende, chlorite, epidote, and felspar. „ 6. 5545. Near Loch i Mhinn. See description, page 56. An example of the Loch Awe grits showing clastic structures. Rounded grains of quartz and felspar lie embedded in a matrix of quartz, felspar, chlorite, Iron oxides, and fine scaly mica. THE GEOLOGY OF MID-ARGYLL. CHAPTER I. Area and Physical Featdres. The area described in this memoir falls almost entirely within the county of Argyll, and is nearly bisected by Upper Loch Fyne and the landward extension of that fiord. To the north-west of this loch are portions of the districts of Argyll and Lome, the dividing line of which is Lochawe ; while to the south-east the northern part of Cowal extends over the remainder of the map, with the exception of a very small tract, amounting to four or five square miles, between Gare Loch and Loch Long, which forms a part of Dumbartonshire, Of the 432 square miles contained in the sheet, about 400 miles are included in the land surface and freshwater lakes, while the remainder is apportioned among the various arms of the sea, of which Loch Fyne appropriates about four-fifths. Inveraray, the capital of Argyllshire, is situated about four miles north-east of the centre of the sheet. The region forms a part of the south-east Highlands, as defined by the Great Glen extending from Moray Firth to the Firth of Lome. That mountainous tract, stretching from sea to sea, exhibits on its coast lines striking differences. While on the one side it presents a bold and unbroken front to the North Sea, its Atlantic sea-board is now represented by a vast archipelago. Even where submergence has failed to produce complete isolation, the incursions of the sea along the weaker lines of the barrier have enabled it to pierce far into the Highlands by occupying the pre-existing valleys. This system of fiords and intervening peninsulas has given rise to some of the most striking coastal scenery in Britain. The district embraced in this sheet forms the central portion of the great belt of country that represents the extreme limits to which these sea lochs have penetrated. The zone enclosing the heads of those fiords may be said to mark the boundary between the Grampians and their less elevated prolongations that deploy towards the Atlantic on the west. In the topography of this picturesque region the centre of interest undoubtedly lies in the phenomena of the fiords, not- withstanding the striking forms of the surrounding hills. 2 Area and Physical Features. The Highland* watershed closely approaches the confines of this region ; it passes through Ben Lomond to the west end of the moor of Eannoch. While the great tract of Highlands to the east of that line drains into the North Sea by a series of large rivers, the western area, which empties into the Atlantic, is too contracted to admit of an impoitant river system. Measuring, however, from the extreme end of the peninsula of Kintyre, the Highland watershed would occupy very nearly a central position. This western area, where the watershed and coast line are in close proximity, has a proportionately steep declivity. From the watershed the drainage is carried to the Atlantic through a series of troughs, dominated by the basin of the Fyne, which has Loch Awe on the one side, and Loch Lomond on the other. The trend of the two former is south-west, while the latter follows an approximately transverse course. Although the water- shed nowhere falls within the sheet under description, it passes within a few miles of its north-east boundary ; but as its position has an important bearing on some of the physical and geological problems with which we are called upon to deal, its salient features cannot be too prominently kept in view. As already mentioned, the most mountainous region is situated in the extreme north-east, where the summit of Beinn Buidhe (3106 feet, in one-inch sheet 45) affords a commanding view of the district. The lower slopes of that mountain extend to the head of Glen Shira, and the easiest ascent of the ridge is by way of that valley. Before the top of that peak is gained, the wide expanse of the Central Highlands, effectually screened from the west by the terminal barrier of the Grampians, breaks gradually into view, and so extensive is the prospect from the summit, that far distant Ben Nevis forms but a central figure in the group of mountains that bounds the range of vision. Viewed from those altitudes the High- land tract appears as an eroded table-land, the broken characters of its surface having been determined by prolonged denudation. West- wards, as far as the eye can reach, is a combination of land and water that forms one of Scotland's fairest pictures. That panorama of which Loch Fyne constitutes the foreground, with the town of Inveraray, lying on its margin, extends beyond the Scottish mainland and past its island fringe to the Atlantic Ocean. Loch Awe dominates the region north-west of Loch Fyne, and Loch Long the south-eastern area. Those sheets of water, with their south-westerly trend, coincide in direction with many of the longi- tudinal valleys and prominent watersheds of the region ; the surface features having been largely determined by the geological structure of that district. The present drainage system, however, is complicated by a still more ancient topography bearing no relation to the existing geolog- ical features, the controlling factor of which has been lost by the destrviction of a pre-existing land surface, together with the geological formations which probably determined its evolution. * The South-Eastem Highlands aa defined by the Great Glen. Area awl Pliysmd Features. 3 The transverse S3'stem is represented in this area by the basins of Loch (-ioil and Loeli Eel':, both of wliich are truncated by those of longitudinal type, including Locli Long, Loch Fyne, and Loch Awe, while the parallel basin occupied by Loch Avieh is sufficiently large to find a jDlace within the same group. While the Highland region beyond the fiords has been undergoing a steady process of degradation by the agencies of sub-aerial erosion, its western barrier has been likewise assailed by the ocean. The investigation of the hydrographic basins of this western region throws much light on their past historjr. That these sea lochs indicate a submergence of pre-existing valleji's is undoubted ; while the ancient terraces at various intervals above the present ^^'ater level constitute an accurate record of the oscillations of the land. But the testimony of those marine terraces is dwarfed by the phenomena of the great freshwater lakes of Loch Awe* and Loch Eck. The latter marks the site of an ancient arm of the sea, between Loch Goil and Loch Striven, which had advanced far into the cleft that dissects the northern part of Cowal. The surface of Loch Eck is 67 feet above ordnance datum, and as the marine terraces considerably^ exceed that elevationf the period of its former junction with the sea maj' be relativel}' defined. The history of Loch Awe, however, is more obscure. While its present surface is 11 6 feet above sea-level its ancient shore-line was at least 1 30 feet higher, at a certain stage in the glacial period, when the water was dammed back by a lobe of ice radiating from the high ground to the north-east. After two futile attempts to force a passage to the sea at its south-western end, those channels have been successively abandoned in favour of the natural cleft which connects the Pass of Brander with Loch Etive, so that this lake exhibits the singular phenomenon of discharging its waters at its head. Moreover, the present outlet is not yet sufficient for the adequate discharge of its waters. Hence, in a wet summer a rise of 8 to 10 feet may be anticipated, which produces a marked change on the configuration of its margins. After a period of excessive rain the peninsula of Eilean Liver is converted into an island, which v?as doubtless its original form vi'hen it received its specific name. On the opposite shores of the loch the ruined Castle of Eincharn occupies a similar site, and one which was evidently selected from the strength of its insular position, but at the present day it forms part of the mainland. The recession of Loch Awe in historic times is not to be attributed to a diminution of rainfall, but to the increasing excavation of its channel of outlet, and its waters will continue to recede until the narrow chasm connecting- it with Loch Etive shall be sufficiently deepened to admit of their junction. * Local tradition ascribed the creation of both lakes to the destructive freaks of a mountain nymph. t A terrace that fringe.s thi.s loch about 20 feet above its present level is considered by Mr. C. T. Clough to represent the 100 feet beach. Area and Physical Features. The rugged and mountainous character of the north-eastern region is exemplified by the wildness of the glens that dissect it. The slopes of Glen Fyne are so precipitoiis that a rise of 2000 feet occurs within the breadth of a mile. Only its lower por- tion falls within this area, the western slopes of which culmi- nate in the ridge of Beinn Ohas (2214 feet). The singularly straight cleft of Glen Kinglass, east of the head of Loch Fyne, is flanked on the north by the conspicuous peak of Binnein an Fhidhleir (2658 feet), and on its southern side by a steep ridge terminating in the more elevated summit of Beinn an Lochain (2955 feet). The road through this glen, connecting Loch Fyne with Loch Long, lies 2500 feet beneath the crest of the latter mountain, and at its summit, reaches an altitude of 860 feet, forming the most elevated highway in the area. Further to the south the pass linking Loch Fyne and Loch Goil is marked by the particularly wild chasm of Hell's Glen, hemmed between the mountains of Stob an Eas (2400 feet) and Oruaich nam Mult (2001 feet), and terminating in the fertile valley which opens on the waters of Loch Goil, that loch being bounded by slopes equally rugged and precipitous. A broad and cultivated strath connects Strachur with Loch Bck, that narrow lake being over- shadowed by steep slopes the summits of which exceed 2000 feet in height. Beyond Loch Bck the scenery gradually softens, and the hills which bound the valley of Glendaruel are not so wild in outline. That remote and picturesque glen presents features that justly entitle it to a foremost place among the glens of the Scottish High- lands. Further to the west lies a sylvan hollow still less frequented, forming the beautiful vale of Strathlachlan. On the other side of Loch Fyne the converging valleys of Glen Shira and Glen Aray are conspicuous features in the landscape. The former, flanked by elevations of 1500 feet, is floored by a fertile strath, amid which the Shira follows a sluggish and sinuous course, while the lake at its mouth is an admirable illustration of a rock- basin in process of extinction.* The river Aray, on the other hand, has an uneven and rocky floor, its sylvan track being marked by the picturesque waterfalls of Linneghlutton and the Miller's Linn, whilst luxuriant woodland clothes the valley slopes. Glen Aray, which truncates the prevailing geological features, belongs to the more ancient valley system to which allusion has been made, and not miprobably marks the course of a pre-existing river of consid- erable size that extended to the Clyde estuary along the hollows of Strachur and Loch Bck. The district of Argyll between Loch Fyne and Loch Awe is dissected by no other hollows of importance, with the exception of the valley of the Add, which forms an open and extensive catch- ment basm drammg the south-western area, and culminating in a floor of fertile terraces where that river deploys upon the plains of Kilmichael. * The Dubh Loch, the depth of which is 42 feet. Area and Physical Features. 5 The uplands of that central district consist of a series of longitud- inal ridges, which gradually decrease in height in a south-west direction. While the summit of Cruach Mhor, above Glen Aray, has an altitude of 1982 feet, the chain of peaks which marks the watershed beyond includes Beinn Bhreac (1723 feet), An Suidhe (1687 feet), Beinn Laoigh (1421 feet), and Cruach Breac-liath (1154 feet), and from the latter, the plateau falls still more rapidly towards the plains of Kilmartin. West of Loch Awe the district of Lome gives fresh diversity to the landscape, in the terraced platforms and mural escarpments formed by the volcanic series of the Old Red Sandstone. Not the least interesting of the data in regard to the denudation of that region is the nature of the ancient terrestrial floor on which the volcanic materials were laid down, from which we learn that the topography of Eastern Lorne in that far distant epoch differed little from the physical features of to-day, into which it imperceptibly blends. But besides these major features of the area, we find, scattered broadcast over the region, minor clefts, usually transverse to the prevailing surface features, which often conceal yawning chasms and wooded gorges, with many waterfalls. These ravines lend a charm to the local scenery that is enhanced in the moorland districts by their presence being unsuspected beyond their immediate precincts. The falls of Blarghour, within easy reach of the main road, are well known to the tourists who frequent Loch Awe, but the upland regions beyond the beaten track abound in cascades equally attractive. The existing type of scenery is the result of a long process of rock sculpture of great geological antiquity. We have already seen that the lavas of the Old Red Sandstone epoch were laid down on an ancient terrestrial surface corresponding closely to the general topography of to-day. But beyond that period further investigation is baffled by the absence of chronological landmarks. Subsequent to that age of vulcanism, however, we are able to trace events which have modified the architecture of the region. After its close the district furnishes no clue to its history during Mesozoic time. Renewed igneous action on a colossal scale ushered in the Tertiary period, and has set its mark on this area by the multitude of dykes whose erosion has given rise to many of the finest gorges. Many of the longitudinal valleys have been excavated subsequent to those intrusions, but we must be on our guard, however, in limiting the age of the larger troughs to that final period of excavation. Erosive agencies in Lorne have laid bare valleys of corresponding trend, of an antiquity exceeding the epoch of the Old Red Sand- stone, which had been buried by the deposits of that period. For all that is known to the contrary, many of our western valleys maj^ be equally ancient, and may have undergone the processes of burial and restoration more than once in their subsequent history. But among the agencies of denudation, ice must be ranked as one of the most important. Glaciers creeping seaward have left cogent memorials of the destruction which attended their march. 6 Area and Physical Features. The effects are most obvious in the main channels now occupied by the western fiords, which conceal beneath their waters deep basins dug out by the advancing ice streams, the bottoms of which are yet tenanted by Arctic mollusca, descendants of the fauna of the Glacial Age. The effects of the ice plane are not restricted to the submarine valleys, for the features of the landscape yet bear the stamp of its action in the rounded outlines of the ridges and the striated surfaces of the rocky slopes. Besides abrading the upland regions, the rock debris was reduced to the consistency of clay, and forced into the hollows and against the mountain foot. While many of these glacial deposits have been removed by subsequent denudation, sufficient have remained to diversify the landscape and to conceal many a rocky hollow. The trail of the ice sheet is still further indicated by the presence of moraines. They occur in great profusion along the central belt extending across the map from Loch Avich to Caol Ghleann, which apparently marks the terminal limits of the mer de glace during a protracted period. Erratic blocks, occasionally hundreds of tons in weight, dispersed alike over mountain ridge and valley slope, frequently form conspicuous objects in the landscape. Often fantastically poised far from their parent source, they furnish an accurate record of the movement of the ice which carried them, and confirm the evidence revealed by the glacial striations. While the lochs and fiords which lave the lower valleys dwarf the minor lake scenery, many an upland tarn, fringed with the floating water lily and tenanted by duck and diver, relieves the wildness of the mountain solitudes. In northern Oowal these elevated lakelets are few and far between, but in the remaining area from Loch Pyne to the Braes of Lome they form a conspicuous and pleasing feature in the landscape. Their peculiar habit of occurring in groups is well exemplified by the chain of tarns around Loch Leacann, north-west of Furnace. Further to the west, Fincharn Loch and Loch Gaineamhach constitute the lateral boundary of a still larger assemblage, while to the north another group occupies the very watershed between Beinn Bhreac and Cruach an Lochain east of Loch Awe. At the foot of the former the waters of Lochan Long reflect the shadows of that crag at an altitude of over 1400 feet, while the hill of Cruach an Lochain has evidently derived its name from an ancient lake beneath its slopes, the basin of which is now concealed by a peaty swamp. The latter may be taken as a typical example of a silted-up lake, many other instances of which are interspersed amongst the moorland scenery. Between Loch Awe and Loch Avich the Cam Loch fills an extensive basin, while Lochan a' Bhruic affords a striking instance of shrinkage from its former margins. To the west of Loch Avich, a most extensive group of tarns softens the landscape between Loch Tralaig and Gleann Domhain. South of Loch Fyne these small mountain lakes are of rare occurrence, but Loch Eestil, at the Area and Physical Features. 7 summit of the pass between Loch Fyne and Loch Long, is a well- known object to the traveller. The lakelet of Curra Lochain, 1000 feet above Loch Goil, laves the hollow which divides the rugged peaks of Beinn Lochain (2806 feet) and Beinn Bheula (2557 feet), while at the eastern foot of the latter hill Lochan nan Cnaimh reposes at an elevation of 1378 feet. In the extreme north of the district a group of tiny tarns form a cluster around Lochan Mill Bhig. Still further to remind us of the frigid climate of the ice-age is the presence of the Arctic hare, which finds a congenial habitat in the moorland regions, while yet higher among the misty summits the croak of the ptarmigan relieves the silence of its desolate retreats. And besides these remains of an Arctic fauna, the flora of the higher lands still retains the lingering relics of a boreal vegetation. The landslips of the Highland regions are conspicuous objects on the steeper slopes, where the piles of naked debris at once attract the eye. One striking example on the eastern slopes of Glen Shira, where the slipped mass abuts upon the Dubh Loch, is so concealed by vegetation that its presence would be unsuspected were it not for the marginal features. The inhabited disti'icts form but a very small proportion of the area, and are almost entirely confined to the coastal fringe and the valleys which diverge from it. The only town is Inveraray, the capital of Argyllshire, with a population of about eight hundred, and the more impoi-tant villages are Strachur, Lochgoilhead, Furnace, and Kilmiohael-Glassary, the last of which was formerly an important centre of population. In the first half of the 19th century the inhabitants of the burgh of Inveraray numbered about 1200, but in spite of this rapid decrease the depopulation of the rural districts has proceeded yet faster. The rural part of the parish of Inveraray with a former population of about 1000 shows a corresponding diminution, notwithstanding that the stone quarries at Furnace give employ- ment to 100 men.* These small farmers or crofters generally prosecuted the herring fishing in addition to their farming. The dispatch of the fish to the markets by steamer has in recent times destroyed the local herring curing (salting) industry. Although some of the lochs and mountain defiles are traversed * As examples of the reduction since the same period we may quote the following : — Auchindrain, formerly 7 tenant farms and 27 cotters, now 4 farmers and 1 cotter. Auchnangoul, ,, 16 farmers and 8 or 10 cotters, now 5 farmers and 1 cotter. Kenmore, ,, 17 farmeps, now 2 farmers. The Rev. P. N. Mackichan of Inveraray, to whom we are indebted for the above statistics, was informed by an old parishioner that he had counted 51 walking from Kenmore to the parish church of Inveraray. As the distance is 5 miles these must have been mostly adults, whereas the entire adult population of Kenmore to-day is 16. 8 Area and Physical features. in the tourist season by a daily stream of visitors journeying to more distant scenes, the influx of summer residents is comparatively small notwithstanding its proximity to the populous districts of the Clyde. With the exception of the brief season devoted to sport, the mountain regions are practically deserted by all save the game- keeper and the shepherd, and their solitude is such that one may roam over the moorlands for days without encountering a single human being. J. B. H. CHAPTER II. Formations and Groups of Eock and their General Distribution. The following shows in tabular form the various formations occurring in the area embraced in the map. fPeat in basin-shaped hollows and on Eecent . < ^ ^^^ ^'^^' i -bresh-water alluvium [ Marine alluvium Sign ON Map. Glacial . ■( Glacial shell beds. Moraines. Fluvio-glacial sands and gravels . . '©^ [Boulder Clay. c, 1 , [ Sandstone and Conglomerate . . c' bandstone J ° MetamorpMc. Graphite schists ....... g' Quartz schists ....... % Schistose grits and greywackes . . ^ Epidotic and Chloritic schistose grits, &c. (" Green Beds ") p^ Schistose limestones ...... \ Garnetiferous mica-schists . ... g Albite schists ... . .1" Slates and phyllites with some quartzite . 1 Graphitic-slates and phyllites with some quartzite . I' Mica-schists of various types ..... g Igneous. Contemporaneous of 1 j^^^^^^^^ p^ ^i Lower Old Eed ^^^^^^ ^^^ Agglomerate . . Ts c> Sandstone age J 10 Formations and Groups of Bock and Distribution. 'Volcanic Necks . Trachyte Basalt and Dolerite No' Tr B Lamprophyre and Mica-trap SM Post-foliation < Hornblende porphyrite Felsite Po F Intrusive < Diorite and Hyperite . Kentallenite D K 'v Granite G f Epidiorite, Hornblende anc I Pre-foliation X Chlorite schist. BG V (^Serpentine . 2 Those schistose rocks which are of sedimentary origin form much the largest part of the area south-east of Loch Pyne. They are divisible into groups which strike N.N.E., and, as the relative ages of most of them are uncertain, they will be described according to their geographical positions, beginning first with that in the E.S.E. part of the map, — which has been named after Beinn Bheula, a hill south-west of Lochgoilhead. The Beinn Bheula group consists chiefly of schistose grits and albite schists, and probably corresponds with the Ben Ledi grit series in the adjoining one-inch map 38. It occupies more than half the area on the south-east side of Loch Fyne, and, with the exception of Beinn Ohas, forms all the highest hills in the sheet. To the W.N.W. of the Beinn Bheula group, the Green Beds, the Glensluan schists, the Loch Tay limestone, the garnetiferous mica-schists, the graphite schists, and the Ard- rishaig phyllite series occur in order. The Green Beds contain various other varieties of schist besides Green Beds proper. The garnetiferous mica-schist group includes some outcrops of Green Beds and of limestone, and the gi'aphite schists may be classed as a subdivision of it. C. T. G. North-east of Loch Fyne the Beinn Bheula beds with albite schists occupy about two square miles of the Glen Fyne slopes between Newton Hill and the river Fyne. These are succeeded hy the Green Bed group, which forms a band about half-a-mile in breadth, that meets the coast of Loch Fyne between Clachan and Cuil. The Glensluan schists, which also contain some Gi-een Beds, con- stitute a band about a quarter of a mile wide lying between the Green Beds and the Loch Tay limestone, which latter band extends from Loch Fyne near Cuil-bheag to the ridge of Beinn Ghas above Glen Fyne. This band is succeeded by the garnetiferous mica-schist group which occupies the watershed, and includes the Brannie, Clachan Hill, and Beinn Chas, the loftiest ridge in the map north-east of Loch Fyne and reaching an elevation of over 2200 feet. Good junctions with the Ardrishaig series can only be seen in the Brannie Bum to the north and on the coast of Loch Fyne in the neighbour- hood of Drishaig to the south. At these junctions the graphite beds are represented by a thin band which cannot be followed in Groups of Rock and General Distribution. 11 the intervening ground. The Ardrishaig series, which succeeds, forms a great central belt across the map. To the north-east of Inveraray it has a breadth of three miles, while to the south-west of that place it expands to a width of six miles. The more siliceous portion of the group averages a mile in width between the Douglas water and Furnace, and has a corresponding width further south- west at Loch Gair. While south of Loch Fyne the Ardrishaig group south-west of Newton Bay is represented entirely by this siliceous band which, however, contains much argillaceous material, in the north-western area the siliceous members are subordinate to the argillaceous, and most of the zone has been coloured as phyllite. Its upper boundary with the Loch Awe group flanks the western slopes of Glen Shira, crosses the lower portion of Glen Araj^ from which it stretches to the watershed between Loch Pyne and Loch Awe. Keeping to the western slopes of the Add basin, it crosses that river a mile above Kilmichael Glen and continues to the south-west corner of the map. The remainder of the sheet, forming nearly one third of the area of the map, is occupied by the Loch Awe group, with the exception of the part north-west of Loch Awe where that group is overlain by the Lome Andesites. The epidiorite group is mainly confined to the north-west of Loch Fyne, and its distribution is very general. It is most strongly developed in the Loch Awe basin, where its superficial extent equals that of the sedimentary members. It assumes almost equivalent proportions in the extensive belt of country between the river Add and the coast of Loch Fyne, extending from Loch Gair to Crarae. South of Loch Fyne it is strongly developed east of the valley of Strathlachlan. These sills extend a little beyond the base of the Green Beds in a south-east direction. Serpentine is confined to the western slopes of Glendaruel, midway between the watershed and the river Ruel. J. B. H. In the area north-west of Loch Avich, bordering the north-west part of the map, the volcanic rocks and associated sediments of Old Red Sandstone age rest unconformably on the metamorphic series. The Glen Fyne granite occupies rather less than two square miles in the north-east corner of the map. Between Glen Fyne and Gleii Shira small bosses of granite, diorite, and kentallenite are scattered, while similar rocks are found in the north-west corner of the map beyond Loch Avich. Porphyrites are extensively developed in narrow sills on the slopes of upper Loch Fyne, and occur in less abundance in that part of the sheet lying to the north-west of Loch Awe. Quartz-porphyries occupy many of the north-western slopes of Loch Fyne from the coast to the watershed and form enormous intru- sions over a zone sixteen miles in length and four miles in breadth. Lamprophyres form a conspicuous belt along the western slopes of Glen Shira and the valley of the Brannie, which diverges from that glen. Over other portions of the sheet their distribution is also general. Basalt dykes are extremely abundant, but are most extensively developed in the south-western half of the sheet. J. B. H. ; C. T. C. ta CHAPTER III. Metamorphic Rocks, i. beinn bheula grits and schists. In this group, besides the predominant schistose grits and albite schists, there are siliceous schists in which no remains of clastic grains are perceptible; micaceous schists in which albite is scarce or absent ; thin flaggy schists, intermediate in character between the more siliceous and the more micaceous schists ; and thin lenticular bands of calcareous siliceous schist. The north-west boundary of the area occupied by this group is markedly sinuous, in consequence of the varying inclinations of the surface and the comparatively gentle dip of the beds. It is also displaced by some faults, the largest of which strike north-west and throw down towards the south-west. From the north-west bound- ary near Strachur to the south-east corner of the map, — a distance in a direction at right angles to the general strike of foliation of about ten miles, — all the schists, with the exception of two thin " Green Beds," seem referable to the types mentioned. In spite, however, of the great breadth of country which these Beinn Bheula schists occupy, it does not seem necessary to suppose that the original thickness of the beds represented by them was very great, for evidences of repeated folding are everywhere visible. The folding and regional metamoi'phism exhibited in this group is discussed in Chapter IX., but the special metamorphism found near the Glen Fyne granite is described in connection with that mass. If we regard the whole area in which these schists occur, those which are of tolerably siliceous character, — that is the schistose grits, — seem somewhat more widespread than the more micaceous bands, including with these last most of the albite schists. But there are wide tracts, for instance on the south side of Eudha nan Boin and along the strike south-westward, in which the most common schists are neither very sUiceous nor yet very micaceous, and perhaps they represent altered sandy shales or shaly sandstone. We shall now proceed to describe first the more siliceous, and then the more micaceous schists and albite schists. In some of the siliceous schists the remains of clastic or allothi- genic grains are very scarce, or perhaps do not exist ; but in most cases careful search discloses some, though perhaps in very subordinate amount compared to the recrystallised granulitic matrix. The clastic grains of quartz are often of an opaque white or pale blue colour and slightly opalescent. Those of felspar are Beinn Bhcula Grits and Schists. 13 either of white or pink colour. No thorough examination of the felspars has been made, but it is believed that most consist of oligoclase, — the felspar composing most of the clastic grains in the corresponding series in the adjoining one-inch map 38. Flakes of secondary mica, both white and black or brown, are generally common, and large enough to be readily discerned by the unaided eye. Most of them lie with their flat sides along some foliation plane, — whether this is parallel to the bedding or not, — or they form coats round the clastic grains. A series of massive siliceous schists with distinct remains of grains or pebbles spreads over large areas on the promontory between Loch Long and Loch Goil, and continues thence in a south-west direction to the south margin of the map. They are well exposed in coast sections on the west sides of the lochs, and between Toll nam Muc and Rudha nan Eoin in Loch Goil they form a band about half a mile broad, on the north side of which there is a thin band of albite schist followed by more siliceous schists. The south-east boundary of this series is not always well defined, but near Rudha nan Eoin it runs tolerably straight, excepting where it is displaced by faults. A little south-east of the boundary various thinner bands of massive pebbly schist were traced for short distances, as shown on the map, and in one of these, about a quarter of a mile N.N.W of Am Binnein, there are pebbles an inch in length. Below the albite schist on the hilltop between Cruach a' Bhuic and Onoc na Tricriche, west of Loch Goil, there is a great thickness of massive siliceous schist with occasional clastic grains. It is noticed in several localities that a bed of schist, which seems for a certain distance to form the top of the siliceous series, becomes overlain a little further north-west by beds in which the proportion of siliceous schist may for a considerable thickness be as great or greater than that of albite schist. This phenomenon may probably be explained by the existence of many small folds with both limbs hading north-west which repeatedly bring up the top bed of the siliceous schists. From Cruach a' Bhuic to the south-west side of Lochan nan Cnaimh the generalised lines adopted as the top margins of the siliceous schists on either side of the watershed keep nearly horizontal, but in their further course they usually incline N.N.W. The line on the west side of the watershed runs from the head of Goire Ealt in a nearly west direction to the north- east side of the Loch Eck valley about half a mile below Inver- noaden, but it is much obscured by drift. On the south-west side of that valley no satisfactory representative of the line was found. The margins of various siliceous pebbly schists or grits wore mapped on Cruach Bhuidhe, Beinn Bheag, etc., but none of these was traceable for any great distance. A gradual loss of definition, presumably in consequence of folding, is well seen three-quarters of a mile north- west of Stuck. , Between the 1250-feet and the 1750-feet contour lines on the east side of Carn Ban, four miles west of Loch Eck, there runs the upper boundary of a thick schistose grit which perhaps 14 Metamm-phic Rocks. represents part of the series described near Toll nam Muc. The boundary keeps tolerably straight between the south margin of the map and the N.N.W. fault which pases through Bealach an t-Saic, but on the north-east side of the fault it is less well marked and alternations of the siliceous and the micaceous schists appear. The top of the thick siliceous schist on the west side of the water- shed south-west of Lochan nan Cnaimh, west of Loch Goil, has been traced with short interruptions nearly as far as Glen Kinglas. It enters Oorra Lochain a little above the foot, and in the burn issuing from that loch the schists are well exposed, where for a distance of nearly half a mile they are chiefly of a siliceous and somewhat pebbly character. Between Beinn Tharsuinn and Hell's Glen, north of Loch Goil, the thick schistose grit is traversed by many faults, most of which strike north-west or N.N.W. and throw down to the south-west. The Hell's Glen fault displaces the bed at least a third of a mile ; on its north-east side there are two lines which might be regarded as the top, but the upper one is rather ill-defined and was not traced far. The lower line leaves the glen bottom about half a mile above Monovechaden, and runs along the north-west side of AUt Glinne Mhoir to Beinn an Lochain. On the south and east sides of this mountain it usually keeps between the 2000-feet and 2500-feet contours, and is somewhat well defined, but on the north side the boundary is not so distinct, there being another rather wide outcrop of similar schist above it. On the north side of Glen Kinglas there is no outcrop of siliceous schist so broad as that on Beinn an Lochain, but a band of considerable thickness was traced from the top of the crags a mile east of Binnein Fhidleir, to the east margin of the map. A little to the south-east of the north-west boundary of the Beinn Bheula group of schists there are in places some very massive siliceous pebbly schists. A bed of this kind is well seen near the head of Glen Ganachadan on the south-east side of the fault about three miles north-west of Lochgoilhead. The outcrop is between 20 and 30 feet thick, and close to the fault it is separated from the Green Beds by less massive and more micaceous schists. Nearly a mile W.S.W. of Binnein Fhidleir, north of Hell's Glen, there are two massive siliceous pebbly schists, each about eight feet thick, one of which lies immediately below the Green Beds and the other rather lower. A little below the lower band there is a leaden-coloured satiny micaceous schist about 30 feet thick. Perhaps the most extensive outcrops of siliceous schist in which no distinct clastic grains were observed, are found near the head of the more westerly of the two burns on the south side of Cnoc Coinnich (east of Lochgoilhead). The rock is well seen in the sides of the burn between heights of 2100 and 1700 feet, and breaks up into angular blocks more readily than most of the siliceous schists, but it weathers with much the same pale brown colour and is mixed with occasional bands in which clastic grains may still be seen. The lenticles and thin bands, rarely more than an inch or two thick, of calcareous siliceous schist which occasionally occur, Albite Schists. 15 weather with an ochreous colour. Good examples are found a little west of Shepherd's Point and on the south-west side of Tom nan Con, west of Finart Bay in Loch Long. Tourmaline is only rarely found in the siliceous schists, but a compact siliceous schist from a burn a mile W.N.W. of Ben Donich, N.N.E. of Lochgoilhead, shows on the foliation planes a set of close parallel rods with small prisms of tourmaline lying parallel to the rods. Thin veins of qiiartz, or quartz and felspar, are generally less common in the siliceous than in the more micaceous schists. The felspar in a specimen (4198) from a vein a quarter of a mile north-east of Mark was found to be albite, and it is believed that this is the species of felspar which is most common. The rocks mapped as albite schist or albite gneiss are highly micaceous or chloritic rocks with grains or crystals of clear secondary albite of varying size and abundance. They occasionally contain folia or lenses of granulitic quartz, but albite of the kind described has not been detected in rocks of a distinctly siliceous character. It must be remembered, however, that even if such albites do occur in these rocks they would be hard to recognise by the naked eye in consequence of the similarity of their colour to that of the matrix. The albite grains generally project somewhat from the weathered face of the rock and are of a clove or pale brown colour, — presumably in consequence of the numerous in- clusions. They are never stretched or deformed, and are thus easy to distinguish from most of the clastic grains. They have often a sporadic mode of occurrence, and in certain exposures they are fare, or not seen, while in other adjacent exposures of the same band they may be very abundant. Our impression is that with careful search albites could be found in all the more micaceous beds, excepting, perhaps, in those within a mile or two of the south-east corner of the map. Albite schists often form thin bands alternating with other more siliceous and occasionally pebbly schists in which no albite was observed. These two types of schist may be seen together with the same inclination and in the same limb of fold, and it seems as if in the great majority of cases the differences which now exist between them must be dependent on differences which existed originally between the beds from which they were formed. It is true that the thin micaceous seams which occur along lines of special strain often contain albites even if the rock on either side be a siliceous schist, but it is doubtful whether these seams are ever more than an inch thick, whereas some of the albite schists make wide bands several hundred feet thick. The albite schists mapped appear to play the same role in the section as the phyllites near the Highland border south of the district now referred to, but whether they were originally of the same character is uncertain.* If they were, a considerable chemical * Mr. E. H. Cunningham Craig has lately (Q.J.a.S., vol. Ix., No. 237, p. 10, 1904) described the albite gneisses in the Loch Lomond district, and has advanced the theory that they have been formed by metamorphism from grits. 16 Metamorphic Rocks. change and a large impregnation with soda must have subsequently taken place So as to allow of the formation of the large amount of albite occasionally found. The following is an analysis executed by Dr. Teall of a specimen of albite schist from the head of Stuck Burn (west side of Loch Eck). Silica 63-4 Titanic Acid trace Alumina 18-1 Ferric Oxide 6-7 Lime 0'9 Magnesia . 1-9 Potash 3-2 Soda . 3-2 Loss on ignition 2-8 100-2 Dr. Teall has calculated that if the whole of the soda in the specimen were used to form albite, that mineral would form 28 per cent, of the rock. The proportion of albite in some of the thin seams is probably greater than this, but on the other hand there are extensive exposures in which it is much less. It is clear from their freedom from deformation that the albites wer£ formed after the cessation of the schist-making movements. In the thin slices the puckered foliation planes are often traceable through the albite by the help of frequent inclusions, consisting largely of black iron ores and some nearly colourless prismatic microlites. Associated with the albites, and crossing the foliation planes, flakes of white mica, or of black or brown biotite, are frequently found, which must also have been formed after the cessation of the movements. Small needles of tourmaline, some of which cross the foliation planes, are also tolerably abundant in the albite schists, and in some bands there are many crystals of magnetite. Near the top of Beinn Bheula we noticed a thin seam, about half an inch thick, nearly half of which consists of magnetite. Ilmenite occurs at some localities, as, for instance, on the north-east side of Lochan Mill Bhig, where it forms plates along the foliation, some of which are an inch long and slightly bent. Thin veins of quartz, rarely more than an inch or two thick, are more abundant in the albite schists than in the more siliceous schists, and many of these veins also contain tourmaline in larger needles than are usually found in the schists themselves, and often forming sheaves two or three inches broad. Two tolerably well-defined bands of albite schist cross the promontory between Loch Goil and Loch Long. The band furthest north-west comes to the shore at Mark, while the other lies about a third of a mile south of this place. The south-east band is in some places as much as 300 yards in breadth. Alhite Schists. 17 Various outcrops of albite schist have been mapped in the valley between Lochgoilhead and Hell's Glen, and around the sides of Ben Donich to the east; and a little east of the N.N.W. faults near Drimsynie, albite schist is the prevalent rock from the 20-feet raised beach to above the 500-feet contour. The north-east spur of Ben Donich, from about 200 yards north of the 2250-feet contour to a little north of the 1750-feet level, is almost entirely composed of albite schist. Below this comes lirst a siliceous schist, the outcrop of which on the east side of the spur is rather more than 100 yards broad, and then another broad outcrop of albite schist. This lower outcrop is probably represented between the two north and south faults on the north-west side of Allt Coire Odhair, about a mile south-east of Ben Donich, by an albite schist, the top of which keeps at a tolerably uniform level of about 1250 feet. Albite schists usually form a considerable proportion of the rock for a breadth of a mile or so from the north-west margin of the Beinn Bheula group of schists. Some of those in Strath nan Lub were mapped for short distances. C. T. C. CHAPTER iV. Metamoephic Rocks — continued. ii. THE GREEN BEDS. This group of rocks is of peculiar interest from its abnormal composition. When the survey of this district was commenced, it was considered probable that this type of rock represented volcanic ash beds which accompanied the deposition of sandstones and shales. As the field work proceeded this explanation became more improbable, for not only is there an absence of distinct volcanic lapilli, but the further study of the igneous rocks of Dalradian age seems to point to their intrusive origin. It is certain, however, that these sediments in their original form departed appreciably from normal deposits of sandstone and shale, and that in districts of intense regional metamorphism the mineral reconstruction has not only obliterated all traces of their original clastic condition, but has produced a type of hornblende schist which cannot always be distinguished, even with the aid of the microscope, from rocks which originated as basic intrusions. It is possible that the sediments amongst which these peculiar Green Beds occur were produced directly from the erosion of an igneous complex, and that the latter result from the disintegration of its more basic portions. North of Loch Fyne. — The Green Beds north of Loch Pyne occur along a belt between Clachan and Tom Gobhair in the north-east corner of the map. This zone is not confined to the Green Beds, but represents an admixture of those rocks with bands of mica- schist in varying proportions. That composite band occurs some- what in the form of an irregular lenticle, in the central portion of which Green Beds and mica-schists are about equally abundant, while at the terminations of the lenticle, the former are represented only by a parallel set of thin attenuated seams. The mica-schists which divide the main Green Bed zone from the Loch Tay limestone contain a few Green Bed bands, some of which are in j axtaposition with the latter ; while the garnetiferous schists which overlie the limestone likewise include these bands, but their presence is exceptional. The Green Beds are best developed along that portion of Glen Pyne which is situated about three-quarters of a mile north of Achadunan. In this area the schists are so highly granulitised that clastic structures in the Green Beds are only exceptionally preserved, and the pebbly bands which are so frequent a characteristic of these rocks in Cowal are almost entirely absent. Two gritty bands of this type have been noted near Tom Gobhair, and another, half a Green Beds. 19 mile north of Achadunan. The latter is a sheared epidotic grit, which, under the microscope (10,559), is seen to contain the remains of quartz and felspar pebbles that are now sheared and broken up into a compound mosaic. These lie in a sheared matrix, which is full of pale yellow epidote with chlorite and iron ores. Hornblende is absent or rare, and there is a little calcite. The rock contains much felspar, that is partly untwinned orthoclase or albite, and partlj- oligoclase and acid plagioclase, much of which is very. clear and fresh. Some of the plagioclase is entirely charged with minute grains of secondary epidote. This pebbly Green Bed is associated with finer-grained chlorite schist in which no clastic structures are preserved, and which may be described as a dark green, thin foliated rock with a brown or bronze coloured mica on the cleavage planes ; narrow pale folia of felspar alternate with dark green folia of chlorite. Under the microscope (10,560) it consists of chlorite, albite, epidote, biotite, iron ores, and quartz. Chlorite forms the mass of the rock, and epidote is very abundant in pale yellow grains. Greenish-brown biotite is present in relativelj'' small quantities, and there are a few grains of sphene. Albite occurs in numerous small rounded crystals embedded in the chlorite. They are never idiomorphic, often untwinned, and often simply twinned, but rarely show repeated twinning. While in the Green Beds of Cowal hornblende* has not been detected, it is common in these rocks in the Glen Fyne area. In this region of intense metamorphism, as already shown, clastic structures are seldom preserved. In addition to extreme granulitisation, this advanced stage of metamorphism has left its impress on the Green Beds by the development of hornblende. While in the south-west of Glen Fyne hornblendic Green Beds are rare, as the north-east of that glen is approached they form the prevailing type. The Green Beds which are conspicuously developed at Druim na Muclaich are mainly of the chloritic type, but contain hornblendic bands. The rock occurring amid alluvium near the river Fyne, f mile north by east of Achadunan (10,564), contains pale green hornblende in long narrow prisms, much chlorite, some brown biotite, blue tourmaline, epidote, quartz and iron oxides. Albite of the type previously alluded to is abundant. Specimens taken from the hornblendic bands nearer the centre of the Green Bed zone are of similar type, but contain less chlorite and more felspar (10,558 and 10,562). Both specimens are characterised by abundant tourmaline. The occurrence of tourmaline in these rocks is peculiar, and that mineral appears to be confined to the hornblendic varieties. Moreover, with the development of tourmaline and hornblende there is a diminution in the chlorite and biotite. Although these Green Beds frequently betray their sedimentary origin by the preservation of their bedding planes, and present a * West of Loch Fyne, in the area north of Tarbert, hornblende is an exceptional constituent but doe.=i not occur in the needle-shaped prisms of the Glen Fyne Green Beds. 20 Metamorphic Rocks. type of feature in the field sufficiently distinctive to the geologist familiar with that group of rocks, it often becomes a matter of extreme difficulty to distinguish them from hornblende schists of igneous origin, especially where all traces of their clastic structures have been obliterated. So far as this area is concerned recrystallization has advanced so far that the occurrence of chlorite in the epidiorites is exceptional. Moreover, the hornblende of the Green Beds never occurs in the stumpy prisms which is so common a habit in the epidiorites, but invariably forms elongated needles of actinolite. The Green Beds are also frequently crowded with the tiny rounded crystals of albite, and this is the case whether hornblende is present or not. The occurrence of tourmaline in the hornblendic Green Beds is still another characteristic. It may be remarked that as the north-east of Glen Fyne is approached, where hornblendic Green Beds are the prevailing type, actinolite has also been developed as a secondary mineral in the mica-schists beyond the Green Bed zone. J. B. H. South of Loch Fyne. — The " Green Beds " in this district are schistose rocks in which the colour is chiefly due to epidote and chlorite, and the quartz veins in them are often coloured by epidote. Many of the beds are very fine grained, but others contain conspicuous grains of quartz and felspar, and are connected with the common types of schistose grit by passage forms in which the green colour is less pronounced than in the typical Green Beds. Many of the Green Beds are tough under the hammer, and form prominent crags which weather into blocks that are somewhat rounded at the edges, while the hill slopes are often slightly drier and greener than those composed of the siliceous or common micaceous schists. During the processes of folding some of the Green Beds appear to have escaped the minor crumplings and later foliation found in adjacent schists. No grains of a distinctly ashy or vesicular character, nor any bands which seem likely to represent interbedded lava flows, have been detected in the Green Beds. The abundance of epidote and chlorite resembles that in the epidotic grits at the base of the Torridonian rocks of Skye, — grits which have derived part of their material from weathered epidotised surfaces of Lewisian gneiss. The Green Beds often contain flakes of secondary black mica as well as of chlorite, and grains and crystals of albite like those in the Beinn Bheula group of schists. Small grains and crystals of magnetite are abundant in some beds, while in others we find many small spots and thin veins of ferriferous carbonate. In a pebbly, slightly calcareous Green Bed in the north-west bank of the big bum in Strath nan Lub, nearly due south of Caol-ghleann house, a coarse white limestone seam, from one to six inches thick, is repeatedly folded into isoclines, but it is possiblyan early segregation. In the north part of the map small pink garnets are not uncommon in the Green Beds, and are seen near the junction of the Cur and r,he Gab. Green Beds. 21 In a little scar 500 yards south-east of Cruacii nam Mult a pebbly Green Bed contains a number of small oval or round bodies varying in length from half an inch to two or three inches. They are harder and seem more epidotic than the matrix, and enclose larger and less deformed clastic grains. Some of the forms show a concentric structure, with an outer shell projecting on the weathered face, then a zone weathering in depression, then an inner projecting shell, and a hollow central core. It seems probable that these forms were in existence before the rock was foliated, and that to some extent they protected from deformation the clastic grains contained in them. They have not wholly escaped deformation, however, and many of them have their long axes in one direction parallel to the foliation. In a broad zone to the north-west of the Beinn Bheula group of schists, outcrops of Green Beds repeatedly alternate with others of schistose grit and mica-schist. Many of the alternations anay be due to folding, but it is possible that they may be due in some cases to original differences in the unfolded sedimentary deposits. A little below the base, and also above the top, of the broad zone just referred to, there are in places various thin outcrops of Green Beds. It appears as if none of those above the Loch Tay limestone can be on the stratigraphical horizon of any in the main zone, unless they are unconformable to some of the schists with which they are in contact. These will be described in connection with the schist groups in which they occur. The base of the zone crosses the south edge of the map about half way between the river Ruel and Garrachra, and runs thence nearly to the north-east comer of the sheet. Near Glen Branter the average width of the zone is between a mile and a mile-and-a- half, but about half the width is on a long dip-slope. On the south-west side of the Hell's Glen fault the breadth is less than three-quarters of a mile, but it increases again north-eastwards. Thin outcrops of Green Beds are seen a little below the base ot the main zone at the following among other places — rather more than three-quarters of a mile south-east of Caol-ghleann, in the burn nearly half a mile W.S.W. of Glen Branter House, rather more than three quarters of a mile W.N.W. of Garnach Mor, and the west side of Binnein an Fhidleir. The thin outcrop near Glen Branter is nearly 200 yards from the broad zone, and separated from it by thin flaggy schists with a considerable proportion of satiny micaceous beds. The outcrop, rather more than three- quarters of a mile W.N.W. of Carnach Mor, is 20 or 30 yards broad, and about 200 yards from the nearest Green Bed on its north-west side. The outcrop becomes broader in a N.N.E. direction, and on Cruach nan Mult it has probably been included with the main zone. Nearly three-quarters of a mile W.S.W. of the Ordnance Station 2658 on Binnein an Fhidleir there is a Green Bed, eight feet thick, about 40 feet below the base of the main zone, £tnd separated from it by massive siliceous schists. The line adopted as the top of the main zone is in many places obscured by drift. In a few places, for instance in the part of 2^ MetamorpMc Rocks. Stralachlan River east of Sron Cruaich, there are outcrops of Green Beds between the main zone and the Loch Tay limestone, and these are probably folded portions of the top bed in the zone. C. T. C. Good sections of Green Beds occur in the following places — north of Loch Fyne in many of the burn sections between Clachan and Tom Gobhair ; south of Loch Fyne in the lower part of Strath nan Lub, Glen Branter, the river Cur, the lower part of Leamha&in, many small streams on the north-west side of Stob an Eas ; and numerous burn sections on the lower slopes of Glendaruel between Kilbridemore and Achanelid. G. T. C., J. B. H. iii. THE GLENSLUAN SCHISTS. In describing the Green Beds, attention has been drawn to the fact that they merge into and occur amongst gneissose grits and mica-schists which are similar to the Ben Bheula schists. In some parts of the area the top of the Green Beds would be more appropriately defined by the boundary of the Loch Tay limestone, and Green Beds even occur above this band, but over the district generally these beds are so feebly developed in the schists which immediately underlie the limestone that the upper boundary of the Green Beds has been drawn further to the south-east. The intervening zone between the Green Beds and the Loch Tay lime- stone has been designated the Glensluan schists. It will be understood, therefore, that they have only a relative value as a distinct lithological band, and that they differ from the Green Bed group solely in their relative proportions of epidotic chlorite-schists. North of Loch Fyne this subdivision has an average width of a quarter of a mile, and includes, in addition to some Green Beds, bands of hornblende-schist, dykes of porphyrite, and a boss of diorite. In the extreme south-west of the map the belt gradually contracts until its lower margin almost coincides with the outcrop of the Loch Tay limestone. J. B. H. Central Area — St. Catherine's and Sbrachur. — The Glen- sluan group in this area includes a considerable proportion of massive, siliceous, and somewhat pebbly schists, associated with thin, shivery, more micaceous bands. Near the top of the group there is in many places, as, for instance, in the burn about a quarter of a mile south of Creag Dhubh, a soft leaden-coloured schist which consists almost entirely of mica. The Glensluan schists in Goire No, and farther north-east, contain small red garnets in tolerable abundance, and in some of the bands in the former locality there are also small needles of black tourmaline. The outcrop of this group is not usually more than 100 yards broad, and in some places, including Glensluan itself, it is less. Near Ardno and the head of Stralachlan River it occurs wholly or in part on dip-slopes, and includes also various thick outcrops of hornblende-schist. At these localities the distance between the two sides becomes as much as a quarter of a mile. Loch Tay Limestone. 23 The outcrop is in most places considerably obscured by drift, and there are very few burns which show the full breadth. Perhaps the best sections occur in the following places — in two little east and west streams near the hamlet of Glensluan ; the burn a quarter of a mile south of Greag Dhubh ; Coire No ; and the burn half a mile east of Ardno. C. T. 0. iv. LOCH TAY LIMESTONE. In the mapping of the south-eastern Highlands, as defined by the Great Glen, no band has played a more important role than the Loch Tay limestone. Stretching across the Highlands in practically unbroken continuity amongst associated rocks, the lithological types of which are equally constant, its stratigraphical importance cannot be over-estimated, occurring as it does amongst crystalline schists so entirely reconstructed that unconformities might easily be masked. In the part of Argyllshire contained in this sheet the lithological bands, into which the crystalline schists have been divided, being parallel to this limestone, there can be little hesitation in regarding them as definite stratigraphical groups. Moreover, its lithological distinctions from the Loch Awe limestone are no greater than those between the rock types with which those limestones are respectively associated. The Loch Awe limestone occurs between black slates and Ardrishaig phyllites entirely distinct from the mica-schists associated with the Loch Tay limestone. Further, the Green Beds which are found near the latter horizon are never found near the former. Moreover, the grit bands carry the divergence a step further, orthoclase which is a conspicuous felspar in the Loch Awe group being practically absent in the gneissose grits on either side of the Loch Tay limestone. J. B. H. Central Area — St. Catherine's and Strachur. — The general colour of weathered surfaces of the Loch Tay limestone is dark blue, but that of the fresh rock is either white or mottled black and white. In the mottled bands the black parts vary in length, and are sometimes as much as two or three inches. They are usually formed of crystals of larger size than those in the pale parts, and in the burn below the road a mile W.S.W. of Cruach nan Capull (S.S.W. of Strachur) their long axes are parallel to one another and to other indications of stretching in the neighbourhood. In many places the limestone is divisible into two parts, an upper and generally thicker part, consisting chiefly of tolerably pure limestone, and a lower, which includes a considerable proportion of calcareous siliceous schist. The average thickness of the whole in places where there is no clear folding is perhaps 40 or 50 feet. Two analyses of specimens of the limestone from a quarry at Glensluan have been made by Mr. Ivison Macadam,* and gave the following results : — * " On the Chemical Composition of certain Limestone Rocks from Ballimore, Ai'gyllshire," Trans. Edin. Geol. Soc, vol. iv., 1883, p. 101. 24 Metamorphic Rocks. I. II. Pe,03 . 1-14 1-24 Al,03 . 0-18 0-42 CaO .... 37-21 47-51 MgO . 0-35 0-25 SiOg and silicates 29-62 12-28 00, . 29-63 87-64 SO3 .... 1-56 0-23 Organic matter and moisture 0-41 0-43 100-00 100-00 Analysis (I.) of a specimen of blue limestone of the common type shows a percentage of 66-44 of carbonate of lime and 0-75 of carbonate of magnesia, while that of (II.) of a specimen of a red band shows a percentage of 84-tf6 of carbonate of lime. We are not aware of the occurrence of a red band at any other locality. Small scattered flakes of white mica are occasionally observed on the foliation planes. Weathered faces sometimes show small grains of quartz, many of which are opalescent and probably represent clastic grains. They are not usually elongated in any general direction, and perhaps during the shearing movements the carbpnate portions of the rock flowed round them and allowed them to remain free from distortion. In some places there are impure bands which show on the weathered faces many small pale straw-coloured or white needles often about -jig-inch in length, with their long axes nearly parallel. The needles in specimen 5547, from a part of the limestone just above the hornblende-schist in the burn nearly two-thirds of a mile N.N.W. of the junction of the Our and the Oab, have been determined to be idocrase. A mineral of the epidote-zoisite group is also probably present. The same specimen shows, on the weathered surface, some small fine-grained knots forming projec- tions with some resemblance to the tops of worm tubes. The knots are about the size of peas, and the foliation has a tendency to wind round them. A band of calcareous quartz-schist from the part of the lime- stone below the hornblende-schist a quarter of a mile E.N.B. of Bathaich ban cottage shows projections which to the unaided eye look like those in specimen 5547, but in the micro-section they are seen to be formed of aggregates of minute colourless garnets, making up in some parts more than half the rock. On some of the bedding planes in the calcareous quartz-schist there are oval projections as much as an inch long, but they have not been examined under the microscope. Where undisturbed by faults the main outcrop of the limestone is generally remarkably straight, and it is not so often increased in breadth by folding as might have been expected. Subordinate outcrops parallel to the main band are not uncommon. Near the head of Stralachlan River there are two exposures of limestone a little north-west of the main band. Near Glensluan. there is some- Loch Tay Limestone. 25 times a thin band of limestone, two or three feet thick, ten or twelve feet above the main band, and separated from it by shivery micaceous schists. A thin limestone, from two to seven feet thick, is frequently seen above the main limestone near Creag Dubh, but the distance between the two varies considerably. Among the best exposures are those at the following localities : — Clachan Burn and AUt na Criche Burn (both north of Loch Fyne), the hamlet of G-lensluan, the burn between half and three quarters of a mile S.S.W. of Glensluan, the crags between a mile and half a mile W.S.W. of Creag Dhubh, various burns between a mile and a tliird of a mile B.N.E. of Creag Dubh, Coire No, the burn a quarter of a mile E. of Ardno, KJilbridemore Burn, and most of the streams on the western slopes of Glendaruel. It is doubtful whether there are any sections south of Loch Fyne showing the whole of the limestone which do not also show a band of hornblende-schist — representing, presumably, an intrusive sill — either in or just below the limestone, and very often there are bands in both these positions. C.T.C. North of Loch Fyne. — North of Loch Fyne the Loch Tay limestone is seen near the coast at Criil-bheag, from whence it may be continuously followed, gradually ascending the hillside to the northern margin of the sheet, where it reaches the watershed at an elevation exceeding 2000 feet. The outcrop is exceedingly regular, from which it appears that the folding has here produced little effect on the stratigraphy. The amplitude of the folds has been neutralised by the high angle at which the band is truncated by the ground surface. Its colour is blue to grey, it is crystalline and thin bedded, and is sometimes micaceous, while occasionally it contains pale grey garnets so closely packed as to form hard bands in the rock. It is usually bounded on its south-east side by a narrow band of hornblende-schist, which is sometimes seen at its upper margin, and occasionally within the limestone outcrop, but in its north-easterly prolongation beyond Beinn Chas, hornblende- schist is rarely associated with it, nor has it been observed in its vicinity. A zone of impure limestone that occurs above the Loch Tay limestone is described in Chajjter V. dealing with the garnetiferous mica-schist group amongst which it appears. As this limestone is of inferior quality and is not associated with hornblende-schist, it is improbable that it is the main limestone band repeated by folding. Stralachlan Area. — On the western slope of Glendaruel the Loch Tay limestone occupies a well-marked horizon for a length of about four miles, and forms a continuous band, of which the central position is Kilbridemore. To the north-east, as far as the extreme limits of the sheet, it has been observed that the limestone forms so regular an outcrop that the intense folding of the district has not been shown on the map. In this part of the district however, the mapping more accurately reflects th# general structure. Not only does the limestone show great divergences in its breadth of outcrop, but it divides into a series of parallel bands, many of 26 Metamorphic Bocks. which can be traced into the main bed. Many of the limestone exposures may be continuously followed across the strike for 300 or 350 yards, while other sections in a corresponding position may be represented by bands of only 20 yards, which may be variously repeated in the section — so that we have a great composite zone made up of alternations of limestone and the mica- schist it adjoins. Moreover, the hornblende-schist with which the limestone is associated shows a corresponding interfolding. The limestone is usually of a grey blue colour, but in the Kil- bridemore Burn it is of the mottled type, by the intermixture of black and white calcite. At its margins the limestone often shades into calcareous schist, and occasionally Green Beds occur in juxta- position with it. J. B. H. CHAPTER V. Metamorphic Eocks — continued. V. GAENETIFBROUS MICA-SCHISTS OVERLYING LOCH TAY LIMESTONE. Between the Ardrishaig group and the Loch Tay limestone a belt of granulitic gneisses occurs, litholngically similar to a great portion of the schists which occupy the area to the south-east of that limestone. That is to say, they form a succession of argillaceous and siliceous bands, the clastic origin of which is often entirely obliterated, except in the beds of coarser grit, by deformation accompanied by re-crystallisation partial or complete. Notwith- standing the complete destruction of the original ingredients, the processes of mineral reconstruction have been of an interstitial nature, and the chemical composition of the original individual beds has perhaps not been materially afiected. Original differences of character, depending upon texture or composition, have been perpetuated in the crystalline schists ; alternations of grits and shales being now represented hj bands of gneisses and miea-schists, and variations in the grit bands themselves being reflected by the coarser and finer grained gneisses. While in the south-east portion of the map between Stralachlan and Glendaruel this group is lithologically indistinguishable from the Beinn Bheula schists, its north-east extension from Stralachlan to the slopes of Glen Fyne is characterised by the development of garnets. Although this mineral is of late formation, and may to some extent be regarded as accessory, its presence along a band which is practically restricted to a definite stratigraphical horizon majy probably be accounted for by difierence in original chemical composition, and may thus possess a stratigraphical importance. North of Loch Fyne. — On the north coast of Loch Fyne they form a continuous section from Cuil to Drishaig, and extend to the north of the map, comprising the watershed between Glen Fyne and the Brannie Burn. They give rise to singularly rugged features, and the slopes above Loch Fyne are strikingly contrasted where the Ardrishaig group has been replaced by the garnetiferous mica-schists. Between Dundarave and Drishaig hard bluish graphitic-looking schists, sometimes calcareous, have been taken as a convenient dividing line between the garnetiferous schists and the Ardrishaig group. This graphitic zone is very narrow, probably not exceeding a dozen yards; it sometimes 28 Metamorphic Rocks. contains grits, and at Drishaig, where it is calcareous, garnets are also seen. On the hill slopes between Loch Eyne and the Brannie Burn the graphitic beds are rarely visible, and the north- west boundary is therefore not accurately defined, but in the Brannie Burn graphitic beds of the same type as on the coast at Loch Fyne occupy a corresponding position. The garnetiferous mica-schists of this area consist of alternations of fine gritty gneisses and greyish to green mica-schists, in which chlorite is often conspicuously developed. The garnets them- selves are more prominent in the micaceous bands, but they are often well developed in the gritty gneisses. These beds are markedly contorted as contrasted with the Ardrishaig group to the north- west, where in spite of intense plication the folded limbs are usually regular. Not only in this garnetiferous group are the limbs them- selves contorted, but the whole series is more indurated, and they weather into uneven features which are singularly gnarled. In this area the clastic structures have been generally completely destroyed, even in the grits, but some bands occur in which the clastic grains may still be distinguished. A band of impure limestone is exposed along the hillside half-a- mile south-east of the Brannie ; only a few feet across at its north- eastern end, it widens considerably to the south-west with a breadth of outcrop of 20 to 30 yards, which may be partly due to flattening. The band is very impure, and is largely intermixed with calcareous schist. Quartz veining is more prominent in this group of schists than iti the Ardrishaig and Loch Awe groups. The veins have usually originated prior to the cessation of the movements, as they generally share in the folding of the enclosing rock bands. Some large quartz masses occur on Clachan Hill which are probably on the same horizon; the largest is 100 yards in length, and from 20 to 30 yards in breadth, the long axis being parallel to the foliation strike of the schists, while a smaller patch, 40 yards by 20 yards, lies with its short axis parallel to the foliation strike. These veins are themselves folded and fractured with the schists in which they occur, so that it is clear that they preceded the latest folding. Other quartz patches are found among this group, but of smaller dimensions than those which have been described. Garnets are found to some extent below the Loch Tay limestone band as well as in the limestone, but their presence is exceptional. The garnets in the schists above the Loch Tay limestone are some- times replaced entirely by chlorite, or the replacement may be partial. Occasionally these chloritic pseudomorphs tail out into lenticles. Chlorite has sometimes replaced actinolite, and in some instances garnets may be seen to penetrate the actinolite crystals. The garnets range in size from a quarter of an inch downwards, and considerable variation attends their dispersion ; zones in which these minerals are closely aggregated being succeeded by bands in which their abundance has considerably diminished, while not infrequently they are absent altogether. In the area north of Mica-schists Overlyincf Loch Tay Limestone. 29 Loch Fyne a few Green Beds are met with in the gametiforous mica-schist zone, but they are comparatively insignificant. Epidiorites are poorly represented. J. B. H. St. Catherine's and Strachur Area. — On the north-west side of the Loch Tay limestone in this district there is a broad belt, averaging about a mile in width, in which nearly all the schists, including hornblende-schist, generally contain garnets in consider- able abundance. The south-east boundary of the garnetiferous belt does not always coincide with the margin of the Loch Tay limestone, nor is the north-west boundary a strict strati graphical line, but it is convenient to use the title garnetiferous mica-schist specially for the belt which comes between the limestone and the most north-westerly of the set of graphitic-schists which have been mapped between Ardno and Leanach. The graphitic-schists will, however, be described by themselves as a subdivision of the zone. The best sections within the zone occur in the following locali- ties : — The shore south-west of Eudha Bathaich Bhain, the various streams that run N.N.W. from Cruach na Capull (St. Catherine's), the lower part of Bas Dubh, and the burn south-east of Leanach. In the last mentioned stream the garnets are almost as abundant and large as in the areas farther north-east, but south-west of the burn they perhaps become less prominent. The most common rock within the zone is a somewhat soft fissile gametiferous-schist containing a considerable proportion of mica and chlorite. The chlorite is less abundant than the mica, and is often very irregularly distributed. Intermixed with these soft schists there are other more siliceous bands which do not often contain many garnets. Some of the siliceous schists contain thin rusty-weathering calcareous lenticles, while others show distinct clastic grains of quartz and felspar. Green Beds are seen within the garnetiferous zone at various places, for instance on the shore half-a-mile south-west of Rudha Bathaich Bhain, half-a-mile north-east of Oreag Dhubh, the head of Coire No, the east side of Creag Dhubh and Gleann Dhubh, and the north sides of Cruach nan Capull, Oreag Dhubh and Meall Reamhar. At the second locality the Green Beds occur immediately above the Loch Tay limestone, but in all the other places they appear some distance above this limestone. With the exception of the more general presence of garnets, these Green Beds resemble many of the beds in the main zone already described, but it does not seem as if they can be repetitions of the latter due to folding, for the Loch Tay limestone makes only a single outcrop near many of the localities. Moreover, no instance has been met with in which a Green Bed lies immediately below this limestone. A schist with grains of albite, like those in the alhite schists ot the Beinn Bheula group, is seen in the garnetiferous zone a quarter of a mile slightly north of west of Meall Reamhar. A Green Bed in the same zone, about half-a-mile south-west of Rudha Bathaich Bhain, also contains albite with, however, fewer inclusions than those in the albite in the Beinn Bheula series. A thin band of limestone, and of calcareous siliceous schist, is •30 Metamorphic Rocks. seen a little above the Green Bed in the burn rather more than a quarter of a mile north-east of Cruach nan CapuU (Strachur), and in the burn half-a-mile south-east of Ardno there is an outcrop of limestone six feet thick. In the district between Strachur and Ardno there is a limestone outcrop a little south-east of the graphitic-schists. In the burn about 300 yards south-west of Ardno the upper part of the limestone, for a thickness of six or eight feet, contains conspicuous clastic grains of quartz, and below this occur limestone (twelve feet thick) and somewhat calcareous siliceous schist. In a little burn a mile W.S.W. of Creag Dhubh the weathered faces of the limestone are of a deep ochreous tint, and many of the foliation planes are black, as if slightly graphitic. No corresponding outcrop is known south-west of Strachur except in one place, in a little bum about a quarter of a mile north-west of An Carr. If the limestone is a folded portion of the Loch Tay band, it is probably lying in the axis of an isoclinal fold. The garnets in this group of schists are often deformed and frequently have a border of chlorite which extends into two "tails" in opposite directions. The extensions on the same plane are parallel to one another, and occasionally also to rod structures in adjacent quartz veins. The larger garnets are often traversed by thin strings of chlorite and broken into pieces. In the most westerly of two parallel burns about 700 yards slightly east of south of Laglingarten, some garnets are pulled out into streaks half an inch long. In various other places, for instance nearly a quarter of a mile south-west of Cruach na Oapull (St. Catherine's), the garnets are ci-ossed and slightly faulted by thin strings of quartz. The average breadth of the chlorite rim varies considerably at different localities, and in some places the garnet substance is entirely replaced by chlorite. Occasionally the garnetiferous schists contain radiate forms, composed of chlorite which is probably pseudomorphous after actinolite. The rays are sometimes three or four inches long. The forms are best seen in the burn a quarter of a mile slightly west of S(5uth of Ardno, and in another stream nearly 300 yards south-west of Ardno. C. T. C. Stralachlan Area. — In the district of Stralachlan the boundary between this group and the Ardrishaig series is not well defined, for not only have the graphite-schists not been traced, but the garnets themselves are dying out in a south-west direction. This group, therefore, does not differ essentially from the Beinn Bheula schists. They are less metamorphosed than the garnetiferous beds north of Loch Fyne, and the clastic structures are better preserved. Green Beds are widely distributed, but they are of very limited extent as compared with those of the main zone. A well-marked Green Bed zone, over a mile in length, has been taken as the upper boundary of the garnetiferous mica-schist group along the eastern slopes above Barnacarry. Serpentine and epidiorite occur above the Loch Tay limestone on the slope above Glendaruel, and epidiorite is occasionally noted in other localities. J. B. H. Graphite Schists. 31 vi. GRAPHITE SCHISTS. ' The graphitic schists are dark grey, blacken the fingers, and are usually intimately mixed with thin bands of dark, partly graphitic, fine-grained limestone and calcareous siliceous schist. They often form various sub-parallel outcrops separated by bands of garnetifer- ous mica-schist of the common type. The number of outcrops and the distance between them vary a good deal at difierent localities. The best sections of the graphitic beds occur in the following places — Bas Dubh, the burn half a mile south-west of Laglingarten, various streams near the shore between Mid Letter and Strachur Bay, and the burn a quarter of a mile north-east of Leanach. It is not known how far these rocks extend to the south-west of Leanach. They are in all places greatly twisted and broken by crush lines trending N.N.E., which are more recent than the folding which accompanied the development of the schistosity. The bands of limestone and calcareous siliceous schist are flaggy, and so contorted and crushed that it is difficult to separate them from the graphitic schist ; indeed, it is possible that in the original succession, beds of limestone alternated with bands of difierent sedimentary materials. At the head of Eas Dubh there is a limestone six or eight thick, and in the burn a quarter of a mile north-east of Leanach there is another outcrop with an apparent thickness of about ten feet. North of Loch Fyne these rocks are very sparsely represented. Their occurrence has already been briefly referred to in the description of the garnetiferous mica-schists of that area, where they have been selected as the north-western boundary of that group. C. T. C, J. B. H. CHAPTER VI. Metamorphic Rocks — cmitinued. vii. ARDRISHAIG GROUP. The Ardrishaig group forms a great central belt across the sheet and is confined almost completely to the basin of Loch Fyne ; that Loch, so far as it falls within this sheet, lies entirely in a trough excavated out of these rocks, except for a very short distance at its extreme head. This group has a stratigraphical position between the Loch Awe group and the garnetiferous mica-schists overlying the Loch Tay limestone. Its relations to the former, which it underlies, are described in Chapter VII. It is separated by the graphite schists from the garnetiferous schists which overlie the Loch Tay limestone, but as the former have not been continuously met with, its junction with the garnetiferous mica- schists is not always well defined. Situated on either side of Loch Fyne it is, nevertheless, very unevenly divided by that loch, so that it reaches a far greater extension on the north-west side. Commencing at the north of the sheet about Glen Shira, with a width of three miles, the band expands south of Inveraray to a breadth of about six miles, the widening being due to the gradual diminution of its general dip. As described in Chapter VII. it reappears on the north-west side of the Loch Awe group and forms a compound basin on which that group is lying. In common with the Loch Awe group it is associated with enormous masses of igneous intrusions, consisting mainly of epidiorite and quartz- porphyry, which are approximately equally apportioned, together with other types of intrusive rocks. The Ardrishaig group consists essentially of limestones, phyllites, quartz-schists, and quartzites. LIMESTONE. — Although limestone enters very largely into the composition of the Ardrishaig series it seldom forms bands sufficiently large to be shown on the map, but occurs rather as thin layers or lenticles intimately mixed with the phyllites and siliceous members, and ranging from a few feet in thickness to the thinnest laminations. The limestone usually effervesces with cold hydro- chloric acid and is generally very impure ; on weathered surfaces the colour is rusty brown, and on a fresh fracture pale yellow to buff and occasionally white. Besides aggregations in bands, much calcareous material is diffused amongst the phyllites and quartzites. PHYLLITES. — Th6 Ardrishaig phyllites are characterised by an exceptionally low percentage of silica with an unusual amount of lime, and they consequently form one of the softest rocks in the Ardrishaig Group. 33 Highlands, and may be readily scratched with the finger-nail. Sufficient chlorite enters into their composition to give them a prevailing green colour, but they are often blue and some- times grey or even pinkish. Grey phyllites are frequently the dominating type in areas where siliceous rocks are most strongly developed. These phyllites are never sufficiently hard to serve as roofing slates, and they lend themselves so readily to erosion that small streams are capable of excavating in them deep gorges. The deep valley of Glen Shira and the valley of Strathlachan have been mainly carved out of the phyllites themselves. The mica that gives the foliated character to the rock is a pale sericitic variety. Biotite, when it occurs, is more sporadic and is confined to those portions which lie in areas of advanced regional metamorphism. Moreover, the white mica itself is only feebly developed in regions where such metamorphism is slight, and the argillaceous members are then in the condition of clay-slates. Where, on the other hand, the metamorphism is most intense the advanced state of crystallisation has been such that the chlorite and sericite have crystallised in coarse plates to such an extent as to impart a mottled appearance to the beds. Further, in areas of intense metamorphism, garnets frequently occur besides biotite, but they are never developed to such an extent as in the zone of garnetiferous mica-schists, either as regards their individual size or closeness of dispersion. Moreover, in the moce calcareous varieties epidote has been set up freely in such metamorphic areas. In such districts the alteration is indicated not only by coarse- ness of crystallisation but also by induration, and to such an extent that instead of commonly giving rise to depressions or smooth slopes they readily form crags as well as the ridges and summits of the highest hills. QUARTZITES AND QUARTZ-SCHISTS. — The siliceous members of the group consist of fine-grained compact quartzites and quartz-schists, the texture of which is so fine that, over large areas, clastic structures are not apparent, and this is especially the case in the more highly metamorphosed districts. In some parts, however, grits are frequent and rarely coarse-grained, but taking the siliceous group as a whole its quartzite character is distinctive. Like the phyllites, with which they are associated, the amount of crushing which they have undergone may show considerable variation, as displayed by the efiects on the clastic grains and the degree of foliation that has been produced. In highly metamorphosed districts the pebbles, when they occur, are elongated and flattened. When foliated the quartz-schists are usually highly siliceous with very thin films of mica along the cleavage planes, and closely resemble fissile quartzites. More normal quartz-schists, however, do occur, and there is, moreover, a gradual passage into argillaceous sediments. Calcareous material is often diffused amongst the siliceous rocks, which is indicated on the weathered surfaces by a rusty colour and by the decomposition of the 34 Metamorphio Books. calcareous matrix, leaving the fine siliceous grains in a state of imperfect cohesion. The quartzites and quartz-schists occur in bands ranging in thickness from two or three feet to an inch or less. Moreover, their occurrence amongst the phyllites is irregular. Outcrops of quartzite and siliceous schist may be met with extending for over a quarter of a mile across the strike, in which argillaceous material is practically absent except as thin partings between the individual siliceous beds, while, on the other hand, over broad zones the phyllites may be similarly represented except that thin bands of quartzite appear here and there. Between these extremes every variation is met with, according as the siliceous or argillaceous members preponderate in the section. In colour the quartzites and quartz-schists vary from pale grey to pale buff. The Ardrishaig quartzites may usually be distinguished from the Loch Awe quartzites by their more uniform texture, the general absence of gritty bands and especially of the coarse basal grit of the Loch Awe series. The sharp alternations due to difference in texture so common in the latter group are absent or rare in the Ardrishaig series. Finally, the dark slate pockets and the dark slate partings between the siliceous bands may be said to be almost strictly confined to the Loch Awe group. A specimen of quartzite from a band one mile south of Inveraray was examined under the microscope by Dr. Teall, who described it as follows : — " The minerals occurring in this rock are quartz, mica, felspar, calcite, rutile, zircon, and pyrite. Quartz is, of course, the most abundant constituent. The comparatively large grains seen when the slide is examined with a hand lens, lose their individuality to a very great extent under crossed nicols. This is doubtless due to the very large amount of recrystallisation which has taken place since the rock was formed. Those grains which polarize uniformly over large areas contain minute liquid inclusions with movable bubbles and also fine needles (rutile ?). Mica is fairly abundant and forms, together with small irregular grains of quartz and the other constituents mentioned above, the matrix in which the large grains of quartz are embedded. There is, however, no sharp demarcation between the large grains and the matrix. Felspar is comparatively rare. Calcite is fairly abundant, and occurs both in the form of rhombs and as irregular grains. Minute prisms and twins of rutile occur. Certain strongly refracting grains have been referred to zircon, but no well-formed crystals of this mineral have been observed." DiSTEiBDTiON AND FIELD RELATIONS. — The general disposition of the Ardrishaig group has already been indicated. The shores of Loch Fyne afford admirable sections for its study, both as regards the relations of its individual members to one another and to the gametiferous mica-schists which succeed it on the south-east side. On a broad scale the south-east part of the group is the most siliceous, but the siliceous belt gradually shades away to the north- east. This zone includes much phyllite, and broad bands might be Ardrishaig Grovup. 36 instanced in which the siliceous and phyllitic material may be about eqiially divided ; and the converse may be stated as regards the phyllite zone to the north-west. But not only can the Ardrishaig group be divided roughly into a siliceous and an argillaceous division as we proceed across the strike from south-east to north-west, but a precisely equivalent lithological difierentiation takes place as we follow the trend of the group from south-west to north-east, which shows a corresponding replacement of siliceous by argillaceous material. That part of the group, for instance, which lies to the north-east of Inveraray is more characterised by phyllites than the south-west extension of the band. Moreover, there is a correspond- ing difference in the texture of the siliceous rocks themselves. In the south-western part, right across the group, fairly coarse grits are commonly apparent, while in the area north-east of Inveraray the bands are very compact and of the finest texture. While these lithological distinctions may be exaggerated by the folding so common among these beds, there is strong ground for the belief that it accurately represents the condition of deposit, for not only do the sediments become finer towards the north-east, but the thicknesses of the individual Ijeds diminish in the same direction. Where the siliceous members are in their greatest development the alternations of bedding are as a whole appreciably thicker than where the quartzose zones are less conspicuously represented. A similar decrease in siliceous material and in magnitude of the constituent grains has been observed in parts of Cowal in the groups that lie to the south-east, and the changes have taken place in a corresponding direction. It must be recollected, however, that the breadth of the band represented by the Ardrishaig group has contracted from six to three miles in a north-easterly direction, so that, from this cause alone, the outcrops of the siliceous and argillaceous zones must correspondingly converge in that direction. The contraction of the siliceous band as drawn on the map is therefore proportionately exaggerated. On the north-west coast of Loch Fyne the Ardrishaig band contains numerous outliers of the Loch Awe group, of which the most conspicuous is the Loch Awe limestone, which is especially well developed along a north and south belt between Minard and Beinn Laoigh. Outliers of the Loch Awe grits also occur, and it is often impossible to distinguish them from the siliceous rocks of the Ardrishaig gronp. South-east of Loch Fyne the boundary to the south-west of Newton Bay is very ill-defined, for not only has the graphite schist zone not been further traced but the garnets which are so conspicuously developed in the garnetiferous schist group to the north-east here almost disappear. The boundary drawn on the Strathlachlan slopes represents the extreme north- western limit of the Green Beds, but in the neighbourhood of Mid Letter, south of Strachur Bay, Green Beds occur within the Ardrishaig group, and as some dark schist, which may correspond to the graphite schist, has been noted here and there on the slopes of Stralachlan, beyond a strong band of Green Beds, it is quite possible that these latter also lie within the Ardrishaig series. In 36 Metamorphic Bocks. this group, represented by the belt north-east of Inveraray, where its boundaries can be fairly accurately defined, Green Beds do not occur. Attention has been drawn to the marked lithological difierences between the Ardrishaig phyllites and the Loch Awe slates. Prom a petrological standpoint they are just as sharply marked off from the Dunoon phyllites, which are not only less calcareous but are distinctly harder and, moreover, have a wider range of colour, and both these and the Loch Awe slates admit of being used for roofing slate. Goast Section — Strone Point to Bundarave Point. — A typical section of the Ardrishaig group will now be described. As the main arm of Loch Fyne beyond Inveraray cuts the strike least obliquely, that coast section has been selected to illustrate the general characters of the group. On the eastern shore of Loch Shira hard grey mica-schists, con- taining thin quartzites, pass towards Strone Point into argillaceous beds with a more pronounced silvery lustre and greyish green colour. The quartzite intercalations reveal the intensity of the folding, and as the section cuts obliquely the pitch of the folds, the folded quartzite bands are seen nosing out, and a lateral replacement of quartzite by phyllite is visible. As the folding is isoclinal, and the dip of the folded limbs and of the foliation planes are both to the north-west, the section exhibits a very regular and evenly stratified appearance, which is well marked when these planes are free from contortion. The foliation planes, at Strone Point, dip steadily to the north-west at 22°, while the rodding, indicating direction of stretching, is more to the west. While the beds are more often evenly inclined, they are sometimes undulating. Argillaceous beds considerably exceed siliceous, and some thin calcareous seams are associated with the silvery micaceous schists to the east of Strone Point. These silvery green micaceous beds, with intercalated quartzites showing the same reduplication by folding, are continuous as far as Auchnatra, where the section is covered by a marine terrace. Associated with them, are some very fine grey silky schists, which, though siliceous, contain so much mica that they have' a marked fissility and split readily along their foliation planes with a silky lustre. This forms a valuable lithological type for purposes of stratigraphy, as it is confined to the Ardrishaig group. Beyond Auchnatra, massive well-bedded quartz-schists appear, which sometimes contain small rusty looking calcareous seams, succeeded by quartzites with intercalations of hard thinner schists. These quartzites and quartzose mica-schists continue along the coast for a mile beyond Auchnatra. The silvery micaceous beds, although not seen on this coast section, probably occur under- neath the beach gravel which separates the siliceous exposures, as they are found on the hill slopes immediately above. The quartzose bands often contain small ovate inclusions that were ascribed by the late Duke of Argyll to annelid tubes.* They are usually some form of oxide of iron, and occur mainly *Proc. Eoy. Soc. Ediii., vol. xxi., p. 39. Ardrishaig Group. 37 along foliation planes. In many instances the evidence points to their having resulted from the deformation of pyrites, and possibly, in some cases, of magnetite. They are often seen in the siliceous members of the Ardrishaig group, but as similar bodies have been noticed amongst the sheared epidiorices their organic origin is open to serious doubt, and we have met with no evidence for differentiating them from the products of mineral deformation by the operation of dynamic stress. The dip along this part of the section averages about 25° to the north-west, and notwithstanding the constant reduplication seen in the quartzites the section is free from minute contortion. While the crystallisation is perceptibly more advanced than at Ardrishaig, the beds do not display the coarser crystallisation accompanied by induration and gnarling seen about four miles farther north at the head of Glen Shira. The sections on the adjoining hill slopes resemble those on the coast, and notwithstanding the ribs of quartzite which are so constantly intercalated with the most micaceous members, these rocks have produced surface features singularly even in contour in spite of the marked declivity of the slope to Loch Fyne. Some bands of green schist, a foot or so in thickness, are seen a quarter of a mile west of Auclmatra, and are visible in a burn section on the hillside, where, however, they are black, possibly due to con- tact metamorphism of an epidiorite which adjoins them. Similar green schists, associated with thin seams of the graphite schist, occur near Dundarave and may possibly belong to the Green Beds. The section just described is fairly representative of the Ardrishaig series along their south-westerly prolongation. As previously indicated, however, siliceous horizons are more strongly developed, and coarse grits are occasionally seen, as, for instance, about thirty- five yards south-west of Creag nam Faoillean where some of the quartz pebbles are nearly two inches long in the direction of stretching, but only a third of an inch broad. Grits are especially well developed in the siliceous bands on either side of Loch Gair. In many sections limestone bands are more numerous than in that just described. A limestone band six or eight feet thick is seen in the Kilblaan Burn farther north-east. At the point entering Loch Gair an impure brownish-white limestone has an outcrop of twelve yards in width amongst siliceous schists, its unusual size being probably due to folding, while in the area south-east of Loch Fyne the thickest limestone on the shore north-east of Newton Bay is only four feet thick and of inferior quality (coast of Loch Fyne, quarter mile north-east of Oreag a' Phuill). Over the group generally there is a marked parallelism between the strike of the foliation planes and that of the folded limbs. These are, however, sometimes slightly oblique to one another, and in such cases the bedding planes may, for instance, trend N.N.B., while the foliation planes lie nearer to the north-east, but there is no constancy in this relationship, as frequent instances occur where the converse is the case. 38 Metamorphio Rooks. The hade of these planes is marked by similar divergences, but, as a general rule, the more the bedding planes approach the horizontal or the vertical position the greater is the angle formed by the bedding and foliation planes, while the further they depart from these extreme positions the more the obliquity diminishes. But as the foliation planes are often constant while the bedding may be undulatory, a single section may present marked variations in regard to such planes. Taking the Strone Point section as typical, the most marked lithological distinctions may result from the varying degree of regional metamorphism. In Cowal this metamorphism diminishes the further we recede from its central axis, which is parallel to the strike of the lithological groups, hence the Ardrishaig group is less crystalline than the series it succeeds. For the same reason the part of the group lying to the south-east of Loch Fyne is more crystalline than the portion situated on the north-west side of that loch. This may be exemplified by the occurrence of biotite in the siliceous schists of the former and its absence in similar rocks of the latter locality. Further north-west towards the Loch Awe basin the degree of metamorphism is still less, so that in the valley of the Add, instead of the crystalline character common to the Ardi'ishaig series, the argillaceous members are in the condition of clay-slates, and the siliceous are represented by fine-grained grits. Besides this diminution in regional metamorphism across the strike there is a similar decrease from north-east to south-west, so that this group is perceptibly more crystalline at Inveraray than at Ardrishaig. Beyond Inveraray, however, the rate of progression is much more rapid, so that at the head of Glen Shira the Ardrishaig phyllites become indurated, gnarled, and very coarsely crystalline, and, instead of a soft green homogeneous phyllite, they crystallise into coarse plates of chloritic and micaceous material in which these two substances have been sharply differentiated, giving a mottled or speckled appearance of bright green and silvery white. Rocks of this type, however, occupy but a limited area on this sheet, and are restricted to a small zone stretching from the head of Glen Shira north-eastwards along the valley of the Brannie Burn and passing thence into sheet 45, where they are extensively developed. A narrow belt of an unusual type of metamorphism occupies a strip between Tom a' Bhuachaille and Clachan Hill, with a southerly extension of one and a half miles from the Brannie Burn, and with an approximate breadth of 150 to 200 yards. It consists of gran- ulitic gneisses having more the appearance of sedimentary than igneous origin, although distinct clastic structures have been detected neither in the field nor under the microscope. On the ground, however, they appear to be in natural association with the normal Ardrishaig sediments with which they are associated in the Brannie Burn ; moreover, they consist of siliceous and argillaceous members, the former occurring as homogeneous felspathic quartz- schist in which the foliation is not a prominent structure, while the latter is a dark green biotite schist or gneiss in which garnets are abundant. Notwithstanding their gneissose character, their lithological appearance recalls the members of the Ardrishaig series Ardrishaig Group. 39 in an unusual condition of metamorphism, while the occurrence amongst them of a thin limestone band similar to one of the Ardrishaig limestones seen in the Kilblaan Burn to the south-west strengthens this opinion. The peculiar lithological condition of these rocks appears to have been brought about by thermal meta- morphism resulting from the presence of a large igneous mass lying close below the surface. Thermal metamorphism not only seems to have destroyed original clastic structures, but by a process of rock- welding characteristic of many hornfelsed areas to have masked in part the planes of foliation, while the recrystallisation which has accompanied these changes has produced a mineral alteration which has still further concealed the original condition. A marked feature is the abundance of felspar, which, in a specimen from the siliceous division (8,471), constitutes nearly a half of the rock. It is largely orthoclase, but there is also much acid polysynthetic plagioclase. The quartz occurs in small irregular grains, but they do occasionally form granulitic augen, which may have been derived from the breaking down of pebbles. The large amount of felspar would favour an igneous rather than a sedimentary origin. The absence of chlorite on the one hand and the introduction of biotite on the other are noteworthy ; probably by thermal action the former mineral has been replaced by the latter. The biotite is of a deep brown colour and occurs as irregular crystals, while a pale green form of the same mineral is also seen. The darker argillaceous portion (4,818) exhibits precisely similar characters, the only diSer- ence being the greater abundance of biotite and the presence of much epidote and a little calcite. It is probable that this rock has been derived from a calcareous phyllite while the former was originally one of the siliceous bands of the Ardrishaig group. This conclusion is supported by evidence close at hand, where sediments that flank the augite-diorite below Clachan Hill present a similar type of alteration, which, however, has not advanced so far as to destroy entirely the original clastic structures. They are in the condition of dark banded hornfels produced by the alteration of an argillaceous grit. A specimen (5,003) is seen under the microscope to consist of alternating folia of quartzose and argillaceous material. The former contains much decomposing felspar, mostly orthoclase, and abundant brown biotite, while the argillaceous bands consist of very fine plates of pale mica with occasional grains of quartz, and scattered patches of brown biotite. Iron ores and apatite are also seen. Dr. Flett remarks that the argillaceous bands have the appearance of a hornfelsed slate. The metamorphic band is associated on its flanks with strong lamprophyres, which in the Brannie Burn have intimately veined it, but these rocks do not approach in size the augite-diorite of Olachan Hill, and are obviously incapable of producing the efiects observed. This zone, however, lies in a belt of country in which numerous plutonic bosses are emerging, so that its peculiar lithological con- dition may very well have been developed by a subterranean plutonic mass. J. B. H. CHAPTEE VII. Met AMORPHIC Rocks — continued. viii. LOCH AWE GROUP. To this group of rocks, situated mainly in tlie basin of Loch Awe, a peculiar interest attaches on account of their singularly- unaltered condition. The marked difference between them and the normal Dalradian schists led originally to the belief that they could hardly form an integral part of the great group of mica- schists which enters so largely into the composition of the south- east Highlands. Presenting, as they frequently do, petrological characters akin to the older Palaeozoic sediments, both as regards the sedimentary bands and the igneous rocks with which they are associated, it was scarcely considered credible that they could bear any relation to the mica-schists of the Grampians. The systematic examination, however, of the Loch Awe district enabled us to demonstrate* that a great group of rocks which extend across the Highlands of Scotland as typical crystalline schists, pass along their strike by a gradual diminution in meta- morphism into rocks lithologically resembling Palaeozoic sediments. The sediments, in their unaltered form, frequently show clastic mica, while their associated igneous bands display a corresponding diminution in metamorphism, ranging from highly foliated rocks to others in which the original structure has been completely preserved. Thus, regional metamorphism can not only be studied from its initial stages in the Loch Awe group, but the effects of its progressive increase along individual stratigraphical horizons is equally open to investigation. GENERAL LITHOLOGICAL CHARACTERS. The Loch Awe grOUp consists of (c) Grits and Quartzite. (6) Black slates (sometimes graphitic). (a) Limestone. While these are the essential constituents, the respective members of the group display marked variations in type, both as regards colour and composition, which will be referred to in the detailed descriptions. (a) The Limestone is very variable in its character and is intimately associated with the slates into which it sometimes merges by insensible gradations. Where it appears next to the * Q. J. G. S., vol. Iv., pp. 470-493, "Progressive Metamorphism in the Region of Loch Awe," by Mr. J. B. Hill. Loch Awe Cfroup. 41 Ai-drishaig slates it partakes of the character of that group by- reason of its intimate interfolding ; but where the black slates (group h) on the other side are represented, its association with more graphitic material produces a corresponding change in its appearance. Sometimes the limestone may be seen in the same exposure associated with both types of slate. More frequently, however, the interfolding has been such that the top and bottom of the band can rarely be seen in the same section, so that, when met with in the area occupied by the Ardrishaig group, the black slates are frequently altogether absent or feebly developed, while well within the ground covered by the Loch Awe group the characteristic Ardrishaig slates are no longer exposed. Moreover, although occupying a general position between the Ardrishaig slates on the one hand and the black slates on the other, the limestone does not rigidly adhere to that horizon, but may occur both within and above the black slate, and it appears probable that it may sometimes occur within the Ardrishaig slates. Further, the black slates associated with the limestone are often feebly developed and may die out altogether, so that we get the limestone and grits in juxtaposition. Speaking generally, it may be stated that when the limestone departs from the normal type it partakes of the character of the sediment with which it is immediately associated. In its normal condition the limestone is blue in colour, but varying to pale grey and even white, and to very dark blue and even blackish hues. It is crystalline, stratified, and generally very thin bedded. It is often foliated, with mica developed along the planes of schistosity. Shading into slates on the one hand and into grits on the other, it presents impure types varying in their composition in proportion to the quantity of argillaceous or siliceous material with which it has been mixed. When the argillaceous ingredient is sufficiently in excess it passes into calcareous slate, and difficulty has frequently been experienced in separating out the limestones which occupy a position on the borderland between limestone and slate. There is a similar transition from the limestones to the grits, the former being often thickly strewn with pebbles of quartz and felspar, which may increase in quantity, while the calcareous matrix correspondingly diminishes. The limestone is frequently sandy, and this is especially the case along the south-eastern margin of the Loch Awe group. The crystallisation of the limestone is often exceed- ingly coarse, and in this condition crystals of black calcite, which sometimes attain a quarter of an inch in size, form a large propor- tion of the mass, while at the other end of the scale the limestone is of so fine a texture that crystals cannot be distinguished. In this latter condition the limestones have a granular or even cherty appearance. The pebbly character, while most common in the coarser limestones, is not confined to them, but frequently occurs in the fine-grained compact masses. These extreme divergences in type may all be met with in the same seam. Compact fine- grained slaty limestone, and the coarse gritty type with pebbles 42 MetamorpMc Bocks. of quartz and felspar an inch long, succeed one another in unbroken continuity. As the limestone is everywhere very much folded no reliable estimate can be made of its thickness. Probably it does not exceed on the average 15 or 20 feet, but it appears to have been a lenticular deposit, and, notwithstanding its intimate plication, must often have considerably exceeded these dimensions. (6) Slates. — The argillaceous members of this group, as already indicated, may readily be distinguished from the calcareous sericite schists of the Ardrishaig series. In their normal condition they are comparatively free from lime, and carbonaceous material enters sufficiently into their composition to impart a darkness of hue quite distinct from that of the Ardrishaig group. Thus the Loch Awe slates range in colour from dark blue to black, while occasionally bands are sufficiently rich in graphite to soil the fingers. Being of more homogeneous composition they are more massive in character, and although they have readily yielded to mechanical stress, the interstitial movements have been less pronounced than in the softer and more composite phyllites of the Ardrishaig group. At first sight the difierences in their superin- duced structures are such as to create doubts whether they could have been subjected to the same stresses even when both members occupy the same area, but examination shows that these may be due to the varying powers of resistance which have been opposed to the forces of stress, which are proportionate both to their relative homogeneity and to the stability of the minerals which enter into their composition. The more uniform character of the Loch Awe slates has permitted a ready adaptation in mass to the dynamic forces, which have developed marked planes of cleavage ; the bedding-planes being altogether obliterated or feebly indicated by variation in colour. Their mineral composition has also afibrded them protection, the presence of graphitic material in apparently insignificant quantity being sufficient to retard the processes of crystallisation. It has been observed in the Loch Awe district, whether we are dealing with regional metamorphism or more local alteration due to contact action, that the dark graphitic sediments are proportionately considerably less affected than the other argillaceous, calcareous or siliceous sediments. No analyses have been made from the black schists that occur within this sheet, but from the adjoining sheet (36) two specimens which appeared rich in graphite yielded on analysis only from 4 to 8 per cent, of carbon. It is evident that a very small admixture of carbonaceous material is sufficient to darken the rock. While these dark slates are undoubtedly the characteristic type, paler slates sometimes occur varying in tint from grey to brown, and resembling the darker slates except in the absence of carbonaceous material. Over the Loch Awe series generally, chlorite is rare, in striking contrast to the Ardrishaig series, the argillaceous members of which are usually green in colour from the presence of this mineral. Occasionally, however, chlorite is strongly developed in the Loch Loch Awe Group. 43 Awe group and imparts quite an exceptional appearance to those sediments. The green slates within this sheet have onlj been found in districts where the regional metamorphism has been comparatively slight, so that they only occur in the form of clay- slates or phyllites, in which clastic micas have been preserved. Notwithstanding similarity in colour they may readily be distin- guished by their lithological characters from the Ardrishaig phyllites. The green slates occur on the same stratigraphical horizon as the darker beds with which they are often associated, but their distribution is more local. Although the slates are frequently highly contorted, the cleavage is often perfectly regular, so that the black slates are sufficiently fissile to produce roofing slate. The slates are often intermixed with siliceous material, and gradually pass into grits. Not only is there a gradual passage into the siliceous members of the Loch Awe series, but these latter, even in their greatest development, are separated by argillaceous bands precisely similar to the main body of the slates, and this holds good whether the slates are green or of the more general dark blue or black type. There is a corresponding passage from the slates to the limestones, as pointed out in the description of the latter. Occasionally the slate bands enclose pebbles of quartz and felspar, some of which are as large as those in the coarsest grit bands or gritty limestones, but on the whole they are of far rarer occurrence than in the limestone beds. (c.) Grits and Quartzite. — The siliceous members form the top of the Loch Awe group, and consist of grits almost coarse enough to be denoted as conglomerates, and fine-grained rocks such as quartzites. They have been variously afiected by dynamic forces, and while some have been cleaved others are entirely free from planes of fissility. In this latter condition they either occur as quartzites, of all degrees of texture, according to the size of the siliceous grains, or in rocks of less compact structure, as grits. Amongst these types there are beds representing every gradation both as regards structure and composition. As previously described, they pass gradually into the slates, and also into the limestones, so that we have rocks of mixed composition which strictly speaking do not find a proper place in either of the lithological divisions, but occur, rather, as connecting links, serving to bind those divisions into a single group. The grit bands are divided by thin partings of slate similar to the main mass of the argillaceous series, so that we are evidently dealing with a single system of deposition uninterrupted by any important break. They are composed of pebbles of quartz and felspar, a large proportion of the former being of the clear blue variety. The felspar is pink or dull white, and consists of both orthoclase and plagioclase, the former being often in excess, while, as a rule, the quartz pebbles preponderate largely over the felspar. Coarser and finer pebbly beds rapidly alternate, the coarse bands, which are generally two or three yards and sometimes twelve yards in width, being composed 44 Metamorphic Bodes. of pebbles as large as almonds. Usually the larger pebbles lie in a finer-grained matrix. White mica is not a conspicuous mineral in the grit division, indeed it is never sufficiently developed to enable the rock to split along cleavage planes in the highly siliceous varieties, except when they shade into more argillaceous rocks. In exceptional cases, vyhere metamorphism has been feeble, clastic micas have been preserved in some of the grits, but in most instances it is a secondary product arising from the breaking down of the felspar. In some localities the grits are chloritic and even epidotic, and in these respects present a certain similarity to the Green Beds. While the epidote, which is not conspicuous, is probably a secondary product, the chloritic ingredients are undoubtedly original. The chlorite is disseminated amongst the argillaceous and finer arenaceous m.aterial, and when it occurs in the grits the green colour of the mass is due to the occurrence of this mineral in the matrix, in which the larger pebbles are embedded, so that in the coarser grits the green tints are light though sufficiently pronounced, increasing in depth of colour as the sediment becomes finer. These green grits are intercalated with green slates, and both are distinctly associated with the normal grey grits and dark slates into which they merge both laterally and vertically, so that their occurrence is of minor importance from a stratigraphical standpoint. The grits associated with the black slates are sometimes also dark in colour owing to a certain amount of carbonaceous material entering into the composition of their matrix. These dark grits, however, are even more local in character than the green grits. True quartzite is only of local occurrence, very few of the bands being sufficiently compact to fulfil the lithological requirements of this rock as generally understood. Likewise the coarser grit zones already alluded to do not fulfil the conditions of a conglomerate as usually implied. Many of the grits, however, are petrologically not far removed from quartzites, and in coarseness some of them may be said to approach conglomerate. The grits present marked variation due to the unequal eSects of dynamic action, but as this subject will be treated in Chapter IX. on regional metamorphism, we are only concerned at present with its lithological results. Every gradation is seen between bands which have sufiered extreme granulitisation and others in which the grains have suffered little or no crushing, and in which even their clastic micas are preserved. But in those which have under- gone the maximum amount of crushing, the clastic origin is everywhere apparent. In the coarser grit bands in the latter condition the larger pebbles are often elongated and flattened while the matrix has been partially or completely granulitised. Usually, however, crushing has been confined to the groundmass, and the larger pebbles show little or no alteration. Some of the finer grits, which in the field present no evidence of crushing, nevertheless, when subjected to microscopic examination, show that they have been modified to some extent by pressure, for they have been partly Loch Awe Group. 45 grannlitised and partly recrystallised. Through successive stages of alteration, however, there is a passage to beds so unaltered that clastic micas ai'e still preserved, and even microscopic investigation fails to detect evidence of crushing. The differences are not confined to mineralogical alteration of the rock ingredients, but structural characters have been induced of some importance. I.*^oliation and cleavage-planes have been set up by the same dynamic forces, and by their unequal distribution produce corresponding differences. A normal grit is often replaced by bands which are cleaved as finely as a slate, and between these two extremes every gradation may occur. Boulder Bed. — In addition to the lithological types which have been described, a rather abnormal conglomerate or boulder bed, which occupies a fairly definite position across the Highlands from Aberdeenshire to Islay, occurs within the Loch Awe series. It has been pointed out that the various members of this group some- times shade laterally into one another, which is supported by the behaviour of the boulder bed that occurs promiscuously throughout the series and includes fragments of each lithological type. As might be expected, the largest boulders are represented by grit and quartzite and the smallest by slate, the fragments of limestone occupying an intermediate position, and the boulders range from two feet down to the smallest dimensions. Although foreign fragments, mainlj'^ of granite, are found in this abnormal deposit elsewhere in the Highlands, they have not been detected within the area of this sheet. It is extremely local in its occur- rence, and most probably marks a phase of crustal oscillation which sufficiently raised the sea floor to admit of a certain amount of contemporaneous erosion, fragments of each variety of rock being torn up and rolled and enclosed in a matrix of like composition, or when transported further have been deposited on sediments of unlike type. On the whole the deposit points to a distribution of the boulders from coast margins to sediments formed in deeper water, the larger siliceous boulders being enclosed in gritty or quartzose argillaceous sediments, and only occurring exceptionally as small boulders in the purely argillaceous or calcareous rocks. Slate boulders occur exceptionally in the grits but are mainly confined to the limestones, while boulders of the latter rock are rarest of all and almost entirely restricted to the limestone itself. The presence of grit boulders indicates that the coarser siliceous deposits had to some extent been formed before the boulder bed, but there appear to be strong grounds for believing that the boulder bed and the coarse grits belong to the same general phase of deposit, for besides the grit boulders, which are, properly speaking, true rock fragments, the slates and limestones likewise contain pebbles of quartz and felspar identical with those entering into the composi- tion of the grits and sometimes attaining the size of two inches, while pebbles ranging from a half to one inch in length are exceed- ingly common. As these large pebbles often occur in the slates and limestones in close association with rock boulders, they may 46 Metmiiorphio Bocks. probably be due to the contemporaneous erosion indicated by the boulder bed. This inference iinds some support in the character of the limestone boulders. As already pointed out, the gritty lime- stone frequently passes into thin-bedded limestone of normal type, but so far as observation has gone it would appear that the limestone boulders are confined to the fine-grained compact type while tlie enclosing rock is frequently a gritty limestone ; indeed, the absence of boulders of the latter type would suggest its subsequent formation, and that it belongs to the epoch of contem- poraneous erosion represented by the boulder bed. The facts, that the grits often contain flattened pockets of black slate up to a foot in size, which may possibly represent patches of clay torn from their original position and rolled within the grits, and that the coarser grit bands themselves occasionally contain small pebbles of slate and limestone, point to the same conclusion. That the boulder bed represents an abnormal phase of deposition due to an elevation of the sea bottom and not an unconformity, may be inferred from the identity in the lithological types among which it occurs, from the limestone in which it lies forming a continuous deposit -with limestone of normal type, and from the fact that its base never rests on the Ardrishaig phyllites, notwithstanding that the latter are traced across the map for a distance of twenty-five miles. DISTRIBUTION AND FIELD RELATIONS. — As described in Chapter II., this group occupies at least one-third of the area under descrip- tion. Its south-eastern margin extends right across the map from the western slopes of Glen Shira to the south-west corner of the sheet. With the exception of a small tract west of Loch Awe, where it underlies the Lower Old Ked Sandstone volcanic rocks of Lome, this group covers the whole of the area in the north-west part of the map. Its junction with the Ardrishaig series from the north margin of the map to Glen Aray is well defined. Along this line the western slopes of Glen Shira truncate the beds at a high angle in a direction transverse to the dip. South-west of Glen Aray, not only is the general north-westerly dip at a lower angle but the slopes towards Loch Fyne are likewise more gentle, so that the physical features truncate the dip at a much lower angle. Conse- quently the isoclinal plications into which the beds have been thrown have resulted in partial alternations of this series with the Ardrishaig group, while the huge protrusions of quartz-porphyry in elongated masses parallel to the strike have still further con- cealed the junctions of the two groups. Between Glen Aray and the chain of lochs bordering Loch Leacann the quartz-porphyry has often been taken as a convenient boundary. Further south- west, however, as these quartz-porphyries die out the boundary admits of a more precise description, but the gentle angles of the general geological dip and the average slope of the ground do not permit of such a sharp definition as in the ground lying between Glen Shira and Glen Aray, The boundary drawn is so far a fixed geological line that it represents the extreme limits of the Ardrishaig series. The only members of the group that have Lvch Awe Group. 47 been differentiated on the map are the limestone and the grits. The slates are as a rule so poorly represented and the isoclinal folds are so steep that the argillaceous members rarely occur in well- defined bands thick enough to be worth representing on the one- inch map. They appear as partings between the grits or intimately folded with the siliceous or calcareous beds with which they are associated. The apparent dip and foliation as seen in the various sections, although comparatively uniform, have no relation whatever to the geological position of the series as a whole, or to the relative positions of the lithological divisions, and for purposes of strati- graphy they must be altogether disregarded. The Loch Awe group rests on a compound trough of the Ardrishaig series, which re-emerges from beneath them in the area to the west of that under consideration. From both margins the dips steadily increase till, along an axis approximating generally to that of Loch Awe, they become vertical. MAEGINAL EELATIONS BETWEEN THE LOCH AWE AND ARDRISHAIG GROUPS. — The stratigraphical position of the members of the Loch Awe group, both as regards one another and their relations to the underlying Ardrishaig group, have already been indicated. As these groups in their passage across the Highlands are in a more advanced state of metamorphism, the order of succession in this comparatively unaltered area becomes of importance in throwing light on the stratigraphy of other districts where the geological relations are more obscure. The marginal relations of the two groups will, in the first instance, be briefly described. The western slopes of Glen Shira, along which the Ardrishaig series is typically represented, are capped by a mountainous platform of grits and quartz-schists belonging to the Loch Awe group, the edge of which makes such a conspicuous escarpment that the two groups are sharply demarcated by this physical feature. Commencing at the north-east, a thin grey limestone is seen mixed in with coarse grits at the edge of the crags above Drimlee, and the limestone occurs again amongst the calc-sericite schists 300 yards below these crags, where it occupies an outcrop of 30 to 40 yards. It is again visible half-a-mile further south-west in the Drimlee Burn amongst calc-sericite schists in a corresponding position, where it appears in two outcrops due to folding. The top side of the upper outcrop is covered by drift, but another band, which is probably its lateral extension, is met with along the bend of the burn half-a-mile to the south-west, where it occurs as a bluish limestone associated with black slate and undoubtedly adjoins the Loch Awe grits. At Stuc na Gaoith, about 300 yards south of this last exposure, the edge of the Loch Awe grit escarp- ment rests nearly flat on clay-slate, beyond which the section is lost beneath soil. Under Creag Dubh a whitish-grey limestone, associated with clay-slate, occurs a hundred yards within the Loch Awe grits, and is repeated at the margin of the series, where it is separated from the grit by a band of hornblende-schist. It is not again seen amongst the Ardrishaig slates below. In the Allt na Scardan Burn a thin bedded limestone is strongly 48 Metamorphic Bocks. represented on either side of a quartz -porphyry sill and occurs next to calc-sericite schists, with which it is intimately interfolded. At the base of the grit crags above, it occurs again as a strong fine- grained blue limestone with argillaceous partings of Loch Awe type. The ground between the two exposures is covered by drift, but the evidence suggests that there is only one limestone associa- ted at its top with the Loch Awe grits and at its base with the Ardrishaig phyllites. Half-a-mile north-west of Maam the limestone adjoining the same quartz-porphyry sill is whitish-grey and intermixed with micaceous beds, but appears as a blue crystalline limestone at its top. A quarter of a mile further north-west at the base of the grit crags, an impure limestone is seen associated with nearly flat quartz-schists and calcareous thin grits. The valley between Stuc Scardan and Three Bridges in Glen Aray follows the margin of the grits. Along this valley the lime- stone is again seen half-a-mile distant from the last exposure. It occurs as a thin band associated with mica slate and some graphitic material, while a quarter of a mile further along the strike, it is seen as a thin limestone, six feet thick, resting on Ardrishaig slate. These limestones in this valley occur close to the margin of the Loch Awe grits, with which they are probably continuous. In the river Aray the junction of the two groups is visible at Eas k Chlabh, where the Ardrishaig phyllites and bedded quartzites are succeeded by thin limestones. The latter are separated from the Loch Awe pebbly grit by six or eight feet of black slate. On the north-west side of the pebbly grit the limestone is again exposed, where it is grey-blue in colour, and associated with some decomposing sandy limestone and with some blue slate, and is succeeded north-westwards by the pebbly quartzite. Summing up the evidence as far to the south-west as Glen Aray, it may be stated tliat limestone almost invariably separates the Ardrishaig slates from the Loch Awe grits. The limestone is often associated in its upper portions with black slate which divides it from the grits, but it is not always present between the two members. While limestone and black slate undoubtedly occupy an intermediate position between the Loch Awe grits and the Ardrishaig slates, it is possible that the limestone may partly transgress these limits and sometimes occur slightly below the top of the latter. Olen Aray to Loch Zeacann. — To the south-west of Glen Aray as far as the watershed above Loch Leacann the marginal relations of these groups are obscured by great intrusions of quartz-porphyry, which occupy a larger portion of the ground than the sediments. The burn (Allt Bail a' Ghobhainn) which flows into the Aray in a direction only slightly oblique to the strike, furnishes good sections of quartzites and sheared grits. A brown and bluish-grey limestone, the width of which is not seen, is associated with a little dark slate, while further along the section dark graphitic slate Loch Awe Growp: "^9 occurs, some of which is closely interfolded with slates of the Ardrishaig type. The limestone and black slate apparently occur between the Ardrishaig slates and the Loch Awe grits, and as the section nearly coincides with the strike it is clear that the various members have narrow outcrops. Between Inveraray and the Douglas Water a limestone near Dalchenna appears next to a quartz-porphyry and associated with a little dark schist, while in the immediate neighbourhood grits are seen with black slate intercalations ; and further south-west in the Douglas Water at Barmore Wood a limestone is associated with quartzites, the argillaceous partings of which are dark slate of Loch Awe type. Further to the south-west, outliers of the Loch Awe group are exposed lying on the Ardrishaig series, — for instance, about a mile south-east of the village of Auchindrain some crags occur made up of coarse grits with pockets of clay- slate, while close below them there is a thin band of contorted lime- stone. These crags are surrounded by the Ardrishaig slates. In the upper part of the Leacann Water above Braleckan, numerous thin outcrops of limestone are found amongst Ardrishaig slates and quartzites, while half-a-niile south-west of that stream a thin bedded limestone appears alongside a gritty quartzite. In the zone of sediments to the south-west of Loch Leacann, between the large quartz-porphyry intrusions of Bheinn Ghlas and Beinn Laoigh, some outcrops of limestone are seen amongst the Ardrishaig slates, one of which, twelve feet in thickness, is associ- ated with black slate. The quartzite and grits bordering Loch Leacann probably belong to the Loch Awe group, and there are other instances, which need not be enumerated, where they appear as outliers in the Ardrishaig series. So'idh-west of Loch Leacann. — In its further passage from the watershed above Loch Leacann to the south-west corner of the map the junction is only slightly truncated by the intrusive quartz-porphyry, which is rapidly approaching its limit in that direction. In the burn half-a-mile to the north-west of Loch nam Breac Buidhe, the limestone is frequently exposed, associated both with grey and with black phyllites. It is again seen on the same horizon near Loch Sitheanach, both with slates of the Ardrishaig type and with dark slates of the Loch Awe type, and in close proximity to Loch Awe grits containing black slate lenticles. At Oruach an Sgliata, within the Loch Awe group and near its margin, a band of black slate, cleaved, and suitable for roofing purposes, is associated with a fine greenish grit. This band of black slate is not accompanied by limestone, but close at hand, at the base of the grits, a little grey limestone is found with fine sheared grit. Following the same horizon between the Dubh Loch and Lochan Breac-liath the limestone is well exposed in large outcrops, and the evidence is clear that it immediately overlies the Ardrishaig phyllites, and that the black slates are either absent or only feebly represented. These limestones, although never belonging to the typical pebbly type, are everywhere sandy. The sandy layers in 50 Metamorphic Bocks. bhe limestone are perfectly interlaminated, and it is obvious that the arenaceous and calcareous material represent a continuous sequence of deposit. The south-eastern boundary of the Ardrishaig and Loch Awe groups has been recently re-examined, and that portion lying in the basin of the Add as far as the Grinan Canal, in particular detail. As a result of that examination it has been found that the limestone all along the line almost invariably exhibits the fine sandy character above described,* and in some cases the clastic grains become coarse and the limestone is of the same pebbly nature as that occurring in the heart of the Loch Awe basin ; nevertheless, this coarse pebbly type is exceptional at the margin of the Loch Awe group. In this south-eastern district the evidence is conclusive that the limestone immediately overlies the Ardrishaig phyllites. Although, however, the limestone is sometimes associated with a little dark phyllite, there is often a natural passage from the Ardrishaig phyllites and overlying limestone to the Loch Awe grits without black slate being represented. When the black slate is present it can, in most cases, be shown to overlie the limestone. An example is seen a quarter-mile south of Oraig of Achnabreck, where the dark and partly sandy Loch Awe limestone is separated from the pebbly grit by bluish-black slates. Again, at Achnabreck some black slate divides the pebbly grit from the limestone, the latter being in part also of the pebbly type. A coarse grit zone, almost conglomeratic in character, occurs three-quarter -mile south of Lecknary and the same distance from the margin of the Loch Awe group, within which it lies. Besides quartz, it contains pebbles of black slate and dark fine-grained limestone, as well as some large patches of black slate of similar composition to the pebbles or boulders. This zone links the coarse conglomer- atic grits with the boulder bed, and is conclusive evidence that the limestone and black slate, which have suffered contemporaneous erosion, and furnished, to some extent, the material of the grit, must necessarily occupy a lower stratigraphical position. The evidence afforded by a study of the marginal relations of the two groups is sufficient to establish an ascending order of succession from the calc-sericite schists of the Ardrishaig group to the siliceous members of the Loch Awe group, and that the limestone and black slates are the connecting links. These two latter are so intimately associated that their mutual position cannot always be precisely defined, but as a general rule the black slate is above the limestone. The superposition of the grits to the limestone is indubitably established. In studying the marginal relation of the two groups between Loch Fyne and Loch Awe, it is significant that the limestone, although usually sandy, rarely exhibits the coarse gritty character * Localities : — half-mile south-west of Loch Dubh — one-mile south-west of same locality. Carron burn — Burn south-west of Meall Reamhar — AUt Breac-liath — quarter-mile north-west of Barr — north west of Craigmurrail — Low Monydrain — Auchnabreck — Achahoish. Loch Awe Group. 51 seen further to the north-west. There is either an entire absence of black slate or it occurs in very limited amount. Further, the boulder bed has not been observed at that margin, so that, speaking broadly, the occurrence of the boulder bed, the gritty limestone, and black slate in appreciable quantity appear to hang together. Moreover, the marginal limestone, where not associated with black slate, is seldom of the dark graphitic hue which it assumes when in association with well developed black slate. It would seem, therefore, that the oncoming of these conditions, bringing about a change in the character of the deposit as a result of the crustal oscillations, to which reference has been made, was less perceptible in the south-eastern portion of the area. Notwithstanding that the deposits more rapidly change their character to the north-west, it has been shown that within the area of this sheet the sequence is conformable. But further to the west, in the Garvelloch Islands beyond the mainland of Argyllshire, Dr. Peach finds an unconformity at the base of the boulder bed. This fact harmonises with the conclusion stated above, since it indicates a variation in the nature of sedimentation progressively increasing from south-east to north-west. GENERAL DISTRIBUTION. — Having described the marginal relations of the Loch Awe group, we may now briefly indicate its general distribution. Besides the quartz-porphyry intrusions already alluded to, this group of sediments is penetrated by much igneous material represented by sills of the epidiorite group. One of these sills forms a band four miles broad, extending from north-east to south- west along Loch Awe, in the heart of that group. Not only is the epidiorite more strongly represented within the Loch Awe group than at the margin of the latter, but the limestone, boulder bed, and black slate are correspondingly of greater development along approximately the same horizon. To some extent this may be accounted for by the gradual flattening of the beds as we approach the centre of the Loch Awe basin, but this cause does not entirely explain the phenomena ; for not only are the foregoing sedimentary rocks more strongly developed, but the boulder bed has so far not been observed at the margin, and the gritty limestones, which are of exceptional occurrence at the boundary, are conspicuously developed on the horizon of this great epidiorite sill. Qlen Aray. — This glen dissects the Loch Awe group from the large epidiorite sill to the south-east margin of the group. On the north-east side of the glen the rocks are almost entirely siliceous and made up of rapid alternations of coarse pebbly grits with finer quartzose-schists and quartzite of the typical Loch Awe type. They contain lenticles of black slate, and are frequently divided by black slate partings too thin to be mapped. They are sometimes so inter- mixed with calcareous matter as almost to constitute a limestone while at other times quartzites may be associated with them. Thin limestone is occasionally interfolded with the grits in bands often too small to be shown on the map. Half-a-mile south-west of Lochan k Mhaidaidh a seam of black slate, twelve feet iji thickness S2 Metamorphic Rocks. occurs amongst the quartzose bands, and some brown slate is also occasionally seen. The siliceous beds are often foliated and granulitised, the pebbles of the coarser bands, even when of the size of marbles, being elongated and flattened, although they are frequently undisturbed. The folded limbs have a steady dip to the north-west, and the foliation planes are similarly inclined — the limbs may be evenly inclined or contorted. The foliation is usually later than the folding, and there is often a slight obliquity in their respective strikes ; for instance near Stuc Scardan, while the foliation strike is nearly east and west the bedded limbs strike north-east and south-west. As the foregoing remarks are applicable to large tracts of country occupied by the Loch Awe group that lie to the south-east of the large epidiorite sill extending from Glen Aray to Kilmichael, that belt will not be touched upon unless special points call for description. Qlen Aray to Loch Awe. — The section along the river Aray for one- and-a-half miles from the margin of the Ardrishaig group resembles those already described, the sediments being almost entirely siliceous with coarse grit bands and very subordinate intercalations of slate and limestone. At Drimfern, however, a marked change appears, and bands of black slate often graphitic, and dark bluish, fine-grained, often granular limestone are strongly developed on the western slopes of Glen Aray along an outcrop extending from Drimfern to the watershed near An Greachan. The limestone occurs either in strong bands within the black slate or between the slate and the grits, or it may be so intimately associated with the black slate that the zone may represent an equal admixture of both. . Further, the grits themselves along this zone are often dark from the graphitic matter in their matrix. So far as the three litho- logical types are concerned, viz., the black slate, the limestone, and the grit, there can be little doubt that they indicate continuous deposition. A little further up the valley, however, between these limestones and the edge of the large epidiorite sill, the boulder bed is first seen about two miles from the margin of the Ardrishaig group. It occurs in two well defined bands with a maximum breadth of thirty yards and contains quartzite boulders enclosed in a hard, dark, quartzose argillaceous matrix, showing evidence of much pressure. They are associated with dark phyllites and grits, some of which are also dark in colour, while at one place the slate adjoining the boulder bed also contains grits. The upper band, for a great part of its course, follows the margin of the large sill, while the lower band occurs entirely amongst grits and phyllites. Towards the watershed these bands converge, and there is little doubt that we are dealing with a duplication of the same bed. Besides these two bands, others less continuous occur, so that at one place as many as four parallel outcrops succeed one another within a breadth of a quarter of a mile. East of Glen Aray, three-quarters of a mile west of Lochan Sheileachan, the boulder bed is seen on a corresponding horizon, and consists of boulders of quartzite over one foot in length in a sandy matrix. Loch Awe Grou^. 53 The slope east of Loch Awe forms but a slight angle with the general dip, so that the boulder bed, instead of forming well defined bands as on the steep slopes of Glen Aray, occurs as a series of broad lenticles. Nevertheless, its south-easterly margin follows a fairly steady south-westerly course in the same relative position to the massive sill of epidiorite. Near the watershed west of Oruach Mhor a large inlier of sediment is almost entirely enclosed by the epidiorite sill, which appears to overlie it. Within this loop the boulder bed is spread broadcast in nearly horizontal masses, forming prominent features on the map. Peat often conceals its margin, but between Cruach Mhor and Cruach an Lochain one of these bands has a well defined south-east margin for upwards of a mile. It occurs in close association with patches of limestone of the dark granular type seen in Glen Aray. In composition it is very similar to the exposures in that glen ; the quartzite boulders, from an inch to two feet, being enclosed either in a dark gritty slate, or else in a matrix identical with the boulders. In one instance a grit boulder, eighteen inches in size, is enclosed in a dark graphitic grit, while north of Creag Dubh, grit boulders, one foot in diameter, are lying in a matrix of black siliceous slate. In the Blarghour Burn the margin of the boulder bed is in contact with the limestone, to which it appears to have been gradually converging. In Glen Aray it is always at some distance from the limestone. After the watershed is reached they appear to be contiguous, while still further south-west the boulder bed forms part of the limestone. The latter rock has hitherto been of the fine-grained granular type, but the Blarghour Burn cuts its way for half-a-mile through a limestone of this character, associated with black phyllites, and passing into a gritty limestone, while still further south-west, midway between that burn and the Kames River, a highly gritty limestone merges into a limestone conglome- rate, boulders and pebbles of blue cherty limestone and blue slate being enclosed in a matrix of gritty limestone. Still further south- west, near the Kames River, and only three-quarters of a mile from Loch Awe, a strong gritty limestone adjoining a coarse siliceous grit contains boulders of fine-grained blue cherty limestone up to a foot in size, some of which are slightly bent. At the northern margin of the sheet in the neighbourhood of the small lochs one-and- a-half miles east of Loch Awe, some lenticles of boulder bed appear as inliers in the epidiorite sill, one of which occurs as a conglome- ratic limestone with pebbles of quartzite up to two inches in size. The large epidiorite sill by a slight bend crosses Loch Awe, and its south-easterly margin for some distance between Barr Phort and Kilmaha, follows approximately the northern coast line, re- crossing Loch Awe near Ardary, and thence following a steady trend to Kilmichael. At Barr Phort a conglomerate adjoins hard green slate, and consists of quartzite pebbles, from a half to two inches in size, embedded in a slaty matrix, while within the conglomerate a band of fine grit six feet broad is seen. Near Kilmaha fine-grained bluish black limestone, associated with black slate, passes into gritty limestone, sometimes conglomeratic in character and enclosing pebbles of quartzite. 54 Metamorphic Rocks. South of Loch Awe to Kilmiehael. — The sill which crosses the loch near Ardary and extends to Kilmiehael almost entirely covers the ground west of that line. It is associated with broad bands of limestone, not only along its margin, but within its limits. Within the horizon of this sill grits and slate also occur, but are subordinate to the limestone. That these broad bands of limestone represent folded repetitions of a single bed is clearly shown by the mapping near Pincharn, where the bands are seen ascending the hill by a series of folds the ends of which can be traced. The predominating feature of these limestones is their gritty character ; indeed, the larger bands may be wholly or chiefly of this nature. No stratigraphical distinction, however, can be made; fine-grained limestone accompanied by black slates passes gradually into the gritty types, the latter usually immediately adjoining the siliceous grits. Numerous fragments of quartz and felspar, the largest of which may exceed an inch in size, are found in the conglomeratic limestones, together with occasional pebbles of siliceous grit. Flattened boulders of slate up to six inches in diameter occur, but it is not certain that these latter are not the products of brecciation. Occasipnally gritty material appears in the slates ; some fine phyllitic beds with pebbles of quartz and felspar up to half an inch in size are seen south-west of Lochan Tor Bhealaich, and a green slate south-east of Lochan Add is similarly studded. How far these conglomeratic limestones may be related to the boulder bed is not clear, but they undoubtedly occupy a horizon which closely corresponds with it; the limestone boulders which further north are embedded in gritty limestone, are here absent, but the limestone itself is identical with the matrix of the limestone boulder bed. While the strong gritty character of the limestone undoubtedly suggests an abnormal phase of deposition, it must be borne in mind that the normal fine-grained limestones are also frequently arenaceous, and that the Loch Tay limestone on the coast of Loch Pyne occasionally contains pebbles of quartz and felspar as large as peas. The evidence, however, is clear that the Loch Awe limestone in its gritty condition is associated with the coarse siliceous grits, which latter rarely contain limestone pebbles. In that part of the district now referred to (south of Loch Awe) the sediments are singularly unaltered, some of the argillaceous members being in the condition of clay-slate. A green slate of this type from Kilbride (5,694) is seen under the microscope to consist of chlorite, pale mica very finely divided, very small angular quartz grains and limonite. It is a very fine slate, and where the section is thinnest the rock is composed very largely of extremely minute scales of mica which are probably original. West of Loch Awe. — This district extends westwards from Loch Awe to a line drawn from Loch na Sringe across Loch Avich to the watershed between Gleann Domhain and Loch Awe, and includes, for convenience of description, a small loop at Ballimeanoch on the east side of Loch Awe at the north edge of the map. The sediments largely occur on the western side of the massive epidiorite sill before noted, and are not only intermixed with Loch Awe Ghroup. 55 epidiorite but in the neighbourhood of Loch Avich are concealed by the Lome andesites which overlie them. In this western district the rocks are much less metamorphosed, and contain zones which are practically unaltered. The limestone is rarely seen, but the green sediments, on the other hand, reach their greatest development. In the neighbourhood of Ballimeanoch, green slates are overlain by greenish grits which are sometimes coarse and contain pink felspars in addition to quartz and a considerable amount of chloritic matter; pebbles also occur scattered in the slates. Blue and dark slates are also met with, but in less abundance than the green. All these beds are comparatively very unaltered. A microscopic section (7,768) of a green felspathic grit with scattered pebbles of quartz and felspar in a finer-grained, dark green, hard quartzose matrix, shows that the felspar pebbles are partly oligoclase and partly perthitic orthoclase — the quartz pebbles are irregularly lenticular, but many of them are still optically continuous through their whole extent, and their margins show a loss of material by crushing, which is blending with the matrix, the latter being composed of quartz and abundant decomposing felspar, together with epidote, chlorite, sphene, and small irregular scales of brown biotite. The parallel structure of the rock may be due to original bedding, but the slide shows clear evidence of shearing. On the west side of Loch Awe, between Inverinan and the river Avich, the sediments are mainly very fine grey grits, associated with blue or pale-grey phyllites. The grits, although very un- altered, are often so well cleaved as to display a marked fissility, while the argillaceous members are in the condition of clay-slate with original bedding frequently well preserved. Near Inverinan the sediments are mainly chloritic quartzites and slate, varying in colour from dark green to paler hues. In the neighbourhood of Loch Avich the green sediments are the predominating type ; chloritic grits (often coarse) with chloritic slate skirt the north-east end of that loch and continue to Drissaig, the grits varying considerably in texture and often including red orthoclase. The green grits contain pebbles and blocks of slate, pebbles of fine grit, and of quartz and felspar up to half-an-inch in size. The grits sometimes form very massive beds, as in the burn above Drissaig, where beds of fine-grained grit, from ten to twelve feet in thickness, are lying nearly flat amongst blue slate and piled on one another with undulating junctions, with foliation or cleavage almost vertical or nearly at right angles to the bedding. South-east of Lochan nan Oaorach the coarser grits of Loch Awe type occur, as 'well as a good example of the boulder bed, which appears to occupy a position between the coarser grits and the finer chloritic grits. In the neighbourhood of Barmaddy the green sediments pre- dominate ; the coarse grits contain fragments of finer grit and quartz pebbles half-an-inch in size, as well as sub-angular felspar, while some of the slate beds also contain pebbles. When the coarse felspathic green grits are associated with green slate they some- 56 Metamorphic Hocks. times contain fragments of the latter. The green slates merge into the dark-blue slates by intermediate stages. South-west of Lochan k Bruic greenish slates contain pebbles of quartz ranging from the size of peas to an inch in diameter; and from here to the watershed the sediments are almost entirely of the chloritic type and are well seen in the country bordering the Cam Loch, where the bedding is slightly inclined to the north-west and the cleavage, is at a high angle. Near Loch 3, Mhinn fine grits alternate with phyllites in a singularly unaltered condition, and clastic micas are apparent. In fact, as seen in the field, these rocks are no more altered than the Silurian sediments of Cornwall. A specimen of fine-grained green grit examined under the microscope (5,545) shows that the clastic structure is perfectly preserved — the pebbles are mostly quartz, but there is a good deal of felspar, both orthoclase and plagioclase; it contains abundant chlorite, some epidote and pale flakes of mica, together with granular sphene and small grains of iron ores which are weathering in some cases to leucoxene — the pebbles, however, are sometimes broken, and they are losing material at their edges, which is blending with the matrix. (Plate VIL, fig. 6.) Between Loch a Mhinn and Creag Luaragain Bheag a conglo- merate is seen consisting of pebbles of quartz and felspar, some- times an inch in length, embedded in a matrix of pale green slate, while west of Dun Dubh a gritty limestone contains quartz frag- ments of similar dimensions. And to the north of Am Barr a bluish- black mottled limestone is charged with pebbles of quartz, felspar, and black slate, ranging in size from a quarter to two inches. J. B. H. District of Olen Bomhain and North-west of Loch Avich.— This area is almost triangular in shape, being bounded on two sides by the western and northern edges of the map, and on the third side by a line drawn from a point on its western margin about one-a-half miles west of the head of Loch Awe north-eastwards to about the middle of Tjoch Avich, and thence northwards to the edge of the map. The area thus enclosed is about twenty square miles in extent, part of which is occupied by sediments and volcanic rocks of Lower Old Eed Sandstone age, and also bj' igneous rocks intruded at difierent periods, but all of later date than the schists. The schists of sedimentary origin, which, in this region, cover a much smaller area than the epidiorite masses, belong, with perhaps slight exceptions, to the Loch Awe group of Mr. Hill. This group here consists of the following members given below in descending order: — (e) Fine-grained quartzites with partings of dark shale and with at least one bed of sandy limestone. (d) Pebbly quartzite passing down into pebbly limestone, which is locally developed as a "Boulder Bed." These pebbly rocks contain fragments of the underlying members of the group, pointing either to local erosion of those rocks or to unconformability. Loch Awe Group — Ardrishaig Fhyllites. 57 (c) Dark carbonaceous limestone generally associated with slates like those in sub-group h. (b) Black slates intercalated with grey silky phyllites and passing down into, (a) Grey and green phyllites and sandy bands. Some of these beds probably represent the top of the " Ardrishaig Phyllites." As the present area is a continuation of that of Loch Awe, all the schists of sedimentary origin entering into it are invaded by the same set of wide-spread sills of epidiorite and are thrown into a series of isoclinal folds, the longer axes of which, however, have, in this region, a more northerly trend than elsewhere in the map. The isoclines are nearly all bent over westwards, so that there is an apparent eastward dip of all the structures. All the rocks are more or less cleaved or foliated, the dip of the planes of cleavage and schistosity being at high angles and varying from south-east to east, and nearly coinciding with that of the axial planes of the folds. In addition to this more intimate plication, the corrugated rocks have been further folded into broad anticlines and synclines, the longer axes of which coincide in direction with those of the minor folds. The south-eastern limit of this area nearly coincides with the crest of a compound anticline of this kind, while the western edge of the map, from a point northwards of latitude 56° 11', obliquely truncates a compound syncline, the north-west limb of which lies outside the limits of the present map. Hence the different members of the Loch Awe group arrange themselves in more or less parallel belts with irregular interlocking junction lines, while inliers of the underlying and outer belt, and outliers of the overlying and inner one, occur within each belt in turn. (Sub-group a). — In accordance with this arrangement, a belt, over a mile wide, of grey and green phyllites and green chloritic sandy beds occupies the north-western slopes of the ridge which extends from Dun Dubh, about a mile west of the head of Loch Awe, through Oarn Duchara, its culminating point, to about the middle of the south shore of Loch Avich. Good sections of these rocks, folded with epidiorite sills, are exposed in the upper part of the Duchara Burn which flows out of Loch a Ghille. They are also well seen in all the streams draining this ridge and flowing into Loch Avich. In the south-west of the area they are seen in the Allt Dearg and the head waters of Allt Ath Mhic Mhartein. Inliers of these rocks occur throughout the tract occupied by the succeeding belts, the largest of which stretch southwards from Loch Pearson, at the western margin of the map, and others occur in that small area of schists that lies nearly surrounded by the Lower Old Eed andesites west of Loch Tralaig. As has already been stated, it is highly probable that some of the members of this subdivision may belong to the Ardrishaig phyllites appearing here on the crest of the compound anticline, though, for convenience of description, they have been provision- ally placed with the Loch Awe group. 58 Metamorphic Bocks, Owing to tlieir constant reduplication there is no means of accurately judging of the thickness of the beds exposed along this line ; but for the same reason it is obvious that it cannot be great. The epidiorite sills with which they are intercalated form, indeed, the greater part of the country-rock. (Sub-groups h and c). — Lying to the north and west of the beds just described, come dark grey and black phyllites of subdivision (h) with bands of dark blue and black limestone (sub-group c), which are so intimately, mixed up with one another by intercala- tion and interfolding— though the main mass of the limestone over- lies the phyllites — -that it is found most convenient to treat them as forming one belt. It lies on the east side of Grleann Domhain, and extends from the west margin of the map, where it is over a mile wide, to An Dam, the stream flowing into the head of Loch Avich, where it does not measure half that width. Good sections of these rocks are afforded by the streams draining into An Dam opposite Maolachy. They are exposed in all the tributaries of the Eas Bhachain, especially that which flows out of Lochan Mhic Guaraig. The section in the main stream is much confused by being traversed by a line of fault-breccia invaded by several basalt dykes, the stream generally following the line of breccia. Excel- lent sections are exposed along the lower part of the Allt Dearg, where it has cut down so as to run on a platform of these highly corrugated rocks, while the overlying epidiorites and the quartzite are seen forming the sides of the gorge and capping the neigh- bouring heights. These beds are also exposed along the lower part of the Allt Ath Mhic Mhartein, near the western edge of the map. Isolated outcrops, probably inliers of these rocks, occur to the north of this belt. Three separate bands, surrounded by epidiorite, are seen on the hill sides and crossing the streams to the north- west of and within half a mile of Maolachy. Considerable areas of these strata occur still further north, round Loch na Salm, between a large outlier of Old Eed Sandstone volcanic rocks and the main mass of the volcanic plateau. They also reappear on the other side of the plateau to the east of Loch Tralaig. These occurrences may be looked upon as inliers within the belt of rock formed by the overlying quartzite. Other isolated areas of these strata occur to the north of Gleann Domhain, near the western margin of the map where, however, they are all much affected by granite intrusions. The slates are bleached and more or less hornfelsed, the impure calcareous rocks are converted into calc- silicate-hornfels, and the limestone is changed into a white crystal- line marble. (Sub-groups d and e). — The quartzite with its local basement beds, the pebbly quartzite, the pebbly limestone, and the " Boulder Bed" much interrupted by sills of epidiorite, Old Red Sandstone volcanic rocks, and intrusive igneous rocks of various ages, occupy the rest of the area. They extend from the western margin of the map a little to the south of Gleann Domhain, cross that glen and pass northwards and disappear under the Old Red volcanic platform of Cruach an Nid, and reappear in Looh Awe Qroup — Summa/ry of the Straligrwphy . 59 an attenuated form east of Loch Tralaig. Outliers of these sub- groups occur within the belt of black schist and limestone to the south-east of Gleann Domhain, where the local basement pebbly limestones are best developed. Thus the pebbly limestone is exposed in many sections where the Allt Ath Mhic Mhartein is truncated by the western margin of the map. It is well seen in the Allt Dearg, about half a mile from the edge of the sheet, where it merges into a pebbly quartzite. Prom this point branch- ing outcrops can be followed south-x/est to the margin of the map. In a side stream joining the first tributary of the Allt Dearg above this point, at about one-third mile south of Drum Buidhe, the pebbly limestone, from the large size of the included fragments of dark slate and limestone, may be looked on as a boulder bed. This outcrop is seen in the cliff overlooking the tributary stream on the one side, and on the other it can be traced almost continu- ously to the north-east into Eas Bhacain. Crossing this stream, which coincides with a line of fault having a downthrow to the north, the pebbly limestone or boulder bed is seen forming a scar at about three quarters of a mile from the mouth of the burn. Thence the bed can be traced at intervals in a north-west direction for two miles, more or less parallel with the pebbly edge of the quartzite. The pebbly quartzite occurs mostly in the outliers and along the south-eastern margin of the belt. The beds of dark carbonaceous limestone (c) are best seen in Gleann Domhain just above and below the mouth of Eas Bhacain, near the foot of that stream, and in Oreag Ohlachach. At a waterfall known as Leum na Lie, on the Barbreck River, which runs into Gleann Dom- hain, at about 150 yards above the mouth of Eas Bhacain, thin bands of sandy limestone are intercalated with the quartzites and phyllites. These beds must occupy the centre of the syncline and are thus the highest strata in the area. The pebbly quartzite and limestone boulder bed occur about two miles further down the stream near Turnalt, in sheet 36, where they form the western margin of the belt. North of Gleann Domhain the belt is interrupted by a mass of granite, where the quartzite is much altered, the accompanying phyllites and impure quartzite being altered into biotite-hornfels- rocks. Strips of quartzite alternating with epidiorite occur from the northern margin of the granite northwards to near the Old Red volcanic platform. Folds of the quartzite reappear near the lasg Loch, and to the east of Loch Tralaig. B. N. P. Summary of the SPratigraphy. In the previous chapters descriptions have been given of the several groups into which the crystalline schists have been divided, together with their stratigraphical relations to the bands they respectively succeed. We shall now consider briefly, the general stratigraphical sequence of these deposits as a whole. The Loch Awe group furnishes the most satisfactory proof as regards the stratigraphical position of its individual members. It 60 Metamorphic Bucks. has been shown that the siliceous members overlie the argillaceous and calcareous members of that group. And, moreover, the lower members of that series merge by a natural passage into the Ardrishaig group. And as there is no evidence of an unconformity in the groups which severally succeed one another to the south-east, there are no grounds for restricting the natural order of succession to the Loch Awe and Ardrishaig groups; on the other hand, in the absence of evidence to the contrary, there is no escape from the natural deduction that this order of succession must be extended to cover the entire field of these deposits, and that there is a natural upward succession from the Beinn Bheula schists to the Loch Awe grits. To invalidate this, order of succession it would be necessary to demonstrate the existence of an uncomformity, or that the groups to the south-east are the metamorphosed equiva- lents of bands further north-west brought into their present position by plication or disruption. But further, it has been shown that the siliceous members of the Loch Awe group, and of the groups which succeed the Ardrishaig phyllite series to the south-east, exhibit differences in composition which may be said to be fundamental. For whereas orthoclase is a common felspar in the Loch Awe group, that felspar has not been definitely detected in the south-easterly groups, the felspar phacoids being restricted to oligoclase. No doubt alkali felspars to some extent were originally present, but the soda-lime felspars unquestionably characterise those deposits, while in the Loch Awe group alkali felspars often predominate. It has also been shown that while the Loch Awe limestone and the Loch Tay limestone differ petrologically, their associated rock bands have distinctive characters, the slates in association with the former being of peculiar composition, which are entirely un- represented on the latter horizon. Moreover, the Green Beds which frequently accompany or occur in close proximity to the latter, are never known to occur anywhere near the horizon of the Loch Awe limestone. It has been pointed out that the group of Green Beds, while conforming to a definite stratigraphical position as regards its base and main zone of deposit, nevertheless transgresses those limits, and in its upward succession has been found in the lower parts of the Ardrishaig group, while below its main base it is met with in the Beinn Bheula group. In other words, in the Loch Awe group alone is that phase of deposit entirely unrepresented. This vertical range of the Green Beds binds the various rock groups into a connected whole, and lends further support to the opinion, based on the isoclinal plication of the area, that the deposits form a stratigraphical succession of no great thickness, out of all proportion to the area which they occupy at the surface. While the varied distribution of the Green Beds might lead an unconformity to be suspected, their field relations negative this hypothesis. Not only is the main Green Bed zone strictly con- fined to a stratigraphical horizon, but the base of tha,t zone forms a well-defined boundary along a length of at least fifty miles in this Summary of the Stratigraphy. 61 part of Argyllshire, always adhering to the same stratigraphical position. Moreover, these Green Beds merge into the other sedi- ments amongst which they happen to be intercalated. Probably other lithological types amongst the mica-schists may have an equally extended range, but the more striking appearance of the Green Beds in the iield leads to their ready detection. . The nature of the rock forming the old land surface, from the erosion of which these sedimentary groups have been derived, is not clearly brought out, the pebbles which enter into their composition being confined to grains composed of a single mineral. True rock pebbles are, however, found in the coarse conglomerate referred to as the boulder bed, but so far as this area is concerned the pebbles are restricted to sedimentary rock fragments which may all have been the products of contemporaneous erosion. In the extension of this deposit, however, beyond the limits of this sheet, pebbles of granite are often of frequent occurrence, and these are the only derivative composite fragments that these groups are known to contain. On the whole, therefore, there are some grounds for concluding that these rocks have been derived from the dis- integration of a plutonic complex, and that the various petrological types which they include found their counterparts in that igneous complex. The direct erosion of an igneous rock ofiers a feasible explanation of such peculiar deposits as the Green Beds, which under the action of extreme metamorphism reassume an igneous character. Moreover, the type of gneisses amongst which these Green Beds occur is marked by the predominance of soda- lime felspars, while similar beds in the upper groups, which ate not associated with these Green Beds, are characterised by the abundance of alkali felspar, together with granite fragments of acid composition. Further, the grits of the entire rock series are distinguised by the presence of clear blue quartz grains, a type often found in the older igneous rocks, as in the Hebridean gneiss and many of the epidiorites. The deposits were evidently laid down in shallow water, there being marked oscillation of the sea floor, but whether they are marine or freshwater the complete absence of definite fossil re- mains leaves in obscurity. That, however, they were deposited under similar conditions to those controlling normal sediments we see no reason to doubt. All that can be said definitely regarding their age is, that they must be older than the Lower Old Eed Sand- stone deposits which unconformably overlie them. What that interval separating them from the Lower Old Red Sandstone was, there is no evidence to show, but the old floor on which the latter were laid down is similar to the surface of the schists as we see it to-day. Every character which regional metamorphism has stamped on them had been impressed before the era of the Old Eed Sandstone which has been built out of their detritus. J. B. H. CHAPTER VIII. Older Igneous Eocks. Epidioeite and Serpentine. This group includes an assemblage of rocks ranging in composi- tion from intermediate to basic and even ultrabasic types. So far as this area is concerned it may be said to embrace all the igneous rocks older than the foliation of the crystalline schists. With the exception of some of the ultrabasic types, these rocks are so far linked together that they probably all originally contained plagioclase felspar and augite. Moreover, in their present condition hornblende or chlorite almost invariably occurs as a dominant constituent. Both hornblende and chlorite schists are included amongst the foliated rocks, while porphyrite and andesitic basalt represent the more massive types. This group is widely distributed over that portion of the district which lies north-west of a line drawn from the head of Loch Fyne to the eastern slopes of Glendaruel, which amounts to three-quarters of the area comprised in the sheet. As their intrusive nature may be inferred from their truncation of the sedimentary bands and from the contact metamorphism they have produced, and, moreover, as there are no volcanic agglomerates or ash beds in the schistose sediments, it is probable that they include no contemporaneous lavas.* A glance at the map will show that these rocks are most strongly developed along its central line, which corresponds largely to the trend of Loch Awe. Stratigraphically they range from the Loch Awe group to a little below the group of the Green Beds. Towards the south-east there is a gradual diminution in the number and size of these sills. Further, there is generally a decreasing acidity in chemical composition in the same direction, for the sills on the horizon of the Loch Tay limestone range from basic to ultrabasic types and those in the Loch Awe group from intermediate to basic. This chemical progression is not absolutely uniform, but it certainly is clear as regards the extreme types. Lastly, their petrological characters, dependent on their metamor- phic condition, show a corresponding progression from the margin of the belt to the central axis. While the outer border of the zone, corresponding to the horizon of the Loch Tay limestone, represents the extreme phase of deformation and recrystallisation, the centre of the belt includes rocks in which the original igneous character is entirely preserved. Between these extremes there is every gradation according to the degree of deformation and to the extent of the subsequent recrystallisation. *In the Summary of Progress for 1903, p. 69, Dr. Peach noted the occurrence of contemporaneous volcanic rocks in the Black Schist and Limestone series, south of Tayvallich, in the adjoining Sheet (36). It is possible, therefore, that contemporaneous lavas may also be included among the Dalradian igneous rocks of this area, although evidence of their existence has not been obtained. .a > c« pj nxj o o m tj o.a Tfi XI J^ (D ^^ S.S 0) w a5 -^ ^ o ^M o r^ -d dJ • > ,/- O T3 . (L) 4-. CD in H g"3 ^ 111 -^3 ^c5'B C Ki ° d " i:; 9-a -g 3 in '^ o i-I ^ c^^ Oh 2 u ji; S in 3 a aj P S'S -t- ,-' O 66^ ^^1 [-<>>" "3 c bo lu Epidiorites. 63 THE LOCH AWE SILL. — The most prominent intrusion on the map is the great epidiorite mass which forms a broad belt along the Loch Awe basin, trending generally north-east and south-west. At its south-west end the band is five miles in breadth, narrowing in the centre, where it crosses the loch at Portinisherrich, to little over a mile, when it again expands towards the north-east. The south-east margin, between Kilmichael and Loch Awe, is a rather sharp boundary, except between Loch Fincharn and Loch Awe, where it merges into a group of smaller sills. Between Portinisherrich and Glen Aray the margin is an irregular complex of folded intrusions, many of which can be similarly traced into the main mass. Its north-western edge shows a corresponding relation. The intrusive nature of this sheet is clearly shown by the fact that it reposes indiscriminately on the different members of the Loch Awe group, and produces contact metamorphism on the limestone, black slate and quartzite in turn. The contact metamor- phism does not extend more than a few feet from the margin of the sill, where the rocks have been indurated and hornfelsed so that they no longer split along their structural planes. Argilla- ceous rocks have been converted into porcellanite, and colour banding has been produced in these and the impure calcareous sediments. Moreover, calc-silicates have been often developed in the limestone to such an extent that from four to six feet from its junction it has been converted into epidosite. In a general sense this intrusion conforms to the horizon of the Loch Awe limestone, which it frequently overlies. The sill often contains infolds of sedimentary material. Reference has already been made to the progressive metamor- phism shown by the sediments of the Loch Awe group. In the case of the epidiorite intrusions, there are also striking variations in metamorphism ranging from massive rocks with original structures through various gradations into crystalline schists. While large portions of this sill consist of normal epidiorite it includes other types. The mass, which is sometimes vesicular, varies in composition from basic to intermediate. The dominant minerals are hornblende and plagioclase felspar, with epidote, calcite, quartz and iron ores as the more important accessories, while chlorite may occur either as one of the principal ingredients or as a subordinate mineral. Augite has not been detected, but pseudomorphs after this mineral have been observed in the less altered andesitic types, and uralitic hornblende is of frequent occur- rence. There is every gradation in structure, from a coarse gabbro- like rock to one of the finest texture. The very coarse types are often foliated, consisting mainly of hornblende and felspar, the former mineral occuring in plates often half-an-inch in length, and the felspar in elongated prisms, while epidote is abundant. Zones of finer texture, composed of hornblende and felspar, also occur, in which the rock is massive and compact, and approaches a fine schistose diorite in character. The prevalent type is a felspar-hornblende-schist in which large porphyritic felspars are scattered. With this variety bands 64 Older Igneous Books. of very fine chlorite schist are commonly seen, in which the porphyritic felspars are equally abundant. Alternating zones occur in which the porphyritic felspars vary in their closeness of aggregation, while they are altogether absent in some bands. Similarly, the groundmass of the rock varies both in texture and ia composition, these divergences being often so sharply contrasted as to produce banding. In the varieties where chlorite takes the place of hornblende the rock often occurs as a green schist or slate, which cannot readily be distinguished in the field from a sediment. The porphyritic felspars occur in idiomorphic crystals, but they are often fractured and sometimes rounded at their margins, yet notwithstanding that they often lie in a highly sheared matrix, they have, on the whole, sufiered very little crushing. They frequently measure half-an-inch, and rarely, two inches in length, and in some zones they are so closely packed that there is hardly any matrix. The hornblende sometimes occurs as a porphyritic constituent, but not so commonly as the felspar. While the rock is usually fresh it is often very decomposed, and the tiny felspars which sometimes remain are the sole indication of its igneous character, but when these are absent the igneous origin is entirely obliterated. In this condition the rock is often charged with calcite, arising from the breaking down of the lime- bearing hornblende and felspar, and partly, doubtless, from the large limestone-masses with which it is associated. That these green calcareous schists are alteration products of the epidiorite there is abundant evidence to show, as cores of normal epidiorite may be seen to pass gradually into them. Bpidote is commonly distributed throughout the sill as grains in the rock, or in a more massive condition in nodules, veins, and strings, intermixed with quartz, and forming epidosite. Cores of such epidosite, from a few inches to a foot or more in their longest diameter, pervade every variety of the rock. Tiny crystals of albite, similar in appearance to the albites occurring amongst the mica-schists in Cowal, have been occasion- ally observed, both in the unaltered rocks of andesitic type and in their metamorphosed equivalents. The country bordering Inverliver on Loch Awe comprises the least altered rocks of the district, both igneous and sedimentary. At this place the sill shows least alteration, the porphyritic felspars have suifered little or no deformation, and the rock has none of the characteristics of an epidiorite, but preserves structures which are truly igneous, and appears in the field as bands of andesite or andesitic basalt divided by others which are more sheared. Seams rich in porphyritic felspar alternate with others not porphyritic, and contain cores of epidosite. In fact, apart from condition of metamor- phism, they are identical in the field with the banded rocks already described. With these compact seams are alternations of the same material but very vesicular, some of the vesicles being filled with quartz and others with calcite, while some cavities are only lined on their walls, and others again are empty. Vesicular types are Upidiorites. 65 not confined to the unaltered rocks, but are equally abundant in the schistose varieties, where they have been noted here and there along the whole length of the sill, and in some of these latter, where these cavities have been filled with blue quartz scattered broadcast over the rock, the resemblance to quartz pebbles is very striking. The passage from these normal igneous rocks to typical crystal- line schists as seen in the field is fully supported by their appear- ance under the microscope. A specimen from Tnverliver (9,274) is seen to be an augite-andesite or andesitic basalt with vesicles filled with calcite, sometimes surrounded by a thin film full of magnetite, which probably represents an original glassy border. It contains idiomorphic phenocrysts of plagioclaso in a ground- mass of felspar microlites, carbonates possibly after augite, a little biotite, chlorite, and interstitial matter containing minute grains and skeletons of magnetite. The rock is entirely unsheared, and except for its decomposition has been otherwise unmodified. (Plate VII., fig 1.) A specimen from a non-porphyritic band (9,278), although much decomposed, points to the rock having been originally a fine- grained, perhaps partly glassy andesite which has undergone practically no shearing, the vesicles filled with granular quartz, chlorite, and acicular actinolite being often perfectly rounded, and cataclastic structures are very inconspicuous. The matrix consists now of chlorite, actinolite, epidote, finely granular felspar, quartz and dusty iron oxides. The vesicles form almost half of the rock. The next specimen (9,277), from a non-porphyritic band in which vesicles are absent, is also an andesite or andesitic basalt with a crystalline groundmass. The following slide (9,276) is a porphyritic andesite of essentially similar type, but in which there are slight traces of shearing ; some of the felspars are broken, while others have their twinning planes bent, indicating a certain amount of mechanical movement. (Plate VII., fig. 2.) In 4,367 the deformation has been carried a step further, the porphyritic felspars being often disjointed and fractured. The groundmass, which is very much decomposed, con- tains lath-shaped plagioclase, chlorite, epidote, and calcite, and so far as regards its mineral composition is very similar to some of the calcareous chloritic felspathic rocks of the normal schistose type. Although these mineral changes may be partly secondary, it is evident that the rock has sufiered more shearing than those just referred to. (Plate VII., fig. 3.) The next specimen, however (5,525), is interesting, because it contains a considerable amount of hornblende in the groundmass. The principal ingredient is plagio- clase felspar in lath-shaped crystals, quite irregularly disposed and very much resembling the groundmass of a non-fluidal igneous rock. Between the felspars there lie a fair amount of chlorite, fibrous or acicular actinolite, and some biotite in irregular scales or clusters. Epidote is fairly common, and there are minute granules which appear to be leucoxene after titaniferous iron oxides. The rock contains large porphyritic crystals of felspar three-eighths of an inch across, which are not represented in the slide except by a few small 66 Older Igneous Rocks. phenocrysts. No steam cavities are apparent. The rock has been originally a holo-crystalline granular porphyrite or andesitic basalt. Evidence of shearing is afforded by the occasional flexure or fracture of the felspars, which is most pronounced in certain bands of the slide. (Plate VII., fig. 4.) The specimen now to be described (4,366), which contains similar felspar phenocrysts, has reached the stage of a hornblende-schist or epidiorite, in which those phenocrysts are almost the sole remnants of the original igneous structure. The matrix consists of lath- shaped plagioclase felspar, chlorite, epidote, and a little hornblende, and has a strongly marked parallel arrangement due to shearing, though fluxion bands may have been originally present. Some large patches of calcite have the appearance of draw^n out steam cavities. The schistose structure has been sufficiently developed in the rock to mask its original igneous character. (Plate VII., fig. 5.) The last specimen to be referred to in this series (5,522) has been still further deformed ; the porphyritic felspars while yet preserving their individuality have been wonderfully shattered by the forces producing the schistosity, and the fragments are now separated by veins and patches of chlorite and carbonates, and a granulitic felspar mosaic. They lie in a finely foliated matrix of granulitic felspar, chlorite, hornblende, epidote, calcite, and granular sphene. In one or two places, however, and more especially in the wake of the phenocrysts, the felspars still retain their original lath-shaped rectilinear form. The felspar phenocrysts contain some epidote and secondary mica. Between the south-east margin of the Loch Awe sill and the coast of Loch Fyne smaller intrusions of epidiorite are everywhere distributed. Towards the south-east, however, they show a gradual diminution, except in a tract between Loch Glashan and Crarae. Their general tendency is to form sill-like bands parallel to the strike of the schists, but they frequently occur in complex shapes. Notwithstanding their frequent variation in petrological type, most of them find their counterparts within the complex of the Loch Awe sill. The more acid types of that sill are, however, of less frequent occurrence, and while rocks of intermediate composi- tion undoubtedly occur, the type in which felspar is the sole porphyritic constituent is restricted to the zone which borders the Loch Awe sill. As we approach Loch Fyne both felspar and horn- blende are porphyritically developed in varying proportions, but over the Loch Fyne belt generally, hornblende is usually the princi- pal and, in many cases, the sole porphyritic constituent. They present every variation both as regards texture and foliation, and many have broken down completely into fissile hornblende or chlorite schists, the latter being frequently calcareous. The coarser-grained rocks may contain crystals of hornblende and felspar up to an inch in size, with crystals of actinolite sometimes two or three inches long, and they have frequently a nodular appearance, due to a crystalline aggregation of hornblende and felspar, in shapes which rudely correspond to the outlines of Epidiorites. 67 hornblende crystals. The coarser-grained rocks are especially well developed in the neighbourhood of Loch Glashan and they extend across Loch Fyne between that loch and the valley of Strath- lachlan. These coarse-textured masses in the Stralachan area range in composition from rocks that were originally gabbros to others which correspond to quartz-diorites. A specimen of very coarse-grained epidiorite of the former type (4,000) shows pale-green actinolite, compact and massive, with irregular outlines, lying in a matrix of felspar which forms a mosaic of granules, partly polysyn- thetic and partly untwinned ; the felspar is filled with epidote and zoisite, and is often penetrated by tiny needles of actinolite. Chlorite, and leucoxene after ilmenite are both present. This rock shows practically no schistosity. An epidiorite (4,005) from the same area contains much quartz, generally of the clear blue variety, and large felspars ; dark green hornblende and brown biotite are both common, and there are large masses of iron ore weathering to sphene or leucoxene, and much epidote. It appears to have been originally porphyritic, and the porphyritic felspars are exceedingly well preserved ; they are turbid with minute cavities and tiny flakes of hornblende, and the felspar is not filled with epidote to anything like the extent of the last specimen (4,000). The groundmass, moreover, is quite granu- litic, but more acid than the preceding, and the rock would correspond to a quartz-diorite. Many of these coarse varieties show evidence of crushing, but the foliation is so very imperfect that their general character is massive rather than schistose. Some of the more acid types contain felspar crystals in Carlsbad twins, which to the naked eye have the appearance of orthoclase. Uralitic hornblende has sometimes been observed, but augite has no\s'here been detected. These coarse rocks of Stralachlan show rapid alternations in composition ; some zones consist almost entirely of stumpy prisms of dark hornblende closely packed together, while in others felspar is the predominating mineral, and, again, other zones may contain equal quantities of these minerals. Epidote is strikingly developed in all these rocks and often forms cores of a bright yellow colour, which are con- spicuous objects in the field, even when they contain a plentiful admixture of hornblende crystals, in which the granular epidote acts as groundmass. In some cases the cores are simply portions of the epidioi'ite, in which epidote is largely in excess, while in the finer-grained hornblende schists they are restricted to epidosite, in which granular epidote is intermixed with a very small proportion of quartz and carbonates. The margins of these very coarse epidiorites are frequently much sheared, and in some cases broken down so completely that the resulting rock is a highly fissile chlorite-schist in which hornblende is almost entirely absent, and presents many points of similarity to some of the Green Beds (4,006). Some of these extremely coarse epidiorites have so far resisted metamorphism that in parts they resemble the post-Dalradian 68 Older Igneous Rocks. diorites, a resemblance which microscopic investigation does not diminish. A small epidiorite boss at Barr an Longairt, half a mile north by west of Castle Lachlan, contains a small acid zone of the nature of a contemporaneous vein which might readily be taken for a later diorite dyke. Bpidote, however, in the condition already described, pervades the vein and enclosing rock. The vein is coarsely crystalline and highly felspathic, with large well developed plagioclase felspars and a smaller quantity of green, rather fibrous, hornblende. Under the microscope (10,566) the rock is seen to be comparatively free from shearing, the plagioclase felspars are very idiomorphic, and though filled with yellow grains of epidote are so fresh as to show perfectly all the details of the twinning. Probably a small amount of orthoclase, less perfectly formed than the soda-lime felspar, is present, and a little quartz. The hornblende, which is not abundant, is fibrous, and is mixed with much epidote. Apatite is common, and iron ores scarce. There is a great abundance of granular sphene. The nature of the sphene, hornblende and epidote is suggestive of the epidiorites, but the idiomorphism of the felspars would otherwise have more closely allied it to the newer diorites. A specimen from the normal epidiorite in close contiguity to the last clearly shows that both types belong to the same phase of intrusion. Under the microscope (10,567) it is seen to be on the whole very similar to the preceding, but more distinctly porphyritic, and contains large idiomorphic plagioclase crystals and horn- blende in a matrix of finer felspar and chlorite. The hornblende is uralitic ; epidote is very abundant, and associated with it is much chlorite, and there is also granular sphene. Another specimen (10,568) is similar, but contains more hornblende and is more sheared, but the original igneous structures are still clear. The south-eastern margin of the epidiorite group, which corresponds approximately to the horizon of the Loch Tay lime- stone, is represented by an assemblage of epidiorite sills of a more uniform basic character, which never attain the same coarseness of texture as the sills which are intruded in the Ardrishaig and Loch Awe groups. The more acid felspathic zones of the latter groups are entirely absent, and when they reach a texture which is comparatively coarse, hornblende is always in excess of felspar, and in some cases is porphyritically developed. These rocks are darker in colour owing to the deeper hues of the hornblende, which is frequently black as seen in the field, in contrast to the brighter green colours of that mineral in the upper zones, where it commonly occurs as tremolite. Chlorite in comparison to hornblende is insignificant in its occurrence, and calcite is rare, while biotite is a common constituent. Garnets, moreover, are often seen in the epidiorites of this horizon.* * Such examples of garnetiferous hornblende-schist may be seen in the Loch Tay limestone on the west side of Ardno Bum ; three-quarters of a mile south-east of Ardno, and in the sill which underlies a limestone slightly south-east of the graphite-schists between Ardno and Strachur. E2Jidiorite and Serpentine. 69 These rocks are usually extremely foliated, and their recon- struction is of greater intensity and diffusion than in the types previously described. In the most schistose types the hornblende is represented by dark needles of actinolite. These sills occur in well-defined bands with a marked parallelism to the strike of the sediinents, and they are singularly free from the irregular margins which distinguish these rocks to the north-west. The Loch Tay limestone is almost everywhere associated with epidiorite intrusions, either as a single band or as a set of parallel bands. That their repetition is due to folding is in many cases clearly brought out by the mapping, as along the M^estern slopes of • Glendaruel. The extreme south-easterly limit of the group is seen near the base of a thick zone of Green Beds in Glenbranter, on the slopes of Cruach nam Mull, and on the western side of Binnein an Fhidhleir. On the western slopes of Glendaruel, serpentine occurs in an extensive sill, with irregular boundaries, determined by the folds of the strata in which it has been involved, so that it is now represented by a series of lenticular masses, the largest of which is one and a quarter miles in length and over a quarter of a mile in width. The total length of the zone exceeds two miles. Its stratigraphical position is slightly above the Loch Tay limestone and near the south-easterly margin of the garnetiferous mica-schist group. While the smaller patches are foliated throughout, the large mass has been sheared only at its upper margins, and it is mostly in the condition of a massive serpentine which, even under the microscope (4,004), shows very little evidence of shearing. The mesh structure is often well exposed, and it is blackened by grains of magnetite. One of the smaller masses however, shows a transition from serpentine to epidiorite (4,368), in which masses and groups of pale green hornblende crystals are lying in a matrix of highly saussuritised felspar. The epidiorite has probably been derived from a gabbro. Although serpentine is restricted to this locality, rocks of extreme basic type occur amongst the epidiorite group further to the north- east, where they have been observed on both sides of Loch -Fy^®' ^^ the vicinity of Inveraray and at St. Catherine's. Some are so unctuous to the touch as to be locally known as potstone, a property due to the presence of talc. An actinolite schist from Three Bridges (Glen Aray) is seen under the microscope (5,520) to consist of long blades of green actinolite lying in a matrix of fibrosis chlorite which is intermixed with talc ; the latter mineral occurs in colourless scales showing straight extinction and very high polarization colours. With the exception of some iron ores, carbonates, and leucoxene, these are the sole mineral constitutents of the rock. It contains no felspar, biotite, or epidote, and in all probability has been derived from an ultra-basic intrusion. A sill of similar chara(;ter (5,521), but containing epidote and a small quantity of quartz and felspar, but no talc, occurs near Dalchenna, and, although not so basic as the last, represents a stage where basic material is verging towards ultra-basic. 70 Older Igneous Bocks. A band of the potstone type at St. Catherine's is a chlorite schist with a slight amount of hornblende in small fibres (984). A colourless scaly mineral yielding high polarization colours is apparently talc. Magnetite occurs both in large and small grains. In addition to these minerals a carbonate is present which was tested both in cold and hot acid and found to be dolomite. Another specimen from the same locality (985) contains abundant horn- blende associated with chlorite, much magnetite, and a few grains which may be felspar, but that mineral is practically absent. It is doubtful whether talc is present, and if so, it cannot be abundant. Dr. Flett suggests that this rock may probably have been some form of pyroxenite, or some peculiar modification of a gabbro. Although these epidiorite intrusions have not been folded to the same extent as the sedimentary schists, they have been sufficiently involved in the earth movements to obscure their original dimensions. The average thickness of the sills on the Loch Tay limestone horizon may be about thirty feet. The Loch Fyne intrusions between Strathlachlan and Loch Glashan must, in many cases, be at least 100 feet, while the Loch Awe sill, which often uninterruptedly spreads over very uneven country for miles, must considerably exceed these dimensions. To the west of Ford a ridge, made up of epidiorite, has been truncated by deep clefts in a direction transverse to the strike and to the axes of folding, leaving a chain of isolated peaks the walls of which furnish a set of gigantic transverse sections that present an unbroken mass of epidiorite with a vertical range of at least 400 feet. At the head of Glen Aray, in the extreme north-east, the epidiorite, which there forms a steep mural escarpment, shows an equal thickness. J. B. H. Glen Domhain and West of Loch Avich. — In this district there - is a large development of sills of epidiorite which can be separated into two main types — (1) a fine-grained variety, rich in chlorite and epidote with porphyritic crystals of plagioclase felspar, which appears to have been derived from a compact basic rock ; (2) a type, with many porphyritic crystals of plagioclase, which may probably have been originally an andesite or andesitic basalt. (1) The sills of this type are usually highly vesicular at their margins, and in many cases the vesicles seem to have been filled in prior to the movements which deformed the rocks, for even in the schistose varieties the vesicles often remain almost globular. These intrusions are usually associated with the phyllites, black slates, and black carbonaceous limestones (sub-groups a,&,c,page 57) of the Loch Awe group, and appear to have produced little or no contact metamorphism on these sediments. For this reason, but more especially from their fine texture and highly vesicular margins, they suggested the idea in the field that they might be lava flows, as the least deformed portions of the rocks closely resemble the diabase lavas of Arenig age of the Southern Uplands of Scotland. No pyroclastic rocks have as yet been found associated with them. Indeed, in this region conclusive evidence on the question of the probable contemporaneous origin of some of E]jidiorites. 71 these vesicular rocks has not been obtained. The thickness of many of the sills and their wide extension point rather to the intrusive nature of the series as a whole. Rocks of this type floor the greater part of the present area, their' typical development being found in the ridge which forms the watershed between Gleann Domhain and the basin of Loch Awe. (2) Examples of the second type of epidiorite intrusions are found in Gleann Domhain in association with the quartzites and limestones. Owing to the marked contact metamorphism produced by the later granite masses and other intrusive rocks, it is difiScult to determine the local alteration that may have been caused by the epidiorite sills. Further south-west, however, away from the influence of these younger intrusions, they develop contact metamoi-phism, especially in the case of impure calcareous bands which they convert into calc-silicate hornfels. A good example of this is seen on the shore at Duntroon Castle on the north side of Loch Crinan. B. N. P. CHAPTER IX. Folding and Pkogeessive Eegional Metamoephism. Amongst the schistose bands within this sheet folding of an isoclinal type may be said to be universally prevalent. The lateral pressure to which these rocks have been subjected has resulted in the production of a system of folds, which on a broad scale display a certain uniformity, by the approximate parallelism of the folding axes with the strike of the bands, and by a corresponding uniformity of the hade of the axial planes in large areas. The variations in the hade of the folded limbs have produced a system of pseudo-anticlines and synclines, that on a large scale affect considerable areas, and on a small scale may form exceptional features within belts of country in which they play but a sub- ordinate part. The general strike of the folding axes is north-east and south- west, and the general dip of the isoclinal folds has a direct relation to two great central axes, one of which corresponds approximately to the south-western end of Loch Awe, and the other to the central belt of the district of Cowal. While along the former axis the isoclines are vertical, and gradually diminish in inclination on either side towards the north-west and the south-east, along the Oowal axis the isoclines are often nearly horizontal, and this broad axial zone has been referred to as the "flat belt." The cleavage planes which have been set up by the same stresses display corresponding variations in dip. The resistance to stress has been unequal, and these two axial lines correspond to lines of minimum and maximum strain, so that while the Loch Awe axis marks a region where the folding and cleavage correspond to a simple set of isoclines and foliation planes, the "flat belt" of Cowal constitutes the central axis of a great zone where the isoclines and foliation planes have been contorted. Moreover, " the flat belt " represents an horizon of extreme metamorphism diminishing in intensity towards the region of Loch Awe, which presents the least alteration. The correspondence between the degree of folding and the degree of crystallisation possesses a significance which will be referred to again. For convenience of description the areas on either side of Loch Fyne will be treated separately. North-west of Loch Fyne. — In that part of the map lying to the north-west of Loch Fyne the Loch Awe group occupies the greater portion of the area. This sedimentary series, which is mainly con- fined to the Loch Awe basin, is resting in a compound syncline upon the Ardrishaig group, its central portion corresponding Folding and Progressive Regional Metamorphism. 73 approximately to Loch Awe. This is brought out by the behaviour of the Loch Awe epidiorite sill, to which attention has already been drawn, which has an outcrop five miles in breadth. Moreover, the Loch Awe limestone, that is associated with this sill between Kilmichael and Loch Awe, occurs in a series of broad bands over an area of at least three miles, some of the individual members of which exceed a quarter-mile in width. That these bands owe their repetition to folding is clearly seen in the neighbourhood of Fincharn, where they are connected by sharp terminal folds. The Loch Awe group is not only isoclinally folded, but the fold- ing shows a fan shaped-arrangement.* In the centre of the group the axial planes are vertical, while on either side to the north-west and south-east, the hade of these planes gradually recedes in opposite directions. But as the Loch Awe group is lying in a gentle but irregular trough, the central portion of the group is, broadly speaking, horizontal, whereas the individual beds are folded into vertical limbs. As we proceed outwards on either flank from this central belt, this extreme discordance between the folding axes and the general bedding gradually diminishes. We find, therefore, in the Loch Awe area, firstly, a compound syncline involving the group as a whole; secondly, a series of isoclines which have afiected the individual beds without seriously modifying the compound synclinal folding of the group ; thirdly, a pseudo-syncline or fan-structure brought about by the arrangement of the isoclines, their axial planes dipping on either flank towards a central axis where those planes are vertical, the latter also to the same extent being a pseudo-syncline or fan- structure of foliation. The foliation dip is of no value either as showing stratigraphical relationship or as an indication of the curvature to which the planes of foliation have been subjected. The folds are shallow, as may sometimes be seen even in com- paratively thin bands where, though the axial planes are perpendicular, the margins of the bands present very little irregularity, and unless seen in cross section the plication would not be suspected. The interfolding is admirably illustrated by the marginal relations of the Loch Awe epidiorite sill with the sediments, the boundary often being represented by a complex set of sub-parallel tongues emerging from the main mass, while the interbanding within the igneous mass itself furnishes similar evidence. The intimate nature of the isoclinal folding is particularly well exemplified by the relations of the epidiorite sill to the adjoining sediments near Arichamish, where instead of a normal junction between the two rocks, the line of demarcation is represented by a complex of minute folding, strips of fine grit alternating repeatedly with strips of porphyritic epidiorite. The bands of alternation, both sedimentary and igneous, range from an inch to a foot in thickness. That the banding is due to reduplication by folding * The fan-shaped arrangement of the strata in the Loch Awe district has been described by Mr, Peter Macnair in his paper on "The Building of the Grrampians," Proc. Roy. Philosoph. Soc. Glasgow, vol. xxxiv., p. 147. 74 Folding mid Progressive Begional Metamorphism. may be readily seen by the " nosing out" of the folds, though the section presents an appearance of perfect regularity with the dip highly inclined. This composite zone extends for many yards, and may be seen in transverse section along the scar faces. About half-a-mile north-east of Loch a' Mhinn a similar banded complex represents the margin between the epidiorite and sediment, where the composite zone is of far greater extent than at Arichamish, and contains bands as narrow as an inch in breadth. The preservation of such minute interfolding is due to the fact that the bedding is approximately horizontal, with steeply inclined folds, so that the original surface of the bed is exposed, and the even " planing " of its contorted margin has furnished in ground plan an exact counterpart of the distortion it has undergone. Such a finely divided complex indicates an extremely shallow type of folding, and in some cases by a gradual diminution of their amplitudes the isoclines may even die out altogether, and horizontal beds have been merely traversed by cleavage planes, as may frequently be seen amongst the grit bands in the neighbourhood of Loch Avich. Even where the isoclines reach their greatest development in the Loch Awe district their amplitude cannot be great, or the individual bands in close association, such as the limestone and epidiorite, would not form such broad outcrops. While to the south-east of Loch Awe both the axial planes of the isoclines and the planes of foliation hade to the north-west, on the opposite aide of that loch both sets of planes lose their com- parative uniformity and may incline in opposite directions. Between Inverinan and Barnaline, where the bedding planes are gently inclined to the south-east, the foliation is often highly inclined to the north-west, while in some instances it hades steeply to the south-east. Near Loch a' Mhinn, while the bedding planes dip at high angles to the north-west the foliation planes are steeply inclined in directions varying from north-west to south-east. Along the Dun Dubh range the foliation varies from vertical to planes inclined at about 70° in either direction. North-west of Loch Awe the isoclines are usually free from contortion and the foliation planes are in a similar condition, although they are sometimes puckered, but never to the same extent as in the area to the south-east. The irregularity in the isoclinal arrangement is apparently due to the varying degree of compression to which the beds have been subjected; where lateral stresses have been most severely felt the axial planes have been most uniformly disposed, while, on the other hand, where stresses have been less intense the compression has admitted of an uneven type of plication corresponding more nearly to normal folds. This conclusion is supported by the regional metamorphism which affects the area. It has already been pointed out that within the limits of this sheet we find every variation from grits and slates, in which the original clastic condition has been com- paratively unaffected by compression, to granulitic gneisses in which the clastic origin has been completely obliterated. Further, Folding and Progressive Regional MetamorpMsm. 75 we see a like variation in the igneous rooks, from those in which their original igneous character has been completely preserved to schists and gneisses which have been entirely metamorphosed. The least altered rocks are found on the horizon of Loch Awe, while the schists and gneisses that occur in the central belt of Cowal are of an advanced crystalline type; the intervening area being occupied by schists in an intermediate condition of crystallisa- tion. While, broadly speaking, the extreme type of metamorphism is found in the flat belt of Oowal, the least altered rocks are con- fined to the belt flanking the north-west of Loch Awe, which corresponds with the pseudo-syncline of foliation ; that is to say, where deformation and crystallisation are least developed the folds have been least compressed, and instead of the uniform synclinal hade seen south-east of Loch Awe the folded limbs have been free to dispose themselves with less regularity, and in some cases the plications are so feeble that they may even die out altogether. On the other hand, the extreme metamorphism of the " flat belt " is seen in beds in which the folds have not only been compressed but in which these earlier flexures have themselves been subjected to renewed movements, which have not only plicated the isoclines with their related foliation planes, but in some cases have almost succeeded in obliterating these structures. In Chapter VII. it has been shown that the sediments of the Loch Awe group include grits and slates which range from rocks resembling those of normal Palaeozoic type, containing clastic micas, to rocks in which the metamorphism has never proceeded sufficiently far to obliterate the original clastic condition, so that true crystalline schists are only exceptionally met with. It has also been shown (Chapter VIII.) that the igneous rocks associated with them have a corresponding range in metamorphic condition, beginning with normal igneous rocks and ending with crystalline schists ; and their transitional stages have been completely demonstrated by a chain of intermediate types supported by microscopic investigation. It has been likewise shown that the Ardrishaig group, which lies to the south-east, represents an advanced degree of crystallisa- tion that is accompanied by a greater degree of folding, by which the original thickness of individual quartzite bands has been greatly modified, according to the uneven distribution of compres- sion over the respective parts of the fold. Still further to the south-east the mineral alteration has pro- ceeded yet farther in the band of garnetiferous mica-schists, for not only are the minerals which constitute the planes of foliation of coarser crystallisation, but garnets are also of sporadic occurrence through the rock, with every variation in their relative size and closeness of distribution. Moreover, the folding also exhibits greater contortion. Finally the members of the Beinn Bheula group show a still further increase both in deformation and degree of crystallisation, and while their clastic origin may still frequently be seen when studied in mass in the field, yet the rocks have been so completely 76 Folding and Progressive Regional Metamorphism. granulitised that tinder the microscope no trace of their clastic condition can generally be detected, and they appear as granulitic gneisses. In this group the garnets no longer occur as con- spicuous constituents, but on the other hand great bands may be traced in which albite plays a similar rdle. Moreover, biotite is more conspicuous than in the groups to the north-west. The Beinn Bheula group, which exhibits the greatest degree of meta- morphism, also displays the maximum amount of folding. In this group we have represented an anticline of foliation, but whether it constitutes a normal anticline of foliation or a pseudo-anticline corresponding to the pseudo-syncline of Loch Awe need not at present be discussed, as it will be referred to in more detail in the section dealing with the south-east of Loch Fyne. Sufficient, however, has been said to show that there is a progressive increase in metamorphism from the centre of Loch Awe to the central belt of Cowal, or in a direction proceeding from north-west to south- east. Further, the former carries us back to the very initial stages of the metamorphic processes, so that we may study regional metamorphism from its very beginning to the highly advanced stage of crystallisation represented by the granulitic gneisses of Cowal. This progressive metamor])hism, however, pursues a direction transverse to the strike of the sedimentary groups, so that its extreme ejHects are not confined to the limits of a single strati- graphical series. In the case of the Loch Awe group the chain is so far complete that comparatively unaltered rocks, sedimentary and igneous, may be followed by a series of transitional stages into crystalline schists, in which the foliation on the whole is not of an extreme type. But in the successive stratigraphical groups which succeed it to the south-east, progressive metamorphism represents an advance in crystallisation from rocks whose least altered equivalents are already in the condition of crystalline schists. Besides, however, a progressive metamorphism %'ery gradual in its advance which obtains in a south-easterly direction from Loch Awe to Central Cowal, we have described also a general increase of crystallisation from south-west to north-east which, so far as it appertains to the geology of this sheet, is subordinate to the former. In a publication previously cited* it has been demonstrated that the Loch Awe group in the adjoining area to the north passes laterally into crystalline schists of an even more advanced stage of metamorphism than that displayed by the extreme type of crystallisation in Cowal, and that the Ardrishaig group shows a similar lateral passage from the very initial stages of metamorphism to a like type of extreme crystallisation. The progressive metamorphism towards the north-east in the area contained within this sheet converges to a focus of extreme crystallisation, located in the north-east corner of the map between *Q.J.G.S., 1899, vol. Ixv. Mr. J. B. Hill on "Progressive Meta- morphism in the Region of Loch Awe." Folding and Progressive Regional MetamorpMsm. 77 the river Fyne and the Brannie Burn, a region which is marked by a belt of plutonic intrusions of a later age than the regional metamorphism of the district. The Loch Awe group in its extreme north-easterly extension between Glen Aray and Glen Shira is some considerable distance from this metamorphic centre, nevertheless it there shows a marked difference in metamorphism from its equivalents on Loch Awe. The coarser grit beds have so far been altered that the individual pebbles are flattened and elongated, and the group as a whole is markedly more foliated. The associated epidiorites are correspondingly affected, so much so that the andesitic rocks of Loch Awe no longer appear. The Ardrishaig group lying closer to the metamorphic focus has suffered more alteration and, as described in Chapter VI., suddenly increases in crystallisation at the head of Glen Shira, the advance in metamorphism being accompanied by puckering of the foliation planes veiy similar to that of the argillaceous beds of the central belt of Oowal. A special belt of metamorphism within the Ardrishaig group has produced a band of biotite-gneiss in which the foliation planes are less conspicuous. These peculiar gneisses, however, probably owe their special character to thermo-metamorphism from under- lying plutonic rock connected with the later intrusions which have already been alluded to. As the remaining groups to the south-east, from the garnetiferous schists to the Beinn Bheula schists, are already in an advanced state of metamorphism south of Loch Fyne, their extension into the north-east district is not accompanied' by such an abrupt increase in crystallisation. The Green Beds, however, form an exception, and perhaps the most pronounced metamorphic variation which this north- eastern region presents is the replacement of the chloritic beds of Gowal, forming the Green Bed group, by hornblendic rocks. In the description of that group it has been shown that on the north side of Loch Fyne the clastic structures have been almost completely obliterated, and that, with this change, hornblende has been developed in gradually increasing amount until, as the north-east of Glen Fyne is approached, the Green Beds are represented by hornblendic schists. Further, these hornblende-schists have assumed a structure so typically igneous that even with the assistance of the microscope individual specimens cannot be dis- tinguished from the Dalradian intrusive rocks. In brief, the entire stratigraphical sequence, which displays such a wide range in metamorphism to the south-west, when brought within this focus, assumes a comparatively uniform crystalline con- dition ; so that the progressive metamorphism which the rocks have undergone has been in inverse ratio to their metamorphic condition beyond that centre of extreme crystallisation. J. B. H. South-east of Loch Fyne — Near the middle of the area occu- pied by the Beinn Bheula series there is a tract of ground in 78 Folding and Progressive Regional Metamorphism. which the foliation is on the whole nearly flat, though much corrugated hj small anticlines, which may be referred to as the flat belt. Its breadth varies considerably in different places, and generally increases in a north-east direction ; on the west side of Loch Eck, for example, it is about half a mile broad, on the east side of the Loch Goil valley it is two or three miles, and on the north-west side of the main belt there are narrower belts in which the foliation is also nearly flat. The axial planes of all the folds in the belt appear to strike N.N.B., and have little or no pitch, but they are not all of the same age, and the strain-slips formed in connec- tion with some of the earlier folds have often been sharply twisted. On the E.S.E. side of the belt the dominant foliation generally dips S.S.B. at angles greater than 45°, while on the W.N.W. side, excepting near the granite at the north-east of the map, it generally dips W.N.W. at lower angles, often about 20°. The belt thus appears to represent the central portion of a great anticline, the axial plane of which inclines rather steeply W.N.W. Very many minor folds within the belt have also axial planes hading rather steeply W.N.W., and their under limbs are often slightly more thinned and altered than the upper. For some miles, too, on either side of the belt, the schists are afiected by small folds of the type described.* The early observers had no doubt of the existence of a great anticlinal fold with an axis running along the flat belt, but it is clear that, even if there be such a fold, it cannot be one of a simple character, and further, that it is an anticline of foliation and not of bedding. This conclusion is evident from the want of correspondence in the beds on either side of the flat belt. There is evidence also on the hilltop south-west of Oarrick Castle of a great isoclinal syncline immediately south-east of the flat belt, and with both limbs dipping south-east. By means of this syncline a thick band of albite schist forms most of the hilltop on Oruach a' Bhuic, Beinn Bhrea, Sgor Ooinnich, and Onoc na Tricriche, being nearly flat in these localities. But where it assumes an E.S.E. dip it gradually thins away and runs down towards Carrick Castle on the one side and Glen Finart on the other, and on its E.S.E. side is apparently overlaid by a series of thick massive grits which closely resemble those which come below the albite schist on Cruach a' Bhuic.f Numerous small flexures, in and near the flat belt, fold not only the foliation planes but also most of the quartz veins and many lines of strain-slip. It is evident that the strata had previously been flexured before the strain-slips were folded, and it is suggested that perhaps the axial plane of the syncline mentioned may also have been folded by the great anticline which appears to have afiected the beds.J * Geology oj Gowal, Plate II. , page 21. ilbid., Fig. 47, page 204. J This view is tentatively advanced in the Memoir on the Geology oj Gowal, p. 86. Folding and Progressive Regional Meta/morphism. 79 It is also possible, however, that the syncline may not be part of a structure which has been bent by an anticline, but that it may be of the same age as the arches on the north-west side of the flat belt, and that the dominant foliation planes which dip south-east may have been produced by pressure from above, acting from a south-east direction. The view that in this locality there is no large anticline, even of foliation, was first advanced, we believe, by Mr. Peter MacNair.* It has the merit of being comparatively simple, and seems to accord well with structures lately discovered by Mr. E. H. Cunningham Craig in the adjoining one-inch map 38, where on the east side of Loch Lomond, and near Loch Katrine, various synclines have been found, the south-east limbs of which are in some places vertical, while in others thej'' dip north-west in an opposite direction to the north-west limbs.-|- In the earlier folds which were in existence on the south-east side of the Carrick syncline before the production of the puckers and folds which are well developed near the flat belt, the axial planes generally incline E.S.B. at nearly the same angle as the foliation, and it is the upper or longer limbs of anticlines that are the most thinned and altered. This is seen, for instance, on the coast near Toll nam Muc and Carraig nan Roin. In most of the * ' ' The Altered Clastic Rocks of the Southern Highlands, their Structure and Succession." Geol. Mag., 1896, pp. 167 and 211. t Since this Explanation of the one-inch map 37 was written, Mr. MacNair has further explained his views in his paper on "The Building of the Gram- pians" (Proceedings Phil. Sec. Glasgow, 1903), and has illustrated them by a map and horizontal section across the Cowal area. In describing a traverse from near the head of Loch Awe, on the north-west side, to Dunoon on the south-east, he says (p. 76): — "As we approach a Une running down the centre of Cowal, through the heads of Loch Goil, Loch Striven, and Loch Ridden, and the hiUs to the north-west of Tighnabruaich, we find the axes of the isoclinal folds approaching the vertical." This statement gives, in my opinion, an incorrect representation of the facts as regards the heads of the three lochs mentioned. If Mr. MacNair's views of the tectonic structure were correct, the axes of the isoclinal folds in the localities mentioned would be more nearly vertical than the axes of the folds further north-west, and he implies that they are so. Such, however, is not in accordance with my ob- servations in the field, and, on pp. 226-227 of the Cowal Memoir, I have mentioned various localities, the examination of which compelled me to sup- pose that the axial planes of the main folds in these localities were approxi- mately horizontal. This is reaUy the crucial point of difference between us. I should like also to take this opportunity to correct a statement made by Mr. MacNair, in the same paper, through some misapprehension of my meaning, that in the text of the Cowal Memoir "32,000 feet is given as the approximate thickness of the beds from the boundary fault at Innellan to the centre of the anticline." It is expressly stated in the passage referred to that this is an estimate ' ' of the approximate thickness of the beds measured across the early foliation planes." In other words, it is an estimate of the thickness of the mass of rock material existing after the beds had been foliated — that is to say, after they had already been greatly folded. This estimate was introduced, not with the view of arriving at the original thick- ness of the strata — a task never attempted by me — but in order to suggest a possible explanation of the high grade of metamorphism found in the fiat belt. It was argued that such may have been due to the earth's internal heat, owing to the load of folded rock material which may at one time have covered the strata now exposed along the flat belt. C. T. C. 80 Folding and Progressive Regional Metamorphism. later puckers and folds, on the other hand, the axial planes incline rather steeply W.N.W., and the under limbs of anticlines are the most thinned. The differences of inclination of the unbent axial planes of fold cannot, however, be relied on as a universal test of age, for among the later folds there are also some — as a rule not very prominent or acute — which have axial planes inclining B.S.B. In these, however, and in those others which have the unbent axial planes inclining rather steeply north-west, it is rare to find quartz veins running along the foliation, whereas in the earlier folds such veins are very common. In some places on the north-west side of the Oarrick syncline, as, for instance, on the hilltop about three quarters of a mile north of Drimsynie (Lochgoilhead), and in Gleann Oanachadan south of the top of Stob Liath, we find strain-slips which are nearly hori- zontal, or which hade north-west less steeply than the strain-slips of the folds which are well-marked near the flat belt. These nearly horizontal strain-slips are occasionally puckered by later move- ments. There is evidence in many places, as, for example, on the east side of the valley a mile or two north of Lochgoilhead, that even in the areas where the dominant foliation is nearly flat, the bedding may still be considerably folded, but along axial planes which are themselves approximately flat. We do not consider it probable that these axial planes were flat when originally produced, but rather that they have been bent into their present positions by later movements— movements of the same age, perhaps, as those which have so often bent the foliation planes into small puckers and anti- clines. On the east side of the valley referred to, there are, on the west side of the N.N.E. fault near Inveronich, almost continuous exposures of very micaceous albite schist between the alluvium and the 400-feet contour line. When followed northwards this schist gradually splits into finger-like branches with their long axes nearly horizontal, and the branches gradually become thinner, and all die out before we reach Polchorkan.* A similar structure is shown on the upper slopes of Ben Donich. If the bedding always kept parallel to the foliation, the thick siliceous schist the top of which has been traced on the north-west side of Allt Glinne Mhoir and on Beinn Lochain, would continue all round Ben Donich, but though there are thick outcrops of siliceous schist on the south- east and east sides of this mountain, these dwindle into insignifi- cance on the west and north-west sides. It can be seen in many sections that the metamorphism exhibited in different beds varies somewhat, according to the position of the band in relation to the folds in the section. For instance, the proportion of mica in the schistose grits is generally greatest in the most attenuated limbs, and in these portions the remains of clastic grains are also less numerous, oj" smaller and more flattened, than near the axes of fold. This is well seen in specimen 5,295 — an isoclinally folded piece of schistose grit of the Beinn Bheula series — obtained from about a mile and a quarter B.S.E. of the head of *Ihid,., Fig. 52, page 226. tu o CO u Folding and Progressive Regional Metamorphism. 81 Loch Eck. It may be concluded, therefore, that the abundance of secondary mica, resulting from the alteration of clastic grains of felspar, depends upon the extent of the shearing of the rock. A phenomenon which also points to the production of mica during shearing processes is the frequent presence in the strain-slips ti-aversing tolerably siliceous schists, of thin bands, rarely more than half an inch thick, which are chiefly composed of mica. Such are especially common in the strain-slips in the earlier folds in the Beinn Bheula series, but they occur also, though generally in less thickness, in the strain-slips in the later folds in, and near, the flat belt. Distinct remains of clastic grains have not been noticed in any of the very micaceous schists. It is possible that in some of these schists the absence of such grains may be due to unusually intense alteration and shearing, but in many sections in which the dip is uniform and where there is no reason to suppose the shearing has been more intense in one part than in another, we still find alter- nating parallel bands in which the clastic grains vary in number and size, in a way that recalls the alternations in some unaltered shales and grits. It seems probable, therefore, that as a general rule those beds which contain the largest clastic grains were originally of a more pebbly character than the others, while the more micaceous schists represent finer sediments. Though much of the substance of the clastic grains of felspar in the gritty beds has presumably been used, as already stated, to form secondary mica, yet other portions have probably been broken down and recrystallised into small water-clear granules, which, together with the grains of recrystallised quartz, form the granulitic matrix that composes so large a part of most of the siliceous schists of the district. The portions of clastic grains that still remain are usually flattened between the foliation planes, and also considerably elongated in the direction of stretching, which is parallel to these planes. The grains generally tail oflfinto granulitic material at the two opposite ends, and they are frequently crossed, nearly at right angles to their long axis, by narrow cracks filled with similar granulitic material. It is to be noticed, also, that as a rule the quartz grains have become granulitic, and have been drawn out into long lenticles, more frequently than the felspar grains in the same beds. Independently of the slight changes of metamorphism which are connected with the folds in particular localities, there is also a o-eneral increase in alteration in the schists of the Beinn Bheula group as we advance north-west towards the flat belt. In the case of the schists north-west of Loch Long this increase is best shown by the alteration in character of the albite schists, but if the siliceous schists in the small area south-east of Loch Long were carefully compared with those in the flat belt we would expect to find a corresponding increase in metamorphism, for in the adjoining one-inch map 29, the siliceous schists on the strike of some of those south-east of Loch Long are decidedly less altered than those 82 Folding and Progressive Regional Metamorphism. further north-west in the flat belt. They contain, for instance, no black mica, or black mica only in small and rare scales, and the clastic or allothigenic grains of quartz and felspar bear a greater proportion to the recrystallized or authigenic portion of the rock than is the case in the corresponding schists in the flat belt. Near the flat belt, and for several miles north-west of it, the albites in the albite schists are often the size of common shot or rather larger, and are quite distinct to the unaided eye, but on the south-east side they are smaller. In the more micaceous beds south of Rudha nan Eoin they are too small to be distinguished without the aid of the microscope, but minute grains certainly occur as far south as the foot of Knap Burn, and probably a little further. Northwards from the foot of Knap Burn to Woodside Lodge, the grains gradually increase in size, and at the samtj time thin veins of albite pegmatite, in which the albite may be crystallised in much larger forms than in the schist matrix, become more numerous. There is ground for the opinion also that along the foliation strike from S.S.W. to N.N.E. the average size of the albites in the albite schists increases slightly. In Glen Kinglas the size is often that of a common pea. Both in the albite schists and in some of the more micaceous schists near the flat belt, and within a distance of two miles E.S.E. of it, small needles of tourmaline are tolerably abundant, the occurrence of which perhaps points to an impregnation with boracic acid partly before, and partly during, the formation of the albites. In some micro-sections tourmaline needles are included in albite, and at certain localities they are considerably bent and broken. As we proceed north-west from the flat belt there seems a decrease in metamorphism, but this is not rapid, and is interrupted by the special metamorphism exhibited by the garnetiferous mica- schists. In the Ardrishaig phyllites south-east of Loch Fyne flakes of biotite are still common, though smaller than those in many of the schists of the Beinn Bheula group. Some bands of albite schist occur in the garnetiferous mica-schist group, but the albites are smaller than many of those found further south-east. Though the rocks in the greatly contorted area in, and near, the flat belt, thus bear evidence of greater regional metamorphism than most of those on either side, which appear in general somewhat less contorted, we cannot conclude that the increased contortion has induced the increased metamorphism. As far as can be judged, the beds at, and a little south of, Rudha nan Eoin are as much contorted as those in the flat belt, and yet the albites are very small, The special type of metamorphism exhibited by the garnetiferous mica-schists becomes more pronounced in a north-east direction and seems of earlier date than that which produced the albites, the garnets being very frequently deformed, and the actinolites always decomposed and often crossed by faint lines of movement along the foliation planes. The area in which garnets are conspicuous gradually increases in breadth from south-west to north-east, and Folding and Progressive Regional MetamorpMsm. 83 the garnets seem larger and more abundant between Leanach and Ardkinglas than sonth-west of Leanach, and near the north-east corner of the map they become still larger. C. T. C. When we consider the physical conditions under which the tectonic arrangement of the rocks and their regional metamorphism were superinduced, it becomes difficult to dissociate the structural features of the rocks in mass from those processes of mineral differentiation which may be regarded as interstitial. It appears clear that, in most areas where compression has been greatest, crystallisation has reached its extreme phase, while, on the other hand, where the folding has been least the degree of metamorphism has diminished. In the Loch Awe area, where metamorphism and compression have been least, the isoclines are at a high angle. Moreover, this area has been invaded by enormous masses of igneous intrusions which are ever diminishing towards the zone of extreme plication and crystallisation, till they die out altogether on reaching the margin of the latter. Over the district as a whole there is no reason to attribute the metamorphism to the thermal action of an underljdng plutonic magma. The sharp increase in metamorphism, however, in the extreme north-east of the sheet coincides with the appearance of a belt of plutonic intrusions which have induced contact action of a strictly limited nature ; but it is probable that those intrusions were injections from a single large subterranean reservoir.* The direction of the folding axes and of the planes of foliation demonstrably indicates that the dynamic stresses acted along a north-west and south-east direction, but the evidence is not sufficiently clear from which of these points the stresses emanated, while it is even possible that compression may have proceeded from both directions. It is evident, moreover, that however uniform those lateral forces may have been, there was a marked absence of uniformity in the resistance which opposed them. Notwithstanding that the crystallisation of the most metamor- phosed rock masses points to a high temperature, and that minerals have been formed that are the common products of thermal action, yet, judging from the distribution of the epidiorite intrusions, and their tendency to concentrate in the least altered rock zones, we are not justified in attributing such thermal conditions to the presence of an underlying magma. Whatever view may be taken of the interpretation of the struc- ture of the "flat belt" of Cowal, the petrological condition of the rock masses there is analogous to the results that would be likely to obtain at depths approaching the crushing point of those rocks. The latter are represented by biotite gneisses, in which granulitic aggregates of quartz and felspar are separated by folia rich in mica. *In the paper previously cited (Q.J.G.P., vol. Iv., 1899) Mr. J. B. Hill has recorded his opinion that the intense regional type of metamorphism found between the granite masses of Glen Fyne and Ben Cruachan " was linked with the same phenomena as those that afterwards resulted in the irruption of the granite-masses." 84 Folding and Progressive Regional Metamorphism. Phacoids of quartz and felspar frequently form lenticular granulitic aggregates with tail-shaped terminations. Moreover, the mosaic of the phacoids often blends with that of the flaser. In addition, transverse structures have been imposed by secondary planes of schistosity, and these banded gneisses are frequently puckered and gnarled. Finally, the phacoids are usually the sole indication of the original clastic structure, and in the absence of these grains all traces of its original clastic condition have been obliterated. Notwithstanding the enormous plication which these rocks have undergone they have been singularly free from fracture, the faults which are now seen having been produced at periods long subse- quent to the schistosity movements. Reversed faults in which rock-flexure finally obtains relief are absent. But in addition to major thrust-planes which disrupt rocks in mass, there is an equal absence of the more interstitial type of brecciation repre- sented by crush conglomerates ; and in this connection it is a significant fact that pseudo-conglomerates are confined to the Loch Awe group, where metamorphism is at a minimum. There appears, therefore, strong ground for the opinion that these rocks have been subjected to lateral compression at a depth in the earth's crust below the zone of fracture, in which the pressure of the superin- cumbent mass has set up difierential movements, resulting in a rearrangement of the mineral constituents. Although thermal action must have played a considerable part in the production of these crystalline schists, it is only as a neces- sary consequence of dynamic forces, which have liberated vast quantities of heat, in conjunction with the greater internal heat natural to such depths. It does not necessarily follow, however, that the production of crystalline schists is confined to the results of dynamic action and depth temperature ; but so far as that part of the Highlands is concerned which lies within the area under description, we have no hesitation in arriving at this conclusion. Brief allusion has been made to the fact that the sudden increase of metamorphism to the north-west of Glen Fyne is probably attributable to thermal action from a subterrannean reservoir. At any rate, there seems very little reason to doubt that some metamorphic zones within that area are the products of thermal alteration from an igneous mass not exposed at the surface. The peculiar dioritic intrusions which invade that area, although not altogether free from crushing, are evidently of later age than the crystallisation of the schists, and belong to a petrological type which is known in the south of Scotland to pierce Upper Silurian rocks. J. B. H. CHAPTEE X. Lower Old Red Sandstone. Excluding the plutonic masses, the rocks of Lower Old Red Sandstone age are restricted to the area in the north-^-est corner of the map, extending from Loch Awe by Loch Avich to the neighbourhood of ijoch Tralaig, where they consist mainly of lavas, intermediate in composition between basalts and andesites. Usually a band of conglomerate intervenes between the lava-flows and the highly eroded platform of schists on which this formation rests unconformably. Area in Nortlv-west Corner of the Sheet. — Some fairly large patches of the andesitio lava-flows may be seen in this portion of the map, separated from one another partly by faulting, and partly by intervening areas of the older schists. One of these patches lies between Loch Tralaig and the northern end of Loch Pearsan, on the western margin of the map, and terminates to the east along a line of fault on the west side of Loch Dubh-mhor. In the extreme north-west corner, the andesites sweep round the low- lying area at the west end of Loch Tralaig, and thence may be followed eastwards in a line of bold escarpments along the north side of that loch, forming the Braes of Lorn. Farther east they extend as far as Cnoc nan Gobhar, and a small outlier may be seen on the east side of Oruach Narrachan. At the base of these lava-flows there is everywhere the usual marked unconform ability with the older schists, which evidently constituted an uneven surface of ridges and hollows at the time of their outpouring. A sedimentary series is occasionally present at the base of the lavas, but it is by no means a constant feature. It is best seen to the east of the north end of Loch Pearsan, and may probably represent the same horizon as the sedimentary strata be- neath the lavas at Oban. In the present area a thickness of nearly 100 feet of conglomerate rests upon the upturned edges of of the older schists. Between the conglomerate and the base of the lavas there intervene about fifty feet of purplish flagstones, but apparently without any representative of the fossiliferous zone which occurs near Oban. The blocks in the conglomerate are on the whole not so well-rounded as in the Oban area, and comprise a variety of rocks, such as andesite, limestone, epidiorite, phyllite, gametiferous schist, and quartzite, but with no granite fragments. Occasional thin beds of finer sediment occur in this thick mass of coarse detritus, but they ai-e not constant. To the south and east 86 Lower Old Bed Sandstone. of Loch Fear, where, however, it is only a few feet in thickness, this conglomerate is well exposed and lies almost horizontally. At the foot of the waterfall below Loch Fear, the basement beds consist of thin purple shales lying underneath the conglomerate and resting upon phyllites and limestones. North of Loch Tralaig, and to the east and west of Innie farm-house, thin beds of conglomerate appear from under the andesites. Again, north-west of Corrielorne, beds of conglome- rate form the floor on which the andesites rest, while east of Loch Tralaig and north-east from the farm-house of Druimnashallag, some red and purplish shales intervene between the lavas and the epidiorites. These are the only exposures of the sedimentary representatives of the Lower Old Eed Sandstone to be met with in this district for a distance of six miles from east to west. The greatest thickness of the series of andesitic lavas in the north-west corner of the sheet is probably not more than 500 feet, the majority of which belong to a basic type of andesite and resemble those seen further east along the northern margin of the map. The basic flows are intermediate in composition between basalts and andesites. They are often dark grey and com- pact without any conspicuous phenocrysts of felspar, but with numerous small specks of a reddish-brown mineral (? iddingsite) which may be seen under the microscope to occur frequently as pseudomorphs after olivine, and to lie in a matrix mainly composed of small plagioclase laths. This type of rock is of common occur- rence in the Lome area, and will be described in the Explanation of Sheet 45. These basic flows usually become slaggy and often brecciated in their upper portions. They are often, moreover, highly decomposed and in a much shattered condition, and this is especially the case in the rocks forming the line of crags to the north of Loch Tralaig, where they are traversed by numerous small basic dykes of Tertiary age. In the more elevated ground to the north of that loch they are succeeded by a more typical class of andesite, which is usually of a purple or grey colour, and contains numerous phenocrysts of plagioclase and greenish pseudomorphs after some ferro-magnesian mineral, probably hypersthene. The rock closely resembles the typical hypersthene-andesites of the area further north in sheet 45. No representatives of more acid types occur in this area, nor deposits of acid tuff, as is the case further north. On the west side of Cruach Maolachy, north-west of the head of Loch Avich, a small outlier of the andesites occurs resting upon a fairly steep slope of epidiorite, which illustrates in an instructive manner the uneven type of surface over which these flows were poured out. But perhaps the most interesting deposits of this area associated with the volcanic manifestations of Lower Old Red Sandstone times are to be found in the masses of agglomerate and felsite to the east of Kilmelfort, and to the south and south-east of Loch Pearsan. A large mass of the agglomerate is seen to the east of Loch a' Mhinn. It is made up of large blocks, chiefly of porphyrite, epidiorite, Volcanic Agglomerate. 87 limestone, phyllite, slate, and quartzite, often with only a small amount of matrix, but occasionally the pebbles are small, and embedded in a compact-looking groundmass. Fragments of a pale felsite are also common, and the blocks are usually subangular. At the crags due east of the loch the agglomerate reposes on black schist, and in one place it passes into a breccia of black schist fragments while in another place it is crowded with fragments of quartzite. To the north-east of Loch nam Ban the agglomerate apparently rests upon felsite, and contains numerous blocks of similar material. On the west side of the exposure the lower part of the mass lies upon limestone, phyllite and epidiorite. It is interesting to note the frequent large proportion of blocks of biotite-porphyrite [9,302, 9,303] indistinguishable from theporphy- rite dykes and sills so common in this area. Similar types of porphyrite, however, are seen traversing the agglomerate itself, so that some at least of these inti'usions are evidently of later date. A small outlier of this agglomerate is well exposed on the western and southern slopes of Barr Kilmhealaird, a prominent hill to the south-east of Loch Pearsan. It is here apparently clinging to a somewhat steep face of epidiorite, and is seen to pass into cracks and fissures in the older rocks in a remarkable manner. The schists consist here of phyllites with calcareous bands, quartzite, and epidiorite, which are all represented in the agglomerate, together with numerous blocks of porphyrite and felsite. They are for the most part subangular, though some, especially the quartzite fragments, are well rounded. Here, as elsewhere, the fragments are of local origin, and none of andesite or granite have been observed. At the foot of the southern slopes of Barr Kilmhealaird a considerable mass of porphyrite is exposed, and in the nearest poi-tion of the agglomerate there are numerous blocks of porphyrite up to two feet in diameter, so that it is not improbable that the intrusion of porphyrite may represent the source of these blocks. The general local derivation of the materials in the agglomerates is well shown all over this area. Thus on the hill side above Loch nam Ban the rock is largely made up of fragments of pink felsite, closely resembling the felsite upon which it rests. The same close rela- tion to the underlying material may be observed further to the east, in the stream which joins the Duchara Burn, near Logalochan. In some places it would appear that the agglomerate passes into a kind of felsite charged with numerous fragments of schist ; at least it is difficult to see any sharp line between rocks of this character and others which are evidently entirely fragmental and in part resemble a fine-grained tuff. Some of the finer-grained and more tufi'-like portions of the agglomerate were examined under the microscope [8,578, 9,299- 9,301, 10,358], and show numerous small angular fragments of phyllite, quartzose-schist, quartzite, and limestone, embedded in a fine-grained fragmental groundmass containing small angular quartz grains, felspars, white mica, and iron ores. Some of the fragments of porphyrite were also examined [9,302-9,304], and 88 Lower Old Red Sandstone. these are seen to consist of biotite-porphyrite often with numerous plagioclase phenocrysts, and closely resemble the normal type of porphyrite so common as dykes and sills in the surrounding area. From the local character of the materials and the mode of occurrence of these sheets of breccia and agglomerate, it is probable that, underneath this platform of volcanic debris, one or more volcanic vents may lie concealed. A neck is actually exposed in the schistose area, about half-a-mile south-east of the shepherd's house of Maolachy, and about one-and-a-half miles south-west of the head of Loch Avich. It is an elongated mass of acid igneous rock, about a quarter-mUe long and half as broad. Dr. B. N. Peach, who mapped this exposure, has reported on it as follows in the " Summary of Progress of r,he Geological Survey for 1899," p. 100 : — " It consists of an acid igneous rock, probably a very acid andesite, or perhaps a rhyolite, in which evidence of flow while the rock was in a very viscid state is everywhere conspicuous, the whole rock being now quite platy and its layers so highly crumpled as to simulate a puckered schist. The planes which separate these plates conform in a remarkable manner with the outer edges of the mass, and are either more or less vertical or dip inwards at high angles. Often near the edge, the mass includes numerous blocks of the surrounding black slates or epidiorite sills. It also contains broken off portions of material similar to itself, which show flow-structure in a marked degree. So numerous are these fragments in some places that the rock looks like a brecciated conglomerate or agglomerate. The nature and the source of this breccia are well exhibited in the stream which skirts its north- western limit, where its contact with the sedimentary rocks is best exposed. Here the line of contact with the dark schists and lime- stones can be followed for about two hundred yards. The rocks are much shattered, and their junction with the igneous mass is somewhat irregular, but the latter rock is seen to have flowed upward, conforming to the unseen surface, and catching up innumerable fragments of the black slates and limestones, as well as bits of its own chilled outer layers, and involving them in the still flowing viscous rock. This is in all probability a neck or vent which may have supplied the material of the more acid lavas and tufis of the volcanic platform of Lower Old Red Sandstone age in Lome." Near Lochan Losgainn Beg several other masses of a similar flow-breccia are to be seen, some parts of which contain more broken up fragments than matrix. But that they are truly of eruptive origin can be shown in many cases where they behave as dykes or sills, and cut both the rocks of sedimentary origin and the epidiorites. They are in turn cut by the basic dykes of Tertiary age. The masses of ejected material, agglomerate, breccia, and tuff, above described, together with their associated necks or vents, in all probability belong to the later phases of the Lower Old Eed Sandstone period of vulcanicity as manifested in Lome. If this be the case, then, as the a,gglomerate rests immediately upon the older schists, this area must have escaped invasion by the earlier more basic lava-flows. H. K. Volcanic Series. ^9 Loch Avich Area. — The Old Eed Sandstone rocks which have been laid down on the truncated edges of the schists are in this district almost entirely confined to the volcanic phase — sheets of lava piled on one another to a height of many hundreds of feet, occasionally associated with volcanic ashes, and pierced by dykes pertaining to that system of eruptions. They are irregularly underlain by a coarse conglomerate, while an insignificant amount of shale rarely separates the lava-flows, in which they have often become involved. The lavas belong to the andesite group, and their normal types are represented by augite-andesite which, however, in this area, by the accession of olivine, pass over into andesitic basalts. In the field the andesites display marked variations both as regards their texture and their readiness to decompose, but over the dis- trict generally they may be difierentiated into two dominant types. Of these the most characteristic have a purple or reddish colour, are generally much decomposed, and often very vesicular ; while the least common variety is dark in colour, more compact in texture, and more rarely scoriaceous. A specimen of the latter obtained from Inverinan (6,930) contains idioniorphic crystals of pale green augite, sometimes forming glomero-porphyritic groups, and black masses of iron oxide pseudomorphous after olivine, while the groundmass is fluidal, with elongated, streaming, lath-shaped plagio- clase felspars, and is somewhat of the pilotaxitic type, though rather more coarsely crystalline than usual. A specimen of the purple lava from the same locality (6,931) is of similar composition, but the only porphyritic constituents are pseudomorphs, some of which are brown iddingsite, while many others are altogether obscured by the presence of iron oxide, but from their outline are probably after olivine. These basic andesites, containing olivine in original phenocrysts, form a connecting link with the basalts. The olivine, however, in this area is apparently restricted to the lavas, an augite andesite dyke from the same locality (6932) containing only augite and plagioclase felspar. Both these minerals occur as phenocrysts, the augite often decomposed to dark green fibrous aggregates of chlorite, while the groundmass, consisting of small lath-shaped plagioclase felspar, is rudely fluidal. The tufis are usually of a reddish colour and the lapilli are generally of small size, but are sometimes an inch in diameter. No sandstones occur in this area, but red shales are of excep- tional occurrence amongst the flows. They are impure shales with ill-defined bedding, containing small angular quartz fragments and tiny scales of mica, bleached and weathered, together with opaque or dark brown fragments stained with iron-ore, which may possibly be of volcanic origin (6,933). Thin strips of shale have often been caught up by the lavas in their flow. The conglomerates are of irregular composition, not only as re- gards the size and character of their constituent boulders, but also in their relative degrees of attrition. This deposit, which sometimes occurs between the andesites and the underlying bod-rock of schist, 90 Lower Old Bed Sandstone. may also be seen interposed between the lava-flows, but only near the base of the volcanic group. In this area, the fragments of which they are composed have been mainly derived from the subjacent schists, and boulders of andesite, though sometimes the only constituent pebbles, are often entirely absent, and even then the matrix is usually andesitic. Where the conglomerate rests directly upon the bed-rock, the presence of a certain quantity of andesitic boulders indicates that it is of later formation than the earliest lava-flows. It often consists of a confused mass of angular blocks, some of which may be four or five feet in size, together with a subordinate amount of fine-grained rounded material, while, on the other hand, the boulders are sometimes well water-worn and the conglomerate assumes a more normal aspect. The evidence is conclusive that the boulders are strictly of local origin, and so much is this the case that it frequently conforms rigidlj'^ to the composition of the underlying platform ; so that this deposit may be said to merge into the rock on which it imme- diately reposes, as some of its constituent boulders are practically in situ. There can be no doubt that it was formed under con- ditions of great terrestrial disturbance, for not only does it fill up former inequalities of the land surface, but contemporaneous fissures have been similarly infilled. A dissected cleft of this nature forms a striking feature at Craignamoraig, standing up as a great mural escarpment, and displaying a vertical section of this coarse agglomerate, the larger blocks of which are practically in juxtaposition with the walls of the fissure from which they have been broken. Moreover, the continuation of this fissure is seen in the adjoining schistose grit, which is brecciated along the fracture. The south-eastern boundary of the andesite group forms an extremely regular margin, extending from Inverinan to Bamaline, parallel to Loch Awe, and in close proximity to its coast-line, while its base is only 120 feet above, the normal surface of that lake. Along this margin the lavas rest directly upon the schists, and the conglomerate is absent. Except as regards its south-easterly boundary, the group is of irregular distribution owing to the exposure of the uneven floor by denudation. Erosion has sometimes revealed the very surface of the old floor on which these rocks have been laid down, as marked by the red ferruginous staining of the schists, from which the overlying Old Eed rocks have been removed. That these deposits formerly covered a larger area may be plainly inferred from the ferruginous staining of the old floor sometimes extending beyond the Old Eed margins, as noted, for instance, on the southern shores of Loch Avich, and by the occurrence of a small outlier of andesite that is seen on the coast of Loch Awe below Arinechtan. The lava-flows dip gently to the south-east and are rarely observed with a reversed inclination, while the intrusive dykes appear to hade to the north-west, so that the centre of eruption was probably also in the latter direction. No volcanic vents have been detected in this area, but they have been observed in the adjoining region farther to the west. Volcanic Series. 91 It is evident that in this area we are dealing only with a portion of the volcanic pile, but, nevertheless, these basal deposits attain a maximum thickness of 750 to 1,000 feet. The conditions under which these eruptions conim_enced clearly point to an old land surface of schistose rock carved into ridges and valleys determined by the trend of its lithological bands, and in which superficial deposits were practically absent. Further, there is no evidence that any part of the volcanic pile in this edge of the Lome basin was accumulated beneath the sea. J. B. H. CHAPTER XI. Intrusive Igneous Eocks. i. gaeabal hill mass. The granite or granitite in Glen Fyne belongs to the igneous complex of Garabal Hill and Meall Breac described by Messrs. Dakyns and Teall.* It contains both orthoclase and plagioclase, the former often in large pink porphyritic crystals an inch or two long, with much biotite but no muscovite, while hornblende frequently accompanies the biotite and is sometimes more abun- dant, than the latter mineral. In a few places, however, near the margin of the complex, there is a dark fine-grained diorite apparently free from quartz and sharply separated from the granite. The interstitial quartz, which is sometimes opalescent, is not always conspicuous in hand-specimens and probably varies considerably in amount. Dark basic inclusions, often a foot or two in diameter, and most of them probably referable to diorite, are very abundant and are sometimes cut by paler granitic strings. The granite is as coarse in grain at its junctions with the schist as in the interior, and where these rocks come together there is often a considerable space occupied by alternating bands of each. Near Cruach Tuirc the area composed of such alternations is about 160 yards in breadth, and most of it lies on the south-west side of the line adopted in the map as the generalised boundary of the granite. The granitic alternations occur partly as vertical bands, sometimes several feet wide, and partly as thinner laminae and strings, which run nearly parallel to the foliation of the schist. The schists near the granite are considerably altered and indurated in places, and weather with a peculiar brown colour. They consist chiefly of micaceous albite schists and more siliceous schists, and belong to the Beinn Bheula group. On the south-east side of Glen Fyne the general strike of the foliation of the schists near the granite is north-west, almost at right angles to that usual in the district, and the dip is north-east towards the granite. The various minerals which are supposed to have been developed by contact metamorphism near the granite are as follows : — sillimanite, cordierite, andalusite, black or brown biotite, horn- blende, epidote, iron pyrites. Both sillimanite and cordierite occur in the andalusite hornfels near the top of Cruach Tuirc, but it is doubtful whether they are common. Andalusite, on the other hand, is very conspicuous in the more micaceous schists and albite * Quart. Jvurn. Oeol. Soc, vol. xlvii., p. 104. Diorite and Hyperite. 93 schists, and forms brown prisms, sometimes half an inch long, which are never bent or broken, and which make a close projecting network on the weathered rock face. The albites in the albite schists lose their usual pale brown colour and become pink, pre- sumably in consequence of the oxidation of the magnetite inclusions. As andalusite has not been seen more than a mile from the granite, while the albites in the albite schists extend at least twenty miles away, we may conclude that the two minerals have been produced by difierent processes of metamorphism. The altered colour of the albite is similar to that observed near some dolerite dykes, and it is probable that the albites were already in existence at the time of the granite intrusion. The hornblende supposed to be due to contact metamorphism, occurs in prominent needles in a siliceous schist with conchoidal fracture in the river Fyne, seventy yards above the bridge situated a mile and a half above Achadunan. In a foliated lamprophyre sheet on the north- west slope of Oruach Tuirc there are also pseudomorphs of biotite after needles of actinolite, which may have been developed in a similar way. Biotite often forms little spots, about the size of small peas, in the siliceous schists, and occurs also on the foliation planes of many of the schists in greater abundance than usual, and has probably replaced a good deal of chlorite in the albite schists. Bpidote, which often occurs in short irregular strings, sometimes mixed with quartz, is well seen on the B.N.E. side of Lochan Mill Bhig, and in the burn north of Meall Beag. The dark fine-grained diorite which is sometimes found in small patches at the margin of the complex is seen in several places in the river Fyne nearly three-quarters of a mile north of the summit of Cruach Tuirc, and in a little burn about a mile and five hundred yards north-east of that hilltop. At the first mentioned locality it shows a parallel structure, perhaps due to original fiow, which is not noticeable in the adjoining granite. 0. T. 0. II. HYPERITE MASS NEAR LOCHGOILHEAD. Hyperite, or more strictly quartz-biotite-hyperite, which is supposed to be of approximately the same age as the Glen Fyne granite, is seen in the burn about a mile and a quarter E.N.E. of Donich Lodge (Lochgoilhead), and in the side streams north-west of that burn. The rock is coarse-grained and usually of a dark grey colour, but the felspar has sometimes a pink tint. Biotite is not conspicuous in the hand specimen. The augite and hypersthene are seen under the microscope to be sometimes edged by hornblende, but it seems possible that the hornblende was formed during the original cooling of the rock. The adjoining schists are considerably altered. In the albite schists the albites become pink, and the matrix is darker than usual and speckled with little strings and grains of pyrites. In two of the side streams draining the south-east slope of Ben Donich, the furthest west of those shown in the one-inch map, and an adjoining one, there is a hard breccia made of pieces of schist 94 Intrusive Igneous Hocks. like those usually found near the hyperite, hut with no fragments of the latter rock, which may perhaps have been formed by breccia- tion in situ almost at the same time as the intrusion. The hyperite is itself much slicken-sided, veined with calcite, and traversed by thin courses of breccia — often nearly horizontal — but it is not clear that these have any connection with the breccia outside. About three-quarters of a mile north of Ben Donich there is a small irregular intrusion of hyperite, sometimes in the form of a sheet and sometimes with vertical walls, which closely resembles the one in Donich Burn. A small exposure in a burn three- quarters of a mile S.S.W. of the head of Loch Restil is also perhaps referable to hyperite. C. T. 0. III. DIORITE AND EENTALLENITE MASSES BETWEEN GLEN FYNE AND GLEN SHIRA. The area between Glen Shira and the granite mass of Glen Fyne is characterised by a varied assemblage of igneous intrusions linked to each other by transitional petrological types, and also to the granite, of which they represent its marginal and dyke phases. In addition to dykes and sills, small plutonic bosses occur along a narrow belt in which the dykes are few. These bosses consist of granite, tonalite, augite diorite, and kentallenite, and frequently exhibit transitional rock types, so that the mass of which they form a part constitutes a plutonic complex. The kentallenite occurs as a broad lenticular mass in the Brannie Burn, half a mile in length, only a portion of which lies within this map. The upper portion of the mass has been pierced by granite, the intrusive character of which is unmistakable. The kentallenite presents the usual characteristics of this peculiar rock, the outer surfaces being remarkably fresh, and pitted by cup-shaped depressions and deep hollows which gives it a singularly rugged aspect. It is dark grey in colour and of so coarse a texture that the constituent minerals can readily be detected with the naked eye. Olivine in fairly large grains is seen amongst the more conspicuous dark augite and brown biotite, while felspar is the least recognisable ingredient. When hypersthene is present it can generally be readily detected without the assistance of a lens. But the most characteristic mineral is the biotite, owing to its ophitic character, by which discontinuous patches are in optical continuity. When the rock is examined under the microscope it appears that plagioclase and orthoclase occur in approximately equal quantities, and that, as regards the plagioclase, acid types pre- dominate. Moreover, the felspars form the interstitial matter in which the olivines and augites are embedded; and in the felspathic groundmass, plagioclase is idiomorphic with respect to the orthoclase, which latter mineral, therefore, represents the ultimate phase of the groundmass. Hence we find felspar ophitically enclosing the basic minerals, and the felspar most characteristic of this ophitic arrangement is orthoclase. Kentallenite Masses bekveen Glen Fyne and Qlen Shira. 95 A specimen from the Brannie Burn (7,414) shows under the microscope olivine in large masses, weathering to magnetite and serpentine : rich brown biotite in large plates, often moulded upon felspar in an ophitio fashion ; very pale, almost colourless augite with frequently markedly idiomorphic outlines. Hypersthene occurs both as small grains scattered through the felspar and also as a rim around olivine, which may possibly represent a reaction border. Orthoclase is very abundant in large shapeless plates, which are sometimes simply twinned, and idiomorphic plagioclase is embedded within it. Orthoclase and plagioclase occur in about equal proportions ; but the augite is not so highly idiomorphic as is usual in the kentallenites. The rock is remarkably fresh. While the rock on the whole is of a remarkably uniform type, this example nevertheless exhibits phases more acid on the one hand and more basic on the other. Within the mass narrow inclusions of more acid and finer-grained material, restricted to a width of about six inches, are observed, while a larger band, six feet in width, occurs within the normal rock, into which it gradually merges, and probably represents an acid segregation. Microscopic examination (8,469) shows this rock to be very much decomposed, but containing idiomorphic felspars which are largely plagioclase, while some orthoclase also appears to be present, together with pseudomorphs and patches of chlorite and calcite after some ferro-magnosian mineral, and grains of iron oxide. On the other hand, the lower portion of the mass in the Brannie Burn is represented by a fairly coarse dark greenish picrite, con- sisting of augite, a little enstatite, olivine, and biotite, a small quantity of interstitial plagioclase and magnetite (9,293). The kentallenite from near the margin with the picrite contains a little enstatite, and the plagioclase is apparently in excess of orthoclase (9,294); while near its lower margin the kentallenite is of fairly normal type, but contains some enstatite. The picrite is the only example of an ultrabasic rock in the district, and must be regarded as the most basic modification of the magma which produced the varied assemblage of bosses and dykes between Glen Fyne and Glen Shira. A large elongated mass, about half-a-mile in length, occurs on the north-west of Glachan Hill on a parallel horizon. This rock, which over the greater portion of its mass is a diorite, possesses strong affinities with the kentallenite, so much so, that at its south-west end, which is the more basic part, a variety of kentallenite replaces the augite diorite, while the more acid part confined to the north- east portion is intermediate between diorite and vogesite. A specimen from this augite-diorite (8,461) contains much pale green pyroxene, similar to that of the kentallenite, but the borders of the augite carry a narrow zone of deep green dichroic hornblende, as in certain of the diorites which will be described in the sequel. Biotite is extremely common, with colour and pleochroism identical with that mineral in the kentallenites, while occasionally it ophiti- cally encloses felspar. The felspar principally consists of idio- morphic plagioclase lying in a small quantity of shapeless ortho- 96 Intrusive Igneous Rorhs. clase. Quartz is fairly common, and there are a few pseudomorphs apparently after olivine. There is a considerable amount of green hornblende, not only surrounding augite, but also in separate individual crystals. This rock has close affinities -with, the kental- lenite in the Brannie Burn, but differs from it in the absence of hypersthene, and in its paucity of ortlioclase and olivine, together with the presence of green hornblende and quartz. The more basic part of the mass, however, is a kentallenite containing some hypersthene (9,296), while a specimen obtained close to the latter, showing idiomorphic augites in the hand-specimen, is a variety of kentallenite approaching augite- diorite and hyperite. The slide (9,297) shows idiomorphic augite and hypersthene in a groundmass of smaller augites, hypersthene, and more or less idiomorphic plagio- clase, with probably some interstitial alkali felspar. Olivine is an accessory often seen as cores in the pyroxene, and biotite occurs in small flakes. The least basic phase of the mass at its north-east end is a grey medium-grained rockof dioritic appearance, which the slide (9,298) shows to be a vogesite, closely allied to diorite. It contains prismatic brown hornblende, biotite, more or less idiomorphic plagioclase, orthoclase, and quartz, with accessory magnetite. The quartz and orthoclase sometimes form micropegmatite. The variation observed in this mass is especially interesting as showing the remarkably close genetic connection between the kentallenites, augite-diorites, and lamprophyres of this area. But that this genetic relationship is not confined to the basic phase will be seen from the next mass to be described, in which augite- diorite of kentallenite type constitutes a marginal condition in a rock of progressively increasing acidity, which finally culminates in a biotite granite. Two small granite bosses that occur half-a- mile south of Beinn Chas, between Glen Fyne and the Brannie Burn, show margins of diorite which are clearly seen in the field ; these dioritic margins, however, are confined to a narrow peripheral zone, so that the greater part of the mass is composed of biotite granite. A specimen from the central portion (7,412) shows the rock to be a distinctly acid type of fine-grained granite, with a fair amount of plagioclase, the constituent minerals being quartz, orthoclase, and plagioclase felspar, and biotite, which occurs both in large crystals and in clusters of small crystals weathering into epidote and chlorite, while some of the biotite shows distortion due to fluxion or crushing. A specimen further from the centre (8,635) marks a phase on the border between granite and diorite. The rock is a biotite tonalite with a little sphene In this rock, plagioclase, orthoclase, and quartz occur in diminishing proportions in the order named. The orthoclase is moulded on idiomorphic plagioclase, as in the kentallenites, but there is no evidence in the slide that augite was present. Near the margin (8,637) this rock passes into hornblende-biotite- diorite. It contains green hornblende, partly primary and partly secondary after augite, and biotite in irregular scales. There is Qranite, Diorite, and Kentallenite. 97 much idiomorphic plagioclase, with orthoclase and a very little quartz filling up the interstices. The slide also contains a crystal of pale augite (malacolite or diopside) similar to the augite of the kentallenites, with borders of hornblende. The following slide (7,413) obtained from a specimen at the margin carries the evidence still further, as this rock is seen to be an augite-diorite similar to the mass described from Clachan Hill. It contains in abundance idiomorphic crystals of nearly colourless augite passing into uralitic hornblende at its margins, while patches of similar hornblende are scattered through the rock together with a little chlorite. Biotite is common, and generally found on the surface of the augite. The felspar is partly plagioclase and partly orthoclase, the former in lath-shaped idiomorphic sections on which the orthoclase, in irregular masses, has been moulded, while quartz is scarce. In addition to this granite mass with kentallenite affinities, other bosses of granite, which correspond to the more normal type, are scattered over the area as represented by the centre of the Beinn Chas intrusion, with variations from diorite to granite, all of which find their equivalents in portions of the larger granite mass of Glen Fyne. Amongst these may be mentioned a series of three lenticular bosses on the east slope of Glen Shira, above EUerig, that probably represent a single intrusion, which are in the main more accurately defined as quartz-diorite but pass into more acid forms that, taken alone, would be better designated as granite. But in addition to these types, which in composition and texture corre- spond to the non-porphyritic phases of the Glen Fyne granite, an intrusion is seen on Clachan Hill which matches the porphyritic type of that mass. The large porphyritic felspar crystals in this rock appear to be plagioclase. It contains also much idiomorphic hornblende in brown and dark green well-formed elongated prisms, and deep brown biotite in idiomorphic plates. Plagioclase felspar is in excess of orthoclase, and there is a fair amount of interstitial quartz, while sphene, magnetite, and apatite occur as acccessories. It is richer in ferro-magnesian minerals and plagioclase than the normal type of the Glen Fyne granite, and the rock corresponds to a quartz diorite, while it is interesting to note that it is some- what intermediate in structure between granite and porphyrite, and to this extent links the plutonic rocks with the porphyrite dykes, in some of which the porphyritic felspars attain almost similar dimensions. This set of inti-usions, ranging from acid to ultra-basic types, would appear, therefore, to be the product of difierentiation in an original parent magma, and their distribution points to this having been the result of a concentration of the more basic oxides in its periphery. Moreover, some of these intrusions have been subjected to a similar process of difierentiation m situ, so that the series exhibits complementary characters as regards not only the dis- tribution of the group, but also in respect to some of the individual members of that group. The magma which formed the source of supply of these rocks 98 Intrusive Igneous Eocks. was probably of intermediate composition, and, had it retained its uniformity, would have crystallised as a diorite — an inference which is confirmed by a study of its dyke phases. The contact metamorphism produced by these masses has been influenced in its effects by the degree of crystallisation of the schists prior to their intrusion. Where regional metamorphism is of an advanced type contact alteration has produced the least effect, and vice versa. But notwithstanding this disturbing element, the evidence is clear that the basic intrusions have been more potent in this direction than the acid. While the quartz-diorites of Glen Shira have usually altered the phyllites in their imme- diate vicinity, corresponding intrusions among the more highly crystallised sediments further north-east have produced insignificant effects. The more basic masses, however, of Glachan Hill and the Brannie Burn have caused extensive modification of the schists, notwithstanding their advanced degree of crystallisation. On the slopes below the Glachan Hill mass, the slates have been hornfelsed and indurated for nearly a quarter of a mile from its margin, and it is probable that the augite-diorite is prolonged beneath the surface, but the argillaceous schists have completely changed their character and been converted into biotite hornfels (5,003). On its south-east and south-west borders the limit of alteration is far nearer to the igneous outcrop, while at its north-east corner, which represents its most acid phase, the hornfelsing is confined to about thirty yards. The metamorphic aureole of the kentallenite in the Brannie Burn^ is restricted to a belt extending about fifteen or twenty yards from its margin, in which characteristic hornfels has been developed. It is possible, however, that the garnetiferous mica-schist in which the latter occurs was originally a more metamorphic rock than the phyllites bordering the augite-diorite, that are situated beyond the garnetiferous zone. Under the microscope (8,470) it is seen to be a fine-grained dark slate which has developed a knotted structure by the formation of clusters of deep brown biotite, and contains a few large pale garnets. J. B. H. IV. GRANITE, DIORITE, AND KENTALLENITE, IN NORTH-WEST CORNER OF THE SHEET. GRANITE AND DiOKiTE. — Several small intrusions of granite and diorite have been mapped in this portion of the sheet, and are confined to the area south of the volcanic plateau and to the north of a line stretching from the head of Loch Avich and down Gleann Domhain to the western edge of the map. The largest mass lies between Gleann Domhain and Loch an Daimh. It broadens rapidly to the west and is continuous with a large intrusive mass, the greater part of which is comprised within Sheet 36. About a quarter of a mile to the north-east of this mass another very small patch is seen on the north-west side of Gleann Domhain, Further to the north a small patch occurs on the north Granite and Diorite. 99 side of Loch a Chlachain, and another small intrusion crosses the road leading from Loch Avich to Kilmelfort to the south-west of Barr Kilmhealaird, on the western margin of the map. All these intrusions are remarkably similar in composition and structure. They consist of a grey diorite, usually fine-grained, and showing conspicuous biotite flakes in the hand-specimen. The rocks are in all respects identical with the more numerous and larger intrusions, which are more typically developed further west in Sheet 36, in the explanation of which sheet a detailed description of them will be given. It will therefore be sufficient here to point out shortly their principal characters and relationships. The normal type of rock is a diorite, which shows under the microscope abundant idio- morphic plagioclase frequently with a zonal structure, biotite, a greenish hornblende, and some interstitial quartz and alkali-felspar. Apatite and magnetite are accessory; sphene, however, has not been noted. A pale, almost colourless, augite often occurs in addition to hornblende in the finer-grained varieties. The horn- blende is occasionally of the brown prismatic variety [8,571] and resembles that characteristic of the hornblende-lamprophyres. The biotite is in ragged plates and never shows idiomorphism. It is seen to be moulded upon the plagioclase, and hence of later consolidation than that mineral. This is a very characteristic feature of the biotite of the diorites associated with the larger granite masses of the Western Highlands, such as those of Ben Cruachan and Glen Fyne, and is also seen in the biotite of the kentallenites, to which these diorites have been shown to be closely allied. The alkali-felspar and quartz are always interstitial and usually only of small amount. The rocks are quartz-mica-diorites of the tonalite type, and closely resemble some of the dioritic modifications of the Ben Cruachan granite and the quartz-diorites of the Crifiel mass in the Southern Uplands, and they belong in all probability to the same period of intrusion as these and other larger and closely allied masses. These intrusions are rarely sufficiently acid in composition to be classed with the granites. A portion of the small mass seen on the western margin of the sheet on the Loch Avich road, is practically a granite in composition, while it is intermediate in structure between granite and porphyrite. It shows under the microscope [8,570] phenocrysts of plagioclase and biotite in a groundmass of smaller crystals of plagioclase, biotite, orthoclase, and quartz. The structure of the groundmass is microgranitic, and the specimen is from the marginal portion of the intrusion. Further to the east, however, a mass of fine-grained granitic rock is well exposed on the north side of Loch a Ohaoruinn which shows under the microscope slightly difierent features to the normal type of this district. A specimen from this locality, when examined under the microscope [8,575], shows much idiomorphic plagioclase and pale, almost colourless, grains of augite, biotite of later consoli- dation than the plagioclase, orthoclase, and some interstitial quartz. The orthoclase occurs in micropoikilitic plates, which play the role pf groundmass, a feature which is so characteristic of the 100 Intrusive Igneous Bocks. kentallenites, and is so well seen in the micro-sections of those rocks from the Ben Bhuidhe area to the north-east, described partly in these pages and partly in the explanation of Sheet 45. The rock may be termed an augite-granite. It is allied to the monzonites (augite-syenites) and to the Icentallenites. The rock, moreover, constitutes an interesting connecting link between the more normal granitic and dioritic intrusions of this area and the kentallenites, which represent the more basic modification of the same magma. The dioritic and granitic masses of this area are invariably accompanied by a distinct alteration of the surrounding schists into which they were intruded. This alteration is of the hornfels type, and resembles that observed round the Ben Cruachan granite and described in detail in the forthcoming explanation of Sheet 45. The alteration here observed, however, is probably not so intense as that connected with the far larger mass of Ben Cruachan. The marginal portion of the intrusions occasionally displays a sharp contact with the contiguous schists, but more frequently it is rather of a complex nature, and shows numerous small tongues and veins of fine-grained material traversing the hornfels, while many pieces of hornfels lie embedded in the marginal portion of the igneous rock. These fragments are sometimes so numerous as to constitute a kind of breccia with a dioritic or granitic matrix. An intermixed type of contact of this kind is well seen in connection with the small mass on the road to Loch Avich on the western edge of the map, where fragments of a dark horn- fels have been caught up and enclosed within the igneous rock. These pieces consist mainly of aggregates of small brown and greenish flakes of biotite and finely granular felspar and quartz. The biotite flakes have often a strong tendency to a parallel arrangement [8,570]. In a specimen showing the contact between diorite and hornfels from the same locality [8,571], the hornfels is slightly difierent in composition and consists mainly of granular aggregates of brown and greenish hornblende, with some biotite flakes and iron-ores, in a very felspathic micro-crystalline matrix. Some small patches of coarser felspar and quartz suggest an intermixture of granitic material. The line of contact between the two rocks is fairly sharp. Another specimen, however, from the contact zone [8,572] shows an intricate mixture of granitic material and hornfels without any sharp line between the two. The hornfels of this locality probably represents the alteration of the dark phyllites which cover large areas in this neighbourhood. When the phyllites alternate with certain impure calcareous zones, a banded hornfels is produced, closely resembling that of the Pass of Brander, and consisting of alternating bands of dark biotite-hornfels and calc-silicate hornfels. The calcareous hornfels is well seen to the west of Loch an Daimh, and especially near the margin of a small tongue from the large mass of diorite to the west, lying on the north side of the small lochan on the edge of the map west of Loch an Daimh. The limits of the contact alteration about the diorite masses are Kentallenites. 101 not easily definable. Hornfels is invariably present about the margin of each of the intrusions, but in some cases it has been observed without the visible presence of any diorite or granite, and at some distance from the nearest intrusion. It is not improbable, therefore, that there may be masses of igneous rock below the surface, which, though they have affected the overlying strata, have not yet been laid bare by denudation. Further, from the mode of occurrence of the dioritic and granitic masses, there seems to be reason to believe that they may form merely the more protruding portions of one great intrusion, and that the separate areas now exposed may in great part be due to incomplete denuda- tion. KENTALLENITES. — Two exposures of kentallenite are known in this area, which are both situated in the tract surrounding Cruach Narrachan, to the north-west of Loch Avich. One exposure occurs on the north side of a small burn, about one-third of a mile south-east of the summit of Oruach Narrachan, and the other about a quarter of a mile E.S.B. of Lochan a' Bhailis and west of Cruach Narrachan. Both these exposures are dyke-like in form and the rock is intrusive into a large epidiorite sill. The breadth of the exposures varies from ten to twelve feet ; the length of the Cruach Narrachan intrusion is about thirty feet, and of that near Lochan a' Bhailis rather less. In their dyke-like behaviour these intru- sions difier from the kentallenite masses of the Beinn Buidhe area, Glen Orchy, and Ballachulish, which are larger and more plug-like in form. A. dyke-like mass, however, also occurs on the shore of Loch Linnhe, near Dalnatrat, four miles south-west of Ballachulish (in Sheet 53). The dyke to the south-east of Cruach Narrachan trends E.N.E. and W.S.W., while that near Lochan a' Bhailis runs nearly north-west and south-east. The characteristic mode of weathering of both these rocks is precisely similar to that of the intrusions of the other localities, above referred to. There is the same dark, rough surface with its numerous pittings and small cup-shaped hollows. In petrological characters these two intrusions are precisely similar. On fracture the rock appears fresh, of a dark grey, almost black, colour, and plates of lustrous biotite are conspicuous to the naked eye. The olivine and augite may also be recognised, contrasting with their dark colour in a marked way with the paler felspathic portions. The actual contact between these rocks and the epidiorite surrounding them is not exposed, so that one cannot examine the margin of the dykes. In general ap- pearance the rock of both intrusions closely resembles that of Glen Orchy, though it is perhaps slightly finer-grained, and this resemblance is still further borne out by a study of the micro-sections [8,287, 8,5d8] under the microscope. These show large augites, of a very pale-greenish colour, fresh, markedly idiomorphic, and often with zonal structure. The olivine is fairly abundant and fresh, though much cracked and occasionally showing an incipient alteration into serpentine. The biotite is of a rich, reddish-brown colour, and occurs in irregular plates without 102 Intrusive Igneous Rochs. any sign of idiomorphism, and groups of detached flakes are often seen to be in optical continuity. It is moulded upon the plagio- clase and is therefore of later consolidation. The plagioclase is more or less idiomorphic, while the orthoclase is interstitial, and the former is probably slightly in excess of the latter. The ortho- clase is well seen in the Cruach Narrachan rock [8,287]. The rocks resemble the Glen Orchy and Ballachulish types of kentallenite rather than that of Beinn Bhuidhe. This is seen in the abundance of olivine and augite and in the relatively small proportion of the orthoclase, and in the absence of accessory hypersthene. The occurrence of kentallenite in this area within a few miles of diorites of the tonalite type and augite-granite of the monzonite type is of special interest. These latter rocks contain a similar type of augite to that of the kentallenite, though the idio- morphism is not so well seen, and the biotite is of the same variety and is in both cases of posterior formation to the plagio- clase. The orthoclase is invariably interstitial, and occasionally, as in the case of the augite-granite, shows the earlier formed minerals embedded in it in a micropoikilitic manner, and this feature is also characteristic of some of the kentallenites, such as those of the Beinn Bhuidhe area. There is thus an evident relationship in this area between the kentallenites, diorites, and granites, which have very probably been derived by a process of differentiation from the same parent magma. The close rela- tionship between similar types of rock has also been established* in other Argyllshire areas, to which allusion has already been made. The occurrence of similar types of rock in these several areas with similar relationships, serves not only to connect them closely together from a petrographical point of view, but is also strong evidence that the associated sets of intrusions in each area are of the same geological age. The granites, diorites, and kentallenites, therefore, of this area may not unreasonably be assigned to the same general period of intrusion as the Ben Cruachan granite, and this has been shown to be of later date than the outpouring of the Lower Old Eed Sandstone lavas of Argyllshire. H. K. SILLS AND DYKES ASSOCIATED WITH THE NEWER GRANITE MASSES. Area South-east of Loch Fyne. — The great intrusions of felsite and quartz-porphyry which are so extensively developed north- west of Loch Fyne, have not a wide distribution on the south-east side of that loch ; indeed, they are almost confined to that part of the watershed of Loch Fyne which lies north-east of Strachur. The hornblende-porphyrites have a still more limited range, and it is doubtful if any occur north-west of a line between Achadunan in Glen Fyne and Butterbridge in Glen Kinglass. But it is some- times difficult, in consequence of decomposition, to distinguish between hornblende-porphyrite and felsite. South-east of Loch Fyne the lamprophyres and mica-traps are very numerous, but only * See Hill and Kynaston. ' ' Kentallenite and its relations to other igneous rocks in Argyllshire." Q.J.G.S., vol. Ivi., 1900, pp. 531-557. Silts and Byhes associated with the Newer Granite. 103 in thin bands. It is doubtful whether any occur within a mile or two of the north-east corner of the map, and they are also rare near its north-west margin. FELSITE.— Near the Loch Tay limestone, between Ardno and Ardkinglas, there is a large sheet of felsite which has in one place been extensively quarried for road metal. In Ooire No there are several bands which may be continuations of this sheet, and one is seen in most of the burns between Coire No and the north-west side of Socach Uachdarach, in the Cur about a mile S.S.E. of Cruach nam Capull, and a third of a mile above the junction of the Cur and the Cab. Further S.S.W., felsite is seen along the lines of crush which run S.S.W. towards Bridgend, north of Loch Eck. The most S.S.W. locality at which felsite has been observed is about three-quarters of a mile N.N.E. of Bridgend. There is some- times more than one sheet of felsite on the south-east side of Beinn an t-Seilich, south of Glen Kinglas, and one of these is cut by a later lamprophyre sheet with stellate groups of hornblende. In the burn two-thirds of a mile S.S.W. of Stob an Eas, one of the sheets is about thirty feet thick, and contains phenocrysts of soda felspar both at the sides and in the middle, pseudomorphs after hornblende (?) and biotite, and some rounded grains of quartz. Further south-west the same sheet is cut by a dark mica trap. HOENBLENDE-POKPHYEITE AND HORNBLENDE-ANDESITE.- On the south-west slopes of Cruach Tuirc, overlooking Glen Fyne, horn- blende porphyrite occurs in several irregular sheets upwards of twenty feet thick. The general colour of the matrix is pink or buff, but close to the margins it is dark grey or chocolate brown ; the red porphyritic felspars and the biotites continue to the very edge of the intrusions ; the quartz blebs are not abundant and are not always present. Within the granite near Cruach Tuirc various thin bands of hornblende-porphyrite, similar to those in the schists, have been noticed which present chilled margins and weather into more sharply-jointed blocks than the granite. A sheet which has not been examined under the microscope, but which is probably a hornblende porphyrite, runs by the foot of Lochan Mill Bhig, about a mile south of Cruach Tuirc, expands in places into an out- crop 100 yards wide, and seems to die out in a south direction soon after crossing Eas Riachain. Another similar sheet appears a little further south-east which has been traced a mile and a half into the watershed of the river Kinglas. On a lower horizon another intrusion is met with, which, in the burn rather more than a third of a mile south-east of Binnein an Fhidleir, is thirty feet thick and has a lamprophyre sheet close above. In one of the sheets of hornblende-andesite, on the shore 250 yards south-west of Ard Gailich, some of the amygdules are more than two inches long and contain barytes as well as a ferriferous carbonate. Near the margins, fluxion structure is prominently developed, the small hornblendes of the groundmass, which are much decomposed, being usually parallel to one another and to the margin, though they change direction occasionally so as to flow round the felspar phenocrysts and some large hornblendes. 104 Intrusive Igneous Bocks. DIOKITE, LAMPROPHYRE, AND MICA-TRAP. — 111 the grOUp of lampro- phyres and mica-traps we include many different rocks, some of which are so decomposed and filled with secondary carbonates that the original characters cannot be determined. Most of them have a reddish colour, owing apparently to a reddish tint of the felspars, but others are dark grey. The term mica-trap is used here to include all the varieties in which biotite is abundant and in excess of horn- blende or pyroxene. A few bands, as, for example, the lower sheet two-thirds of a mile N.N.B. of Stob an Bas, are evidently related to the kentallenites, and contain small grains of olivine, together with some orthoclase. In the bands in which augite is abundant and which may be called dolerite, or mica-dolerite, the augite differs from that of the later dolerites and basalts in being of the pale variety called malacolite, and generally also in occurring in porphyritic forms which are idiomorphic in respect to the felspar. Augite of a pronounced ophitic character has never been observed. The augite crystals are often the size of peas, and in some bands remarkably abundant, as for instance on the coast near Leak, where they form about half the mass of some thin sheets. In various nearly east and west dykes on Ben Donich, and north of that hill, hornblende is more conspicuous than augite, and the rock may therefore be called augite-diorite. In the intrusions in which hornblende and augite are both found, the hornblende occurs both as inclusions in the augite — which gives rise to lustre- mottling — and also surrounding the augite ; hence it seems likely that both these minerals were developed during the original consolidation. A reddish-brown north-east dyke which descends to the raised beach on the south side of Mark contains many porphyritic crystals of hornblende, some of which are an inch long. A striking feature in some of the lamprophyres is the presence of spherulitic structures near the margins. In a band nearly a, quarter of a mile slightly south-east of Monovechadan these structures are about the size of small shot, show radiate fibres, and have a red tint, while the matrix is almost black. A thin dyke in a burn above some scars about a mile south-west of Ben Donich, contains spherical structures which are built up of concentric layers difiering somewhat in colour. The amygdules are in most cases prominently elongated along planes parallel to the margins of the bands, and they help us to distinguish these bands from the later dolerites and basalts, in which the amygdules are nearly always spherical. On the weathered face the rim surrounding the amygdule often projects slightly, and may be of a red tint even when the rest of the rock is not. The matrix always grows finer in grain near the original cooling surface. In the mica-traps the mica flakes become gradually smaller near the margin and ultimately cease to be visible, and the matrix loses the usual red colour. But in the bands which contain augite the latter mineral occurs sometimes in as large forms at the margins as in the interior. Many of the augite-diorites and dolerites weather into spheroidal lumps, but in other bands the Sills and Dyltes associated with the Newer Oranite. 106 prominent joints are genercally parallel to the cooling surface and split the rock into coarse slabs. Inclusions of schist and vein quartz are common, and in certain portions of some bands, as for instance in a sheet on the north and west sides of Beinn Eeithe, they perhaps make up a quarter of the whole mass. Most of the dykes run N.N.B. or else nearly east and west. Those with the former direction are nearly always somewhat crushed, and there are very few of the long N.N.E. faults which are not accompanied in some place by lamprophyre dykes. It seems likely that many of the lamprophyres are later than the faults referred to, and that they followed those pre-existing lines of weakness, but they themselves may have been crushed by subsequent movements. The sheets that make small angles with the horizon, the paths of which appear often to have been deter- mined by pre-existing joints, are more abundant than the dykes, but rarely exceed twenty or twenty-five feet in thickness. They do not keep to the bedding or foliation planes of the schist but repeatedly cut across them, and it is clear that in most districts, if not all, the movements which accompanied the development of schistosity had ceased before the intrusion of these schists. In spite of their thin- ness, the sheets are of considerable importance in working out the geological structure of the district, because they are older than the majority of the faults, and so help us to determine their effects. Excellent examples of faulted sheets are to be found on Clach Beinn, the west slopes of Beinn Mhor, and the south-east side of Stob Liath. No great alteration is usually perceived near the sheets or dykes, but near a dark basic sheet in Kinglas Water, rather more than 600 yards below Butterbridge, the albite schists are hardened and impregnated with iron pyrites, and the albites become red. Between Loch Fyne and the adjacent margin of the granite there are dykes cleaved or foliated in places, which have some resemblance to the diorites of the lamprophyre group. They generally strike nearly east and west, and some of them are cut by hornblende-porphyrites. Their cleavage or foliation planes are but slightly developed compared with those in the adjoining schists, and they have never been observed to continue into these schists ; yet they are not always strictly parallel to the sides . The foliation planes in the schists are always sharply cut by the dykes, and at the time of the intrusion they appear to have been as prominent as they are now. The most southerly of these foliated dykes occurs in the burn rather more than 600 yards north-east of Achadunan, and is a cleaved greenish rock six or eight feet thick. The cleavage planes are nearly parallel to the side of the dyke and are slightly lustrous, but show no mica flakes which can be seen by the unaided eye. The rock contains many white calcareous spots, about the size of grains of rice, which in most places have their long axes parallel to the cleavage or foliation planes. In one place where the cleavage is not well developed, the matrix shows a crowd of little interlacing needles, apparently of hornblende. In the south branch 106 intrusive Igneous Mocks. of Bas Riachain the dyke is represented by two parallel bands, and in one place there is hardly any cleavage, and the rock weathers in a massive form with slightly projecting knobs. Most of the dioritic dykes in the area north of Achadunan show more or less foliation. On the north-west side of Cruach Tuirc there is a thin foliated basic sheet that differs somewhat from the other rocks as it contains distinct flakes of mica and radiate structures in which the arms are sometimes nearly an inch long, and composed of scales of biotite, probably replacing needles of actinolite, which may have been developed by contact meta- morphism near the granite. C. T. C. Luch Fyne to Loch Awe. — West of Loch Fyne the sills and dykes associated with the newer granite masses form three natural divisions, each of which is characterised by the distribution of its own peculiar phase of intrusion. The first of these divisions extends from Glen Fyne to the western slopes of Glen Shira, and consists of two extreme types, viz., an acid phase of diorite-porphyrite, which occurs in great abundance in the eastern portion of the tract largely confined to the slopes of upper Loch Fyne, and a group of lamprophyres, distributed in equal profusion along the western edge of the area and more particulary restricted to the western slopes of Glen Shira. The central division occupies the slopes from the coast of Loch Fyne, between Inveraray and Crarae, extending westward to the watershed between that loch and Loch Awe. It consists of great sill-like intrusions of quartz-porphyry of an extremely uniform type, which, gradually converging towards Glen Aray, die out on the slopes of Glen Shira exactly where the closely packed lampro- phyre group first makes its appearance, so that on this hillside we see one group suddenly replaced in a lateral direction by the other without mutual transgression. Westward, this central division is replaced by an area in which the intrusions are but sparingly distributed on the Loch Awe slopes. This area is characterised by the occurrence of porphyrite sills of more basic composition than the diorite-porphyrites of Glen Fyne, while lamprophyres of Glen Shira type are of occasional occurrence. Glen Fyne to Glen Shira. — Near the Glen Fyne granite, and in the altered aureole in which granite veins are conspicuous, sills of felsite occasionally occur. But two miles to the south-west of that granite mass a great zone of thin sills and dykes commences, with an approximate north-east and south-west trend, which continues as a belt of closely aggregated intrusions for a distance of five miles as far as Dundarave. Of the sills which compose this group the most common type is characterised by the presence of large crystals of porphyritic felspar, some of which are a square inch in section, and resemble the porphyritic felspars of the Glen Fyne granite. In these sills and dykes, however, the porphyritic crystals are more usually plagio- clase, while in the granite these individuals are mainly orthoclase. Sills and Bylces associated ivith the Newer Granite. 107 These sills are often micro-granitic as regards their matrix, and quartz may be easily distinguished with the naked eye. Although many wonld be more correctly designated as granite porphyries, they so frequently pass over into diorite-porphyrites, in which hornblende represents the ferro-magnesian constituent, that the group more properly lies within the porphyrite division. A sill of the more acid type from Olachan (4,803) is seen under the microscope to be a granite porphyry containing phenocrysts of idiomorphic felspar, in which orthoclase preponderates over the plagioclase, a few highly corroded crystals of quartz, and numerous idiomorphic phenocrysts of biotite, now represented by pseudo- morphs of chlorite and limonite. The matrix is micro-granitic, and consists of untwinned felspar, quartz, and a little plagioclase. Secondary muscovite is present in the groundmass and in the porphyritic felspars. With the addition of hornblende, a relative excess of plagioclase, and a diminution of quartz, we reach rocks of diorite-porphyrite type, which represent perhaps the most common phase of these intrusions (8,452, 8,453, 8,454). The hornblende phenocrysts are idiomorphic, vary in colour from green to brown, and are sometimes zonal, and resemble in this latter respect some of the hornblende of the diorites. They are often pale in the centre, and may have had at one time a nucleus of pyroxene, and a limited amount of quartz is always present. Associated with these hornblende porphyrites in the eastern edge of the group, we find sills of almost identical composition, but in which the original hornblende phenocrysts are now represented by biotite, in irregular aggregates of small imperfectly shaped plates, mixed with magnetite, some chlorite, and occasional needles of dark green, very dichroic, hornblende (4,805). In the slide these are seen to be grouped together so as to yield the outlines in trans- verse section of large hornblende crystals. These biotite-porphyrites, which are easily recognisable in the field, are older than the hornblende-porphy rites, which sometimes cut them. Their peculiar characters would appear to have been brought about by thermal metamorphism on the original hornblende, which after being decomposed into chlorite was affected by contact alteration, con- verting the chlorite into biotite — a mode of alteration which is actually seen to have been induced on a lamprophyre by a granite intrusion on the northern flanks of Beinn Buidhe. Although no plutonic masses are exposed sufficiently near to account for the phenomena, yet, as has been already pointed out, the schists in this area contain zones which appear to have been acted upon by thermo- metamorphism produced by some underlying plutonic mass. A sill near Dundarave that lies beyond the more metamorphic zone occupied by the biotite porphyrites, but which appeared in the field to represent that type of intrusion, was examined under the microscope (8,458). It is seen to correspond exactly to the biotite-porphyrites except as regards the alteration of the horn- blende. In this rock the decomposition of the hornblende is complete, the original phenocrysts being now replaced by chlorite, '■^° Intrusive Igneous liochs. limonite, and calcite, the decomposition being entirely normal in its character. In all probability this rock represents the biotite- porphyrites prior to thermo-metamorphism. In addition to the various types of porphyrite, this group includes rocks that apparently correspond to orthophyres, which have been observed mainly on the nortii-west coast of Loch Fyne near Drishaig. These sills are much decomposed, so that it is not always possible, even with the aid of the microscope, to decide whether the felspar is orthoclase or plagioclase. The peculiar feature of these rocks is the occurrence of large Carlsbad twins of altered orthoclase, similar in form and size to the crystals of sanidine in the Drachenfels trachyte.* In the field neither quartz nor biotite is seen, but white mica, which appears to be a secondary product, is conspicuous. Under the microscope (4,806) the idiomorphic phenocrysts of orthoclase and plagioclase are very largely decomposed into sericitic mica and other products, and pseudomorphs apparently after biotite occur consisting of pale, bleached highly-refracting mica, calcite, iron-ores, and chlorite. The matrix is filled with scaly secondary muscovite, but appears to have been formerly orthoclase, while quartz is present in small irregular grains. The lamprophyre sills, which succeed the porphyrites to the west, are conspicuously developed both along the western slopes of Glen Shira and in the Brannie Burn, which branches off in an easterly direction from the head of that Glen. Like the porphyrite group, the lamprophyres have a wide petrological range, and include types from minettes to kersantites, and from vogesites to spessar- tites. Usually in an advanced state of decomposition, it is often impossible to determine, either in the field or under the microscope, to what particular family they belong, and they are all shown on the map under their group name as lamprophyres, without any attempt at difierentiation. They are often rudely foliated, and very commonly include fragments from the schists, which are usually well rounded. They vary in width, and are seldom continuous for more than a short distance. Their mode of weathering is very characteristic, from the large amount of carbon- ates that arise from their decomposition. A common type of lamprophyre in this area resembles in the field a decomposing basalt, in which small green phenocrysts, forming pseudomorphs after the principal ferro-magnesian constituent, are conspicuous, while felspathic ocelli in small red patches are especially charac- teristic of the lamprophyres of this class, and biotite is frequently present. The ferro-magnesian constituents disappear more completely than the felspars ; but in this area it has been noticed that the biotite resists weathering longer than augite and hornblende, while olivine has in all cases altogether broken down and is represented only by pseudomorphs. The characteristic idiomorphism of their constituent minerals, and especially the porphyritic habit of the ferro-magnesian ingredients are usually * Appendix to Otology of Cowal, p. 299, Petrological remarks by Dr. Teall. Sills and Bi/kes associated with the Newer Granite. 109 retained in their pseudomorphs, so that the rock types may often be identified after the mineral destruction has been complete. In most cases, however, owing to the porphyritic habit above noted, the ferro-magnesian minerals lend themselves to identification more readily than the felspars, so that it becomes impossible to differen- tiate between vogesites and spessartites on the one hand, and between minettes and kersantites on the other; and where an unequal uncertainty attends the ferro-magnesian constituents, they cannot even be discriminated within these limits. In this district the augite and hornblende-lamprophyres are more prevalent than the mica - lamprophyres, and plagioclase is more abundant than orthoclase, so that the prevailing types are spessartites, followed by vogesites and kersantites, while the minettes, which mainly represent this family in the West of England, are here infrequent. Olivine, replaced by calcite, is of rare occurrence ; it has been noted in the Glen Shira area in hornblende-lamprophyres (4,811-4,813), as well as in a kersantite from the Stralachlan district (4,011); while in a kersantite from Portinisherrich large pseudo- morphs after this mineral, in calcite and quartz, show a meshwork of dusty magnetite (6,692). This latter rock appears to contain a fair proportion of orthoclase, which would indicate a passage to the minettes. Vesicles have been observed in a lamprophyre from Stuc Scardan, which are now filled with calcite and chlorite (4,815), while a coarse variety of kersantite from Strathlachlan (4,012) contains a miarolitic cavity filled with quartz and carbonates as regards its centre, passing outwards into a pale felspathic patch, in which the felspar shows a somewhat radiate disposition, and converges towards the central portion. One of the porphyritic lamprophyres already alluded to, in the Brannie Burn, is seen under the microscope (8,466) to contain large phenocrysts of a pale-green augite weathering to a network of chlorite, and of hornblende with a pleochroism ranging from dark chestnut to clear yellow brown. These lie in a groundmass of felspar and chlorite pseudomorphs apparently after hornblende. The felspar is much decomposed, and appears to have been plagio- clase, while quartz is absent. The rock is filled with calcite, chlorite, and other secondary products, and appears to have been originally a spessartite. The inter-relation of the dykes of this area and the plutonic masses is clearly established. Even the comparatively acid quartz diorite of Glen Shira (4,802) shows affinities both to the porphyrites and to the lamprophyres, which are seen in the peculiar colour of the hornblende and in the idiomorphism of that mineral and of biotite, so that the study of the dykes still further supports the hypothesis of magmatic differentiation which has been already advanced in connection with the plutonic intrusions. The foliated lamprophyres which have been described in the area east of Loch Fyne are represented in this district only at one locality, near thej unction oftheMerk Burn with the river Fyne.* Hereadark * About one mile N.N.W. of Achadunan. 110 Intrusive Igneous Bocks. green, fine-grained lamprophyre-schist about three feet wide is bounded by a fault trending north- west. The foliation planes of the dyke are slightly oblique to this fault and almost at right angles to the schistosity of the highly metamorphosed mica-schists in which it has been intruded. The dyke contains inclusions of mica-schist, but its foliation penetrates neither these fragments nor the adjoining crystalline schists, their mutiial foliation planes being sharply defined. On either side of this sheared dyke a smaller parallel intrusion about one foot in breadth occurs, the nearer of which shows a slight tendency to cleave parallel to the fault. These lateral dykes are unsheared, but under the microscope reveal evidence of crushing. This foliated dyke displays under the microscope conspicuous effects of shearing, and is distinctly brecciated, the veins being filled with calcite. Biotite is much decomposed, but is still recognisable in large idiomorphic plates, often bent and deformed, and in some parts of the slide they are arranged in a streaming fashion, as if brought about by interstitial movements; the other ferro- magnesian minerals have been entirely converted into chlorite and calcite. The felspar has suffered similar alteration, being repre- sented by micaceous material, calcite, and granular quartz, while in some instances it is replaced by a kind of granulitic mosaic (5,005,5,006). The dyke with a tendency to cleave occurs within a yard of this lamprophyre-schist, and consists of pale-green hornblende in elongated irregular prism S; and much decomposed felspar filled with secondary mica and calcite ; a few idiomorphic phenocrysts may be traced, .the original nature of which is doubtful, as they have been replaced by aggregates of green hornblende. A greenish-brown biotite in small quantity forms aggregates of clustered scales. The mineral constituents resemble those in the sheared lamprophyre, and this rock has undergone a similar alteration but to a much less extent (5,008). The other dyke, which shows no trace of schistose structure, is a hornblende-lamprophyre, but appears, nevertheless^ to have undergone a certain amount of modification due to crushing ; the hornblende is entirely of a green variety, like that of the schists, and there is no trace of primary biotite or augite (5,007). The modifications of these lamprophyres appear to hg,ve been brought about by the stresses which culminated in the fault already alluded to. The local metamorphism, however, which has been induced, while sufiicient for the complete deformation of the lamprophyre, has produced no apparent effect on the crystalline schists of an earlier generation. Moreover, this local metamorphism, strictly limited in degree, has succeeded in completely modifying the felspathic constituents, which, in rocks of the lamprophyre type, often remain comparatively fresh long after the other minerals have been destroyed. QUARTZ-POEPHYKY GROUP. — This group of rocks, to which attention has already been drawn, plays a conspicuous part in the geology of the western slopes of Loch Fyne. They occur in large, irregular, elongated masses, with a strike pa,rallel to the schistose bands and to the porphyrite and lamprophyre djrkes already described, Pm -.1^ ao ■■}^- .s is £ '*. o " <-^ ;>> >^ 42 & ;-H o & S! k! a O" y ^*-H i,- O -o 3 Xi < .M0^ •am--; >-. rs^ C^ S&. '*'°i . \\ /p — v_^^ ' S\ V 'A LOCH ECK. tVi^''Y^^^ SCAL£ //■'." ';7 SV PERMISSION OP ~ 1 /'. 'V SrR JOHN MURRAY K.CS. ly '■:]* LAURENCE PULLARFRSE S , '^.'' ■ ^ SunvEYtD gy V •'' ' ,L , _^ na YOUNC.MA, JOHN MCWITT.BA,, /' / / .,/ ^"^V.. SBUNDiNGs IN fcc:r (< 1 / ;) '"^"^ A\""''" ':] 1 "^■^--\ % Roeh Basins. 129 ice-streams, one proceeding from the Black Mount along the basin of Loch Awe, and the other advancing from the elevated range of Beinn Buidhe. Moreover, O'wing to the closer proximity of the latter centre of dispersion, this stream doubtless possessed the greater energy, so that it not only largely contributed to the deepening of Loch Awe, but overflowed westwards and excavated the basin now filled by Loch Avich. These basins are not confined to the floors of the fiords, although alluvial deposits along the mainland valleys have usually concealed their fornier presence. The Dubh Loch in Glen Shira, however, affords an example of a rock -basin in process of being silted up. A \Jl —''^^ V / DUBH LOCH 30U/VOWGS IN F£er ^_ f/fOM .si/Ryer ar / ■' \^ Off. H. /?. MILL . /,/>' ) ) N /:':>/' f/?.i/_ -- '^ / f--'^ ^ ^ /■ \ !?/' \^ / • / '°<> BO^Tk \ ■ ■' 1 * ^'^*"^— -V HOL/S£\':'- •■. ;«■ \ '■■■.;■. \. •»' It^ \ TLM/L reference to the diagram, for which we are indebted to Dr. H. R. Mill, shows that the deepest soundings, viz., 42 feet, are near the point of exit, which is of course due to the rapid silting now taking place at its upper end, while the central ridge has probably been produced by the enormous landslip which has taken place on the hill slope that immediately abuts on this loch. On the vexed question of the origin of rock-basins generally, space does not admit of discussion, but so far as this region is con- 130 Glacial a-)id Recent Deposits. cerned, in which these features are well displayed, we are unable to dissociate such phenomena from glacial conditions, with which they appear so intimately bound up. J. B. H. 1. DIRECTIOX OF ICE-MOVEMENT. Central District — Inveraray, Dalavich, KUiaichael-Glassary, and Stralachlan. — The course of the ice-stream from north-east to south-west is indicated by abundant striae over the district between the shores of Loch Fyne and Loch Awe and on the extreme summits of the intervening watershed, the altitudes of which exceed 2000 feet. This evidence is confirmed by the roches moutonnees and by the dispersion of the erratics. The most important deflections indicated by the strife have been noted in the south-western district, where, as already described, part of the ice-sheet that advanced along the basin of Loch Fyne was deflected towards the Sound of Jura, along the Orinan hollow. That the diverging mass was of large size is shown by the occurrence of westerly ice-markings at elevations of 1000 feet, while at lower levels north-westerly striae have been preserved which conform more precisely to the trend of the hollow. It is not implied that the ice which filled the bed of Loch Fyne departed from that channel and ascended heights of 1000 feet on its passage to the Sound of Jura ; indeed, there is evidence from the striations of the Loch Fyne slopes that the lower portion of the mass, extending to 400 feet above the present loch level, pursued its south-westerly course, while the westerly divergence was confined to the ice above that elevation, except at the mouth of the transverse hollow at Loch Gilp. As these transverse striae are oblique to the topographical features which have guided the final glaciation of the district, they are associated with markings indicating that later ice-movement. In places the two sets of striations intersect one another, and it can be seen that the westerly grooves have been crossed by those trending south-west. But in addition to the flutings caused by this defiected ice- stream at the period of maximum glaciation, similar striation has been produced at a subsequent period by the glacier which occupied the Loch Awe basin. This mass, on approaching the lower end of the loch, pressed lobes of ice into the north-westerly clefts, which have dissected the line of hills to the west of Ford ; so that the summits of those peaks retain striations parallel to the course of Loch Awe, while the intervening hollows record a transverse series. Other deflections of ice-movement over the district are usually readily understood from the nature of the local topography ; the valley of Glen Aray, for instance, which runs nearly north and south, exhibits striaa of this direction on its slopes. J. B. H. Loch Avich, Loch Tralaig, and Glen Domhain District. — The ice-movement over this area during the period of maximum Direction of Ice Movement — Boulder Clay. 131 glaciation, as shown by striae and rock surfaces, appears to have been in a general W.S.W. direction, with local deviations according to the shape of the ground. On the hills between Loch Awe and Loch Avich and Glen Domhain the striae point towards the south- west, while on the high ground above Loch Tralaig, in the north- west corner of the Sheet, they show that the ice moved there a little to the south of west. It is difficult, however, to determine how much of the striation is due to the maximum glaciation, for most of the phenomena exhibited by the area belong to the later glaciation, which seems in this region to have moved much in the same path as the older ice, and to have obliterated most of its traces. Thus the boulder clay, if originallj' laid down within this area, has been almost entirely removed. B. N. P. South-eastern District. — On the south-east side of Loch Fyne the striffi in the deep glens and near the sea-lochs have nearly the same direction as the adjacent glens and lochs. Thus near Loch Fyne and the upper part of Glendaruel the direction is nearly south-west or S.S.W., while near Loch Goil, Loch Eck, and Loch Striven it is S.S.E. Near the tops of the higher hills, as, for instance, Sgor Coinnich, Beinn Bheula, Gnoc Coinnich, Beinn Eeithe, Garnach Mor, the two Beinn Lochains, and Binnein an Fhidleir, the direction is south-east or S.S.E. , and in some cases at a considerable angle to the adjacent glens. Except in valleys, the south-east direction is not found at levels much below 1500 feet, and in some places, as, for example, on the north-west side of Stob an Eas and half a mile east of Binnein an Fhidleir, a south-west direction is shown at levels considerably above 1500 feet. It is evident that the ice which produced the noi'th-west and south-east striations must have come from the north-west, for near some of them boulders of rocks are found which occur in situ on the north- west but not on the south-east side. Thus about 300 yards north of Cruach nam Miseag we find pieces of garnetiferous mica-schist and quartz-schist like rocks found five or six miles further north- west, but there are no similar rocks on their south-east side. These blocks occur at a height of about 1900 feet — an elevation greater than that of the rocks from which they have probably been carried. Throughout the district there is also a total absence of pieces of the less altered phyllites and schistose grits which occur near the south- east boundary of the Highlands in One-inch Maps 29, 30, and 37. On the south side of Drynain Glen there are two directions of striation, one south-east and the other north-east, and the latter is perhaps due to a movement from the high ground on the south-west at a time subsequent to the general glaciation. C. T. 0. ii. BOULDER CLAY. Central District. — The drift of the valleys is usually a greyish blue, stiff till studded with boulders, the upper surface of which has often been oxidised to yellowish and brown tints. Intercalations of sand and gravel are occasionally observed, and the deposit is 132 Glacial and Recent Deposits. sometimes laminated and contorted, but as a rule it consists of a stony, unstratified clay. In the more elevated valleys and on the upper hill slopes the drift when present is less cohesive, and forms a loose deposit in which angular rock fragments are more abundantly strewn, and is in many cases largely made up of morainic material. While it is possible that its looseness may sometimes be attributable to the disintegration of the till and the removal of its matrix, this upland type is generally an original ''blocky" deposit corresponding to the moraines of the lo\^■er ground. Drift of this type associated with moraines occupies a great expanse in the valley of the Brannie burn, on the slope which corresponds to the tail of the ice-drag. A typical deposit of the valley till, on the other hand, is seen on the slope of the hollow occupied by Loch Leathan, conforming to the same " crag " and " tail " arrangement. In Glen Aray drift of the upland type associated with moraines is found in its upper limits ; along its central division glacial deposits are comparatively absent ; while its lower extension, be- tween Three Bridges and Inveraray, is covered along its flanks by deep deposits of stiff grey till. The eastern slopes of Loch Awe between Blarghour and Durran are largely strewn with extensive deposits of the loose upland drift, intermingled with morainic heaps, which extend eastward to the Loch Fjrne slopes as far as Auchnangoul. Extensive tracts of till occupy the basin of the Add between Carron and Gleann Airidh, and drift deposits are spread over the elevated moorland marked by Loch Sitheanach and Loch Gaineamhach, while thin spreads of boulder clay largely conceal the solid geology between Beinn Laoigh and Brenachoil, and also the lower slopes of Loch Fyne above Kenmore. To the west of Loch Awe the low-lying belt of country fringing that loch from Barnaline to Cruachan is largely covered with a stiflf blue boulder clay, overlain by numerous moraines, good sections of which are afforded by the Allt Mdr burn. Similar till fills the hollows to the north of Loch Avich, but a section of drift for a depth of 20 feet in the valley north-east of Drissaig consists of a loose accumulation of fine angular and sub-angular fragments, rudely stratified, and surmounted by a few feet of coarse gravel, and in this section the typical till is absent. Further to the south-west the slopes on that side of Loch Awe are very partially covered with drift, which is mainly confined to the hollows ; a strip fringing Loch Awe a mile south-west of Inverliver, and extending to a height of at least 60 feet above the loch surface, is laminated and intermixed with sand and gravel. In the Stralachlan district, south-east of Loch Fyne, glacial deposits are only sparsely distributed ; a considerable covering of boulder clay occurs in the valley of Kilbridemore, on the slopes of Glendaruel, and the higher ground descending to Loch Fyne is strewn with a thin deposit of the coarse blocky drift already described. Stiff blue boulder clay is occasionally seen on the lower slopes, often supporting the raised beaches, as at Barnacarry and Boulder Clay. 133 Lepliinmore, while at the latter locality the till at a height of 40 feet above sea level is both laminated and contorted. The local derivation of the drift is evident, as instances occur where junctions of the till and underlying bedrock show transitional stages fi'om one to the other, analogous to the passage from solid rock to the deposit of " head " in the West of England. While the latter, however, has been the result of superficial " creep" alone, the former has been brought about in the same way under pressure, which has carried the deformation a stage further. In the Strone Point peninsula, at the head of one of the small burns emptying into Glen Shira, the micaceous beds pass into blue clay with masses of the solid rock left as lenticles. Again, in the burn half-a-mile south of Claonairt, a blue boulder clay is apparently derived directly from the underlying blue slate ; the till is laminated and intercalated with fine sand, and shows great contortion. Similar phenomena are seen in the Eas nan Tarbh burn, a quarter-mile from its junction with the Douglas Water, where bluish mica slate supports five or six feet of rudely laminated drift in which large portions of the underlying mica slate are embedded. This deposit is somewhat of the nature of" head," and is overlain by large angular boulders slightly waterworn, which are covered by fine sand showing contortion. Glacial shell beds have seldom been noticed. In Loch Gair, a little to the north of Gallanach, shells are seen both in the underljdng glacial clay and in the overlying 20-feet beach, those of the former being broken and brittle, while in the latter deposit they are solid and fresh. In Union Bay shells have been noted at the junction of the glacial clay with the overlying gravel of the raised beach. J. B. H. South-eastern District. — The deep red boulder clay so common within the limits of this sheet, the source of the colouring matter of which is doubtful, is found near the head of Glen Pinart, in the Loch Eck valley near Rudha Garbh, and in Strath nan Lub, but none has been noted in the drainage area of Loch Eyne, where the general colour of this deposit is pale grey or pale buff. In the boulder clay in the glen west of Oreag Dhubh (Strachur) and in a burn 700 yards south of the pier at St. Catherine's, and at some other localities, a faint lamination is observed inclining nearly with the hill slope. Near St. Catherine's the clay contains granite boulders. At two localities on the side of Loch F^j-ne the boulder clay con- tains broken shells. In one of these, 90 yards south-west of Ardna Gailich, where granite boulders occur in the deposit, the shell fragments are very friable and rarely more than half an inch long, but on one piece distinct glacial striations were observed. At the other locality, about 200 yards south-west of the centre of the bay at Airidh a' Ghobhainn, the shelly boulder clay also shows a lamination dipping north. It is suggested that the shelly boulder clays are later than the glacial shell clays, to be subsequently described, in which the shells seem to be in the position of growth. If these 134 Glacial and Mecent Beposlts. glacial shelly clays are of the same age as those in the Kyles of Bute, and if the shell fragments in the boulder clays have been derived from the former, then there must in this district have been at least two periods during which boulder clay was formed, for near Toward boulder clay is seen below the glacial shelly clay. In Ooire No, nearly a mile south-east of Ardno, the boulder clay is about 60 feet thick, perhaps as thick as anywhere on the south- east side of Loch Fyne ; below it, we find 30 feet of laminated sand and gravel, and above there is morainic material. 0. T. C. iii. MORAINES. Central District. — -Morainic mounds are almost entirely con- fined to the north-eastern division of this district, the boun- dary of which extends from Loch Avich to the Gaol Ghlean, m Cowal. Moreover, a broad belt along this termination is characterised by a profusion of these deposits not met with in the north-east, which appears to mark the extreme limits of the ice over a protracted period. To the west of Loch Awe this morainic band is extensively developed between Barnaline and Cruachan ; while between that loch and Loch Fyne the belt can be followed from Braevallich, across the watershed to within one and a half miles of the coast of Loch Fyne. On the slopes above that loch they are especially conspicuous between Loch Leacann and the Douglas water, which approximately demarcate the breadth of the belt ; while to the south-east of Locli Fyne they are strongly developed at the head of the Gaol Ghleann. Moraines are especially well seen from the high road near Auchindrain, which skirts a long line of these heaps. Further up the hill, in the valley occupied by a stream issuing from Lochan Dubh, they form a series of parallel transverse ridges, flanked bj'' lateral moraines of less definite shape. They are also profusely scattered over the hummocky watershed to the north and west of Loch Leacann, and are conspicuous features in the peat flat which is dissected by the upper waters of the Abhain Bhealaich burn, while still further to the north these mounds are associated with spreads of morainic drift in the upper part of the Blarghour bum, and in the Allt Mor burn. On the slopes of Loch Awe they extend down almost to the very margin of the water, fringing the ancient beach about eighty feet above its present surface, while one example occurs in the delta of the Abhain Bhealaich burn only twenty feet above the loch, and the valley occupied by this stream is fringed with moraines for upward of a mile from its mouth. On the west side of Loch Awe they extend almost to the margin of the water in the neighbourhood of New York, while north of Loch Avich a horse-shoe moraine with a south-west convexity, traverses the hollow continuous with Gleann Meashan, and the alluvial deposit fringing Loch Avich at Drissaig contains a circular mound in its centre. Moraines are also observed along the valley of Glen Aray, being more numerous near its head, while further down the glen a group of these mounds almost abuts upon the high road in Moraines. 135 the neighbourhood of Low Balantyre. Along the valley of Glen Shira moraines are only occasionally observed, but they become more prominent in the upper part of the glen, and in the branch to the east occupied by its tributary, the Brannie burn, these mounds are exceedingly numerous ; they likewise abound, but in lesser profusion, in the upper basin of the Kilblaan burn, which joins the Shira lower down the valley. On the coast of Loch Fyne, ^^•ithin four miles of its head, a moraine occurs at Auchnatra abutting against the landward margin of the fifty-feet beach, which latter shows contortion. Moraines ai'e scattered along the bottom of Glen Fyne, and are especially abundant in the neighbourhood of the shooting lodge. To the south-west of the great morainic belt hitherto noticed, moraines are comparatively rare. They are seen, however, in tolerable abundance in parts of the Add basin, and a few have been noted at the head of its lower strath. The range of hills to the west of Ford contains moraines in its deeply dissected transverse clefts, and they are likewise distributed along the western foot of that range. Moreover, on the south-side of Loch Fyne, between that loch and Glendaruel, they are equally limited in their distribution, being mainly confined to that part of Strathlachlan lying immediately south of Feorline and to the valley formed by Gaol Ghleann and its extension to Glendaruel as far as Kilbridemore. J. B. H. Lock Avich, Loch Tralaig, and Glen Domhain District. — At one period of the Later Glaciation this whole area appears to have been enveloped in the great confluent glacier formed by the ice which emerged from the Highland glens to the north and east of Loch Awe. The distribution of the moraines and morainic debris over the area shows in a remarkable manner the decrease of this mer de glace. Thus at several places on the watershed between Loch Awe and Glen Domhain moraines, made up in great part of granitic rocks and schists like those of the Oruachan massif, occur in such a manner as to preclude the possibility of their having been deposited by any local accumulation of ice, as they often lie in rampart-like masses athwart the natural slopes of the ground. At one period in the general decrease of this mass, lobes of ice seem to have been pushed down Glen Domhain, westwards through the hollows between Glen Domhain and the edge of the sheet, and north-westwards through the hollow occupied by Loch Tralaig. It is owing to the further decrease of these lobes that the fine set of moraines seen below Lag a Lochain in Glen Domhain, and those round Loch a Losgainn Mor, and below Loch Tralaig are due. Successive barrier-like ramparts of moraine enclosing alluvial plains which represent silted up lochans are admii-ably displayed in the area between Loch Tralaig and the north-west corner of the map, pointing to pauses during the shrinkage of the ice. B. N. P. Soxdh-eastern District. — The drift at the surface is often morainic and disposed in steep ridges and heaps, but their long axes vary considerably in direction, On the north side of Kinglas 136 Glacial and Recent Deposits. Water near Butterbridge, and elsewhere, the moraines consist almost entirely of great heaps of boulders with very little fine material be- tween. In Ghlinne Mhoir, north of Lochgoilhead, the upper limit of the conspicuous moraines is fairly well defined and gradually descends westwards from 1200 to 900 feet in height. Hell's Glen is comparatively free of drift, but moraines appear at its head and extend a considerable distance along the road to St. Catherine's and up the west slope of Stob an Eas. On the south-west side of the road the morainic banks run north-east, and near Coire No the south-west margin of the moraines is well defined. On the east side of Glen Branter, north-west of the head of Loch Bck, the long axes of some of the moraines are almost at right angles to the course of the glen, while further south-west, in Strath nan Lub, they are usually nearly parallel to the direction of the Strath. The valley, which continues in the same direction as Strath nan Lub, S.S.W. from the junction of this strath with Garvie burn, is occupied by moraines, or by irregular heaps of sand and gravel, as far south as the peat moss a third of a mile E.S.B. of An Cruachan. On the north side the moss is bounded by a long semicircular ridge with the convex side facing south, which has evidently dammed back the drainage on the south side and led to the growth of the peat. The ridge is not an ordinary moraine, being composed to a large extent of steeply-inclined layers of fine yellow sand, and the banks and heaps a little further north are also composed in part of similar material mixed with coarse angular gravel and boulders. South of the semicircular ridge there are no banks or heaps of sand, though loose sand is seen in places overlying deep red boulder clay. On the north-west side of Gleann Dubh (Glensluan), south of Strachur, the boundary between the morainic mounds and the drift covering the higher and smoother slopes is tolerably distinct from the head of the glen to half a mile south-west of the hamlet ; but lower down there are moraines extending to higher levels than on the north-west, which become gradually smaller and less definite on the high ground. Mr. 0. Maclaren (Udin. New Phil. Jour., vol. 1, p. 189, 1855) has described the mounds near Glensluan as terminal moraines belonging to the glen. The watershed at Strachur between Eas Dubh, which flows into Loch Fyne, and the Our, which flows into Loch Eck and thence into the Clyde, is very low, and occupied partly by a peat moss and partly by drift, with one or two small rock exposures near the north end of the moss. The drift generally is greeni.sh-grey boulder clay, but there are also some sandj^ mounds. Similar but more con- spicuous mounds are seen on the east side of the Our near Ballemeanoch, and extend south to Invernoaden. There are moraines at the sides of Loch Bck, which are referred to by Mr.C. Maclaren as remnants of lateral moraines (pp. cit.). Smaller moraines are found in the corries east of Beinn Mhor. 0. T. 0. Erratic Blockn. 137 iv. EKKATIC BLOCKS. Central District. — The dispersion of the erratic blocks over the area supports the conclusions respecting the course of the ice sheet indicated by the rock strise. In a district containing various types of rock restricted in their distribution, as in the area iinder description, the erratics, if in sufficient abundance, furnish perhaps more definite evidence regarding the ice-movement than rock striations. The most numerous erratic blocks are, as might be supposed, epidiorite, not only from the profusion of these rocks over the region but from their habit of forming ridges and peaks. To the north-east of this area the flanks of Beinn Buidhe contain peculiar types of that rock with a coarse gabbro structure, huge blocks of which have been stranded on the hills between the Brannie burn and Strone Point. The largest of these blocks* exceeds 250 tons, while two smaller massest are found one and a half miles further to the southward. Erratics of the same material, but of less dimensions, extend as far as the Strone Point peninsula. An epidiorite block of large size is seen on the western slopes of Glen Aray,t and a huge boulder of the same type stands out of the peat to the west of Loch Glashan. Erratics derived from the granite of the Glen Fyne and Beiim Buidhe slopes are common on the Brannie hillsides, and have been noted as far to the south-west as Lochgilphead ; a similar block has also been observed south-west of Loch Glashan, while boulders, apparently derived from the granite of Ben Gruachan, have been recorded to the north of Loch Avich, § and a similar erratic exceed- ing three tons in weight is seen seven miles farther to the south- west || at an elevation of 800 feet. A boulder of augite-diorite, four feet across, occurs one-quarter of a mile to the south-west of the parent-mass below Olachan Hill, while the quartz-porphyry intrusions, which are restricted to their own belt west of Loch Fyne have furnished numerous boulders that have been distributed over the country to the south-west as far as the margin of the sheet. Boulders of garnet-mica-schist, confined in this area to the extreme north-east, are similarly dispersed ; a large boulder lies west of that zone, one-quarter mile north-west of Olachan Hill, while a block exceeding sixteen tons is seen in Glen Aray.H Some rounded boulders of actinolite- scliist which have been observed on the coast of Loch Awe ** have probably been brought down by the Awe glacier from the hills behind Dalmally ; while an erratic which has been noted in the * i-mile N. of Tom a' Bhuaohaille at an elevation of 1300 feet, t In the valley of the Kilblaan buru. I 140 yards south of Stronmagachan. g l^-mile S.W. of Craignanioraig, resting on epidiorite. II |-mile S.W. of Loch a Mhinn. IT 1 mile east of South TuUich. ** Boulders of actinolitic and garnet-schist are common on the shore near Budha na lie Moire. 138 Glacial and Recent Deposits. hollow of Locli Leathan* may possibly have had a like origin. A large boulder of typical albite schist was seen on the shore of Loch Fyne, a mile north-east of Dundarave ; and a gigantic grit block over 300 tons in weight is lying on the lower slopes of the Add.t In not a single instance have erratics been observed which could not be traced to a north-eastern source. In this connection it may be noted as a significant fact that the Lome Andesites which are strongly developed west of Loch Awe have not contributed a single boulder to the region south-east of that loch. J. B. H. South-eastern District — Though the ice movement near the higher hill tops in Oowal was at one time south-east, it is uncer- tain whether any boulders have been carried from the north-west side of Looh Fyne to the south-east side. On the south-east side of that loch no blocks of the andesites of Lome have been found, and pieces of quartz-felsite are somewhat uncommon, and may perhaps all have been derived from bands in that area. It is possible that when the lower part of the ice sheet was moving down Loch Fyne and the areas near it, the higher portion flowed ofi' in a south-east direction. The granite boulders derived from the Garabal Hill and Meall Breac complex are extremely abundant on the shores of Loch Fyne, Loch Goil, Loch Long, and Loch Eck, and the hillsides near Loch Restil, Socach, and Caol-ghleann. On the shores of Loch Fyne the granite erratics are associated with boulders of diorite, altered albite schist, and a schist with spots of white opaque felspar — an assemblage which could only have come from the Garabal complex and adjacent areas to the north-east. The following list gives some of the highest localities at which granite boulders have been noticed, viz., 240 yards north-east of the Ordnance Station on Binnein an Fhidloir, at a height of 2600 feet; 170 yards north-west of the same station, at a height of 2500 feet ; close to the top of the watershed 600 and 700 yards north-west of Mullach Coir a' Chuir ; the south-west side of Beinn Tharsuinn (Lochgoilhead), at a height of 1760 feet ; 260 yards south-west of Gruach nan Capull (St. Catherine's), at a height of 1800 feet ; nearly a third of a mile slightly north of west of Sitli an t-Sluain, at a height of little more than 1000 feet. The highest locality at which the granite occurs in situ is on Meall Breac (2115 feet), in the north-west corner of One-inch Map 38, and it is clear that the boulders on Binnein an Fhidleir, east of the head of Loch Fyne, must have been pushed upwards to higher levels. There are several boulders of the dolerite of Sith an t-Sluain about a quarter of a mile south-west of that hill ; none have been noticed in other directions. Boulders of albite-schist are found at various localities north- west of the main outcrops of albite-schist — for instance, on the shore at Tigh Claddich four miles from the head of Loch Fyne, in *^-mile N.W. of Socach, resting on limestone. + 300 yards S.B. of Achnagatearlach. Fl'uvio-glacial Gravels. 139 the burn a third of a mile S.S.E. of Laglingarten, by the road a little more than a third of a mile S.S.W. of Ardno, and eighty yards above the road a little more than half a mile east of Socach. Hence it has been suggested that there was once a transport from tiie albite-schist hills in a north-west direction. But this does not seem necessary, for the same ice which carried the granite boulders in a south-west direction would cross outcrops of albite-schist. C. T. 0. V. FLUVIO-GLACIAL GRAVELS. District of Loch Awe and the Add. — These deposits are mainly confined to the basin of Loch Awe and' to the lower part of the valley of the Add where that river deploys upon the plains. On the Loch Fyne slopes the 100-feet beach seen at Auchgoyle Bay appears to be wholly a glacial deposit. Its upper surface is irregular, and consists of a thickness of ten or twelve feet of gravel resting on boulder clay, a section of which is seen on the Allt an Tosdach burn. But in the Auchgoyle burn just above the higher margin of the gravel there is a deep section consisting of loose stony drift intercalated with beds of sand and gravel, which is underlain by a foot of coarse conglomerate lying in brown clay and resting on the normal stiff blue till ; the junction between the two latter being contorted. The unconformity between these upper drifts, and the basal till is clear, and the former appear to corres- pond to the gravels fonuing the 100-feet beach lower down ; more- over, the boulder clay forming the margin of that beach further south is seen to be full of perfectly rounded stones. The slope that fringes the Add between Duchaman and Achna- shelloch is covered with spreads of sand and gravel, in which boulders are sometimes embedded. Similar deposits are seen in the valley of Kilbride, where some high level gravels overlie the drift which are clearly not referable to the stream deposits of that valley. They are very coarse, and include blocks of epidiorite and quartz-porphyry as large as two feet; the latter have evidently been derived from the upper part of the Add basin far to the north-east. These gravels, however, attain their greatest development on the margins of Loch Awe, where they are clearly associated with the oscillations of level that mark the later history of that loch during the close of the Glacial period. These oscillations have been con- tinued so far that instead of the natural discharge of that loch at its south-western end that marked its early history, the exit is now situated in the Pass of Brander, where it empties into Loch Btive ; so that this loch presents the singular feature of discharging its waters at its upper end, from which it derives the main source of supply. It formerly overflowed through the hollow now occupied by Loch Leathan into the Add valley above Kilmichael. At a subsequent period, after a partial recession, it effected an outlet at a lower level through the naiTow gorge of Oreagantairbh on to the plains of Kilmartin ; while still later, when the surface of the loch stood below the level of that gorge, the cleft along the 140 Glacial and Becent Deposits. Pass of Brander was sufficiently deepened to admit of its overflow into Loch Btive. It is probable that the Creagantairbh channel existed at a still earlier epoch than that alhided to, and was choked v/\th glacial deposits when the loch stood at its highest level. These gravels form a serifes of well-marked terraces in the neighbourhood of Ford, the most perfect of which extends from J'ord Pier to Loch Ederline, with an average height of seventy feet above the level of Loch Awe.* Its eastward extension is interrupted by the Ford Eiver, the dissection of which has exposed gravel cliffs thirty feet in height. The western terrace in the neighbourhood of Ford Pier possesses a smooth natural surface with a gentle declivity towards the loch ending in a cliff slope ; but between Ford and Loch Ederline it is broken into a series of mounds, the upper surfaces of which are of uniform height. Its counterpart on the eastern side of the Ford River is much more uneven, partly due to its having enclosed islands of epidiorite, but mainly to the presence of basin-shaped hollows in the gravel, one of which is occupied by a small lochan.f Between the hollows, hillocks of gravel appear, the heights of which are not only uniform with one another, but with that part of the terrace which is unbroken. When these terraces were formed the loch discharged its overflow into the valley of Kilmartin through the Pass of Creagantairbh, the summit level of which is between thirty and forty feet below their upper margins. That they were formed, before the close of glacial conditions will be seen presently from the evidence at Durran, and their modification is probably diie to the fact that the outlet was partially choked with ice that descended to the loch level in the more mountainous region to the north-east, the subse- quent melting of which has given rise to the hollows. Before tracing further the line of these deposits along the valleys of outlet, it will be convenient to describe the terraces along the margins of Loch Awe, many of which fringe the eastern shores. At Fincharn there is a well-defined gravel terrace at a height of 210 feet,+ which extends up the course of the stream to about the 300 feet contour line, and apparently marks the delta formed at the mouth of that burn when the loch was at a higher level ; while between Fincharn and Braevallich relics of that former rise of level of Loch Awe are preserved by small fringes of gravel as high as 300 feet. The valley of Braevallich is covered with a series of well-marked terraces extending to the same height. One of these gravel slopes, which reaches a height of 200 feet at Durran, extends as a strip parallel to the loch margin for upwards of a mile at a uniform height of 170 to 180 feet, and corresponds, therefore, to the * The surface of Loch Awe is 116 feet above ordnance datum. t The hollow J-mile further east was the site of a lakelet at the time of the Naval Hydrographical Survey in 1861. \ These heights, unless otherwise noted, refer to ordnance datum and not to the level of the surface of Loch Awe, which, as previously stated, is 116 feet. Fluvio-glacial Gravels. 141 terraces at Ford. Along the valley at Braevallich the terraces have been moclified by the erosion of the stream, but their shape and arrangement along the hill slopes point to their beach origin. The highest of these terraces half-a-mile south-east of Braevallich is evidently associated with glacial conditions. It is flat topped, but is capped by a spread of blocky detritus brought down from the high ground above; these blocks rest unconformably on fine-grained wavy sand, which is supported by fairly rounded coarse gravel, the latter being underlaid by a fine sand containing a few angular blocks, while the very bottom consists of a wavy laminated sandy clay with large isolated blocks. The beaches, corresponding in height to the Ford terraces, are fringed with moraines, some of which are apparently lying on them, while one moraine is seen among the gravel beds near Durran only twenty feet above the surface of the loch. Having furnished evidence, therefore, of the former elevation of Loch Awe to a height of at least 300 feet (nearly 200 feet higher than at present), and that glacial conditions prevailed at that phase in its history, we may, in the light of this evidence, examine the channels of its ancient outlet. The terraces of Ford hithei-to described are confluent with a set of alluvial deposits which partially line the valley between Loch Ederline and Loch Leathan. The surface of the latter, which constitutes a basin on the very summit level of the Pass connecting those lochs, is about 240 feet high, that is to say, sixty feet lower than the former level of Loch Awe, and about the same height above the terraces at Ford. The alluvial plain of Lon Glas, now mainly covered with peat, in which a few isolated gravel mounds are emerging, attains on its flanks a height of about 200 feet to within half-a-mile of Socach, an elevation only slightly greater than that of the Ford terraces. For half-a- mile, however, in the neighbourhood of Socach gravel terraces fringe the valley to a height of 250 feet, and the intervening ground to Loch Leathan is a peat flat representing the former extension of that loch. On the margin of Lon Glas beyond Glasvar, relics of a gravel terrace are seen corresponding in elevation to the deposits of Socach. But the evidence that the waters of Loch Awe actually overflowed from th6 summit level of Loch Leathan into the strath above Kilmichael is not restricted to these topographical data. Although Loch Leathan now drains to the northward (into Loch Awe), the valley to the south of that loch has been deeply excavated in the drift by an ancient river channel into which a small stream from the north-west has since been diverted; while to complete the evidence, the hollow lower down (about one mile from Loch Leathan) is lined by a gravel terrace which shares the declivity of the valley. This terrace in its upward extension along the hollow reaches a height of 200 feet, and occurs in small strips and patches along the valley of the Add which have been eroded by the action of that river. These terraces frequently display very irregular surfaces, while at Lag they are associated with mounds of gravel containing large rounded blocks. While the ancient exit of Loch Awe was by way of Loch 142 Glacial and Recent Deposits. Leathan, this channel was no longer available on the retreat of its waters below a level of 240 feet, when it had recourse to the present hollow of Oreagantairbh, which diverted its overflow to the plains of Kilmartin. The alluvial terraces of Ford are continued past Loch Ederline into that hollow, the summit level of which is about thirty feet above the surface of Loch Awe, and rather more than that measure- ment below the top of those terraces. Beyond the summit level at Glennan the river terrace makes a flat feature, and the large size of some of its boulders points to the rapidity of the stream, while its volume is still further shown by a gigantic pothole excavated in a massive epidiorite which crosses the valley at Gurach. Moreover, on issuing from the narrow gorge of Oreagantairbh, its presence on the plains of Kilmartin is attested by a continuous array of flat- topped terraces. Before quitting the consideration of the chronological events that attended the close of the glacial epoch, so far as relates to Loch Awe, it will be interesting to note that this loch retains no traces of marine submergence, notwithstanding that a depression of about 140 feet would necessitate an inroad from the Sound of Jura and convert it from a lake to a fiord ; and moreover, no glacial shells have been found among its clays. It seems probable that its discharge through this comparatively low pass of Oreagantairbh was coeval with the formation of the marine beaches that mark the fifty-feet level, one of which, in Loch Fyne, is associated with a moraine and shows contortion. This lower channel was, of course, sealed when the loch stood at the higher elevation, and the valley of Loch Leathan marked the site of its overflow ; and as the ice-sheet must at that period have retreated from at least the lower part of the Loch Awe basin, this epoch may probably be represented by the formation of the 100-feet beach. It is evident, therefore, that a general submergence of the area sufiicient to make Ijoch Awe an arm of the sea would, in the light of this reasoning, necessitate a depression of at least 250 feet. If such submergence took place, it must have been prior to the later stages of the glacial pei'iod above narrated, and no traces of it have been detected. J. B. H. District of Loch AvicJi. — The shrinkage of the confluent glacier during the later glaciation of this district is probably indicated by a lake terrace 200 feet above the present level of the loch which is found round the south-west or upper end of Loch Avich. This terrace is of such a character and in such a position that it must have been formed when the lake drained to the south-west towards the head of the Barbreck River in Glen Domhainn. The gravels and sands of this terrace have been traced to the actual watershed at about one-third of a mile to the south-west of Maolachy, on the road between Kilmelfort and Loch Avich, where the road traverses a small rocky gorge evidently cut by the overflow from the lake. The fine silts and sands of the terrace can also be seen close to the roadside near the same spot. The present outlet of the lake shows Fluvio-glacial Gravels — Shelly Marine Clays. 143 no evidence of a barrier of moraine stuff or rock of such depth having been removed since glacial times, and the only explanation of the position of the terrace, which is in every way comparable to one of the terraces of the " Parallel Koads of Glen Rojr," is that the barrier was due to the lower end of the lake being occupied by ice when the terrace was formed. B. N. P. District South-east of Loch Fyne. — Some fluvio-glaciaJ sands and gravels spreading high up the hill slopes between the Our and the Cab, which have no clear relation to the present streams, are seen in a little stream that comes into Liogan a little above its junction with Oriogan, about three miles east of Strachur, and similar beds are common in the district south-east of Loch Fyne, but they have not been systematically mapped. In the little stream referred to, some of the laminae contain a black cement, probably the black oxide of manganese, and in fluvio-glacial beds three quarters of a mile E.S.E. of Socach, a black cement has been proved to be mainly composed of this substance. On either side of Glen Finart below Coille Mheadhonach there are patches of sand and gravel in the form of mounds. A section on the roadside thirty yards south of the burn which flows into Loch Goil a mile and a quarter north of Oarrick Oastle shows, underneath a gravel which probably belongs to the raised beach, three or four feet of fine sand in which the bedding has been folded into isoclines, probably under the influence of advancing ice. 0. T. 0. vi. SHELLV MARINE CLAYS. At various localities on the coast there are beds of laminated clay containing glacial shells which appear to occupy the positions in which they were originally embedded. A few feet above high- water mark, in the little stream by the Lodge about 700 yards E.N.E. of Rudha No, and about three miles from the head of Loch Fyne, there are a few feet of greenish-grey sandy laminated clay dipping gently down stream, and about thirty yards above the Lodge the same clay is exposed more clearly and contains some stones six or seven inches long, and shells, both singly and collected into thin layers. When the ground was surveyed, a greenish-grey laminated clay with shells was exposed on the foreshore a third of a mile and also nearly half a mile north-east of Oreag nam Faoileann, south of Strachur Bay, but old exposures are liable to be hidden and new ones formed by the shifting of the shore gravel. In one of the exposures the laminas dip south-east at 30°, while in another towards the north-west, and there are indications of contortion. This deposit was described by Mr. W. Ivison Macadam,* who gave * "Preliminary Notice of a Clay Shell-bed between Newton and Stracliur, Loch Fyne, Argyllshire," Trans. Edin. Geol. Soc, vol. iv., 1883, p. 94; and "Further Notice of the Tigh-na-criche Shell-bed, Loch Fyne, Argyll- shire," op. cit., p. 232. 144 Glacial and Recent Deposits. lists of the shells collected by him, and also an analysis of the clay. The analysis is as follows : — Ferric oxide, FejOg, 16-08 Aluminic oxide, Al.O.,, - 22-83 Calcic oxide, Ca 0, 2-55 Magnesic oxide, Mg O, 1-96 Silica, etc.,vSi 0,, - 56-58 100-00 The lists of shells collected by Mr. Tait are given in an appendix. Close to Douglas Pier, not marked in the One-inch Map, but about 130 yards south-west of Corriesyke, Loch Goil, a laminated stoneless clay was noted dipping steeply seawards, and a similar clay was observed also in two places north-west, and in one south- east of this exposure, but no shells were noticed in the clay.* C. T. C. vii. RAISED BEACHES. Along the coast of Loch Pyne the twenty-feet beach forms a more or less continuous fringe, the individual deposits being separated by no considerable interval. The fifty-feet beach, on the other hand, is of less frequent occurrence, while the higher shelf, designated by the general term of the 100-feet beach, is yet further restricted. These ancient beach deposits usually have a bouldei'-clay floor, and are conspicuously developed at the mouths of the valleys, the indentations of which have protected them from the scour of the currents, and in such localities they merge naturally with the deltas of the present streams. Shells are of rare occurrence in these deposits, an example of which has already been adduced.t That the older beaches were formed before the close of glacial conditions has already been referred to in connection with the 100- feet gravels of Auchgoyle. Further evidence in support of this conclusion is found at Auchnatra, two miles to the north-east of Inveraray, where both the beach deposit and the underlying boulder clay are contorted, while a morainic mound which abuts against its landward margin points unmistakably to the origin of the disturb- ance. This beach, consisting of sand and gravel, is sloped unevenly and ascends the hillside to a height of fifty feet. The evidence, therefore, is conclusive that a glacier occupied the head of Loch Pyne while that beach was in process of formation. Amongst the more important raised beaches, the one situated between the Dubh Loch and the head of Loch Shira, extending past Inveraray almost continuously as far as the Douglas Water, may be cited. (Plate I.) At the head of Loch Shira the deposit reaches a height of sixty feet. At Inveraray the fifty-feet beach is involved with the alluvial gravels of the River Aray, while the 100-feet * The glacial shell beds on the north coast of Loch Fyne at Lochgair and Minard have already been referred to. t A little to the north of Gallanach in the inlet of Lochgair. Raised Beaches. 145 beach is mainly represented by a rock shelf, and west of Dalchenna the fifty-feet beach is mostly indicated by its marginal ledge. At the mouth of the Douglas Water it has been modified by the delta deposit of that river, and from thence the twenty-feet beach occurs in a series of isolated patches as far as Furnace, where a more exten- sive series of gravel platforms is seen; the terraces, however, above the fifty-feet contour, which at Goatfield reach a height of 100-feet, are carved out of boulder clay. To the north of Orarae these deposits are insignificant, but between that place and Union Bay they fringe the coast almost continuously with a varying height from forty to fifty feet, except in the neighbourhood of Auchgoyle, where gravel beds ascend to 100-feet, and, as hitherto remarked, merge into glacial deposits. The ancient beach on which Minard Oastle has been built forms a conspicuous platform, attaining a height of forty feet. The twenty-feet beach encircles the inlet of Lochgair and extends to the southern portion of the sheet beyond Kames. On the south-side of Loch Fyne the 100-feet beach is not repre- sented, but the fifty-feet beach is seen at Newton Bay, Lephinmore, Lephinchapel, and Goirtein, while a narrow fringe of the same deposit borders the valley of Strathlachlan. The twenty-feet beach is likewise of sporadic occurrence between Newton Bay and the southern margin of the sheet beyond Goirtein. J. B H. SoutJi-easiern District. — The raised beaches in many places, particularly in Loch Goil and Loch Long, are represented hy shelves of erosion merely, but considerable spreads of sand and gravel, either of marine origin or formed of slightly rearranged marine materials, occur near the mouths of many of the larger burns. No relics of the 100-feet beach have been observed on the south-east side of Loch Fyne, in Loch Goil, nor in Loch Long. In various places there are raised beaches that pass quite gradually into terraces the tops of which, when traced inland, reach 100 feet in height or even more, but the materials at that high level have probably been deposited in fresh-water. On the west side of Loch Eck, however, there is a rude terrace, which extends northward for nearly 300 yards from the south margin of the map, and which may represent the 100-feet beach. The absence of the 100-feet raised beach in the sea lochs perhaps indicates that while this beach was being formed in the less mountainous areas the upper parts of these lochs were filled with ice. On the other hand, it should be remembered that glacial shelly clays of the type usual in the Clyde area are well exposed in Strachur Bay, and that some authorities consider these clays to have been formed contemporaneously with the 100-feet raised beach, but in deeper water. It is possible, therefore, that this beach once extended to the head of the loch, and has been deformed by later ice-action. There is a marked gravel terrace at Ardnahien (Loch Goil) at a considerable distance from any large burn, which attains a height of about forty feet at the landward margin. The highest alluvial flat in Glen Finart is a little over the forty-feet level, and the high terraces close to the sea at Corrow, Lochgoilhead, and Sti'achur are only a little lower. K 146 Recent Deposits. The twenty-feet I'aised beach is represented almost continuously either by sand and gravel flats, or by shelves of erosion. Eock knolls surrounded by raised beaches, or by these and the present beach, which must once have formed islands, are found, in Loch Long at the foot of the Knap burn and rather more than three-quarters of a mile N.N.E. of Shepherd's Point; in Loch Goil in the Carrick Castle Hill, and a mile south-east of Cruach nam Miseag ; and in Loch Fyne at Leak, Stucreach, a little north-east of Creag a' Phuill, and St. Catherine's. C. T. C. Recent Deposits. alluvial terraces. In Glen Fyne the mouth of the valley for two ndiles is occupied by broad spreads of alluvia traversed by the Eiver Fyne ; in the upper reaches of that glen, however, they are both meagre and discontinuous. The valley of the Shira is floored by an alluvial strath about four miles long, in which the river winds. Com- mencing at its upper end as a narrow wedge it widens at a uniform rate until at its mouth it forms an expanse half a mile in breadth. In its upper limits the fall is fairly rapid,* and gravel has been deposited, but over the greater part of the valley the declivity is slight,! and the alluvium largely consists of fine silt. The river empties into the Dubh Loch, which is a rock basin in process of being infilled ; and as the greater part of the river channel is marked by an absence of solid rock it is possible that this strath forms the site of other rock-basins concealed below the alluvium. In Glen Aray alluvial deposits form a broad strath between Inveraray and Carbonan Mill, beyond which they are inconspicuous. An alluvial deposit has been formed by the Douglas water at Claonairt as well as at its mouth, and by the Leacann waters below Brenachoil and at Furnace. A well-marked strath confains the winding course of the River Add between Kil- michael and Lecknaiy, while the upland phase of that river is partially fringed with alluvium along the valley of Gleann Airidh. On the western edge of the map a broad alluvial deposit covers the valley of Kilbride. The fluvio-glacial gravels between Loch Awe and Loch Leathan have been dissected by the stream which connects those sheets of water, and the rearrangement of these deposits is gradually filling the rock-basin occupied by Loch Ederline, which attains a depth of nine fathoms. Along the shores of Loch Awe alluvial deposits mark the mouths of the principal streams, one of which below Inverliver connects Eilean Liver with the mainland, except when there is an abnormal rise in the waters of that loch. This deposit forms a delta in which the former channels of the river are still visible, while the existence of others is marked by the indentations of the coastline, which cor- respond to their mouths. Formerly the mouth of the river was situated in the bay on the west side of Eilean Liver, while at the * 30 feet in half a mile. t For about 3^ miles the fall is less than 10 feet per mile. Peat. 147 present it is north of that peninsula. The Inverinan burn is flanked by well-marked terraces, the uppei'most being shelves of" boulder clay.* The margin of Loch Awe is sometimes skirted with a narrow strip of gravel, well seen between Inverinan and New York, a great part of which is covered on the rise of the loch.f Alluvial flats, which have filled depressions marking the site of ancient lochans, occur in the neighbourhood of Barmaddj)-, while the basin now occupied by Lochan k Bhruic is in process of being silted up, the loch having been reduced to one-half its original size. In the district lying to the south of Loch Fyne, an alluvial terrace lines the picturesque vale of Strathlachlan. This strath, however, is dwarfed by the alluvial plains of Glendaruel. As the declivity of its floor scarcely exceeds ten feet in a mile, when the rainfall is excessive the plains are inundated and receive a fresh supply of alluvia, which contribute to their fertility. t J. B. H. Soidh-eastem District. — On the south-east side of Loch Fyne the widest terraces occur in the Lochgoilhead valley, the valley be- tween Strachur and Loch Eck, and in Glen Finart, but in none of these is the breadth much more than half a mile. The alluvial flat on the east side of Polchorkan above Lochgoilhead adjoins a peat moss on the same level ; the latter is perhaps older than the alluvium, and in part covered by it. Part of the flat ground between Balle- meanoch and Strachur is also occupied by a peat moss, and part by the alluvium brought down by the River Cur. The rise between the north end of that moss and the adjacent flat in the Loch Fyne watershed by Strachur ,is slight and for the most part drift covered, and perhaps the Cur may once have discharged into Loch Fyne. Near Ballemeanoch the alluvial flats rise to greater heights above the bed of the adjacent part of the river, and the river must once have had a more rapid fall than at present. Within the more hilly area its course has no doubt been eroded and deepened, while in the Strachur and Loch Eck valley it has perhaps been raised somewhat by material brought down from the hills. At the side of Loch Eck an obscure horizontal feature of erosion, about two feet above the ordinary level of the loch, is seen at various localities between Dornoch Point and the foot of Ooire Ealt. C. T. C. PEAT. The most extensive peat moss in the district is that of Moine Ghlas, the superficial expanse of which exceeds 2000 acres ; situated * Other gravel deposits ' occur on Loch Awe at the entrance to burns at Ballimeanoch, AUt Beochlioh, Blarghour, Ardchonnel, Karnes, Eredine, Durran, Fincharn, Druimdarroch. t Less continuous strips are seen at the following localities — Ooille Chi\il, between Ardchonnel and Barr ; Eredine, and Kilneuair. \ Only the upper portion of this strath, for about 2 miles, lies within the confines of this map. In the autumn of 1888, when the geological survey of this glen was in progress, an inundation extending right across the valley came under our own observation in the vicinity of the Olachan. 148 Recent Deposits. at an elevation averaging 400 feet, it occupies a great upland tract between Loch Glashan and Loch Blackmill, three miles east of Kilmichael. In the extreme south-east a flat expanse of peat flanks the Crinan Canal, which is now under cultivation, but within the memory of the oldest inhabitants was the site of a swamp. The north end of Loch Leathan, N.N.E. of Kilmichael, is bounded by a peat strath extending as far as Socach, which doubtless marks a former extension of that loch ; while inter- mediate between Socach and Loch Ederline the peat basin of Lon Glas indicates the site of a former large sheet of water. Loch Sitheanach is dwarfed by the peat flat into which it merges, the margins of which testify to the magnitude of the original lake, while the site of former lochs is indicated by the peat basins, the localities of which are enumerated in the footnote.* Peat often forms longitudinal strips underlain by di'ift, between the more prominent rock features, and this is especially the case when the streams leave their normal course and enter these ' parallel hollows t so that their current is sluggish until they rejoin the natural drainage system of the district which is transverse to the strike of the prominent rock bands that have determined the topography. But in addition to its occurrence in well-defined flats, peat occupies great expanses of the higher ground. The watershed lying to the west of Cruach Mhdr (above Glen Aray) is extensively covered with thick peat, and similar growths are especially well developed in the district bordering Loch Leacann, and in the valley of the Abhain Buidhe. The remains of trees preserved in these deposits point to the aflbrestation of the country at altitudes considerably in excess of that pertaining at the present day. J. B. H. South-eastern District. — In this region only the peat which occurs in basin-shaped or tolerably well-defined hollows has generally been mapped. In most of the area south-east of Loch Fyne the amount of peat on the hills is not large, their sides being steep and their tops narrow. Considerable spreads of hill peat are, however, found in the following localities : — between Dunans burn and Caol Ghleann, on one side, and Strath nan Lub, Eas Davain, and the heads of Glen Branter and Glean Dubh, on the other; north of the head of Eas Dubh. Certain of the peat mosses, for instance, those about a mile south of Carrick Castle, east of Tom * I mile N.W. of Loch Gaineamhach, 1| miles West of Crarae, and immediately to the S.E. of Cruach an Lochain. This last example is still a swamp, and, from the name of the hill, was evidently a loch within a recent period. t Localities — 1 mile S.E. of Loch Leathan— Feorlin (parish of Kilmichael Grlassary) — The upper waters of the Abhain a Bhealaich burn and of the Allt ath Mhic Mhairtein — Between Cruach an Eaglaich and Cnoc Sgliatach — The lower part of Allt a Ghlinne covering an alluvial spread at the northern extremity of Loch Avich, and three parallel straths north-east of Kames Bay A conspicuous peat ilat flanks the main road between Auchindrain and Kilian, while a parallel flat of like extent occurs on the slopes midway between that high road and Kenmore, Landslips. 149 Molach (east; side of Loch Goil), and a third of a mile north-west of Oruach nan Milseag, probably mark the sites of old lochs. 0. T. 0. LANDSLIPS. Many of the largest landslips, as, for instance, those on the north-west slope of Beinn Mhor, the south-east sides of Caol Ghleann and Onoc Coinnich, and near Upper Olasheoin and Achadiman, are on declivities, inclined in the same direction as the foliation planes in the schists — a situation which must have favoured the development of the slip. There seems, however, an equal number of examples on hillsides which slope at right angles to these planes. The head of the landslip half a mile south-east of Lochgoilhead, pai'ts of the north-east side of the slip a quarter of a mile north- east of Upper Olasheoin, and the north-east side of the slip on the west slope of Carnach Mor, lie along lines of crush or veins. C. T. 0. A landslip situated on the eastern slopes of Glen Shira, immediately above the Dubh Loch, has involved that hillside for over a square mile. A quartz-porphyry sill which has shared in the disruption has contributed a large amount of debris to the lower slopes. From the contour of the bed of Loch Dubh it appears probable that a pile of the wreckage was accumulated in the middle of that loch. Not improbably this colossal landslip may be referred to the time of the melting of the ice at the very close of the glacial period. J. B. H. CHAPTER XIV. Economic Resources. In spite of the profusion of igneous rocks of all types and ages which characterise the geology of this region, there is a disappoint- ing absence of those mineralised zones which almost invariably indicate a concentration of the metallic ores. Fissure veins are suJSBiciently abundant, but the veinstone is in most cases quartz or calcite singularly free from impurities, and with metallic oxides poorlj'' developed. Moreover, the quartz and calcite veins are devoid of structure* and exhibit none of those parallel layers which so frequently characterise those veins that contain the mineral ores. This uniformity in the building up of the veins is illustrated by the huge masses of vein quartz situated on Clachan Hill, the largest of which is 100 yards in length, and from twenty to thirty yards in breadth, which are singularly pure notwithstanding their location in the vicinity of plutonic intrusions. A calcite vein of even larger proportions occupies the pass between Glen Liver and Gleann Domhain,t filling a north-west fissure, the magnitude of which admits of its being mapped as a limestone, while the occurrence of swallow holes marks its outcrop. In addition to quartz and calcite small quantities of ferriferous carbonate are occasionally seen forming thin veinsj and in unim- portant patches along the planes of fissure. While these veins rarely exhibit a sufficient admixture of mineral ore to constitute a gossan, they occasionally contain small quantities of lead, copper, and blende. Moreover, the galena is sometimes combined with silver in appreciable quantity. In a few localities mining operations have been carried on to a limited extent, all of which have been abandoned. At the head of Loch Fyne, near Glachan-bheag, trials have been made in the Loch Tay limestone, presumably for lead, and as that mineral is not seen in appreciable quantities it may be inferred that it was sought for its argenti- ferous contents. Nickel has been extracted in the past from two localities lying to the south-west of Inveraray. One of these mines is situated at Coille-bhraghad (one and a half miles south-west of Inveraray * Of course such crystalline structure as may be visible in the building of the vein or structure due to mechanical deformation is not here alluded to. + Bealaich na Croise, If miles N.W. of Inverliver. I A ferriferous carbonate vein about 6 inches thick, running E.N.E., was observed J raile N.N. E. of Oarraig nan Roin (Loch Long), and thin "flats" of the same mineral with a similar thickness occur near a northwest fracture i mile E.S.E. of the Saddle. Economic Resources. 151 Castle), the other at Oraignure, along the. coui'se of the Allt Aoill bum, locally known as the Oraigerrine mine (one and three-quarter miles south-west of Brenachoil). The latter mine was originally worked for copper, but when the ore of the Ooille-bhraghad mine was found to contain nickel, the former was re-examined, and it was ascertained that while the copper ore had been collected, the nickeliferous ore had been thrown aside as refuse. The nickeliferous ore is magnetic pyrites (pyrrhotite) in which nickel occurs to the extent of from ten to twelve per cent. Although cobalt is an exceptional constituent, it has been detected in both localities, but in quantities less than one per cent. At Coille-bhraghad about one per cent, of copper was contained in a specimen analysed from the bottom of the shaft. Attention appears to have been directed to the presence of nickel about the year 1851 at Coille-bhraghad, but the mine was only worked for a few years. The older copper mine at Craignure does not seem to have been seriously worked at all for nickel, although the nickeliferous ore found amongst the refuse appears to have found a ready sale. The ore in this mine near the surface is in seams interbedded with quartz-schists, some of the sedimentary alternations having been completely mineralised. They are dip- ping at a low angle to the north-west, and have a north-east trend in parallellism with the quartz-porphyry sills which are so common a feature of the district. It is said, however, that when these veins leave the strata they become much steeper. Moreover, the basalt dykes are said to have produced considerable effects on the nickel veins, so that near the sides of those dykes the veins are generally found thicker and the ore of superior quality, whereas at a distance the vein is generally more attenuated, breaks into strings, and the ore contains less nickel and more sulphur. Considering, however, the limited nature of the mining operations, and the undoubted association of the nickeliferous ore with the quartz- porphyry intrusions, the association of basalt dykes with high-grade ore is probably accidental. Pyrrhotite in association with iron pyrites occurs in the hornfels zone which bounds the quartz-porphyry mass of Garbh Achadh. So far as can be ascertained from an examination of the outcrop no well-defined vein is visible, but the ore is evidently of the same nature as the nickeliferous ores already mentioned, and appears to contain small quantities of both nickel and copper. Midway between Garbh Achadh and High Balantyre trial workings have been carried on amongst pyritous ore, also in the vicinity of quartz-porphyry, but although this locality is richer in copper we did not observe any nickeliferous pyrites, and the specimens collected do not contain pyrrhotite. The occurrence of this nickeliferous ore in the aureole of meta- morphism of the quartz-porphyry and within the margin also of that intrusion, and the absence of mechanical deformation, clearly separates it from the older basic intrusions of the district which are schistose. This association of nickel with the acid intrusive rocks constitutes yet another example of the irregular affinities of 152 Economic Resources. the metallic ores, that element being a characteristic associate of the ultra-basic rocks. Thin strings of argentiferous galena are said to have been found in a N.N.E. fracture near Bridgend,* and a few strings of galena occur near the probable course of the same fracture in Coire No. On the east side of Glen Fynet small veins of barytes contain thin strings of galena, together veith small specks of copper pyrites. In the vicinity of Mark J (Loch Long), thin mineral strings which cross the foliation of the schists contain quartz, specular iron, magnetite, pyrrhotite and chalcopyrite. A pyritous schist in the neighbourhood of Strachur,§ in the Ardrishaig phyllite series, contains small proportions of galena, blende, silver, and gold ; and the results of an assay of what appeared to be a sample of the richest stone are given below. || The pyritous schist is chiefly of a siliceous character, and is un- usually rich in pyrites, some bands as thick as six feet being com- posed almost entirely of that mineral, together with some galena and blende, while other portions only contain scattered grains of pyrites, or thin streaks of that substance running with the folia- tion. The highly pyritised bands are sometimes folded along axial planes parallel to the foliation. The total breadth of this pyritic zone is about eight yards, and it appears to have no connection with an adjacent fault. A pyritised schist which has not been assayed occurs a little to the south-east of Oreggans Point, in which the pyrites lumps exceed an inch in size. Schists discoloured with cupriferous impregnations are seen adjacent to the high road at the head of Lochgair. The existence of slag heaps on the hill slopes has given rise to the opinion that iron oi'es pf the district have been smelted in the past. From the distribution of these old bloomeries without reference to the more ferriferous localities, this view is improbable. While it is not impossible that some of the thicker veins of ferriferous carbonate may have been used for smelting purposes, we found no indications that any mining had been done on the veins observed ; and it is most probable that ores were imported and smelted with charcoal from the native woods, just as in later times similar, but more extensive, smelting operations were carried on at Furnace, which appears to have had its origin with this industry, as its name sufiiciently implies. * J mile N.N.E. of Bridgend, the crush is about 6 feet wide, hading west, and is accompanied with a felsite dyke. t ^ mile slightly S. of W. of the Eagle's Pall. The barytes veins do not exceed 8 inches and the galena strings one inch in thickness. X 120 yards NE. of Mark, the strings are flattish and about an inch thick. S 70 yards N.N.E. of McPhun's cairn. II Analysis made at South Kensington by Mr. Ernest A. Smith : — Dwts. Qrs, Silver 11 18l„...^^^^^^f22^Qj^,^_ Gold ^} \h^'' Insoluble residue, mainly silica with a little mica, 24'50°/„ ; zinc, 3"57o ; iron, 30'0% ; lead, 3'0%; sulphur, 30'67o ; together with alumina and oxygen. Sample in one lump. Total weight, 13,700 grains. Economic Resources. 153 Analyses by Mr. W. I. jMacadam of specular iron ore from Strachur are given in the footnote.* The poverty of mineral is not confined to the concentrations of the metallic ores which are the products of mineral veins, but there is an equal sterility of those minerals which enrich rocks in a state of diffusion. Iron is, of course, a prominent constituent of the basic rocks, and the rusty decomposition of the basalts is especially liable to give rise to erroneous impressions of their percentage of that mineral, which is nowhere sufficiently con- centrated as to constitute an ore. The graphitic-bearing rocks doubtless contain large quantities of graphite, but very small proportions of this mineral are sufficient to darken a rock and even to yield its characteristic streak. From analyses which have been prepared from graphitic rocks in the adjoining sheet to the westward (Sheet 36), it is doubtful whether the richest portions of these deposits would yield gi-aphite to the extent of more than four to eight per cent. Moreover, the chemical composition of these rocks being determined by phases in their deposition, the aggregation of this mineral in payable quantity is not likely to obtain ; and until the processes of extraction have been considerably cheapened such deposits cannot be said to possess any economic value. There is reason to believe that oxide of manganese is more frequently associated with the oxides of iron than their surface indications would suggest. Chemical analyses usually give traces of this mineral, but its occurrence in payable quantity has never been observed. Nodules of oxide of iron containing sufficient manganese to be detected with the blowpipe test are frequently found within the peat, the mineral, of course, having been extracted from the underlying rock by the peat acids. A quantitative estimation of manganese by Dr. Pollard of one of these concre- tionary nodules from the hill peat, at the head of Glen Shira, gave 1-21 percent. MnO. Sir John Miirray, investigating the origin of the manganese in the Clyde sea area, attributed the source of this mineral to the disintegration of the rock masses bordering that region. In the streams manganese was found in solution in combination with carbonic and humic acids, the manganese being more abundant in solution towards the head waters of the streams, especially near deposits of peat. The lesser quantity at the mouths of the rivers being due to its deposition as dioxide on the stones in the river * From Trans. Geol. Soc. Edin., vol. iv., 1881, p. 95. Specimen (I.) was picked ore as pure as possible, and specimen (II.) was the ore as found in the veins. The analyses are as follows : — (I.) (II.) Fe,0„ 88-83 37-51 CaO trace 0-28 MgO trace trace SO, 16 0-25 SiOj and Silicates 10-92 61-79 99-91 99-83 164 TJaonomic Resources. bed, coatings and aggregations of manganese dioxide having been observed by him in some of the streams falling into Loch Fyne.* Limestone of good commercial quality is abundant, but the calcining of lime is not as prevalent in this district as formerly, and notwithstanding its abundance, lime is imported to meet local requirements. In the course of the survey the only kiln in active use was that situated in Glen Aray, the property of the Duke of Argyllf. Notwithstanding the abundance of peat, its use for fuel is very limited on account of its distance from the sites of the dwelling- hoiases. South-east of Loch Fyne this form of fuel is seldom taken advantage of, while in the area between Loch Fyne and Loch Awe it is most extensively consumed in the crofting villae'es of Auchin- drain and Auchnangoul. In recent years experiments have been made on the Poltalloch estate, on a process for converting the peat into charcoal, to meet a demand for that material in connection with the iron and steel industries. Although this district is replete with a rich variety of rock suitable for biiilding purposes, with every facility for its trans- portation, stone quarrying has been confined to local requirements, except in the single instance of the quartz-porphyry mass on the coast of Loch Fyne, between Furnace and Crarae, which has been extensively quarried at both those localities for shipment to Glasgow, where it has been utilised for paving setts. The quarries at Orarae are now closed, but operations are still active at Furnace. The quartz-porphyry masses on the north side of Loch Fyne are singularly uniform in character, and suitable for architectural purposes, both as regards beauty and durability. The closeness of grain is naturally accompanied by closeness of jointing, so that large blocks required for foundations and other purposes are not likely to be obtained. On the other hand, the absence of minerals liable to be quickly attacked by acids, and especially the small proportion of iron oxides, together with the evenness of its grain, enables it to withstand decomposition. The red varieties are best adapted for polishing, not only from their beauty, but the hue being due to discolouration by iron oxide, a certain amount of decomposition has been set up, so that their durability is less than that of the paler grey types. While highly suited for building, this rock is not sufficiently tough to constitute a superior road-metal. The granite of Glen Fyne, though more than three miles distant from the sea, is yet located in a favourable position as I'egards transport, should that rock ever be wrought, its removal by rail * "On the Manganese Oxides and Manganese Nodules in Marine Deposits.'' By J. Murray and Robert Irvine Trans. Boy. Soc. Edin., vol. xxxii., p. 721. + The Loch Tay limestone at the hamlet of Glensluan was, a few years ago, occasionally burnt for agricultural purposes. The main band of that lime- stone contains 65"44°/^ of carbonate of lime. The complete analysis is given on page 24. Economic Hesources. 155 being facilitated by the gradient of the glen. This rock is eminently suitable for architectural purposes, the porphyritic type being especially adapted for the latter, while its hornblendic varieties present a combination of hardness and durability, with an increased specific gravity, which would recommend its employment for submarine structures. The epidiorites have been frequently utilised both for building purposes and road-metal. For the former, the fissile varieties, which are readily dressed, have generally been selected, while the niore massive types of far greater durability have been neglected, being more refractory to work. The talcose variety, locally known as potstone, is well adapted for building purposes, not only being soft and pliable to the dressing tools, but experience has shown it to be exceptionally durable. This stone, which has been wrought at St. Catherine's and Oreaggaii, has been utilised for the construction of Inveraray Castle, and an inspection of that edifice shows a wonderful preservation of the original mai-ks of the chisel. For use as road-metal the unsheared and finer-grained epidiorites are the most serviceable, and they probably furnish the best ■ material for this purpose to be obtained in the district, although not so extensively employed as the basalt dykes, which are more readily broken. The Glendaruel serpentine, while sufficiently massive to yield excellent stone, is not likely to be profitably worked on account of its comparatively inaccessible situation. Basalt has been seldom taken advantage of for building stone, as its system of jointing is not sufficiently regular to be profit- ably wrought, but where used it forms, nevertheless, excellent building material, and when its sombre hue has been relieved by red sandstone mouldings and quoins, the combination is attractive. The numerous dykes have been largely taken advantage of for road-metal. Amongst the sedimentary rocks the " green beds" undoubtedly supply the best building material, the more massive varieties being shaped with comparative facility. Most of the stone emploj^ed in the construction of Dunans Castle was procured from a pebbly " green bed."* The limestones have seldom been utilised for building, but have occasionally been used for road-metal. In spite of their low durability, they readily bind and form a good surface, while their absorption is such that these roads quickly dry, and in wet seasons are seldom in bad order. The finer and compact types of the Loch Awe grits have often yielded excellent stone, and the Ardrishaig quartzites have also been quarried. The dark slates of the Loch Awe group yield roofing slate which have been rai.sed in former years, but no quarries are active in this district, although to the south-west of this area slate is at present being quarried.f This slate, however, cannot vie with * In the N.W. bank of the burn § mile below Caol-ghleann. + On the Ross estate on the shores of Loch Sween. 156 Economic Resources. that prodticed from Cumbei'land, Wales, and Cornwall ; and even for local requirements slate is imported, some of which, however, is supplied from the quarries of Basdale in the adjoining sheet (36), which represent the westerly prolongation of the Loch Awe slates. If this industry is ever further developed, the parish of Kil- michael-Glassary affords the most promising field for its prosecu- tion within the limits of this map. The deepest soils are formed on the alluvial and drift deposits, the former of which have been largely laid under cultivation, and the latter to a more partial extent. The alluvial soils are the richer, but owing to the openness of the deposit, which is mainly of gravel, a large proportion of the manures are carried below the subsoil by percolating ■ waters which materially impoverish the fertilitj'- of the ground. The drift, on the other hand, is largely confined to the boulder clays, only too retentive of moisture, and in spite of an extensive system of drainage the soil is too cold to be highly productive in a district with an annual rainfall varying from sixty to ninety inches. The crops raised are mainly restricted to oats and hay ; the former cereal is grown not so much on account of the small pro- portion of organic matter contained in the soil, as from its ability to ripen with a minimum of sunshine. While, for the same reason,* combined with the moistness of the climate, the grass crop requires so long to dry after being mown that the quality of the hay is frequently impoverished by rains before it can be finally stacked. The greater part of the district is hill pasture allotted to sheep farms, of which the arable land represents but an insignificant proportion, and is principally situated in the valleys of the Ruel, Add, Cur, Finart, Fjme, Aray, and Shira, to which may be added the arable and pasture lands of the valley opening into Loch Goil, the valleys which are confluent with Loch Ederline, and those marginal strips of the sea lochs which are occupied by the raised beaches, as well as the corresponding deposits of Loch Awe, the most extensive of which are situated at Fincharn, Durran, Blarg- hour and Inverliver. The fertility of the hill pastures, except where deposits of drift and peat occur, is subject to much variation from the diversity in chemical composition of the rock which produces the subsoil. The limestones support a luxurious vegetation, and the basalts likewise disintegrate into fertile soil, but the superficial area of these rocks is not great. Although the epidiorites commonly project in bare hummocky ridges, the less meta- morphosed areas furnish a rich soil from the decomposition largely of their lime-bearing minerals ; this is especially brought out by the rich green slopes of the lower end of Loch Awe in the vicinity of Inverliver. The quartz-porphyry intrusions west of Loch Fyne stand out in steep ridges, and support but a thin soil, while their slopes are often too precipitous to sustain any vegetation. * i.e.. Insufficient sunshine. Economic Resources. 157 The Lower Old Red Sandstone formation that occupies the country to the north of Loch Avich, is largely composed of volcanic rocks, the soil of which is sufficiently fertile ; but the rapid disin- tegration of the rocks does not favour its accumulation, and this is especially the case as regards the slopes, which are often represented by piles of rocky debris. Of the great schistose groups which traverse the area, the most important from an agricultural standpoint is the Ardrishaig series. This group, largely composed of calcareous slate, owes its fertilising properties to the abundance of lime which enters into its composi- tion. Moreover, at the head of Glen Shira, where these rocks become indurated and stand out as bold crags forming the largest part of the elevated range of Beinn Buidhe, they support a vegeta- tion that is unsurpassed amongst the hill pastures of this part of Argyllshire. It would appear that the metamorphism to which these rocks have been subjected in the latter area has produced sufficient induration to check the too rapid dissolution of the lime- bearing minerals. The schists which lie to the south-east of the Ardrishaig series are frequently bare, and their soils are less fertile ; nevertheless the green beds, probably richer in lime and the alkalies, frequently support a verdant vegetation,* while the Loch Tay limestone, with its generous soil, occupies but insignificant tracts. Speaking broadly, the fiord of Loch Fyne divides the country into districts of different agricultural value, the vegetation being most profuse in the area north-west of that loch. The water supply of the district is only too abundant, and the country suffers from an excessive rainfall, while its humidity is a serious drawback to the climate. In periods of drought, which are rare, water issues from the springs of the quartz-porphyry masses west of Loch Fyne when other sources of supply are exhausted, the joints of these rocks serving as effective reservoirs. Notwith- standing the water-power available it has seldom been turned to account, but formerly extensive powder-works were in operation at Furnace, in which the Leacann water was utilised as the motive power, t The native woods that formerly covered the lower slopes of this district have been largely destroyed for the manufacture of charcoal in connection with those smelting operations to which we have already called attention. The extent to which the land- scape has been despoiled by the removal of its ancient woodlands may be instanced by the example of Gaol Ghleann, which about eighty years ago was the site of an old forest,+ that has been *For examples we may mention the hill-slopes S.E. of Ardkinglass wood, and near Cairndow and Upper Clasheoin, t Water is the motive power used for the electric light machinery for the illumination of Inveraray Castle. Corn mills employing water-wheels were formerly in use but are now abandoned. I Memorials of Argyllshire, by Archibald Brown, Esq., who states that " it was levelled to the ground, charred, and conveyed, for smelting iron, to Furnace on the west bank of Loch Fyne." 158 Economic Resources. described as " an impassable jungle, the covert for native vermin, and in the winter a shelter for the red deer." Subsequent planting, however, mainly restricted to the pine, has in some measure replaced those sylvan tracts. The most extensive plantations ai'e seen at Inveraray, where the profusion of woodland enhances the natural beautif-s of Glen Aray, while the variety and dimensions of its trees indicate the suitability of the district for afforestation. Moreover, as the slopes of that glen are clothed with wood up to an altitude of 800 feet,* the enormous area in the Loch Fyne basin available for forestry will be apparent. The natural capacity of the district for afforestation is one of its most valuable economic resources, the development of which would still further beautify the landscape of this picturesque region. J. B. H. * This elevation does not mark the limits of forest growth as the remains of trees in the peat mosses of the higher ground sufficiently indicate. APPENDIX. Part I. — PALiEONTOLOGICAI.. Part II. — Bibliographical. PAL^ONTOLCIGICAL. List of Fossils from Green Clay between tide marks 200 yards south of the sharp bend of the shore at CrEAG a' PhUHX, SOUTH-EAST SHORE OF LoCH FyNE ; ABOUT '2 MILKS SOUTH OF InVERARAY. EchinoderTnata. Echinocardium (Amphidotus) oordatum, (?) Pennant. Echinus neglectus, Lam . LameUibranchiata. Astarte elliptica, Brown, Astarte compressa, Mont. Cardium echinatum, (?) Linn. Cyprina islandica, Linn. Mya truncata, Linn. Pecten islandicus, Linn. Tellina calcarea, Ghemn. Oasteropoda. Natica alderi, (?) Forbes. Pleurotoma pyramidalis, Strom. 160 Appendix. BIBLIOGRAPHICAL. List of Weitings and Maps hefeering to the Geology of THE I)lSTEICT. 1799. St. Fond, B. Faujas. "Travels in England, Scotland, and the Hebrides." Translated from the French. 2 vols. London. Vol. I., Inveraray, p. 259. 1840. Macctilloch, J. A. " Geological Map of Scotland, with Memoir on ditto." (Published several years after Macculloch's death.) 1844. Nicol, James. "Guide to the Geology of Scotland." 8vo. Edinburgh. 1851. Murchison, Sir R. I. "On the Silurian Rocks of the South of Scotland." Qioart. Jour. Oeol. »S'oc., Vol. VII., p. 168. 1851. Hopkins, W. " On the Granite Blocks of the South Highlands of Scotland." Quart. Jour. Geol. Soo., Vol. VIII., p. 20. 1853. Ai-gyll, Duke of. "Granitic District of Inveraray." Quart. Jour. Geol. Soc, Vol. IX., p. 360. 1855. Maclaren, C. "Notices of Ancient Moraines in the Parishes of Strachur and Kilmun, Argyllshire." Edin. New. Phil. Jour., Vol. I., new series, p. 189, and Proc. Roy. Soc. Edin., Vol. III., p. 279 (1850-1857), Brit. Assoc. Rep., Vol. XIX., p. 90. 1856. Gregory, "W. "On a black Tertiary deposit containing the Exuvife of Diatomes from Glen Shira." Proc. Roy. Soc. Edin., Vol. III., p. 241, and Part II., p. 358. 1857. Argyll, Duke of. " On a Roche Moutonnee on the summit of the range of hills separating Loch Fyne and Loch Awe." Proc. Edin. Roy. Soc, Vol. III., p. 459, and Edin. New. Phil. Jour., new series, Vol. VI., p. 153. 1858. Nicol, James. " A Geological Map of Scotland." 1861. Jamieson, T. F. "On the Structure of the South-west High- lands of Scotland." Qua/rt. Jour. Geol. Soc, Vol. XVII., p. 133. 1862. Jamieson, T. F. " On the Ice-worn Rocks of Scotland." Quart. Jour. Geol. Soc, Vol. XVIII., p. 164. 1862. Murchison, Sir Roderick, and Geikie, (Sir) Archibald. " First Sketch of a New Geological Map of Scotland, with Explana- tory Notes." 1863. Geikie, (Sir) Ai-ohibald. "On the Phenomena of the Glacial Driift of Scotland." Trans. Geol. Soc. Glasgow, Vol. I., Part II. Appendix, 161 1865. Geikie, (Sir) Archibald "The Scenery of Scotland Viewed in connection with its Physical Geology." London and Cam- bridge. 1865. Crosskey, Rev. H. W. " Glacial Deposits of the Clyde District." Trans. Oeol. Soc. Glasg., Vol. II., p. 45. 1868. Argyll, Duke of. " On the Physical Geography of Argyllshire in connection with its Geological Structure." Quart. Jour. Oeol. Soc, Vol. XXIV., p. 255. 1871. Young, J. W. "Miscellaneous Notes on Chemical Geology." Trans. Geol. Soc. Glas., Vol. III., p. 28. (Analysis of Foliated Chlorite from St. Catherines, Loch Fyne.) 1873. Argyll, Duke of. " On Six Lake Basins in Argyllshire." Quart. Jour. Geol. Soc, Vol. XXIX., p. 508, and Geol. Mag., Vol. X., p. 374. 1876. Argyll, Duke of. "On the Physical Structure of the Highlands in connection witla their Geological History." Brit. Assoc. Rep., p. 81, and Nature, Vol. XIV., p. 435. 1881. Macadam, W. I. "Preliminary notice of a Clay Shell-Bed between Newton and Strachur, Loch Fyne, Argyllshire." Trans. Geol. Soc Edin., Vol. IV., p. 94. 1881. Macadam, W. I. "Notice of veins of Specular Iron Ore at Strachur, Argyllshire." Trans. Geol. Soc. Edin., Vol. IV., p. 95. 1881. Macadam, W. I. "On the Chemical composition of certain Limestone Eocks from Ballimore, Argyllshire." Travis. Geol. Soc. Edin., Vol. IV., p. 101. 1882. Macadam, W. I. " Further notice of the Tigh-na-Criche Shell- Bed, Loch Fyne, Argyllshire." Trans. Geol. Soc. Edin., Vol. IV., p. 232. 1886. Argyll, Duke of. " Our Highland Mountains." Good Words for 1886, pp. 32 and 119. 1889. Argyll, Duke of. "On certain Bodies, apparently of Organic Origin, from a Quartzite Bed near Inveraray." Proc Roy. Soc Edin., Vol. XXL, p. 39. 1892. Geikie, Sir A. "Geological Map of Scotland, reduced chiefly from the Ordnance and Geological Surveys, with Explanatory Notes." Edinburgh. 1895 Argyll, Duke of. "Two Glens and the Agency of Glaciation." Trans. Roy. Sue Edin., Vol. XXXVIII.,, p. 193. 162 Appendix. 1897. Guiin. W., C. T. Clough, and J. B. Hill. "The Geology of Cowal." Memoirs of the Geological Survey of Scotland. Edinburgh. 1899. Hill, J. B. "On the Progressive Metamorphism of some Dal- radian Sediments in the Region of Loch Awe." Qtiart. Jour. Geol. Soc, Vol. LV., p. 470. 1900. Symes, R. G. "The Schistose Rooks in the district between KUmelfort and Loch Avich." Summary of Progress of the Geological Sm-vey of the United Kingdom for 1899, pp. 60-65. 1900. Symes, R. G. "The Old Red Sandstone Sedimentary and Volcanic Rocks of the same area," ibid., pp. 96-100. 1900. Hill, J. B., and H. Kynaston. "On Kentallenite and its Relations to other Igneous Rocks in Argyllshire." Quart. Jour. Geol. Soc, Vol. LVL, p. 531. 1901. Hill, J. B. "On the Crush-Conglomerates of Argyllshire." Quart. Jour. Geol. Soc, Vol. LVII, p. 313. 1901. Kynaston, H. "Volcanic Neck at Loch Avich." Summary of Progress of the Geological Survey of the United Kingdom fw 1900, p. 73. 1901. Geikie, Sir Archibald. "The Scenery of Scotland Viewed in connection with its Physical Geology." Third edition, pp. 154 and 205. 1901. Macnair, P. "On the Occurrence of a Zone of Annelid Burrows across the Southern Highlands of Scotlar Proc Perth Soc Nat. Sei., Vol. III., p. 131. 1901. Home, J. " The Geology of the Clyde Territory." Handbook on the Natural History of Glasgow and the West of Scotland [prepared in connection with the meeting of the British Association in Glasgow], p. 40. 1901. Macnair, P. "The Crystalline Schists of the Highlands," ibid p. 414. 1901. Heddle, M. P. "The Mineralogy of Scotland." {Edited by J. G. Goodchild.) 2 vols. Edinburgh. 1903, Macnair, P. "The Building of the Grampians." Proc. Roy. Phil, Soc Glasg., Vol. XXXIV., p. 147. INDEX. AcHADUNAN, 18, 19, 93, 102, 105, 106, 109, 115-117, 124, 149. Add basin, 4, 38, 127, 132, 135, 138- 141, 156. Add River, 11, 50, 120, 146. Agglomerate, 86-88, 90. Albite schists, 15-17, 93. AUt Ath Mhic Mhartein, 57-59, 148. Allt Dearg, 57-59. AUt Glinne Mhoir, 14, 80. Allt Mor Burn, 132, 134. Alluvium, 146, 147. Am Barr, 56, 123. Analyses, chemical, 16, 24, 122, 144, 151-153. Andalusite hornfels, 92. Andesites, 85, 86, 89, 90. Andesitic basalt, 62, 65, 89, 118. Aray River, 4, 52, 117, 144, 156. Area and physical features, 1-8. Ardcastle, 119, 120. Ardkinglass, 83, 103. Aidno, 22, 23, 25, 29, 30, 68, 103, 139. Ardrishaig, 38. limestone, 32, 37, 39. phyllites, 32-39. quartzite, 33-36. Argyll, Duke of, 37, 154. Arichamish, 73, 74. Auchgoyle, 144, 145. Auchindrain, 7, 49, 134, 148, 154. Auchnangoul, 132, 154. Auchnatra, 36, 37, 135, 144. Augite andesite, 65, 66, 89, 118. diorite, 39, 94-97, 104. granite, 100. lamprophyre, 109. porphyrite, 113. Avich River, 55. Ballachulish, 101, 102. Ballemeanoch, 136, 147. Ballimeanoch, 54, 55, 146. Barbreok River, 59, 142 . Barmaddy, 55, 147. Barnaoarry, 30, 132. Barnaline, 74, 90, 132, 134. Barr Kilmhealaird, 87, 99, 114. Basalt dykes, 115-121. Bealach an t-Saic, 14, 125, 126. Beinn an Lochain, 4, 116. Bheula, 7, 16, 131. group, 10, 12-16. Buidhe, 2, 102, 107, 129, 137, 157. Chas, 4, 10, 25, 96, 97. Ghlas, 111. Laoigh, 5, 35, 132. Lochain, 7, 80, 131. Mhor, 105, 136, 148. Reithe, 105, 131. Tharsuinn, 14, 138. Ben Cruachan, 83, 99, 100, 137. Donich, 15, 17, 80, 93, 94, 104, 125. Binnein an Fhidhleir, 4, 21, 69, 103, 131, 138. Biotite gneiss, 77. hornfels, 98, 100. porphyrite, 87, 88, 107, 113, 114. tonalite, 96. Blarghour, 132, 146, 156. Burn, 53, 134. Falls, 5. Blende, 150, 152. Boulder bed, 45, 56. clay, 127, 131-133. Braes of Lorn, 6, 85. Braevallich, 128, 134, 140, 141. Brannie, 10, 11, 28. Burn, 27, 28, 38, 39, 77, 94-98, 108, 109, 132, 135, 137. Brenachoil, 132, 146. Bridgend, 103, 152. Brown, A., 157. Building stones, 154-156. Butterbridge, 102, 105, 136. Cab River, 20, 24, 103, 143. Calc-silicate hornfels, 100. Cam Loch, 6, 56. Caol Ghleann, 6, 21, 117, 118, 134, 135, 138, 148, 157. Carnach Mor, 131, 149. Carraig nan Roin, 79, 150. Carrick Castle, 78, 116, 143. Castle Lachlan, 68. Charcoal, 152, 154, 157. Chlorite schist, 62. 164 Index. Clachan, 18, 22, 116, 119, 120. Burn, 25. Hill, 10, 28, 38, 39, 95, 97, 98, 13V, 160. Claonairt, 133, 146. Clouoh, C. T., 3. Cnoc Ooinnich, 14, 131, 148. na Tricriche, 13, 78. Cobalt, 151. Coille-bhragad, 150, 151. Coire Ealt, 13, 147. No, 22, 23, 25, 29, 103, 125, 134, 136, 152. Conglomerate, 85, 86, 89, 90, 114. Contact metamorphism, 63, 98, 100, 106, 112, 118. Copper, 150-152. Cowal, 3, 38, 72, 75-78, 83, 120, 123, 127, 134, 138. Craig, E. H. C, 15, 79. Craigerrine Mine, 151. Craignamoraig, 90, 124, 137. Craignure, 151. Crarae, 11, 66, 106, 111. Creagantau'bh, 123, 139, 140, 142. Creag Dhubh, 22, 23, 25, 29, 30, 133. Dubh, 47, 53. nam Faoillean, 37, 143. Crinan, 130. Canal, 50, 147. Cruach a' Bhuic, 13, 78. an Lochain, 6, 53, 148. Mhor, 5, 53, 148. na CapuU, 29, 30. nam Miseag, 131, 146. Mult, 4, 21. nan CapuU, 23, 29, 30, 103, 138. Narrachan, 85, 101. TuLrc, 92, 93, 103, 106. Cruachan, 132, 134. Crush-conglomerates, 123. Cuil-bheag, 10, 25. Cur River, 20, 22, 24, 103, 116, 136, 143, 147, 166. D Dakyk«, J. R., 92. Dalchenna, 49, 69. Diorite, 11, 22, 92, 94, 96-99, 104. porphyrite, 106, 107. Dolerite, 104, 115-121. Donioh Burn, 94, 116. Douglas Water, 11, 133, 134, 144, 146. Drimsynie, 17, 80, 126. Drishaig, 10, 27, 28, 108. Drissaig, 55, 132, 134. Dubh Loch, 7, 124, 129, 144, 146, 149. Duchara Burn, 57, 87. Dun Dubh, 56, 57, 74, 125. Dunans, 122, 126, 148, 155. Dundarave, 27, 36, 37, 107, 138. Durran, 132, 140, 141, 146, 156. E Eas Dubh, 29, 31, 120, 125, 136, 148. Riachain, 103, 106. Economic resources, 150-158. Eilean Liver, 3, 146. Epidiorite, 29, 30, 57, 62, 7i. Ei)idosite, 64. Eredine, 128, 146. Erratic blocks, 6, 137-139. F Faults, 123-126. Felsite, 86, 102, 103, 106, 115. PeorUne, 123, 135. Fincharn, 3, 6, 54, 73, 140, 146, 156. Flett, Dr. J. S., 39, 70. Fluvio-glacial gravels, 139-143. Folding, 72-84. Ford, 70, 125, 128, 130, 140-142. Formations and rock groups, 9, 10. Furnace, 7, 11, 111, 146, 152, 157. Fyne River, 10, 77, 93, 109, 146, 156. G Gabbbo, 70, 120. Galena, 150, 152. Gallanach, 133, 144. Garabal Hill, 92, 138. Garbh Achadh, 112, 151. Gare Loch, 1. Garnetiferous mica-schists, 10, 27-30. Garrachra, 21, 125, 126. Glaciation, 5, 127-131. Gleann Airidh, 132, 146. Domhain, 6, 54, 56, 58, 59, 70, 71, 98, 131, 135, 142, 150. Dubh, 29, 126, 136, 148. Glen Aray, 46, 48, 51-3, 63, 70, 77, 106, 111, 112, 117, 130, 132-134, 137, 146, 154, 158. Brancer, 21, 22, 69, 125, 136, 148. Canachadan, 14, 80, 126. Finart, 78, 133, 143, 145, 147. Fyne, 10, 11, 19, 27, 77, 83, 84, 92, 96, 99, 106, 123, 124, 135, 137, 146, 152, 154. Kinglass, 4, 14, 82, 102, 116. Orchy, 101, 102. Shira, 4, 11, 32, 33, 37, 38, 46, 47, 77, 97, 98, 106, 108, 109, 111 124, 133, 135, 149, 153, 157. Sluan, 22-25, 136. schists, 10, 22. Glendaruel, 4, 11, 22, 25, 30, 62, 69, 117, 120, 131, 135, 147. Gold, 152. Granite, 11, 92, 94, 96-98, 114. porphyry, 107, 112, 113. Granitite, 92. Index. 165 Graphite, 153. schists, 10, 27, 31. Green Beds, 10, 18-22, 60, 61, 77. Grits, Loch Awe, 43-45. H Hbll's Glen, 4, 14, 17, 125, 136. Hill, J. B., 40, 56, 76, 83, 123. J. B., and H. Kynaston, 102. Hornblende andesite, 103. lamprophyre, 109, 110. porphyrite, 102, 103, 105, 107. schist, 22, 25, 26, 62, 66. Hornfels, 71, 92, 98, 100, 101, 113. Hyperite, 93, 94, 96. Hypersthene-andesites, 86. I Inverakay, 1, 2, 7, 32-38, 69, 106, 112, 127, 132, 144, 146, 158. Castle, 155, 157. luverinan, 55, 74, 89, 90, 146. Inverliver, 64, 65, 132, 146, 156. luvernoaden, 13, 136. Irvine, R., 154. Isoclinal folding, 57, 71, 84. K Kames, 53, 145, 146, 148. Kenmore, 7, 132, 148. Kentallenite, 11, 92, 94, 95, 101, 102, 104. Kersantite, 108, 109. Kilblaan Burn, 37, 39, 135, 137. KUbride, 54, 139, 146. Kilbridemore, 22, 25, 26, 117, 132, 135. Kilmaha, 53, 128. Knmartin, 5, 139, 140, 142. Kilmelfort, 86, 142. Kilmichael, 4, 7, 52-54, 63, 73, 119, 124, 139, 141, 146. Kinglas Water, 103, 105, 136. Knap Burn, 82, 145. Kynaston, H., and J. B. Hill, 102. Laglingarten, 30, 31, 139. Lamprophyre, 11, 39, 93, 102-109, 111, 113, 115. schist, 110. Landslips, 7, 148, 149. Larach Hill, 116, 126. Lavas, 85, 89. Leacann Water, 49, 111, 146, 157. Lead, 150. Leanach, 29, 31, 83, 125. Lecknary, 50, 146. Lephinchapel, 117, 118, 145. Lephinmore, 117, 118, 120, 123, 132, 145. Limestone, 10, 23-26, 32, 40-42, 154. Loch a Mhinn, 56, 74, 86, 137. an Daimh, 98, 100. Loch Awe, 1-3, 11, 46, 51-54, 62, 63, 71-76, 85, 90, 106, 113, 117, 124, 127-142, 146, 156. group, 40-59. Avioh, 3, 6, 11, 54-58, 74, 85, 88-90, 100, 113-115, 121, 124, 129- 134, 137, 142, 148, 157. Blackmill, 120, 147. Bck, 3, 4, 78, 81, 125-128, 131, 136, 138, 145, 147. Bderline, 140, 141, 146, 147, 156. Etive, 139, 140. Fincharn, 6, 63. Fyne, 2-4, 10, 11, 27-38, 46, 62, 66, 67, 71, 76, 77, 82, 102, 105, 106, 110, 111, 116, 117, 119, 120, 125, 127, 128, 130-1.39, 142-145, 154, 157. Gaineamhach, 6, 117, 120, 132, 148. Gair, 11, 37, 133, 144, 145, 152. Glashan, 66, 67, 70, 137, 147. Goil, 3, 4, 13, 16, 78, 117, 126- 128, 131, 138, 143, 145, 156. Leacann, 6, 46, 48, 49, 134, 148. Leathan, 132, 138-142, 146-148. Lomond, 2, 79. Long, 2-4, 13, 16, 81, 138, 145. na Sringe, 54, 122, nam Ban, 87, 114, 115. Pearsan, 57, 85, 86. Restil, 6, 94, 116, 117, 125, 138. Shira, 36, 116, 144. Sitheanach, 49, 132, 148. Striven, 3, 127, 131. Tralaig, 6, 57-59, 85, 86, 121, 131, 135. Lochan 4 Bhruic, 6, 147. Dubh, 120, 134. Mill Bhig, 7, 16, 93, 103. nan Cnaimh, 13, 14. Loohgoilhead, 7, 17, 80, 93, 116, 145, 149. Logalochan, 87, 115. Lon Glas, 141, 148. Lome, 1, 5, 86, 122, 124, 138. andesites, 11, 55, 121, 138. Lower Old Red Sandstone, 56, 61, 85- 91. M Maam, 48, 116, 124. Macadam, W. I., 23, 143, 153. MaoKichan, Rev. P. N., 7. Maolaren, C, 136. Macnair, p., 73, 79. M°Phun's Cairn, 120, 152. Manganese, 153. 166 Index. Maolachy, 58, 88, 142. Mark, 15, 16, 104, 152. Meall Breao, 92, 138. Reamhar, 29, 50. Metamorphism, regional, 71-84. Mica-dolerite, 104.' trap, 102, 104. Mid Letter, 31, 36. Mill, Dr. H. R., 129. MiUer's Linn, 4. Minard, 35, 120, 127, 144, 145. Mines, 150, 151. Minettes, 108, 109. Moine Ghlas, 120, 147. Mouovechadan, 14, 104. Moraines, 6, 134-136. Murray, Sir J., 153, 154. N New York, 134, 146. Newton Bay, 11, 35, 37, 120, 145. Hill, 10, 123. Nickel, 150, 151. O Olivine dolerite, 119, 121. gabbro, 120. Orthophyres, 108. Pass of Brander, 3, 100, 139, 140. Peach, Dr. B. N., 51, 62, 88. Peat, 147, 148, 154. Picrite, 95. Player, J. H., 122. Polchorkan, 80, 125, 147. Pollard, Dr., 153. Population, 7. Porphyrite, 11, 22, 62, 66, 87, 109, 113-115. Portinisherrich, 63, 109. Potstone, 69, 70, 155. Pyroxenite, 70. Q Quartz diorite, 97, 109. porphyry, 11, 46, 102, 106, 110, 111. veins, 28, 150. Quartzites and quartz-schists, 33, 43- 45. R Raised beaches, 144, 145. Rhyolite, 88. Road-metal, 154, 155. Rock-basins, 127-129. Rudha nan Eoin, 12, 13, 81. Ruel River, 11, 21, 156. S St. Catherine's, 69, 70, 133, 136, 145, 155. Serpentine, 11, 30, 69. Sgor Coinnich, 78, 131. Shelly marine clays, 143-144. Shepherds' Point, 15, 145. Shira River, 4, 135, 146, 156. Silver, 150, 152. Sith an t-Sluain, 120, 138. Slate, 42, 43, 155, 156. Smith, E. A., 152. Sooach, 117, 118, 120, 138, 139, 141, 143, 147. Soils, 156, 157. Sound of Jura, 127, 130, 142. Spessartite, 108, 109. Stob an Eas, 4, 22, 103, 104, 131, 136. Liath, 80, 105. Strachur, 4, 7, 12, 30, 68, 102, 116, 136, 143, 145, 147, 152. Stralachlan, 27, 36, 67, 109, 120, 123, 124, 132. River, 22, 24. Scrathlachlan, 4, 11, 33, 67, 70, 120, 135 145 147. Strath nan Lub, 17, 20, 22, 125, 133, 136, 148. Strone Point, 36, 116, 133, 137. Stuc Soardan, 48, 52, 109. Stuck, 13, 16, 127. Summary of the Stratigraphy, 59-61. Syenite porphyry, 112. Taohylite, 120, 121. Tait, D., 144. Teall, Dr. J. J. H., 16, 34, 92, 108, 112, 115. Thermal metamorphism, 39, 77, 107. Three Bridges, 48, 69, 117, 132. Timber, 157, 158. Toll nam Muo, 14, 79. Tom a Bhuachaille, 38, 137. Gobhair, 18, 22. Tonalite, 94. Trachytes, 121 122. Tuffs, 89. U Union Bay, 133. Upper Clasheoin, 115, 148, 149, 157. Vogesite, 96, 108, 109. 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