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 The geology of the country near Oban and 
 
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45. 
 MEMOIHS OF THE GEOLOGICAL SURVEY. 
 
 SCOTLAND. 
 
 THE GEOLOGY 
 
 OF THE COUNTRY NEAR 
 
 OBAN AND DALMALLY. 
 
 (EXPLANATION OF SHEET 45) 
 
 BY 
 
 H. KYNASTON, B.A., and J. B. HILL, R.N. ; 
 
 WITH Contributions by 
 
 B. N. PEACE, LL.D., F.R.S.; J. S. GEANT WILSON; 
 
 H. B. MUFF, B. A. ; and E. B. BAILEY, B.A. 
 
 WITH 
 
 PETROLOGICAL NOTES 
 
 BY 
 
 J. J. H. TEALL, D.Sc, F.R.S.; and J. S. FLETT, M. A.,D.Sc. 
 
 PnBLISHBD BY ORDER OF THE LORDS COMMISSIONERS OF HIS MAJESTY S TREASURY. 
 
 GLASGOW: 
 
 PRINTED FOR HIS MAJESTY'S STATIONERY OFFICE 
 
 By jambs HEDDERWICK & SONS Ltd. 
 
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MEMOIRS OF THE GEOLOGICAL SURVEY. 
 
 SCOTLAND. 
 
 THE GEOLOGY 
 
 OF THE COUNTRY NEAR 
 
 OBAN AND DALMALLY. 
 
 (EXPLANATION OF SHEET 45.) 
 
 BY 
 
 H. KYNASTON, B.A., and J. B. HILL, R.N. ; 
 
 WITH Contributions by 
 
 B. N. PEACH, LL.D., F.R.S. ; J. S. GRANT WILSON ; 
 
 H. B. MUFF, B.A. ; and E. B. BAILEY, B.A 
 
 WITH 
 
 PETROLOGICAL NOTES 
 
 BY 
 
 J. J.H.TEALL,D.Sc.,P.R.S.; and J. S. FLETT,M.A.,D.Sc. 
 
 PDBLISHEl) HY ORDER OP THE LORDS COMMISSIONERS OF HIS MA,IESTY's TRBASUUY . 
 
 GLASGOW : 
 
 PRINTED FOR HIS MAJESTY'S STATIONERY OFFICE 
 
 By JAMES HEDDERWICK & SONS Ltd. 
 
 At "The Citizen" Press, St. Vincent Place. 
 
 Aud to be purchased from 
 
 W. & A. K. JOHNSTON, Ltd., 2 St. Andrew Square, Edinbubgh ; 
 
 E. STANFOED, 12, 13, and 14 Long Acre, London ; 
 
 HODGES, FIGGIS & Co., Ltd., Grafton Street, Dublin ; 
 
 From any Agent for the sale of Ordnance Survey Maps ; or through any 
 
 Bookseller from the Ordnance Survey Office, Southampton. 
 
 1908. 
 Price, I'mo Shillings and Sixpence. 
 
PREFACE. 
 
 This Memoir describes the geology of the area included in Sheet 
 45 of the one-inch map of Scotland, which embraces the district on 
 either side of Loch Awe near Kilchrenan, the tract between Oban 
 and Dalmally, and the mountainous ground between Loch Creran 
 and Glen Orchy. The prominent geological features of the region 
 are (l)the development of various subdivisions of the metamorphic 
 series of the Eastern Highlands and their associated igneous rocks, 
 (2) the succession of contemporaneous lavas and tuffs that form the 
 volcanic plateau of Lome, (3) the large area of plutonic rocks 
 that stretch from Ben Cruachan northwards to the margin of 
 the map. 
 
 The central portion, comprising a broad belt crossing the sheet in 
 a north-east and south-west direction, was mapped by Mr.Kynaston, 
 It includes the region occupied by the plutonic masses north of 
 Ben Cruachan, the eastern portion of the volcanic plateau of Lome, 
 and the metamorphic rocks on either side of Loch Awe near Kil- 
 chrenan, in the Pass of Brander, in the higher reaches of Glen 
 Strae, and in Glen Orchy. Mr. Hill surveyed a tract extending 
 from Glen Fyne north-westwards to Dalmally and also the lower 
 portions of Glen Strae and Glen Orchy. 
 
 The western portion, from Loch Creran southwards across 
 Benderloch, by Connel Ferry to Loch Scamadale, was mapped by the 
 late Mr. Symes, and Dr. Peach revisedapart of the volcanic plateau 
 east of Kilmore. Mr. Wilson surveyed the area on the north side 
 of Loch Creran, Mr. H. B. Muff the part of the island of Lismore 
 included in the sheet, and Mr. Bailey revised the belt of low ground 
 that stretches southwards to the headlands west of Ardmucknish 
 Bay. 
 
 The Memoir has been mainly written by Mr. Kynaston and Mr. 
 Hill, with contributions from Dr. Peach, Mr. Wilson, Mr. Muff, 
 and Mr. Bailey, 
 
 The petrographical examination of the rocks has been done 
 partly by Mr. Kynaston, Dr. Flett, and myself, and partly by Mr. 
 Hill and Mr. Bailey. 
 
IV PBEFACE. 
 
 Mr. E. T. Newton, F.R.S., determined the tossils from the shelly 
 marine clays on the south shore of Loch Oreran and furnished the 
 list given in the Memoir. 
 
 The list of publications referring to the geology and mineralogy 
 of the district has been prepared by Mr. Tait. 
 
 The photographs reproduced in Plates I. to III. and V. to VII. 
 inclusive have been taken by Mr. Lunn and those in Plate IV. by 
 Dr. Flett. 
 
 The Memoir has been edited by Dr. Home. 
 
 J. J. H. TEALL, 
 Director. 
 Geological Survey Office, 
 22 Jermyn Street, 
 
 London, February 27th, 1908. 
 
CONTENTS. 
 
 Page 
 
 Ohaptbr I. 
 Area and Physical Features, 1 
 
 Chapter II. 
 Formations and Groups of Rock — Distribution of Rock Formations, 6 
 
 Chapter III. 
 Metamoi'phic Rocks — 
 
 i. Schists in south-east corner of map, 10 
 
 ii. Green Beds, . 12 
 
 iii. Loch Tay Limestone, . .14 
 
 iv. Garnetiferous Mica-Schist, 15 
 
 Chapter IV 
 
 Metamorphic Rocks {contimied) — 
 
 V. Ardrishaig Phyllites, 19 
 
 vi. Loch Awe Group, ... 23 
 
 vii. Granulitic Schists of the Moine type, 38 
 
 Chapter Y. 
 Older Igneous Rocks — 
 
 i. Epidiorites and Hornblende-Schist, . . 41 
 
 ii. Oontact-Metamorphism produced by Older Basic 
 
 Masses, . . 45 
 
 Chapter VI. 
 Folded and Regional Metamorphism, . 47 
 
 Chapter VII. 
 Old Red Sandstone, . 62 
 
 Chapter VIII. 
 Newer Igneous Rocks, • . • 82 
 
 Chapter IX. 
 Sills and Dykes, 111 
 
 Chapter X. 
 Oontact-Metamorphism produced by the Newer Igneous Masses, . 139 
 
VI CONTENTS. 
 
 Ohaptee XI. 
 Carboniferous Formation, . 
 
 Chapter XII. 
 Faults, 
 
 Page 
 154 
 
 156 
 
 Ohaptee XIII. 
 Glacial and Recent Deposits, . . ^'^^ 
 
 Chapter XIV. 
 Economic Geology, '■' ^ 
 
 APPENDIX. 
 Bibliography, • ^'^ 
 
 LIST OF ILLUSTRATIONS. 
 
 FIGURES IN TEXT. 
 
 Fig. 1, Section from Oban across the volcanic plateau to Kilbride 
 
 Burn, ... .... 71 
 
 „ 2. Composite Dyke exposed on south shore of Loch Fiochan, 
 
 four miles south from Oban, ..... 135 
 
 PLATES. 
 
 Plate I. The Pass of Brander, Frontispiece. 
 
 ,, II. Boulder Bed, .shore of Loch Awe, near Manse, Kil- 
 
 chrenan, . . to face page 30 
 
 ,, III. Altered and Folded Limestone, Barr Dubh, a 
 quarter of a mile east of Socach, three miles east 
 of Dalmally, . . to face page 53 
 
 ,, IV. Crush Conglomerate, on shore Port Selma, three 
 hundred yards west of Benderloch Railway 
 Station, . . .to face page 59 
 
 „ V, Ooire Creachainn in Granite, east side of Ben 
 
 Oruachan, 3| miles W.N.W. of Dalmally, 
 
 to face page 82 
 „ VI. Boulder of Kentallenite, g-mile west of Kilchrenan, 
 
 to face page 92 
 
 ,, VII. Aplite Veins in Granite, Ben Oruachan Granite 
 
 Quarries, three miles W.N.W. of Dalmally, 173 
 
EXPLANATION OF SHEET 45. 
 
 CHAPTER I. 
 
 Area and Physical Features. 
 
 Area. — The district represented in Sheet 45 of the Geological 
 Survey Map of Scotland is situated almost wholly in the county 
 of Argyll, and embraces an area of 416 square miles. The 
 noi'thern boundary of the sheet extends from the north-east end of 
 Lismore, eastwards across Glen Creran and Glen Etive to the 
 eastern portion of the Blackmount Deer Forest. On the south, 
 the margin of the map extends from the high road through Glen 
 Gallain, two miles south-west of Loch Scamadale, across Loch Awe 
 to the upper part of Glen Fyne. Oban is the only town of any 
 importance on the map, and is situated on its western margin, 
 while the ceaatral portion is occupied by Ben Cruaehan and the 
 mountainous ground lying between Loch Etive and Glen Strae. 
 
 The Callander and Oban branch of the Caledonian Railway 
 enters the eastern portion of the map in Glen Lochy, and passes 
 through Dalmally and the Pass of Brander to Taynuilt, whence it 
 follows the southern shore of Loch Etive to Conn el Ferry and 
 Oban. The principal high road on the map follows the course of 
 the railway, while other high roads run from Dalmally along the 
 south-east side of Loch Awe, branching southward at Cladich for 
 Inveraray ; from Taynuilt to Kilchrenan ; from Oban southwards 
 past Kilmore, being the road to Kilmelfort and Ardrishaig ; and 
 from North Connel Ferry in a northerly direction to communicate 
 with the district about Loch Creran and Appin. 
 
 Physical Features. — The most elevated area on the map occupies 
 nearly the whole of its northern half, and is sharply defined from 
 the comparatively low ground to the south-west by the well-marked 
 line of feature coinciding with the Pass of Brander and Glen 
 Salach. The most conspicuous and commanding feature of this 
 mountainous region is the imposing granite mass of Ben Cruaehan, 
 which forms a well-marked east and west ridge, culminating 
 towards its central portion in two sharp peaks, the highest of which 
 rises to 3689 feet above sea-level. The western slopes of Ben 
 Cruaehan stretch down to the banks of the river Awe at Bonawe. 
 To the south it directly overlooks the Pass of Brander, and extends 
 eastwards in diverging ridges and spurs as far as the head of Loch 
 Awe. On the north-east side of Ben Cruaehan a succession of 
 deep glens, separated by mountain groups, forms a wild and some- 
 
2 Area andj Physical Features. 
 
 what inaccessible coantry as far as the northern margin of the 
 map. In this area no less than eight mountains rise to over 3000 
 feet above sea-level, the most elevated group being situated to the 
 north of Glen Kinglass, and constituting a large portion of the 
 Blackmount Deer Forest. On the west side of Loch Btive a 
 ruo-o-ed group of granite hills, rising to over 3000 feet in Beinn 
 Sg'uiiaird, forms the watershed between Loch Etive and Loch 
 Creran. 
 
 To the east of the great granitic mass of Ben Cruachan and the 
 Blackmount, the hills are of more rounded form and smoother 
 outline, and seldom rise above 2500 feet. Further south the 
 metamorphic area to the east of Loch Awe rises gradually towards 
 Beinn Bhalgairean, 2085 feet, and reaches its greatest eleva,tion in 
 Beinn Laoigh, on the Perthshire boundary, and just outside the 
 eastern margin of the map. Further south another elevated area 
 is situated between Glen Shira and Glen Fyne, the highest point 
 being Beinn Bhuidhe, 3106 feet. 
 
 The greater part of the western portion of the map consists of 
 comparatively low ground, the highest hill in the Lome volcanic 
 area being Beinn Ghlas, 1691 feet above sea-level, and situated 
 near the central portion of that area. The scenery of this region 
 is characteristic and entirely dependent upon the geological 
 structure, the forms of the hills and lines of crag and escarpment 
 being directly due to the character and mode of behaviour of the 
 lava-flows or masses of fvagmental volcanic rock of which they 
 are composed. The glens of this region are comparatively small 
 and narrow, though frequently deep and bounded by steep craggy 
 slopes. To the west of the volcanic area of Benderloch, the harder 
 bands of the metamorphic series stand out as well-marked ridges; 
 running north-east and south-west, above low ground for the most 
 part covered by peat-mosses and deposits of the raised beaches, 
 which prior to the elevation of the country must have formed an 
 archipelago of rocky islets. Wherever we leave the volcanic 
 plateau the form of the ground changes according to the character 
 of the rocks beneath. The epidiorites form the rugged hills and 
 elongated ridges about Loch Awe, sharp lines of ridge being also 
 occasionally formed by the later porphyrite sills. The quartzites 
 stand out, where well developed, in bold lines of crag, while the 
 limestone usually forms smooth grass-covered slopes, and the 
 black schists and phyllites occupy the hollows. The higher ground 
 and bolder aspect of the hills to the east and north-east of Loch 
 Awe is directly connected with the more highly metamorphosed 
 condition of the rocks composing this area. ■ 
 
 Lakes and River Systems. — The area included in the present 
 map, comprising as it does some of the most typical aspects of 
 West Highland scenery, is not lacking in both the fresh and salt- 
 water lochs which give that scenery perhaps its greatest charm. 
 The long, narrow, fjord-like sea-lochs of the West Highlands are 
 here typically represented by Loch Etive. A few miles north of 
 Oban this loch runs inland in an easterly direction for eight miles ; 
 it then bends round to the north-east and stretches away for over 
 
Area and Physical Features. 3 
 
 ten miles almost to the northern edge of the map. The principal 
 rivers which di:ain into Loch Etive are the river Awe, entering at 
 Bonawe, the river Etive at the head of the loch, and the river 
 Kinglass, about halfway between the two, and on the east side of 
 the loch. The Kinglass drains a large portion of the granite area 
 of the Blackmount, and rises in the high ground E.S.E. of Beinn 
 Starav. The river Awe flows from Loch Awe through the remark- 
 able gorge known as the Pass of Brander. This gorge corresponds 
 with a line of fault extending from Glen Salach on the north-west 
 to Loch Awe. It is owing to the gradual excavation of this feature 
 that the waters of Loch Awe are now drained by the river Awe, 
 instead, as one would expect, of finding their way out at the south- 
 west end of the loch. That the loch was formerly at a considerably 
 higher level, and drained out at the south-west end, may be proved 
 by the occurrence of the old loch-beaches, seen at various places 
 along its shores, and especially well developed in the neighbour- 
 hood of Ford at the far end, and there is strong evidence of the 
 existence of the ancient waterway between Ford and Kilmartin by 
 which the loch was at one time drained. This point is referred to 
 in the Explanation of Sheet 37. 
 
 At its north-east end Loch Awe is fed by the waters of the river 
 Strae and the Orchy. The Strae rises among the quartzose flag- 
 stone hills on the north side of Beinn Mhic-Mhonaidh, and has a 
 straight south-westerly course, which coincides with a well-marked 
 line of fault crossing the southern portion of the Blackmount and 
 the Moor of Eannoch to the north-east. About nine miles of the 
 river Orchy are seen on the map, with about four miles of the river 
 Lochy, its principal tributary, which enters it a mile and a half 
 above Dalmally. Up to the junction of the Lochy the Orchy has a 
 westerly course, above which it runs parallel to the river Strae. 
 The two principal streams which flow into the east side of Loch 
 Awe, namely, the Cladich river and the Teatle Water, have a general 
 westerly course, and drain the metamorphic area between Loch 
 Awe and the watershed dividing it from Glen Shira and Glen 
 Fyne. 
 
 In the eastern portion of the Lome volcanic area the streams 
 flow off the high ground near the central part of the area, on the 
 one hand northwards to Loch Etive, and on the other hand south- 
 eastwards to Loch Awe. On the west side they have for the most 
 part a general westerly direction towards the sea-lochs situated to 
 the south of Oban. The whole area is dotted here and there by 
 lochs and small lochans, the two largest being Loch Nell and Loch 
 Scamadale. The Black Lochs form a chain of small lakes -lying 
 north-east and south-west, and situated about two miles south-east 
 of Connel Ferry. These lochs drain into, Loch Etive by a stream 
 which flows out at their south-west end and curves round till it 
 takes a northerly course to Connel Ferry. It would seem probable, 
 however, that we may have here also a reversal of the original 
 state of things, owing to the blocking up ot the north-east end of 
 the lochs by glacial accumulations. Numerous small lochs occupy 
 small basins in the metamorphic rocks in the area about Kil- 
 
4 Area and Physical Features. 
 
 chrenan. The chief of these are Loch Nanfc, which drains to the 
 north into Glen Nant and so into Loch Etive, and Loch an Le6id 
 and Loch an Droighinn, discharging eastwards into the Kilchrenan 
 bum, which flows southwards into Loch Awe. Many of the 
 smaller lochans among the hills are now partially silted up by the 
 accumulations of peat-forming plants, and the peat-covered sites of 
 previous lochans are frequently met with in the hollows among 
 the hills. 
 
 In the north-western corner of the map we see the whole of 
 Loch Creran, one of the shorter and less important of the sea-lochs, 
 with its somewhat sinuous form, entering Loch Linnhe opposite 
 the north-eastern end of Lismore, and contributing to the remark- 
 ably irregular outline of that portion of the sea-coast. 
 
 Population, etc. — The distribution of the population over this 
 area has to a large extent been determined by its geological 
 structure, and the natural conditions coincident with a wet climate 
 and large areas of elevated ground. Nearly all the land in this 
 region, situated above 1000 feet, is destitute of population and 
 given up to deer-forests or sheep-grazing. The population of so 
 mountainous a region is of necessity, therefore, scanty and 
 scattered, and mainly confined to the glens and the low-lying 
 ground situated along the shores of the lochs. The most populous 
 area is undoubtedly to be found in the western portion of the 
 sheet, in the district about Oban, and in the low-lying portions of 
 western Benderloch and Appin to the north. Further east, the 
 shores of Loch Etive as far as Bonawe and Taynuilt support a 
 moderate population, engaged in sheep-farming, cattle-rearing, 
 agriculture, and local industries. The volcanic rocks which cover 
 this area form a somewhat stony soil, but often fairly well adapted 
 for arable purposes, and good for pasture. Considerable tracts on 
 the south side of Loch Etive are covered by natural wood of birch 
 and hazel, and in some places the landowners have raised plan- 
 tations of coniferous trees, which are doing well. The higher 
 ground between Loch Etive and Loch Awe is well adapted for 
 heather, and is given up to grouse-moors and sheep-grazing. 
 
 The mountainous area to the north-east is divided into large 
 sheep-farms as far as Glen Kinglass. The soil is poor and the 
 ground is mainly covered by coarse grass. To the north of Glen 
 Kinglass the granite is richer in quartz, andthelower slopes of the 
 hills are often covered by short heather. All this portion of the 
 area, together with a large part of the metamorphic area to the 
 east, is deer-forest ground. 
 
 The district of Loch Awe-side and the Strath of Orchy is well 
 adapted for supporting a rural population, and is peopled by 
 numerous small farmers, shepherds, gamekeepers, etc. A good 
 deal of the low ground is covered by drift, which, together with the 
 alluvial ground, supports a good arable soil, well suited for crops 
 or hay growing. Nearly the whole area to the east of Loch Awe 
 consists of large sheep farms and grouse-moors, a part of the more 
 elevated portion between Glen Shira and Glen Pyne being deer- 
 forest. Towards the upper reaches of the glens the population 
 
Area and Physical Features. 5 
 
 invariably becomes very scanty, the only dwellings being those of 
 shepherds or gamekeepers, often situated at long distances from 
 one another. Formerly, however, the whole of this area was far 
 more thickly populated, chiefly by the crofter class, than at pre- 
 sent, the numerous ruined buildings and patches of plough-marked 
 ground affording abundant evidence of abandoned homesteads and 
 deserted lands. H. K. 
 
CHAPTER II. 
 
 Formations and Groups of Eock. 
 
 Before entering upon a geological description of the area, it will 
 be convenient here to state in a tabular form the various formations 
 and groups of rock which are included in the present sheet. 
 
 a 
 
 ID 
 O 
 
 Ph 
 a 
 
 03 
 
 a 
 
 (33 
 i— I 
 
 Peat .... 
 
 Eiver and lake alluvium 
 Eecent marine muds and sands 
 Eaised beaches . 
 Erratic blocks 
 Sands and gravels 
 Moraines . 
 Boulder clay 
 Glacial striee 
 
 <i-.^0> 
 
 -e^ 
 
 Carboniferous. 
 
 Sandstones and shales of the 
 Bridge of Awe . 
 
 OldEed \ 
 Sandstone. [ 
 
 l' Conglomerates, sandstones. 
 Lower. ^ shales, associated with 
 l^ volcanic series 
 
 o 
 
 o 
 
 T3 
 
 a 
 
 -a 
 
 'Albite schist ..... 
 Garnetiferous mica-schists 
 
 " Green Beds " 
 
 Clay slates, phyllites, and calc-sericite schists with thin 
 
 limestones and quartzites (Ardrishaig series) 
 Black slates and schists .... 
 
 Quartzites and schistose grits 
 Pebbly Limestone .... 
 
 Granulitic schists of Moine type, with both muscovite 
 
 and biotite ....... 
 
 and 
 
 
 the 
 
 
 
 C^ 
 
 
 I'' 
 
 
 9 
 
 hhin 
 
 P 
 
 jiiiii 
 
 I 
 
 
 I' 
 
 
 X 
 
 . 
 
 X 
 
 m 
 
a 
 o 
 
 3 
 o 
 
 c 
 
 Groups of Rock and Distribution. 
 
 ' ' : A. CONTEMPOEANEOUS. 
 
 Of Lower Old f Andesites . . . . 
 
 Eed Sandstonej Ehyolitic felsites 
 Age. [ Agglomerates and tuffs 
 
 B. — Inteusive. 
 
 Poc^ 
 Tsc^ 
 
 Tertiary. 
 
 Lower Old 
 Eed Sandstone. 
 
 L 
 
 Post-foliation. 
 
 Dolerites and basalts 
 
 Trachytes 
 
 Acid dykes (?) rhyolites 
 
 Volcanic vents 
 
 Volcanic vents 
 
 East and west dolerite dykes 
 Granite . 
 Diorite . 
 Kentallenite 
 Porphyrite and andesite 
 Quartz-porphyry, felspar-por- 
 phyry, and felsite 
 Lamprophyre . 
 
 Pre-foliation. 
 
 Epidiorite, chlorite-schist, 
 hornblende-schist 
 
 B 
 
 Tr 
 F 
 
 Ni 
 
 Nc' 
 
 B 
 G 
 D 
 K 
 Po 
 
 F 
 
 Bg 
 
 Distribution and De'oejojjment of the various Roclc Formations and 
 Groups. — The area represented on the map may perhaps be said to 
 be almost equally divided between the metamorphic rocks, the 
 Lower Old Eed Sandstone lavas, and the Ben Cruachan granite. 
 On first glancing at the map, the eye is immediately arrested by 
 the great mass of granite which occupies all the mountainous 
 ground north-east of the Pass of Brander, and on both sides of 
 Loch Etive above Bonawe Ferry. This mass comprises the well- 
 known granite of Ben Cruachan and Bonawe, and an inner, 
 more acid, and slightly younger granite which forms the wild hills 
 on both sides of the upper portion of Loch Etive. Some subordinate 
 intrusions of granite- and diorite are seen in the north-east 
 corner of the map, and the south-east corner includes a portion of 
 the granite of Glen Fyne. 
 
 The line of fault which coincides with the remarkable feature of 
 the Pass of Brander (Plate I., frontispiece) and with Glen Salach 
 separates the granitic area of Ben Cruachan from the volcanic area 
 of south-western Lome. Thus by far the greater portion of the 
 ground lying to the south-west of the Pass of Brander and Glen 
 Salach is occupied by theandesitic and felsitic lava-flows, with their 
 
8 Groups of Bock and Bistrihution. 
 
 associated tuffs and agglomerates, of the Lower Old Red Sandstone 
 volcanic series. 
 
 A well-marked zone of hornfels accompanies the granite margin 
 from the head of Loch Awe, forms the steep slopes on the north- 
 east side of the Pass of Brander, thence follows the granite in a 
 northerly direction towards Loch Oreran, and forms the hill slopes 
 to the south-east and east of the head of that loch. 
 
 The rest of the map is occupied by the metamorphic group 
 and associated igneous rocks, with numerous small intrusions, 
 mainly in the form of dykes, belonging to the later igneous series. 
 The south-western portion of the mountainous area, stretching 
 from the Strath of Orchy to the north-east corner of the map, 
 consists to a large extent of massive quartzite, often pebbly, while 
 the north-eastern portion is chiefly composed of granulitic schists 
 of Moine type, together with a zone of flaggy quartzite, and 
 another zone of garnetif erous ' mica-schist. The area about Loch 
 Awe and between Loch Awe and Glen Fyne exhibits an interesting 
 variety of rock type, structure, and metamorphism. It comprises 
 the entire metamorphic series from the Loch Tay limestone, green 
 Deds, and albite schists of Glen Fyne up to a massive quartzite, 
 which is frequently pebbly (Schiechallion). The phyllites of 
 Ardrishaig type have the larger share of this ground. Striking 
 north-east in a broad belt from Glen Shira, they spread out over 
 the hills south-east of Dalmally and sweep round westwards to the 
 head of Loch Awe. To the east of Dalmally they thin out into a 
 comparatively narrow belt, rich in calcareous bands, and run east- 
 wards into Perthshire. The black schists and limestone (of Loch 
 Awe type) are perhaps best seen in the area about Kilchrenan, on 
 the north-west side of Loch Awe. The limestone is again well 
 develbped in Lismore, in the north-west corner of the map, and 
 forms a well-defined belt on the west side of Glen Oreran. 
 
 The quartzite and schistose grits are well seen on the south-east side 
 of Loch Awe, between Bovay and Glen Shira. The boulder bed 
 is also seen at two or three places near the south-east shore of the 
 loch, but the best exposures are to the west of Kilchrenan. 
 
 The igneous rocks of pre-foliation age are seen at a glance to 
 reach their maximum development about Loch Awe, and here also, 
 as will be seen in the following pages, they show their greatest 
 diversity of composition and structure. The intrusions of epidiorite 
 and hornblende schist evidently die out to the east and north-east 
 of the Loch Awe area. 
 
 The distribution of the later dykes of porphyrite and quartz- 
 porphyry shows the remarkable development of these intrusives as 
 a broad belt extending from the south-west corner of the map to 
 Ben Cruachan, while they tend to die out on either side of this 
 belt. To the east of Loch Awe the predominant type of later dyke 
 is lamprophyre, this type being very common about Glen Shira. 
 
 The Tertiary dykes are almost entirely confined to the south-west 
 of a line drawn from Oban to Inverinan on Loch Awe, and they are 
 exceedingly numerous in the area about Loch Scamadale. 
 
 Superficial deposits do not occupy any very considerable areas 
 
G-rowps of Sock and Distribution. 9 
 
 upon the map. The alluvial flats and terraces and the raised 
 beaches are of small extent owing to the mountainous nature of 
 the country and the consequent steep slopes of the glens. There 
 are no large peat-mosses, with the exception, perhaps, of that on the 
 north side of Connel Ferry. The hill peat nowhere covers large 
 areas, but is scattered in patches chiefly over the volcanic area. 
 
 The low ground about Strath Orchy is extensively covered by a 
 magnificent series of moraines, amid which the railway threads its 
 course at Dalmally. 
 
 The present map shows a diversity of rock type and structure, 
 combined with avariety of scenic feature, which is seldom seen in any 
 other area of equal extent. There is thus not only a great variety of 
 schistose rocks and metamorphic phenomena, but a very extensive 
 development of igneous intrusions and lava-flows. So varied an 
 area can hardly fail to constitute a wide field of geological interest 
 and inquiry, and to afibrd the materials of problems, metamorphic 
 and igneous, structural and physical, of which we will now proceed 
 to give some account in the following chapters. H. K. 
 
CHAPTER III. 
 
 Metamorphic Rocks. 
 
 i. Schists South of Green Beds in South-east Corner of 
 
 Sheet. 
 
 1. LitJiological Characters. — This small area is made up of the 
 north-easterly prolongation of a group of rocks which have a wide 
 extension in Cowal, and have been described in the Explanation of 
 Sheet 37 as the Ben Bheula group. . They consist of a series of 
 gneissose grits and granulitic gneisses showing a high degree of 
 metamorphism. While in mass they retain sufficient traces of 
 their sedimentary character to leave little doubt of their origin, in 
 individual rock fragments these indications are seldom so 
 apparent. 
 
 In this area their deformation and mineral reconstruction have 
 been so complete that the original clastic character has often been 
 entirely destroyed. They consist essentially of a granulitic 
 admixture of quartz and felspar lying between folia of sericitic 
 mica and biotite. Chlorite, iron ores, and carbonates form the 
 accessory minerals, while albite, andalusite, and actinolite are of 
 more exceptional occurrence. The remains of clastic grains of 
 quartz and felspar are represented by phacoids, which merge into 
 the granulitic matrix, thus obliterating the boundaries of the larger 
 clastic fragments, so that the outlines of the pebble-like grains are 
 much less marked under the microscope. The quartz phacoids, 
 however, often break up into a mosaic coarser than the matrix of the 
 rock. The quartz phacoids are white and blue, the latter being 
 sometimes slightly opalescent. The felspar phacoids are pink and 
 white, the latter being predominant. The clastic grains have been 
 drawn out and frequently fractured, and although the felspars have 
 more successfully resisted deformation than the quartz fragments, 
 their stretching has sometimes been accompanied by fractures, 
 the cracks being now infilled by secondary quartz. Moreover, 
 the lenticles of quartz and felspar are bounded by tails of 
 granulitised material. 
 
 The felspar of these gneisses is almost entirely plagioclase, but 
 some untwiuned felspar is occasionally seen, and no doubt 
 orthoclase entei'ed to a slight extent into their original composition. 
 In this area the nature of the plagioclase has not been determined, 
 but, judging from the extension of this group in adjoining districts, 
 it is probable that it is largely restricted to oligoclase. The mica 
 is mainly confined to the planes of foliation, but biotite also 
 frequently occurs scattered broadcast through the rock. 
 
 These rocks present every gradation in texture from fairly 
 
Schists south of Green Beds. 11 
 
 coarse-grained gneissose grits to highly micaceous schists, the 
 latter, however, having been evolved from fine sandy shales 
 rather than purely argillaceous beds. In this district the more 
 siliceous types distinctly predominate. Moreover, the close 
 interbanding of beds of divergent texture and composition as the 
 result of uneven sedimentation reveals their aqueous origin in the 
 finer and more compact schists, where no clastic fragments are 
 preserved. 
 
 An important feature in this group of rocks is the occurrence of 
 secondary albite in tiny crj^stals, which have been produced 
 subsequent to foliation. The albite grains are of a dull brown 
 colour, and contain numerous inclusions. They sometimes are so 
 abundant as to form distinct zones af albite-bearing rock, but there 
 is every gradation in their relative abundance. In the more 
 chloritic and micaceous zones this mineral is most strongly 
 developed, and is of rare occurrence in the more siliceous 
 members. 
 
 In all the schists of this group the segregation of quartz, and to 
 a smaller eitent of felspar, has been considerable, so that veins of 
 quartz and occasionally of pegmatite are commonly distributed, 
 many of which have been formed prior to the period of rock 
 deformation. 
 
 2. Distribution and Field Relations. — This group occupies an area 
 of less than a square mile as an irregular strip of a north-north-east 
 trend along the valley of the Fyne in the neighbourhood of 
 Inverchorachan. To the east this band abuts against the Glen 
 Pyne granite, the intrusion of that mass having determined the 
 irregularity of its boundary. It lies between that granite and the 
 Green Bed group, with which it forms a junction remarkably even. 
 Situated within the granite aureole, it is not only frequently veined 
 by granite, but has been subjected to contact action in addition to 
 the regional metamorphism of the district, the effects of which are 
 more particularly described in Chapter X. as regards the former, 
 and in Chapter VI. as regards the latter. 
 
 In the hollow at An Caorachan there is a small intrusion of 
 granite and two felsite dykes, which, together with the smaller 
 granite veins, are the only igneous rocks contained within the 
 group. 
 
 The schists of this area are of the siliceous type, and massive, but 
 their dominant characteristic in the field is their highly contorted 
 and gnarled structure, which has given rise to rugged features that 
 form precipitous slopes which extend from the bed of the River 
 Fyne. On the east side of that river they form a hillside over a 
 thousand feet in height, with a breadth of only a quarter of a mile. 
 They attain a like altitude on the western side, but the lower slopes 
 are there largely occupied by granite. In this latter area 
 andalusite is in many places very abundant, and blades of actinolite 
 are frequently seen. Owing to the contortion the dip varies 
 considerably, but notwithstanding the disturbing element of the 
 granite mass in the immediate vicinity, there is . a general north- 
 westerly hade, in spite of occasional reversals. In an area, how- 
 
12 Metumorphic Rooks. 
 
 ever, so intensely folded, the dips are of no stratigraphical 
 importance, but merely indicate the inclination of the folded 
 limbs. 
 
 The only fault of importance runs along the bed of Glen Pyne as 
 far as Inverchorachan in a S.S.W. direction, and on the deflection 
 of the Eiver Fyne at tliat point it pursues its even course by 
 ascending the hillside of An Caorachan. This line of fracture has 
 there diverted the drainage of the upper slopes, which has excavated 
 a deep hollow along the Allt a' Chaorachain burn. This fault, 
 which extends from Perthshire to Loch Fyne, is in this area closely 
 restricted to the junction of this group with the green beds, and may 
 possibly account for the even nature of that junction south-south- 
 west of Inverchorachan, where a rise of 800 feet from the bottom 
 of the valley has not diverted the boundary. Minor faults parallel 
 with and probably related to this fault occasionally give rise to 
 straight features. 
 
 ii. The Gtkeen Beds. 
 
 1. Lithological Characters. — The schistose grits, granulitic gneisses, 
 and mica schists just described are succeeded to the north-west 
 by schists of corresponding lithological character, but which contain 
 in addition certain bands of epidotic, chloritic, and hornblendic 
 schists that are known in the Highland sequence as Green Beds. 
 In areas where regional metamorphism has not proceeded so far, 
 the clastic origin of the Green Beds has been indubitably 
 established. In the district, however, under description regional 
 metamorphism is of such an advanced type that, as already de- 
 scribed, the original clastic structures have often been completely 
 obliterated, and sediments have been converted into holocrystalline 
 rocks. The Green Beds in this area have suffered similar defor- 
 mation and reconstruction, and are for the most part represented 
 by hornblendic schists that are practically indistinguishable as 
 regards their structure from igneous rocks. 
 
 The Green Beds, from their basic composition, differ so markedly 
 from normal sediments that their inclusion among the former is a 
 problem not easy to solve. While it is possible that they originated 
 as volcanic ash, which was deposited on the sea floor during the 
 accumulation of the normal sedimentary deposits, the absence of 
 volcanic lapilli throws doubt on this hypothesis. Moreover, the 
 igneous rocks on their horizon appear to belong to the intrusive 
 group, and so far as the evidence goes volcanic rocks are absent 
 in the area. It is possible, however, that the Green Beds, in 
 common with the associated schists, were derived from the disinteg- 
 ration of an igneous complex, the more basic portion of which has 
 furnished the materials for this abnormal deposit. The Green Beds 
 are not sharply defined from the associated schists, but on the 
 other hand the two types blend imperceptibly into one another, 
 so that with the gradual diminution of the basic minerals the 
 peculiar characteristics as gradually diminish ; in such cases the 
 demarcation of the Green Bed type is necessarily an arbitrary 
 
Green Beds. 13 
 
 distinction. In this area these rocks are very finely foliated, and 
 the hornblende is practically confined to actinolite, so that they 
 appear in the field as green and dark, finely-linear schists. Besides 
 actinolite they contain chlorite, epidote, biotite, felspar, quartz, iron 
 ores, and occasionally tourmaline, but these minerals are not 
 always present together. The felspars are albite and oligoclase 
 (acid plagioclase). The albite is usually present in small rounded 
 crystals. It is never idiomorphic, and is a secondary mineral 
 similar to that which enters into the composition of the albite 
 schists, where it plays the same role as the garnets do in the 
 garnetiferous mica schist group. Biotite is frequently an 
 important constituent, but is more plentiful in association with 
 chlorite than with hornblende. The occurrence of tourmaline in 
 these rocks is peculiar, and that mineral appears to be confined to 
 the hornblendic varieties. The Green Beds in this area difier 
 from the epidiorites in the persistence of a more or less uniform 
 structure. Whereas the hornblende of the epidiorites frequently 
 occurs in stumpy prisms or in blades which are frequently of large 
 size, in the green beds small needles of actinolite take the place of 
 the hornblende. The constancy of the actinolitic character, and the 
 absence of variation in the grain of the rock, are features which 
 distinguish them from the epidiorites when other methods of 
 distinction are wanting. 
 
 2. Distribution and Field Relations. — In the explanation of the 
 map (Sheet 37) which lies immediately to the south, it was pointed 
 out that the main Green Bed zone which had been traced across 
 Cov^al diminished considerably in the north-east of that sheet ; so 
 that, instead of the Green Bed material being about equally 
 apportioned with the associated mica schists, it was merely repre- 
 sented by a parallel set of attenuated seams. In that memoir it 
 was stated that the Green Beds were not confined to the main 
 group, but that similar bands were more sparsely distributed in 
 the schists dividing the main group from the Loch Tay limestone, 
 and in the garnetiferous mica schists which succeed that limestone. 
 Beyond the main zone, however, these Green Beds were not suffi- 
 ciently represented to be differentiated on the map. In their north- 
 easterly prolongation into Sheet 45 the Green Beds of the main 
 group have so diminished that they are not so strongly represented 
 as those intercalated in the schist zones lying to the north-west, 
 and which have not been differentiated. Hence we find them 
 more strongly developed in the garnetiferous mica schists than in 
 the schists south-east of the limestone. To some extent their 
 feeble distribution in the latter may be due to the contraction of 
 the band caused by the intrusion of the Glen Fyne granite, and to 
 the action of the Glen Fyne fault. 
 
 In the northern part of Glen Fyne shown upon this map, for a 
 distance of a mile. Green Beds occur in contact with the limestone, 
 but have not been observed eastof that bed, which is only separated 
 from the granite by a distance of about 200 yards. Along this line, 
 however, narrow green bands of fine-grained hornblende schist are 
 frequently found associated with that limestone. In the Alt na 
 
14 Metamorphic Bocks. 
 
 Paing, near Inverchorachan, a few thin seams of folded horn- 
 blendic schists of this type occur east of the limestone ; the 
 most distant seam from that mass, however, is only separated from 
 it by 15 yards. To the south of Inverchorachan only one small 
 green bed has been observed east of the Loch Tay limestone. 
 
 In the garnetiferous mica schists, Green Beds, although more 
 abundant, are yet only feebly developed. They occur on the 
 eastern boundary of the map near the head of Glen Fyne as 
 hornblende schist, retaining no traces of clastic structure, but 
 preserving traces of original sedimentation as shown by banding. 
 It is a fine linear foliated hornblende schist, and is seen under the 
 microscope (5683) to consist, of green actinolitic hornblende, 
 highly dichroic, and clear yellow green to deep green and bluish 
 green. Felspar is exceedingly abundant, and its decomposition has 
 given rise to secondary micas. Quartz is comparatively scarce, 
 and there is a good deal of iron oxide in irregular patches. In 
 that part of the garnetiferous mica schist, for a distance of Ij miles, 
 extending south to the Alt na Faing, exposures of Green Beds 
 are found which exceptionally have a breadth of outcrop of from 
 30 to 40 yards, a band of which occurs in that burn one-third 
 of a mile distant from the limestone. Actinolite is fairly abundant, 
 but does not enter largely into the mineral composition of the 
 band. The other Green Beds are found nearer to the limestone, 
 rarely exceeding 200 yards from it. For instance, in the burn 
 half a mile north of Inverchorachan a band 40 yards wide of 
 massive hornblendic green bed occurs, which contains small 
 garnets. In the small stream immediately to the south, which 
 flows likewise transverse to the strike of the schists, two very fine- 
 grained hornblendic schists of this group are seen, one of which is 
 twenty and the other ten yards broad, while in the immediate 
 neighbourhood there are a few impure seams — that is to say, 
 transitional beds between (Jreen Beds and mica schists, sufficiently 
 charged with chlorite and epidote to link them with the former. 
 
 iii. Loch Tay Limestone. 
 
 1. Lithologieal Characters. — This important member of the 
 Highland sequence forms a persistent band that stretches right 
 across the Highlands. It consequently possesses a stratigraphical 
 value that cannot be over-estimated in its relations to the crystal- 
 line schists with which it is associated. It divides the schistose 
 group just described from the garnetiferous mica schist group 
 which succeeds it. Moreover, the parallelism of both groups with 
 the limestone confirms the conclusion that they occupy definite 
 stratigraphical horizons. 
 
 The rock is holocrystalline and varies in colour from blue to grey, 
 and is thin -bedded. It sometimes contains white mica and 
 occasionally pale grey garnets. 
 
 2. Distribution and Field Relations. — This band enters the 
 eastern margin of the map at the bottom of Glen Fyne, and pursues 
 a south-south-westerly course along the western slopes to the 
 
Loch Tay Limestone — Garnetiferous Mica Schist. 15 
 
 southern margin of the sheet under the ridge of Beinn Chas, where 
 it occurs at an altitude of 2000 feet. Like the schists with which it 
 is associated, it is much folded, but the folded limbs have a prevail- 
 ing north-north-westerly dip. In spite of this plication its strike 
 is remarkably even, and it would appear that the amplitude of the 
 folds has been neutralised by the high angle at which the band is 
 truncated by the ground surface. For about a mile along the 
 extreme north-east it is frequently associated with green beds, as 
 may be seen in the sections of the small tributaries of the river 
 Fyne which dissect it transverse to the strike. It has been faulted 
 by the Glen Fyne fault at the head of that glen, and occasionally 
 by transverse or north-west faults further to the south-west. In 
 the small stream, about 1500 yards north-north-east of Inver- 
 chorachan the limestone has been thrown to the east by a north- 
 west fault, so that the section exhibits two limestone bands divided 
 by a green bed. About 500 yards still farther to the north-east 
 a burn section likewise shows two limestone bands probably due to 
 faulting rather than plication. Its widest outcrop is seen in the 
 Alt na Faing above Inverchorachan, where it is very hard and 
 thin-bedded, with a dip of 50° to the north-west, and has a breadth 
 of about 35 yards. In this area such a breadth of outcrop is 
 unusual, as it rarely exceeds a dozen yards and is often much less. In 
 many parts of its course it has been much altered and indurated, 
 presumably by contact metamorphism, as it occurs close to 
 the granite margin. As far to the south-west as Inverchorachan 
 it can be easily traced crossing the numerous burns, and is some- 
 times exposed on the slopes that intervene, but beyond that locality 
 it has not been so continuously traced, due to surface debris. On 
 its reaching, however, the higher ground to the south-west, where 
 the rocks are bare, the band can be easily followed. 
 
 iv. G-ARNETIFEROUS MiCA SCHIST. 
 
 1. Lithological Characters. — This group, forms a band on the 
 north-west side of the Loch Tay limestone, separating the latter 
 from the Ardrishaig phyllites. It consists of an assemblage of 
 gneissose grits, granulitic gneisses, and mica schists, differing mainly 
 from the group of schistose rocks south-east of the green beds 
 (described in Section i.) by the strong development of garnets. 
 The band has suffered a similar degree of regional metamorphism, 
 and has often undergone, therefore, a complete mineral reconstruc- 
 tion. On the whole it is, perhaps, less siliceous than the band 
 below the limestone, and in part contains more chlorite. In the 
 iield, however, extensive developments of gneissose grits are seen 
 in which the original clastic character is obvious, not only by the 
 partial preservation of the clastic grains, but by the interbanding 
 of beds of diverse composition and texture that marked the original 
 deposit. 
 
 The most striking characteristic of these rocks is the remarkable 
 development of red garnets scattered broadcast over the group. 
 They are not, however, uniformly distributed, but bands thickly 
 
16 Metamorphic Rocks. 
 
 charged with this mineral alternate with others in which it is compara- 
 tively rare. In size the garnets range from a quarter of an inch down- 
 wards. Notwithstanding that the garnet is a metamorphic product, its 
 introduction, in part at least, preceded the close of the schistose epoch, 
 as its partial destruction by the schistose movements makes clear. 
 Not only do the foliation planes tail round them, but they are 
 frequently crossed by fractures which have been infilled with 
 quartz. Moreover, they are often partially or entirely replaced by 
 chlorite, and these pseudomorphs often form lenticles that tail at 
 their extremities parallel with the foliation of the rock. Garnets 
 are most numerous in the softer beds which are characterised by a 
 preponderating development of micaceous and frequently chloritic 
 material, in which they are so thickly aggregated as to form a 
 large proportion of the mass. This is more especially brought out 
 where they have been replaced by pseudomorphs of chlorite, which 
 give a striking green hue to the rock. Garnet has not been 
 observed to occur massive as in some of the" garnetiferous green- 
 stones of Cornwall, but occurs as individual crystals more or less 
 idiomorphic. 
 
 Actinolite is of frequent occurrence in these rocks, but although 
 like garnet it has sometimes been replaced by chlorite, this 
 mineral appears to have been introduced after the cessation of the 
 movements. In some instances, however, chloritic pseudomorphs 
 after actinolite have been penetrated by garnets, and in all proba- 
 bility the period in which the latter were formed was of longer 
 duration than that attending the production of actinolite. 
 
 The garnets in this group of rocks seem to play a similar role to 
 that of the albites in the schists below the Loch Tay limestone. 
 As the albites, however, show no mechanical change they are 
 clearly of later introduction, taking them as a group, but may not 
 have been later than the formation of some portion of the garnet- 
 iferous zone. The restriction broadly of these minerals to strati- 
 graphical bands appears to point to original chemical differences in 
 their composition. It must not be understood, however, that they 
 are strictly confined to stratigraphical horizons. The garnets, for 
 instance, sometimes extend to the upper portion of the schists below 
 the Loch Tay limestone. They are also found in the overlying 
 groups of the Ardrishaig and Loch Awe series, but in these latter 
 their occurrence is more local, whereas in the garnetiferous mica 
 schist group the presence of garnets is the characteristic feature 
 over the larger part of their extension in the Highlands. In the 
 Explanation of Sheet 37 it has been pointed out that along their 
 south-westerly extension the garnets gradually die out, or are only 
 microscopic, but this feature is general the further they are traced 
 to the south-west, where regional metamorphism has diminished. 
 
 2. Distribution and Field Relations. — This group occupies a strip 
 of about four miles on the eastern margin of the map, from the 
 slopes of Beinn Laoigh to the head of Glen Fyne, from which the 
 band extends to the southern portion of the sheet along the 
 precipitous slopes of Beinn Bhuidhe. In the Geology of Oowal and 
 the Explanation of Sheet 37 it has been shown that, whereas the 
 
Gametiferous Mica Schist. 17 
 
 schists have been folded to a high degree, the boundaries of the 
 larger rock bands or groups follow a course singularly regular. The 
 gametiferous mica schist group in its extension from Cowal into 
 Sheet 45 preserves this regularity until it approaches the head of 
 Glen Fyne, when its junction with the Ardrishaig phyllites is 
 curved until it leaves the sheet above Glen Lochy. The curved 
 junction forms a double fold, the southern apex of which brings 
 the junction half a mile to the westward in the valley between 
 Geann Garbh and Meall nan Tigearn. The margin then, after 
 looping round to the eastward of the latter hill, sweeps round to the 
 north-west, crosses the Allt a' Ohabrain burn at a distance of 1^ 
 miles from Socach, where it bends round to the north-east along 
 the flanks of Beinn Laoigh, that peak and its upper slopes being 
 formed of gametiferous mica schist. It is only along the eastern 
 slopes of Glen Fyne that both edges of the band are seen in this 
 map. In the southern part of the sheet the group has a width 
 slightly exceeding a mile, but the north-eastern portion of the 
 band at the head of Glen Fyne has contracted to half that amount. 
 To the south of Meall nan Tigearn the boundary has been shifted 
 by a fault a quarter of a mile to the north-north-west. 
 
 The group contains some green beds which have been already 
 described. It is comparatively free from igneous rock. It contains 
 a few bands of epidiorite near its western margin below Beinn 
 Bhuidhe. A boss of kentallenite occurs on the southern edge of 
 the sheet, and a few dykes of lamprophyre and basalt are scattered 
 over the area. 
 
 This group is less siliceous as the Ardrishaig phyllites are 
 approached, and where they are also very chloritic it is often diffi- 
 cult to define the boundary between the two groups. The difficulty 
 is accentuated when the chloritic and micaceous zones of this 
 group are not gametiferous. In the sheet to the south (37) a 
 small zone of graphitic schist has been adopted as the boundary ; 
 but on leaving the coast and tracing the beds into the mountainous 
 district represented in this sheet (45), the graphitic schist is too 
 feebly developed to preserve its individuality. In the area between 
 Beinn Bhuidhe and Beinn Laoigh, not only has regional metamor- 
 phism sharply advanced, but the same district has also been 
 subjected to a higher degi'ee of mechanical disturbance ; as a 
 consequence, the graphitic schist has been, as it were, enveloped in 
 the gametiferous schist, and is represented in the latter for a 
 breadth of from a quarter to half a mile by a series of infolds. It 
 is seen in the bum which forms the head waters of the river 
 Fyne, about one mile N.N.B. of Ceann Garbh. It is also seen in 
 the bum sections on the slopes below that peak and Beinn Bhuidhe 
 and in the upper part of the Brannie burn. 
 
 At An Caoran, and on the slopes of Beinn Laoigh, there is a 
 large development of quartzose teds, and both garnets and actin- 
 olite are often replaced by chlorite. This is seen also at Meall 
 Eeamhar, but at this locality fresh idiomorphic garnets are seen 
 within the pseudomorphosed actinolite, their boundaries often 
 extending beyond the margin of the latter mineral. Here also 
 
18 Metamorphic Bocks. 
 
 garnets are frequently seen with chloritic rims, which form tail- 
 like extremities. In the Alt na Faing burn the chloritic pseudo- 
 morphs after garnet are so extensively developed as to give a 
 prevailing green colour to the rock. Moreover, some of the 
 pseudomorphs show as lenticles with tails, and chlorite has also 
 replaced actinolite. 
 
 Along the slopes below Ceann Garbh garnets are often strongly 
 developed in massive quartzose beds, as may be seen half-a-mile to 
 the north of Inverchorachan. Near the same locality (|-mile 
 N.N.W. of Inverchorachan) zones of schist a foot in width are 
 almost entirely composed of garnets. The schists immediately 
 below the limestone are garnetiferous in the Alt na Faing burn, 
 and on the hillside further to the southward above An Caorachan. 
 Quartz segregations are numerous, and as they have become 
 involved in the folding the intense plication of the schists is well 
 marked. 
 
 This group of rocks has undergone a vast amount of contortion, 
 which has involved not only the bedding, but the planes of folia- 
 tion also. Moreover, the larger folded limbs have been crumpled, 
 and, as already shown, the earlier quartz veins have shared in this 
 plication. As a result of this extreme contortion, combined with 
 an advanced type of metamorphism, they have given rise to 
 singularly rugged scenery, which is well brought out on the 
 watershed north-east of Beinn Chas. There the contortion is so 
 pronounced that in the absence of deep sections there is no indi- 
 cation of the general dip. Moreover, the folded limbs have been 
 squeezed and flattened and the quartz segregations of the veins 
 have have been reconstructed. At Meall Eeamhar, at the head of 
 Glen Fyne, the rocks are in a similar state of contortion. As a 
 general rule, however, in spite of contortion there is a pre- 
 vailing dip rudely perpendicular to the strike of the band, and 
 varying with its sinuosity. This dip is to the N.N.W. from the 
 southern margin of the map to the head of Glen Fyne, when the 
 boundary with the Ardrishaig phyllites is folded, as already 
 described. Henceforward the dip, while still inclining generally 
 towards that boundary, is more variable. J. B. H. 
 
CHAPTER IV. 
 
 Metamorphic Eocks— continued. 
 V. Ardrishaig Phyllites with Thin Limestones and Quartzites. 
 
 This group, which succeeds the garnetiferous mica-schists and 
 lies between the latter and the Loch Awe group, is extensively 
 developed on this sheet south of Glen Lochy, the Strath of Orchy, 
 and the head of Loch Awe. On the eastern margin of the map it 
 forms a narrow strip only a quarter of a mile in breadth on the 
 slopes of Beinn Laoigh. Following a direction a little to the 
 south of west as far as Socach, it then bends round to the south- 
 east, following the fold of the garnetiferous mica-schist group 
 already described. It increases in breadth until at the apex of 
 that fold it has a width exceeding a mile. Again contracting 
 from that apex, the south-eastern limb of that fold is reduced to 
 a breadth of half a mile for a very short distance, when it is 
 suddenly replaced by a compound fold, which has the effect of 
 bringing the Ardrishaig phyllites to the surface over a broad 
 stretch of country extending from Beinn Bhuidhe to the head of 
 Loch Awe. There the boundary again swings back from Ea Creag 
 in a west-north-westerly direction to Dalmally and the delta of the 
 Orchy. From thence it follows the north-eastern shore of Loch 
 Awe, crosses the loch in the Pass of Brander, enclosing a 
 small area in Lome south of that Pass, and recrosses the loch 
 in the neighbourhood of Inistrynich. From that point, after 
 following an easterly direction for over three miles along the 
 basin of the Teatle Water, it suddenly deflects to the S.S.W. 
 near the hill of Bealach nan Cabrach, and pursues a fairly 
 uniform course to the western slopes of Glen 8hira on the 
 south of the map. The south-eastern boundary of the group 
 from the slopes of Beinn Laoigh to those of Glen Fyne 
 coincide with the boundary of the garnetiferous mica-schist 
 series already described. The group, therefore, lies in a 
 tract forming a great loop of comparatively low-lying country to 
 the west, while its eastern extension forms a mountainous country 
 that includes the peaks of Beinn Bhuidhe (3106 feet), Ceann 
 Garbh (2635 feet), and Meall nan Tigearn (2423 feet). 
 
 These peaks lie just within the eastern margin of the group, and 
 constitute the most elevated ground of that portion of the sheet 
 lying south of the Strath of Orchy and the Pass of Brander. From 
 that eastern range of hills to the shores of Loch Awe, a distance 
 of nearly eight miles, the declivity is far from uniform. At a 
 distance of about three miles from the eastern boundary there is a 
 line of hills parallel to that boundary, the peaks of which attain 
 
20 Metamorphie Bocks. 
 
 an approximate altitude of 2000 feet, viz., Beinn Bhalgairean 
 (2085 feet), Beinn Bhdidheach (1933 feet), and Beinn an Sithein 
 (2088 feet). That line of peaks approximately divides the group 
 into two equal portions. From their base to Loch Awe it occupies 
 comparatively low-lying country in the basin of the Teatle Water, 
 and for a distance of three miles from Loch Awe the maximum 
 elevation does not exceed 700 feet. Attention has been particu- 
 larly directed to the partition of the group into elevated and low- 
 lying divisions, as these also form areas of distinct lithological type 
 due to marked differences of metamorphism. The western or Loch 
 Awe area having been metamorphosed but slightly, whereas the 
 eastern or mountainous area has undergone an extreme degree of 
 metamorphism, the lithology of the group varies considerably. 
 Not only are these distinct types represented by phyllites on the 
 one hand and by coarsely crystalline schists on the other, but 
 transitional forms connect these two extremes. 
 
 In common with the Loch Awe group, it is associated with large 
 masses of igneous intrusions, consisting mainly of epidiorite and 
 allied rocks, that are fairly uniform in their distribution over the 
 group. A small intrusion of granite occurs on the ridge connect- 
 ing Beinn Bhuidhe and Stac a Chuirn ; a boss of kentallenite is 
 seen near the head of Glen Shira ; and dykes of basalt, lampro- 
 phyre, and porphyrite are sparingly distributed over the area. 
 
 The group consists essentially of calcareous phyllites, with small 
 bands of impure limestone and some quartz-schist and quartzite. 
 These Ardrishaig phyllites contain a very low percentage of silica 
 and a large amount of lime, and in their normal condition consti- 
 tute one of the softest rocks of the Highlands, being readily 
 scratched with the finger-nail. They are usually of a pale- green 
 colour, but are often blue and sometimes grey or even pinkish. 
 They are too soft to form roofing slate, and lend themselves readily 
 to erosion. Li areas of extreme metamorphism, however, induration 
 has produced a rock exceedingly tough, that resists weathering to 
 such a degree as to form high and mountainous country. The 
 rugged range of Beinn Bhuidhe, forming the highest ground 
 between Loch Awe and Glen Fyne, is carved out of these rocks. 
 
 Sericitic mica enters mainly into their composition, except in 
 areas of slight metamorphism, where the rock is almost a clay 
 slate. In areas, on the other hand, of extreme crystallisation, 
 chlorite and muscovite have difierentiated into large plates, so as to 
 impart a mottled appearance to the rock. The green plates may 
 not always consist entirely of chlorite, but, in any case, the 
 chlorite, instead of being uniformly difiiised through the rock, has 
 aggregated into plates of which it must be the principal if not 
 the sole ingredient. Biotite is sporadically developed in areas of 
 increasing metamorphism, and where the latter is of a high degree 
 garnets also occur. They are never so highly developed, however, 
 as in the group of garnetiferous mica-schists, either as regards size 
 or aggregation. Epidote is a common constituent in the meta- 
 morphie districts, especially in the more calcareous varieties. 
 
 Although the Ardrishaig phyllites are highly calcareous, lime- 
 
Ardrishaig Fhyllites. 21 
 
 stone bands of any size are rare. They occur as fine bands inti- 
 mately mixed with the phyllites and siliceous members, and 
 ranging in thickness from a few feet to the finest laminations. 
 The limestone is very impure, and although dolomitic in appear- 
 ance, usually efiervesces with cold hydrochloi'ic acid. The weathered 
 colour is rusty brown, while a fresh fracture varies from bufi" to 
 pale yellow, and exceptionally white. Besides forming bands, 
 much calcareous material is diffused amongst the phyllites and 
 quartzites. 
 
 The quartz schists and quartzites are very compact, and the 
 texture so fine that clastic structures are often not to be detected, 
 even in areas of slight metamorphism, whereas in highly altered 
 districts they are never present. 
 
 The quartzose bands, when foliated, are highly siliceous, with very 
 thin films of mica along the cleavage planes, and closely resemble 
 fissile quartzites. More normal quartz schists also occur, and there 
 are transitional types connecting the siliceous and argillaceous 
 members. The matrix of the siliceous members is often to some 
 extent calcareous, and when the lime has been removed by solution 
 the imperfect cohesion of the siliceous grains leaves the rock friable 
 and easily decomposed. The siliceous bands vary in thickness from 
 two or three feet to an inch or even less. Moreover, their occurrence 
 among the phyllites is irregular, and they can seldom be 
 differentiated on the map. They vary in colour from pale grey to 
 grey buff. They maybe distinguished from the siliceous members 
 of the succeeding Loch Awe groups by their more uniformly fine 
 texture and the usual absence of clastic grains, and especially by 
 the absence of a coarser basal member. Finally, dark slate inclusions 
 and partings, while characteristic of the Loch Awe group, are of 
 rare occurrence in this series. 
 
 In describing the Ardrishaig phyllites of this area, it will be most 
 convenient to commence with the less altered beds of Loch Awe, 
 and from thence to trace the changes in their crystalline condition 
 as they are followed easterly to the flanks of Beinn Laoigh. The 
 beds occurring on the western side of Loch Awe, in the corner 
 formed by that loch and the Pass of Brander, are comparatively 
 unaltered. These rocks in their-least altered forms can be best 
 seen between Tervin and Hayfield. The beds are much folded, 
 with dips of folds at various angles ; yet the series as a whole is 
 nearly horizontal, so that one particular zone may occupy a large 
 area. Along the Pass of Brander, within the aureole of contact 
 metamorphism of the Ben Cruachan granite, much hornfelsing has 
 taken place which has been described in Chapter X., so that towards 
 that Pass from Kilchrenan the change of crystallization is sudden 
 and rapid, but much of that alteration may with certainty be 
 assigned to contact metamorphism. From this western area, how- 
 ever, to the eastern, the change has been more gradual and uniform, 
 and we are dealing with that type of metamorphism which is 
 understood by the term regional. In their least altered state the 
 Ardrishaig phyllites are well displayed on the southern coast of the 
 Pass of Brander, at New Inyerawe. Here they occur as grey and 
 
22 Metamorphic Bocks. 
 
 greyish-green phyllites, lying nearly flat, with the strata sometimes 
 rolling. They are made up of thin interlaminations of phyllite and 
 limestone, in which the former substance predominates, and in 
 places the phyllites very closely approach the condition of clay- 
 slates. They split along cleavage-planes which display a lustrous 
 surface due to the development of sericitic mica. The latter mineral 
 does not occur in definite recognisable crystals, but is suflSciently 
 abundant to produce on a fresh surface a distinct lustrous sheen. 
 On weathered surfaces, however, mica is not visible, and the rock 
 cannot be distinguished from a clay-state. Notwithstanding the 
 cleavage, the bedding is perfectly clear, colour banding being 
 sufiiciently prominent, even when the rock is in its most homo- 
 geneous condition. The surfaces of the cleavage planes are smooth, 
 and usually quite free from puckering. Besides the thin lime- 
 stones, very fine-grained quartzites and quartz-schists are inter- 
 calated, and are well displayed in the burn sections immediately 
 west of Tervin. As we proceed along the strike, the group is now met 
 with in Eilean Beith and Fraoch Bilean, two small islands on the 
 north of Loch Awe, opposite to the entrance of the Pass of Brander. 
 Here it is composed of green, highly calcareous slates, associated 
 with very fine-grained quartzites and hornblende-schist, the 
 sediments being in precisely the same unaltered condition as at New 
 Inverawe. JPollowing the line of strike still further eastward, they 
 reappear on the eastern shore of Loch Awe at Lag-na-Luinge as 
 greyish-green phyllites associated with thin limestones, and still 
 in a similar condition of metamorphism to their counterparts on the 
 western side of the loch. As the group approaches Dalmally, while 
 some zones still retain their unaltered appearance, the beds on the 
 whole are more crystalline. In place of the uniform greyish- green 
 tint on a fresh surface as seen at Inverawe, the colour is less 
 uniform, consequent on the development of scales of chlorite and 
 the difierentiation of that mineral from the mica. A mile still farther 
 east, in the burn that passes the Dalmally Hotel, crystallization 
 has advanced so far that they are in the condition of mica-schists, 
 with white sericitic mica well developed on the planes of foliation, 
 and that mineral together with green chlorite well indi- 
 vidualised. In addition, biotite appears in long blades and 
 irregular blotches, of later age than the schistose movements, 
 which have not afiected them. Still farther eastward, the 
 micaceous beds assume a more silvery appearance by the 
 increased development of white mica. The bedding has now 
 become intensely puckered, steady dips being no longer discernible. 
 Although the group as a whole persists in a fairly even course, the 
 plication within the zone is enormous. The beds are also assuming 
 a more indurated condition, weathering out into crags, instead of 
 the smooth features further to the west. Approaching the neighbour- 
 hood of Beinn Laoigh, these beds are not only intensely puckered 
 and more coarsely crystalline, but they are also considerably 
 hardened. At Socach, about three miles east of Dalmally, these 
 processes of deformation have proceeded so far as to recall the 
 advanced type of metamorphism in the anticlinal district of Cowal ; 
 
Ardrishaig Phyllites — Loch Awe Group. 23 
 
 the crystallization is very coarse, large flakes of white mica 
 and chlorite or green mica being interleaved. They contain here a 
 large amount of segregated quartz, which is both folded and crushed. 
 Unaltei-ed biotite is also abundant, especially in the calcareous 
 zones, and small garnets occur in the indurated calcareous sericite 
 schist. An excellent section of this coarsely crystalline schist is 
 seen in the Eas a Ghaill burn near Socach. From here this group 
 has been traced to the north-east into Perthshire, where they have 
 been mapped along the South-west Highlands as calc-sericite 
 schists, in some portions of which the metamorphic condition is 
 such that the sedimentary character has been entirely destroyed. 
 
 The intense plication of the areas of extreme crystallization is 
 well brought out on the map in the mountain range marked by 
 the peaks of Beinn Bhuidhe and Oeann Garbh. In this area the 
 calcareous sericite schists, with a few quartzites and an occasional 
 thin limestone, besides having a highly indurated and gnarled 
 surface, are so squeezed and contorted that over a large tract no idea 
 can be obtained of the dip. This tectonic confusion is admirably 
 illustrated by the epidiorite intrusions. These rocks over the area 
 generally follow well-defined bands parallel to the sedimentary 
 groups ; moreover, when these igneous bands have been repeated by 
 folding, the duplicated limbs still tend to conform to that 
 parallelism. On the mountainous tract of Oeann Garbh, however, 
 there is no such orderly arrangement. Notwithstanding that the 
 epidiorites there are of a particularly massive type, which in areas 
 less metamorphosed have resisted the stresses, they have been rolled 
 up and squeezed and forced to accommodate themselves to the most 
 irregular plication. Instead, therefore, of presenting a longitudinal 
 system more or less parallel, these rocks assume fantastic and 
 irregular shapes, with numerous isolated lenticles and constantly- 
 changing forms of the individual exposures. 
 
 A portion of the ridge to the south-west of Beinn Bhuidhe has 
 been hornfelsed. Between that peak and Stac a Chuirn a small 
 intrusion of granite is laid bare, which, in all probability, forms 
 part of a very much larger mass that is lying close beneath 
 the surface, and to which the contact metamorphism can be 
 attributed. 
 
 vi. Loch Awe Group. 
 
 AREA SOUTH OF LOCH AWE. 
 
 Lithological Cha/raders. — This group is divided into two 
 parts by the compound fold into which the Ardrishaig 
 series has been thrown, by which the latter, after following 
 a north-easterly direction, has been deflected westerly to Loch 
 Awe. The main portion of the group in this area extends for 
 about eight miles along the coast of Loch Awe from the southern 
 edge of the sheet to near Inistrynich. From that place, after 
 stretching easterly as far as Bealach nan Oabrach, it bends back to 
 the western slopes of Glen Shira. The other area occupied by the 
 
24 Metamm'phic Hocks. 
 
 group lies on the northern side of the boundary of the Ardrishaig 
 series between Dalmally and Ra Chreag. 
 
 As was pointed out in the last section dealing with the 
 Ardrishaig group, the petrological characters vaiy considerably by 
 the unequal action of regional metamorphism. The latter group 
 on its easterly passage from Loch Awe increased rapidly in crystal- 
 lization. The Loch Awe group, likewise, that lies south of the 
 Ardrishaig band differs markedly from its folded extension which 
 appears on the north side of that band. The continuity of the 
 group, however, on the east side of Loch Awe being cut off by the 
 deflected Ardrishaig group, the increase in metamorphism can only 
 be traced from the southern to the northern portion of the Loch 
 Awe group by passing over a portion of the latter. Hence the northern 
 and southern divisions, instead of blending into one another by a 
 gradual passage, are of different metamorphic characters. In the 
 northern area, where the crystallization of the Ardrishaig group is 
 of an extreme type, the associated Loch Awe group exhibits corres- 
 ponding metamorphism, just as in the southern area the two 
 groups partake equally of a lower degree of metamorphism. 
 
 The southern area will be described first, as the group is there 
 more typically represented. This group, which is also strongly 
 developed in that part of the Loch Awe basin lying to the south of 
 this sheet, has proved of the greatest interest in unravelling the 
 genesis of the crystalline schists of the Southern Highlands. Its 
 systematic examination has enabled us to demonstrate* that a 
 great series of rocks that extend across the Highlands as typical 
 crystalline schists pass along their strike by a gradual diminution 
 in metamorphism into rocks lithologically resembling Palasozoic 
 sediments. The sediments, in their unaltered form, frequently 
 contain clastic mica, while their associated igneous bands display 
 corresponding diminution in metamorphism, ranging from highly 
 foliated rocks to others in which the original structure has been 
 preserved. 
 
 The Loch Awe group consists of — 
 
 (a) Limestone. 
 
 (&) Black slates (sometimes graphitic). 
 
 (c) Grits and quartzite. 
 
 The limestone is usually blue in colour, but varying to pale 
 grey and even white, and to very dark blue and sometimes blackish 
 hues. It is crystalline and stratified, generally very thin-bedded. 
 It is often foliated with mica developed along the planes of schis- 
 tosity. Shading into slates on the one hand and into grits on the 
 other, it presents impure types vaiying in their composition in 
 proportion to the amounts 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 been experienced in difi'erentiating these impure limestones, 
 
 * Quart. Jour. Geol. Soc, vol. Iv. "Progressive metamorphism in the region of Loch 
 Awe," by Mr. J. B. Hill. 
 
Loch Awe Group. 25 
 
 which occupy the border-land between limestone and slate. There 
 IS a like transition from the limestones to the grits, the former being 
 often thickly charged with pebbles of quartz and felspar. The 
 limestone, moreover, is fi-equently of a fine, sandy type, tiny sandy 
 laminations of original deposit being closely intei-bedded with the 
 limestones. The limestone is frequently coarsely crystalline, but is 
 in many instances of a fine granular or cherty type, and this is 
 especially the case when associated with much black slate. All 
 the various types may be met with in the same seam ; slaty lime- 
 stone, compact fine-grained, and the coarse gritty types with 
 pebbles, an inch in length, succeeding one another in unbroken 
 continuity. 
 
 The slates may readily be distinguished from the Ardrishaig 
 phyllites. In their typical condition they range in colour from 
 dark blue to black, while occasionally they contain sufficient 
 graphite to soil the fingers. They are more homogeneous, harder, 
 and more massive than the Ardrishaig phyllites, and in their 
 normal condition are not calcareous. While these dark slates are 
 undoubtedly the characteristic type, paler varieties sometimes 
 occur ranging from grey to brown. 
 
 Over the Loch Awe group generally chlorite is rare, while in 
 the Ardrishaig phyllites this mineral hns imparted generally the 
 colouration. Occasionally, however, the slates and grits of the Loch 
 Awe group are green, and the clay slates can then only be 
 distinguished from the Ardrishaig phyllites by their lithological 
 characters. The green varieties, however, are only of local occur- 
 rence and do not occupy a definite horizon. Although the slates 
 are frequently highly contorted, the cleavage is often perfectly 
 regular, and they have been wrought for roofing slate. 
 
 The slates are often intermixed with siliceous material and 
 gradually pass lithologically into grits. Not only is there a gradual 
 passage into the siliceous members of the Loch Awe group, 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. 
 
 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 calcareous beds. 
 
 Qrits 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, some having 
 been completely foliated while others are not even cleaved. As 
 previously described, they pass gradually into the slates and also 
 into the limestones, so that we have rocks of mixed composition 
 which serve to bind the divisions into a single group. The grit 
 bands are divided by 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 
 
26 Metamorphic Hocks. 
 
 break. They are composed ot pebbles of quartz and felspar, much 
 of the former being of the clear blue variety. The latter is pink 
 or dull white, consists of both orthoclase and plagioclase, the 
 former often in excess, while usually quartz preponderates largely 
 over the felspar. Coarser and finer pebbly bands rapidly alternate, 
 the coarser, which are usually two or three yards and sometimes 
 twelve yards in width, being made up of pebbles as large as 
 almonds, which generally lie in a finer-grained matrix. White mica 
 is not a conspicuous mineral except in the highly metamorphosed 
 bands. In exceptional localities of feeble metamorphism, clastic 
 micas have been preserved. The grits associated with black slate 
 are sometimes dark, and those associated with green slate are green, 
 but both are strictly of local occurrence. 
 
 True quartzites are rare in the Loch Awe region ; many of the 
 grits, however, are not far removed from quartzites. Every grada- 
 tion is seen in the grits from bands which have suffered 
 extreme granulitization to others in which the grains have suffered 
 little or no crushing, and in which even the clastic micas are 
 preserved. In the coarse grit bands the larger pebbles are often 
 elongated and flattened, while the matrix has been partially or 
 completely granulitized. Usually, however, crushing has been 
 confined to the ground mass, 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 micro- 
 scopic examination show that they have been modified to some extent 
 by pressui'e. Through successive stages of alteration, however, 
 we arrive at last at beds so unaltered that clastic micas are still 
 preserved, and even microscopic investigation fails to detect 
 evidence of crushing. 
 
 The Boulder Bed. — In addition to the lithological types already 
 described, a rather abnormal conglomerate or boulder bed which 
 occupies a fairly definite position across the Highlands from 
 Aberdeenshire to Islay occars within the Loch Awe group. It is 
 extremely local in its occurrence, and most probably marks a phase 
 of crustal oscillation which sufficiently raised the sea floor to admit 
 of a cei'tain amount of contemporaneous erosion, fragments of each 
 varietjr 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 coastal 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 ai'e 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 prior to the boulder bed, 
 but there appears strong grounds for believing that the boulder 
 bed and the coarse grits belong to the same general phase of 
 
Loch Aive Cfroup. 27 
 
 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 
 composition of the grits, and sometimes attaining the size of two 
 inches, while pebbles ranging from a half to one inch in length are 
 exceedingly common. Foreign fragments in the boulder bed of 
 this area are rare, but granite fragments have been exceptionally 
 met with. These will be further alluded to in dealing with the 
 local distribution of this series. 
 
 That the boulder bed represents an abnormal phase of deposition 
 due to an elevation of the sea bottom and not an important 
 unconformity may be inferred from the identity of the lithological 
 types among which it occurs, and from the limestone in which it 
 lies forming a continuous deposit with limestone of normal type. 
 Moreover, the boulder bed lies within the Loch Awe group itself, 
 and has not been found resting on the Ardrishaig phyllites. 
 
 In studying the marginal relations of the Loch Awe and 
 Ardrishaig groups south of Shira, it is significant that the lime- 
 stone, although usually sandy, rarely exhibits the coarse gritty 
 character seen further to the west. Moreover, the black slate 
 may be entirely absent or present 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 
 appears, therefore, that the oncoming of these conditions, bringing 
 about a change in sedimentation as the result of crustal oscillations, 
 was less perceptible in this area on the eastern margin of the group. 
 
 GLEN SHIRA TO LOCH AWE. 
 
 General Distribution. — A very large part of this area is occupied 
 by epidiorite, an extensive sheet of which practically covers the 
 whole of the district west of the main road running south from 
 Cladich and extending to the shores of Loch Awe. East of that 
 line epidiorite, although not so extensively developed as the 
 sedimentary deposits, occurs in considerable abundance. 
 
 Limestone is also represented in a series of parallel bands 
 associated with phyllite over a belt of country extending from 
 Glen Shira to Loch Awe, as far south as a line running from 
 Cladich to Shira. To the south of that line grits and quartzites 
 form the elevated tract that culminates in the peak of Beinn Ghlas. 
 On the eastern margin of those quartzites along the slopes of Glen 
 Shira limestone and black slate are but feebly developed, and in 
 parts the latter may possibly be absent. On the other hand, the 
 steep angle at which the hill slope truncates the bedding would to 
 some extent account for the apparent diminution. In the vicinity 
 of Shira and Sron Mor, where the ground is flatter, limestone forms 
 large exposures in association with Ardrishaig phyllite. Excel- 
 lent sections of the limestone are seen in the burn at Acurragh, 
 
28 Metamorphic Bocks. 
 
 intimately mixed with Loch Awe phyllite, quartz-schist, and grit. 
 In the vicinity of Lochan na Cruaich, and again one mile S.S.W. 
 of that locality, the limestone in association with epidiorite is 
 highly indurated and baked by contact metamorphism. Over this 
 area the Loch Awe slates are so poorly represented, and the iso- 
 clinal folds are so steep, that they rarely occur in well-defined 
 bands thick enough to be worth differentiation on the one-inch map. 
 Consequently, the only two members of the Loch Awe group that 
 have been represented are the limestone and the grits, the 
 slates usually appearing either as partings within the grits or 
 intimately folded with the siliceous or calcareous members 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, 
 and for purposes of stratigraphy they mustbealtogetherdisregarded. 
 
 The base of the quartzite is well seen on the south side of Allt 
 Fearna, to the east of Inistrynich, where it rests upon grey silvery 
 phyllites with limestone bands. The actual base is here often 
 coarse and pebbly, and contains numerous fragments of limestone 
 identical with that exposed immediately below it, and the junction, 
 in the opinion of Mr. Kynaston, has all the appearance of an. un- 
 conformity. 
 
 The boulder bed is exposed in the bed of the burn just east of 
 Upper Sonachan,' and again about a mile south of Cladich, in the- 
 latter locality lenticular grit boulders being embedded in a slaty 
 matrix. About one and a half miles W.S.W. of Beinn Ghlas 
 lenticles of gritty quartzite about a foot long are lying in a matrix 
 of similar composition. In the vicinity of the small lochan three 
 quarters of a mile south-east of Tom Bharra, some exposures of 
 boulder bed are seen as inliers within the large epidiorite, one of 
 which occurs as a conglomeratic limestone with pebbles of quartzite 
 up to twd inches in size. 
 
 STKATH OECHY AREA. 
 
 The Loch Awe group which here succeeds the Ardrishaig 
 group to the north is in a more advanced metamorphic con- 
 dition than in the area just described. The quartzites are often 
 flaggy ^^^ strongly granulitized, but the coarse pebbly bands, 
 in which the original outline of the pebbles is still fairly distinct, 
 are not uncommon, and are well seen on the north side of Glen 
 Lochy near Socach. Apart from metamorphic condition, the 
 principal feature in the group is the comparatively strong develop- 
 ment of dark and sometimes graphitic schists which ai'e interposed 
 between the limestone at the top of the Ardrishaig series and the 
 Loch Awe quartzites. They are well developed in Strath Orchy, 
 but in their easterly continuation into Sheet 46 they are confined 
 to a narrower zone, which may be traced along the south side of 
 Glen Lochy. There are numerous infolds of the black schists 
 within the quartzite and the latter is often folded with the black 
 schists. J. B. H. 
 
Looh Awe Group. 29 
 
 AREA NORTH' OF LOCH AWE. 
 
 The area now to be described, which is occupied by the black 
 schists, Loch Awe limestone, boulder bed, and quartzites on this 
 side of Loch Awe, forms the roughly oblong strip extending from 
 the south-east end of the Pass of Brander and the promontory of 
 New Inverawe to Loch Nant. To the north-west of this strip the 
 schists disappear beneath the Lower Old Eed Sandstone lavas. The 
 black schist is perhaps more typically developed in this area than 
 in any other part of the map. It is very little altered, as far as 
 composition is concerned, and occurs mainly in the form of black 
 slates, which were formerly quarried for roofing purposes at 
 Kilchrenan. The cleavage is not often at a higher angle than 45°, 
 and lamination indicating the original bedding planes is often 
 well marked, and lying frequently almost horizontal. The laminte 
 do not show nearly such a high degree of puckering as in the 
 more highly metamorphic areas. 
 
 The folding of the whole schistose series is of the isoclinal 
 type, the folds being sharp and of no great amplitude. 
 
 There is no large area of black slates, and it has not been 
 possible to map them out as definite zones, owing to the frequent 
 infolds of limestone and quartzite,andthe numerous intrusive sills and 
 occasional boss-like masses of epidiorite. Perhaps the best exposures 
 are in the small burn about a quarter of a mile south-west of 
 Kilchrenan village. They occur again as a thin strip lying below the 
 blue limestone south-west of Achnamady and between Aehnarnady 
 and Achnacraobh. Numerous outcrops occur between Loch Tromlee 
 and Loch Nant, showing a very distinct lamination and frequent 
 intercalations of calcareous bands. 
 
 The Limestone, known in this area as the Loch Awe limestone, 
 has been mapped out as a well-mai'ked zone, with occasional bands 
 of graphitic slate, from the high road one mile north of Kilchrenan, 
 in a south-westerly direction past Achnacraobh and Achnamadj^ 
 until it disappears, about a mile south-west of the latter locality, 
 beneath the later volcanic rocks. The same belt appears again 
 from beneath the volcanic rocks in a small inlier on the west side 
 of Maol Mor, associated with the black slates. Between Kilchrenan 
 and the Pass of Brander there are numerous outcrops of the 
 limestone associated with black slates or grey phyllites. In the 
 neighbourhood of Tervin numerous thin limestones occur associated 
 with pale phyllites of the Ardrishaig type. 
 
 The limestone near Kilchrenan is quarried for road-metal, and 
 in some places was formerly burnt in kilns, but these have now 
 fallen into disuse. This is usually a fine-grained bluish grey 
 crystalline limestone, but is often much intermixed with impure 
 and more argillaceous bands, which appear to increase towards the 
 Pass of Brander. When these impure bands occur the numerous 
 small overfolds and puckerings are brought out with great clearness. 
 When traced towards the Pass of Brander, the impure limestones 
 are seen to have been afiected by the Ben Cruachan granite, and 
 o-radually assume the character of hornfels, and banded 
 
30 Metamorphic Boclcs. 
 
 calc-silicate hornfels is well seen along the line 
 of crags between Lochan an Ouaig and the base of the Lower 
 Old Eed volcanic series. The action of the granite upon these 
 rocks will, however, be described more fully in one of the following 
 sections. 
 
 To the east of Kilchrenan thin limestone zones are frequently 
 seen folded beneath the epidiorites and chloritic schists ; in fact, 
 these later sills would appear to have been intruded at about the 
 horizon of the limestone and black slates. The limestone, where 
 in contact with a sill, has been distinctly metamorphosed, and is 
 often full of chert-like bands. 
 
 The Boulder Bed is well developed in this area. It is seen on 
 the shore of Loch Awe just below the Kilchrenan Manse, and again 
 forming a well-marked line of outcrop to the west and south-west 
 of Kilchrenan, where it is seen to rest upon the black slates. It 
 is full of pebbles and boulders up to two feet in diameter, arranged 
 with their longer axes parallel to the lines of cleavage in the 
 matrix. The boulders are mostly of fine and medium-grained grits 
 and quartzites, and occasionally of limestone and fragments of 
 black slate. In the exposure on the shore near the Kilchrenan 
 Manse fragments of limestone and dark schist are exceedingly 
 numerous in a gritty matrix, and in the burn just west of Kil- 
 chrenan village the lower part of the boulder bed consists in places 
 almost entirely of fragments of black slate. Derived fragments of 
 igneous rocks also occasionally occur. Thus small pebbles of 
 granite and granophyre were found in this bed to the east ot 
 Achnamady, and pebbles of a pale grey cleaved foliated rock 
 occur in some quantity in the shore section near the Manse. 
 (Plate II.) 
 
 All the boulders and pebbles, with the exception of those of 
 black schist, are well rounded, as if water- worn, and the boulder 
 bed evidently represents a normal conglomerate which has under- 
 gone considerable stresses from earth-movement. The boulders, 
 while accommodating themselves to the strain, have shown greater 
 resistance than the matrix, which has, as it were, flowed round 
 them and past them under the shearing action. The boulder bed 
 does not appear to be continuous or constant in its behaviour, but 
 while it is almost invariably associated with the base of the 
 quartzite, it would appear to rest sometimes upon the black slates 
 and sometimes upon the limestone. In fact, the boulder bed or 
 some type of pebbly bed so frequently accompanies the base of the 
 quartzite and contains fragments of the underlying rocks that we 
 have undoubtedly strong evidence that the quartzite series rests 
 unconformably upon the limestones, black schists, and phyllites. 
 Thus in the Kilchrenan area we have not only the boulder bed 
 resting upon the black slates and containing numerous fragments 
 of them, but, interfolded with the limestone close to the base of the 
 quartzite, we frequently have strong pebbly bands composed of 
 fragments of the limestone and small pebbles of quartz, felspar, and 
 sometimes quartzite, and we have already referred to the pebbly 
 bands at the base of the quartzite overlying the pale limestones and 
 
:a 
 
Loch Awe Group. 31 
 
 grey phyllites of Ardrishaig type ip the area south-east of Loch Awe. 
 A pebble-bed of an analogous character occurs two miles south of 
 Port Sonachan, but the relations of the bed are not seen owing to 
 the surrounding area being occupied by epidiorite and chloritic 
 schists. The matrix of this bed is highly calcareous, and the 
 pebbles, which are usually small, consist of hard, well-rounded, fine- 
 grained quartzites and fragments of felsite, with smaller fragments 
 of pink felspar and grains of quartz. Further to the east, the base 
 of the quartzite is seen overlying the grey phyllites of Ardrishaig 
 type along the slopes on the west side of Glen Shira. Thus the 
 general behaviour of the quartzite, with the frequent boulder bed 
 or pebble bed at its base, has strongly the appearance of an 
 unconformity and overlap. 
 
 Description of some of the Boulders and Pebbles in the Boulder Bed. 
 — Two of the quartzite boulders from the Boulder Bed, when 
 examined in thin slices under the microscope [9059, 
 9060J, were found to consist mainly of quartz grains, 
 mostly well rounded, with a few turbid grains of both 
 striped and unstriped felspar, enclosed in a fine-grained, 
 usually quartzose matrix, sometimes exceedingly turbid, 
 possibly from decomposed felspar. Some chlorite is also occasionally 
 present, and small zircons are sometimes seen in the quartz grains. 
 The quartz often shows strain shadows, but there is scarcely any 
 appearance of schistose structure in the matrix. If an average 
 quartzite boulder from the boulder bed is compared with the 
 schistose grit or quartz-schist, with which the boulder bed is 
 associated to the south-west of Kilchrenan, the two rocks will be 
 observed to present on the whole a similar composition, but to show 
 a marked difierence in structure. Whereas there is little, if any, 
 schistosity in the boulders examined, a section of the " Loch Awe 
 grit" [9061] shows much stronger strain shadows in the quartz 
 grains, a matrix of clear, finely-granulitic quartz, and partial 
 granulitisation of some of the quartz grains, which act as "eyes" in 
 the finer material, as it sweeps round them. The rock also contains 
 two felspars, and what appear to represent small drawn-out frag- 
 ments of black slate. The boulders evidently escaped to a great 
 extent the action of the dynamic forces, owing to the shearing 
 strains being carried ofi", as it were, by the plastic matrix in which 
 they were embedded. 
 
 The granitic pebbles from the boulder bed, referred to above, are 
 seen under the microscope to consist mainly of a somewhat coarse 
 type of micropegmatite, with some plagioclase and some 
 interstitial chloritic alteration products [9286]. Another pebble 
 from the exposure near Upper Sonachan was a fine-grained 
 granophyre. 
 
 Sections were also prepared of some of the felsitic pebbles from 
 the exposure near the Kilchrenan Manse and from the pebble-bed 
 south of Port Sonachan. A section from the former locality [9058] 
 shows phenocrysts of alkali-felspar in a pale brownish, finely- 
 schistose matrix. 
 
 The felspars are turbid from alteration, but often have good 
 
<i2 Mfitamorphic Bocks. 
 
 outlines and show Oarlsbrad twinning, while they tend to lie with 
 their longer diagonals parallel to the planes of schistosity in 
 the matrix. Partially surrounding the phenocrj'sts is a finely 
 granular mosaic, which, together with the felspar crystals, forms 
 well-marked "eyes^" Occasionally the felspars show cross 
 fracturing. Some ovoid patches of granular quartz are also seen, 
 which may possibly represent original vesicles. The ground-mass 
 is foliated, exceedingly fine-grained, and has a somewhat fibrous 
 appearance, owing apparently to the development of minute 
 parallel flakes of biotite. The rock may very possibly have been a 
 felsitic lava. 
 
 The pale felsite-like fragments from the calcareous pebble-bed 
 south of Port Sonachan are seen under the microscope [9057-9060] 
 to be cleaved felsites of a similar general type to those of 
 Kilchrenan. They show small felspar phenocrysts, which probably 
 include orthoclase, in a finely felsitic and more or less banded 
 matrix, containing a few small quartz grains. 
 
 The occurrence of these igneous pebbles in the boulder bed is an 
 interesting point, and serves as additional evidence in connecting 
 this bed with the Port Askaig conglomerate of Islay and boulder 
 bed of the Isles of the Sea. 
 
 The Quartzite. — The relations of the quartzite to the black 
 schists and limestone have already been pointed out. 
 In the Kilchrenan district it is rather in the condition of a 
 schistose grit, and forms a well-marked band on the north-west side 
 of the boulder bed close to Achnamady. Here the rock is 
 frequently of a dark-grey colour, owing to the abundance of 
 included fragments of black schist. From Achnamady it may be 
 traced in a north-easterly direction towards Lock Tromlee. It is 
 well seen in places roiind the shore of that loch and for a distance 
 of about a mile to the north-east. Near Loch Tromlee it is often 
 strongly cleaved. It is usually of a medium-grained texture, and the 
 original grains of quartz and felspar may be clearly seen. 
 
 iEEA BETWEEN CRUACHAN GRANITE AND WATERSHED ON 
 N.W. SIDE OF LOCH CRERAN. 
 
 The larger portion of this area consists of black schists 
 and phyllites with intercalated quartzose bands. Black schists 
 and fine quartzites extend from the south side of Loch Creran 
 up to the margin of the Ben Cruachan granite. All the more 
 argillaceous portion of this series has been convei'ted into 
 homfels by the granite. The exact limits of the hornfels area 
 are naturally difficult to determine. Marked alteration into 
 hornfels may be noticed at a distance of about a mile from the 
 appearance of the granite at the surface, this phase gradually 
 shading off into less altered rocks as the distance from the 
 granite increases. The alteration is even perceptible along the 
 shore of Loch Creran, near Creagan Ferry, a distance of two and a 
 half miles from the nearest granite outcrop. 
 
 The black schists resemble on the whole those of the Loch Awe 
 
Loch Awe, Group — Ardrishaig Phyllites. 33 
 
 area, but the most striking feature of this district is the large 
 amount of quartzose material with which they are associated. In 
 fact, the black schists and their associated quartzose beds constitute 
 frequently a series of banded rocks, consisting of alternating bands 
 and laminae of argillaceous and quartzose material, and often 
 occupying a considerable area. The rock may sometimes be a 
 quartzose schist banded with black schist, or again a black schist 
 full of quartzose bands. A. hand specimen frequently shows the 
 banding, often highly puckered, in a very perfect manner; while in 
 the field the bands vary from a few inches up to broader zones 
 which can be represented upon the map. These banded rocks are 
 especially well seen on Ben Bhreac, on the east side of Glen Salach, 
 and on the hills on the north side of Glen Dubh. The alteration 
 into hornfels tends to accentuate the banding. 
 
 A well-marked zone of limestone of the Loch Awe type occurs 
 at the mouth of the river Creran, on the north side of the loch, and 
 may be traced from this point up the north-west side of Glen 
 Creran. Above this limestone is a band of typical black slates, 
 followed by a series of fine-grained quartz-schists and quartzites 
 with zones of black schist. Further to the west along the ridge 
 and western slopes of Beinn Churalain, the argillaceous zones 
 increase in frequency and consist of grey phyllites ; and gradually, 
 with a still further decrease of quartzose material, we pass into a 
 series consisting almost entirely of greenish and grey phyllites with 
 merely subordinate siliceous bands. When traced to the north- 
 east these phyllites appear to assume more the character of black 
 schists, as if the one passed laterally into the other. 
 
 To the east of the head of Loch Creran the limestone is well 
 exposed in Allt Buidhe, where it is associated with black and 
 banded schists, changed into hornfels, and accompanied by thin 
 quartzite bands. It is here a highly crystalline, bluish-grey or white 
 marble, while the less pure portions are strongly banded and 
 puckered, and converted into calc-silicate hornfels. 
 
 H.K, 
 
 AREA NORTH OF LOCH CRERAN. 
 
 Ardrishaig Phyllites.- — Between the white limestone at Port 
 Appin and eastwards to near the farm of Strathappin the ground 
 is occupied with Ardrishaig phyllites with a general north-east and 
 south-west strike. This belt is composed of dark-grey green 
 phyllites with some lighter bands. It stretches to the south-west 
 and composes the rough ground on both sides of Glaceriska Bay, 
 and a continuation of the same beds also forms the island of Briska. 
 Between Strathappin Farm and Ledgrianach the white limestone 
 is found at intervals. This belt of Ardrishaig phyllites between 
 Port Appin and Shian Bay seems to occur as a compound 
 anticline. 
 
 The limestone at Port Appin is a brown brecciated dolomitic rock 
 which at the Free Church passes into the adjacent phyllites 
 through a series of thin limey bands with phyllitic partings 
 These beds are continued to the south along the east side of Airds 
 
34 ' Metamorphio Rocks. 
 
 Bay, and are also found on the opposite side of Loch Oreran on, the 
 Briska shore. This limestone lies at the top of the Ardrishaig 
 phyllites, and is locally called the Appin limestone. 
 
 Specimens of the limestone from the knolls immediately north 
 of the Free Church Manse at Port Appin were examined by Dr. 
 Flett, who reports that the rock is a dolomite schist containing 
 dolomite in grains and small imperfect rhombohedra, much granu- 
 lar quartz and scaly sericitic mica, a little chlorite, iron oxides, 
 pyrites and sphene. The amount of dolomite varies considerably, 
 and the rock passes gradually into a pale grey or greenish-grey 
 phyllite with accessory carbonates. 
 
 Black Schists. — The black schists lie on the east side of the line 
 shown on the map between Strathappin farm and the head of the 
 raised beach at Shian Bay, and extend eastwards as far as the west 
 side of Loch Oreran. The black schist is associated with a large 
 amount of quartzose material which is so interfered with it that it 
 is quite impossible to separate the two in a satisfactory manner. 
 When the quartzose material attains any thickness it becomes a fine- 
 grained grey banded quartzose rock, and the alternating series 
 has a strong resemblance to the black schist group of Dalmally. 
 The relation of these beds can best be studied in the railway 
 cuttings around Gregan, where they form a dark, rusty grey rock 
 composed of alternating laminae of argillaceous quartzose material. 
 Here they occupy the whole of the south shoulder of Beinn 
 Churalain. In some sections the rock is a quartz-schist banded 
 with black schist, or again it may be a black schist full of quartzose 
 bands. This " grey quartzite" is totally unlike the Schichallion 
 or Banffshire quartzite. It is a fine-grained biotite quartzite, and 
 its position has long been recognised as lying close to the base of 
 the black schist group. Over this portion of the country its 
 apparent thickness is largely due to isoclinal folding, although its 
 original thickness was no doubt greater than further to the north- 
 east. The little black limestone of Perthshire and Ballachulish is 
 exposed on the cliff' to the north-east of the Ruadh Garbh, and also 
 in the railway cutting to the west of Oregan Station. 
 
 Quartzite. — On the north side of Loch Oreran the Central 
 Highland (Schichallion) quartzite is well exposed on the bare cliffs 
 which surround the small area to the south-west of Port Appin. 
 Here the rock is a white felspathic quartzite, very pebbly and 
 brecciated with well-marked original bedding planes. The pebbles 
 are well rounded and consist of white and blue quartz. 
 Immediately to the south of Barriemore this quartzite rests 
 unconformably upon several bands of phyllite and limestone. On 
 the shore to the east of this point the white coarse pebbly quartzite 
 contains included fragments of the adjacent limestone. On the 
 south side of the Barriemore and Port Appin road the junction 
 between these two rocks is clearly a line of fault. 
 
 J. S. G. W. 
 
 The quartzite at the roadside quarry above Port Appin pier is 
 a granulitic aggregate of quartz with many clear grains of fresh 
 microcline, a less number of orthoclase crystals, and a few small 
 
Ardrishaig PhyUites — Easdale Slate Gfroup. 35 
 
 zircons and grains of iron oxide. G-ranulitic structure has almost 
 completely replaced the original clastic character of the rock. 
 
 J. S. F. 
 
 BENDERLOGH AKEA. 
 
 Ardrishaig Phyllite Group. — On the Benderloch area the meta- 
 morphic rocks form the basis of the plain lying to the west 
 and north of the Lower Old Red Volcanic plateau. They form 
 long and often abrupt and isolated ridges which still retain the 
 appearance of an archipelago of rocky islets, although now united 
 by the deposits of the raised beaches. The members of the 
 Ardrishaig phyllite group compose the western half of the penin- 
 sula of Ardmucknish. In common with the other metamorphic 
 rocks, they are intensely folded upon north-north-east and south- 
 south-west axes, and are either vertical or highly inclined. 
 
 As developed in this region, the group is chiefly interesting on 
 account of its division into two parts, the one being phyllitic while 
 the other consists mainly of limestone and dolomite, constituting 
 the Appin limestone. The former, which is much the larger 
 portion, consists essentially of phyllites and slates of a grey-green 
 colour and well-banded character, the different laminae being 
 distinguishable both by colour and texture. Beds of this type are 
 exposed in the headland of Fionn Ard. The Appin limestone 
 consists of several beds of white or yellow limestone or dolomitic 
 limestone. Although much more massively developed in this 
 region, these calcareous beds at once recall the thin bands so well 
 known as occurring elsewhere in the Ardrishaig phyllite group. 
 It is important to note that the Appin limestone differs widely in 
 character from that of Lismore, which will be described later. 
 
 In Ardmucknish the Appin limestone is exposed along the 
 north-eastern extremity of the coast and again in Camas an Fhi,is, 
 from which bay it may be traced inland by means of limited 
 exposures. 
 
 Black Schists or Easdale Slate Group. — The striking feature of 
 the black schists in the Benderloch district is the frequent inter- 
 calation of quartzite bands in the group. This is, perhaps, best 
 exhibited in the sections exposed along the shores of Loch Oreran 
 eastwards from Ruadh Garbh. The quartzites are generally fine 
 in texture, but occasionally they are pebbly and contain grains of 
 blue and white quartz, and also of felspar. Pebbly beds with a 
 dark argillaceous matrix also occur. The quartzite usually forms 
 bands of considerable thickness, which are almost always separated 
 by thin partitions of slate. Thicker intercalations of slate and 
 others of dark limestone also occur with the quartzite, while 
 gradual transitions of quartzite by interstratification with black 
 slate may be studied in many clear sections. There is, probably, 
 much repetition due merely to folding of the strata, but there 
 can be no doubt that the alternation of sediments is mainly due to 
 original deposition. Inland the quartzite bands frequently project 
 as ribs on the hill slopes, and are well exposed in the rock cuttings 
 along the railway. 
 
36 Metamorphic Rocks. 
 
 The black slates are lustrous and graphitic, with abundant cubes 
 of pyrites, and closely resemble those wrought at Ballachulish and 
 Easdale. Its thin limestone bands are in many cases similar to 
 the little beds of black limestone which are a characteristic feature 
 in the slate quarries of Easdale and Belnahua to the south-west, 
 and Ballachulish to the north, of the present area. Occasionally 
 the limestones are more sandy and light-coloured. 
 
 The correlation of this rather heterogeneous group with that of 
 the black slates of Easdale and Ballachulish is strengthened by the 
 fact that the Loch Creran exposure is one of several which serve 
 to connect up the two typical areas. 
 
 Inland exposures of the members of this group are not so satis- 
 factory as that just described, especially as regards the black slates, 
 which are largely concealed by superficial deposits. The thicker 
 intercalations of quartzite give rise to ridges, several of which 
 have been cut through in the construction of the railway that 
 traverses the area. 
 
 Members of the group appear from beneath the escarpment of 
 the Old Eed Volcanic and sedimentary rocks, and in a very disturbed 
 and crushed condition form hillocks on the east side of Ardmuck- 
 nish Bay, one of which marks the site of Beregonium or Selma, a 
 stronghold of the early Irish colonists. On the west side of 
 Ardmucknish Bay the black slates form a narrow fringe along the 
 shore of Garbh Ard. Most of this last-named headland is, however, 
 composed of quartzite whose relations are obscure. The quartzite 
 is a white, fine-grained rock arranged in thin beds. Pebbles are 
 very rare and intercalations of black slate are almost unknown, 
 but greenish beds of a somewhat phyllitic textuj-e are not at all 
 uncommon. It is, therefore, apparent that in character it does not 
 agree with the quartzite intercalations in the black slate group 
 which are so well developed along the shores of Loch Creran. On 
 the western side of the Garbh Ard the margin of the quartzite, 
 where exposed upon the foreshore, is separated from the Appin 
 limestone by a few feet of black slates and limestone, but, unfortu- 
 nately, thei'e are signs of considerable movement having taken 
 place near the junction line. Inland, however, by the roadside 
 west of Lochan Dubh, the same junction is exposed where there 
 is little sign oi movement, but the black slates and limestone are 
 absent. 
 
 On the eastern side of Garbh Ard the quartzite is in contact 
 with the black slates of the Easdale group, and though there is a 
 slight movement and brecciation near the junction, the amount of 
 displacement may be inconsiderable, and the transition between 
 the two rock types appears to be gradual. Elsewhere south of 
 Loch Creran the junction is hidden under beach deposits. 
 Although the evidence is inconclusive upon the point, it seems 
 preferable in the meantime to regard the Garbh Ard quartzite as 
 merely a particularly pure example of the quartzites of the Easdale 
 slate group, and not a representative of the quartzite which has 
 been described in various parts of the Highlands, e.g., Islay, as 
 constituting a separate unconformable group. E. B. B, 
 
Metamorphic Bocks. 37 
 
 LISMORE ISLAND. 
 
 The Limestone Schist. — The portion of Lismore Island which is 
 included within this sheet, is composed almost entirely of a bluish- 
 grey, finely-crystalline limestone. The bedding is made apparent 
 in the cliff- sections by the difference in weathering of bands of an 
 impure argillaceous limestone and thin quartzose layers, which 
 frequently alternate with beds of purer limestone. The rocks are 
 thrown into numerous sharp, closely-packed folds, the limbs of 
 which are rippled by smaller folds. The axes of plication trend 
 north-east to south-west, paBallel to the length of the island. The 
 axial planes are vertical or dip to the south-east, but occasional 
 folds thrown over to the south-east were observed. On the western 
 side of the island the bedding is generally vertical, and the folds 
 appear to be of greater amplitude than those seen elsewhere. The 
 limestone is traversed by an inconspicuous cleavage, the planes of 
 which are vertical or dip at high angles to the south-east. The 
 argillaceous limestones show a slight puckering due to an incipient 
 strain-slip cleavage. Scattered cubes of pyrites are common 
 throughout the limestone. 
 
 The Black Schist crops out chiefly on the west side of the island, 
 but also half-a-mile north of Srdn na Oraoibhe and again below 
 Cachilardrishaig on the east side. At the latter place it is graphitic. 
 The outcrops run along the base of the cliffs beneath the limestone, 
 and extend up into the cliffs in folds. On the neck of the penin- 
 sula lying to the north-west of Fennacrochan, the limestone 
 completely arches over a series of folds in the black schist. In 
 several other sections on the coast to the south-west of this the 
 limestone may be seen arching over the black schist, the tops of 
 whose folds reach from 3 to 4 up to 30 or 40 feet above high-water 
 mark. In fact, wherever the relation between the two rocks can 
 be made out, the limestone appears as the upper member. 
 
 Bilean Ramsay and the neighbouring islets are composed of 
 limestone and black schist which have the same characters as on 
 Lismore. 
 
 Eilean Dubh, lying between Lismore and Benderloch, is formed 
 of isoclinally-folded beds of white and yellow quartzite, dipping at 
 high angles to the south-east. Crushed quartz-pebbles are re- 
 cognisable in some of the beds. At the southern end, where the 
 strike swings round from its normal north-east to south-west 
 direction to nearly due north to south, thin green phyllites with 
 some pale quartzose limestone bands are intercalated in the 
 quartzite. H. B. M. 
 
 Inliees of Metamorphic Eocks within the Volcanic Plateau 
 
 OF LORNE. 
 
 Oban, Glen Grutten, and Achavaich Inliers. — In the Oban region 
 the old floor of metamorphic strata upon which the Lower Old Red 
 Sandstone and its volcanic rocks rest is repeatedly brought to the 
 surface by means of a series of step faults aided by denudation, as 
 
.38 Metamorphic Hocks. 
 
 may be seen by glancing at Sheet 45 and the horizontal section 
 (Fig. 1, p. 71). 
 
 : The Tdcks which there «nter into this old floor consist of black 
 - sktes, and limestones of the Basdale slate group. The beds are seen 
 to be constantly repeated by folds of no great amplitude, which 
 have a tendency to be isoclinal, as all the steeper limbs face the 
 north-west, as shown by the limestone bands, while the strain^slip 
 cleavage planes which affect the slates all dip steeply towards the 
 , south-east and S . S.E. The limestones are dark and slightly crystalline 
 and micaceous, but the grade of metamorphism is nowhere high. 
 The folding arid cleavage of the slates and ^their relation to each 
 other are admirably displayed on the shores at Oban, 
 land also in the cliff at the south end of the ' town, 
 where the road bends a'ound into Glen Crutten. At 
 this latter point the limestone bands which have been 
 quarried help. to show the type of folding. The limestones were 
 also formerly wrought in a quarry a little to the south of the road, 
 and also in knolls standing above the raised beach deposits a little 
 -further up Glen Grutten. The railway cuttings south of Oban 
 Station also expose excellent sections of the contorted slates in 
 .places cut by sills and dykes of dolerite of Tertiary age. 
 
 A considerable inlier of these slates and limestone bands also occurs 
 at Achavaich, between Oban and Oonneli its presence there being 
 evidently due to the same system of faulting as that which has 
 -produced the exposures at Oban. 
 
 Kilbride Inlier. — North-eastwards from Kilbride, about five miles 
 south-east from Oban, there is an inlier about a mile long and a few 
 hundred yards in breadth occupied by silvery-grey phyllites, black 
 slates, and limestone, surrounded by volcanic rocks of the Lome 
 platform. The limestone bands, which are thin, dark, crystallirie, 
 and riiicaceous, are repeated by innumerable small synclinal 
 folds, the axial planes of which dip uniformly towards the N.N.W. 
 The dark slates and limestones probably represent the base of 
 the Basdale slate group, and the silvery phyllites the top of the 
 underlying Ardrishaig phyllites (fig. 1). B. N. P. 
 
 vii. Gkanulitic SchiSts of Moine Type with both Muscovite 
 AND Biotite. 
 
 This series occupies an extensive area in the north-eastern 
 portion of the map on the east side of the Ben Gruachan granite. 
 It is associated with two other zon.es of metamorphic rock, 
 namely, (1) flaggy quartzite or quartz-schist, and (2) garnetiferons 
 mica-schist. 
 
 (1) By far the larger portion of the area referred to is occupied, 
 however, by the granulitic schists of Moine type. In Strath 
 Orchy it is not difficult to separate these rocks from the quartzite 
 further south-west. The , quartzite- has undergone extertsive 
 granulitisation, but, still, frequently shows original quartz and 
 felspar grains of large size. .It contains little or no biotite, while 
 on the other hand thg granulitic schists of liloine type show a 
 
Cfranulitio Schists of Maine Type. 39 
 
 considerable development of biotite. These latter schists maintain 
 a uniform character over a large area, and consist of fine-grained 
 granulitic rocks in v^hich nearly all trace -of the original clastic 
 structure has been lost. They vary in colour from pale-grey or 
 pink to dark-grey, the darkness of the shade depending on the 
 amount of biotite present. The pinkish colours are apparently 
 due to the large amount of felspathic material occasionally 
 present. 
 
 A quartzite of pronounced flaggy character and without any 
 distinct original clastic grains is well seen near the foot of Allt 
 Broighleachan, in Glen Orchy. It appears to be separated by 
 granulitic schists of Moine type from the more massive and often 
 pebbly quartzite which occurs lower down the glen, and in most 
 field-sections it can be distinguished therefrom with comparative 
 facility. On one of its margins it is bounded by these 
 granulitic schists, while on the other it gives place gradually 
 to the garnetiferous mica-schist (2), to be subsequently 
 described; 
 
 Throughout nearly the whole of the area occupied by the 
 granulitic rocks and flaggy quartzite small veins of pegmatite are 
 common. They occur chiefly as small veins and strings of coarse 
 red felspar, usually associated with quartz, and often running 
 along the joint planes and other lines of weakness in the schists. 
 Some of the veins consist mainly of quartz. Besides distinct veins, 
 isolated pockets and strips of felspathic material are sometimes met 
 with, and these appear to be more common in the mdre highly 
 felspathic schists on the west side of Glen Strae. As to the origin 
 of these felspathic veins and patches, it can hardly be considered as 
 probable that they are due to the direct contact action of the 
 granite, since they are frequently seen at distances of several miles 
 from the contact, nor can they be considered as direct injections 
 from the granite magma. It is possible, however, that they may 
 perhaps be due to some kind of " granitisation," accompanied by 
 the introduction of felspathic material, which may have thus 
 aSected the rocks at a considerable depth. On the other hand, 
 there is no doubt that these granulitic rocks originally contained a 
 considerable quantity of felspar which could have supplied the 
 material for these structures. It may thus be more reasonable to 
 suppose that they were formed by segregatory action afiecting the 
 original felspathic constituents, and accompanying the cataclastic 
 stresses and reconstruction of the rocks. 
 
 (2) In certain parts of this north-eastern portion of the map 
 highly micaceous schists, often with both black and 
 white micas, and frequently garnetiferous, are not uncommon. 
 Thus, garnetiferous mica-schists may be seen in Glen Orchy just 
 below the Falls of Orchy (near the foot of Allt Broighleachan), and 
 they again form a, well-marked zone to the east and south-east of 
 Loch Dochard in the Black Mount. They also occur on the south- 
 eastern slopes of Beinn na Sroine, between Glen Orchy and Glen 
 Lochy. Lithologically, they often bear a strong resemblance to the 
 garnet-schists which occur below the calc-sericite group to the 
 
40 Metamorphic Bocks. 
 
 south-east of Glen Lochy and in the Glen Pyne area, but they appear 
 to occupy a different stratigraphical position. 
 
 The following section of rather over a mile in length, passing 
 through these schists, is seen as one ascends the burn situated 
 1^ miles south-east of Loch Dochart, in the north-east corner of the 
 map: — 
 
 1. A greenish homblendic schist or gneiss, the nature of 
 
 which is somewhat doubtful. It has provisionally been 
 classed with the later epidiorites, but it is not impossible 
 that it represents a foliated impure limestone. 
 
 2. On the south side as we ascend the burn are dark grey 
 
 biotite schists, the biotite often developed in elongated 
 patches, apparently representing pseudomorphs after 
 actinolite, irregularly distributed over the foliation planes. 
 This passes into a schist with two micas, often gneissose, 
 and with some small pegmatite veins. 
 
 3. Garnetiferous mica-schist, with two micas and numerous 
 
 garnets, constituting a zone nearly half-a-mile in 
 breadth. 
 
 4. Pale grey flaggy quartzites — a narrow zone. 
 
 5. About a quarter of a mile of silvery-grey mica-schists, 
 
 with two micas and small garnets. These rocks are often 
 very quartzose and contain knots and bands of quartzose 
 material, and are well exposed at the east end of Loch 
 Dochart. 
 
 The biotite schist (2) is seen under the microscope [8618] to be 
 fine-grained in texture, and to consist mainly of biotite and 
 quartz, with a small proportion of felspar, and occasional eyes of 
 granular quartz and small garnets. The garnet mica-schist [8619] 
 is considerably coarser, but intensely puckered and well foliated. 
 It is rich in quartz and contains both white and brown micas, the 
 former being slightly in excess, and a certain amount of felspar. 
 The garnets have often been broken up and show but little of their 
 original substance, being mainly represented by chloritic alteration 
 products, biotite flakes, and granular quartz. 
 
 It is highly probable that the granulitic schists of Moine type 
 are the equivalent of the Struan flags of the Central Highlands. 
 The recent progress of the Survey in the Central Highlands and in 
 Eastern Ross-shire has shown that the peculiar granulitic structure 
 and composition of the Moine schists of the North- West Highlands, 
 with their intercalated bands of muscovite-biotite gneiss, are 
 reproduced among the rocks of the Central Highlands, of which 
 the Struan flags form a characteristic portion. If, as may well be 
 the case, we have here in Argyllshire the representatives of the 
 Moine-schists, we might with good reason compare the garneti- 
 ferous mica'Schists with the muscovite-biotite gneiss. H. K. 
 
CHAPTEK V. 
 Older Igneous Rocks. 
 
 Under the term of older igneous rocks will be described in this 
 chapter those igneous masses which are of earlier date than the 
 earth-m.ovenients which caused the folding and foliation of the 
 rocks of the Highlands. These rocks will be seen from a 
 glance at the map to be almost entirely confined to that portion 
 of the metamorphic area which is occupied by the grey phyllites 
 of Ardrishaig type, the black schist, limestone, and quartzite. 
 They reach their maximum development in the area on either 
 side of Loch Awe, and die out in a north-easterly direction. In 
 the Glen Creran area igneous rocks of this class are also only 
 developed to a very small extent. These rocks are almost 
 invariably intrusive, and occur in the form of sills, now con- 
 sisting of epidiorite, chlorite-schist, hornblende-schist, and 
 similar types. Occasionally masses showing a more boss-like 
 behaviour may be met with. 
 
 i. The TSpidioriies arid Hornblende-Schists, &c. — Numerous 
 masses, behaving as sills, of epidiorite, chlorite-schist, &c., have 
 been mapped among the metamorphic rocks in the area 
 surrounding Loch Awe. Petrologically there is a large variety 
 of these rocks, and several variations both in composition and 
 structure may be met with in a single sill. The rocks are 
 frequently markedly schistose in structure, especially towards 
 the margins of the sills, though certain of the larger and coarser 
 masses have retained their original structure in a reinarkable 
 degree. Often a single sill may be represented by several 
 detached masses, owing to repetition by folding, or to the form 
 of the ground combined with a flat type of isoclinal folding. 
 Occasionally a mass of foliated basic igneous rock may cover a 
 considerable area, as, for instance, on the east side of Loch Awe, 
 on the soTithern margin of the map, but they more usually form 
 elongated masses standing out as conspicuous ridges above the 
 softer types of schist, though, on the other hand, irregularly- 
 shaped and complex intrusions are often met with. The rocks 
 may be fine-grained or coarse, and in colour are various shades 
 of green, and occasionally almost black. A considerable variety 
 of rocks have been included under the general term of epidiorite 
 for convenience in mapping. A short description, therefore, of 
 the relations and petrological characters of some of the principal 
 types occurring in the Loch Awe area will be of some interest 
 in this connection. 
 
 Bescription of the Principal Types. — A large elongated mass 
 of chloritic schist occurs along the west side of Loch Awe to the 
 
42 Older Igneous Bocks. 
 
 S.S.W. of Kilchrenan. It appears to have been intruded at 
 about the horizion of the black slates and blue limestone. The 
 rock appears to be thoroughly schistose throughout. Porphyritic 
 felspars are common, and in some places small nodules of 
 epidote, about the size of peas and closely resembling 
 amygdules, are exceedingly numerous. Frequently these 
 amygdule-like bodies show scarcely any signs of deformation, 
 although the surrounding matrix of the rock may be well 
 foliated. Irregular patches of crystalline granular epidote are 
 also of somewhat frequent occurrence. A specimen from near 
 the central portion of the mass, about half a mile north of 
 Fernoch, shows under the microscope large crystalline aggre- 
 gates of epidote and a few traces of felspar phenocrysts, in a 
 schistose matrix of plagioclase felspar, chlorite, and fibrous 
 amphibole. Traces of felspar laths are abundant in the matrix. 
 The chlorite and amphibole are mostly confined to those parts 
 along which the movement has been most intense. The slide 
 shows amygdaloidal cavities filled with carbonates and quartz- 
 mosaic. The rock may be called a schistose epidiorite. 
 
 In the area lying between the road leading from Kilchrenan 
 to Grlen Nant and the eastern end of the Pass of Brander the 
 epidiorites are represented by a more massive and coarser type, 
 which appears to behave frequently as a more boss-like type of 
 intrusion. A similar type of rock is also seen on the opposite 
 side of Loch Awe at Bovay, near Cladich. This may be termed 
 the dolerite or diabase type. A hand specimen shows a coarse 
 or medium-grained doleritic structure, though specimens from 
 the margins of the masses are often schistose and finer-grained. 
 A good examjjle of this type occurs as a large mass situated three- 
 quarters of a m.ile N.N.W. of Ballimore and within half a mile of 
 the Pass of Brander. The rock is decidedly coarse, and shows clear 
 felspars of elongated coluinnar habit, unorientated, in a dark 
 greenish "matrix. A similar rock, but somewhat finer in grain, 
 occurs a quarter of a mile north-east of Ballimore. Under the 
 microscope [9049, 9050] the plagioclase, which is slightly cloudy, 
 is seen to be frequently cracked and broken, the fractured portions 
 being often shifted, as shown by the discontinuity of the twin 
 lamellee. The interspaces between the felspars are filled almost 
 entirely by secondary minerals, viz., greenish hornblende, 
 chlorite, a pale greenish biotite, epidote, leucoxene, and car- 
 bonates. Apatite is accessory, and may occur in relatively large 
 crystals with an elongated columnar habit [9049]. In the 
 coarser and fresher of the two specimens a small quantity of 
 what may possibly be original augite is present [9049]. This 
 type of rock was doubtless originally a plagioclase- augite rock, 
 closely allied to the dolerites and gabbros. Another exposure 
 of epidiorite, a quarter of a mile south-west of Ballimore, shows 
 large porphyritic felspars, measuring sometimes over two inches 
 in length. 
 
 These doleritic types of the Kilchrenan, area frequently show 
 a considerable quantity of biotite in addition to green hornblende 
 
Older lyneous Bocks. 43 
 
 and chlorite [5523 and 5687, |-mile east of Kilclirenan, and 
 6921 and 6922, near Barbreck, Kilchrenan]. In some cases, 
 as, for instance, in the rock exposed at Bovay and at Taycreggan, 
 the biotite tends sometimes to be in excess of the other ferro- 
 magnesian constituents [5526, 8674, 8675], and resembles 
 somewhat the brown mica so characteristic of rocks altered by 
 large igneous masses. In this case, however, the biotite could 
 not have been formed by contact action, as there is no later 
 igneous intrusion of any size in the immediate neighbourhood. 
 The biotite has more likely been the result of thermodynamic 
 action, and has evidently replaced an earlier ferromagnesian 
 constituent. 
 
 On the opposite side of Loch Awe occurs a large mass of 
 igneous rock belonging to the " epidiorites," with a breadth of 
 about four miles. The rock belongs, however, to the fine- 
 grained type, and is schistose throughout its entire mass. 
 Portions of the mass are conspicuously porphyritic, while other 
 parts again show the vesicular or amygdaloidal structures 
 already described from the rock near Femoch. A specimen, 
 from two miles S.S.E. of Portsonachan, of a dark, almost black, 
 schistose porphyritic rock shows under the microscope [9052] 
 phenocrysts of plagioclase, rather tiirbid, often broken, and 
 occasionally showing a partial granulitisation. The phenocrysts 
 have a strong tendency to a similar orientation. Some grains of 
 green fibrous hornblende also occur, and amygdules of epidote, 
 occasionally with chlorite in addition. The ground-mass is 
 markedly schistose, and appears to consist to a great extent of 
 minute lath-shaped crystals of plagioclase, though it is a good 
 deal obscured by secondary products, and by an opaque material 
 in long streaks and strips probably consisting of disseminated 
 iron-ore. There are also some large grains of iron-ore, partially 
 replaced by leucoxene. The rock may probably have been 
 originally a basic type of andesite. 
 
 Not far from this locality there is a zone of rock, 
 forming part of the same mass, which consists of 
 numerous lenticular patches of a highly amygdaloidal 
 rock embedded in a fine-grained greenish schistose matrix. 
 The amygdules are filled with quartz and stand out conspicuously 
 on the weathered surface. Under the microscope [9053] 
 numerous amygdules, occasionally measuring ^-inch in diameter, 
 but usually smaller, are seen imbedded in a brownish turbid 
 matrix. The amygdules are sometimes almost spherical, though 
 often more or less elongated, so as to appear oval or almond- 
 shaped. They are filled with crystalline quartz, forming 
 usually a rather fine-grained mosaic, which becomes coarser 
 towards the central position. Some of the amygdules contain 
 calcite in addition to quartz, and occasionally some grains of 
 epidote in their marginal portions. The ground-mass is very 
 turbid, but appears to consist of very small felspar laths in a 
 dark brownish matrix. No phenocrysts are seen. The rock has 
 strongly the appearance of a highly vesicular basic lava. 
 
44 • Older Igneous Rocks. 
 
 Another -well-marked Tesicular yariety occurs as a narrow bed 
 associated with the calcareous pebble-bed two miles south of 
 Portsonachan. A section shows fairly num^erous amygdules, 
 filled with calcite, and mostly more or less drawn out. The 
 main mass of the rock consists of larger and smaller lath-shaded 
 plagioclases, unorientated, grains of iron-ore, some greenish 
 alteration products, and interstitial matter. The structure is 
 that of a very fine-grained dolerite or basalt. 
 
 We thus have varieties of the so-called " epidiorites," which 
 in hand specimens and in thin slices under the microscope are 
 strongly suggestive of true lavas, and similar rocks have been 
 mapped in Sheet 37 to the south, and are described in the 
 explanation of that sheet. (See also Summary of Progress of 
 Geological Survey for 1900, p. 42, also for 1899, p. 62.) Some 
 of these vesicular rocks resemble the Lower vSilurian lavas 
 associated with the radiolarian cherts of the Southern Uplands, 
 but there is no field evidence in support of this view in Argyll- 
 shire. On the contrary, the evidence has always hitherto 
 tended in a contrary direction, and has shown, in most cases 
 conclusively, that the "epidiorites" are intrusive sill-like 
 masses. 
 
 In the area further east, about the head of Glen Shira and 
 between Glen Shira and the lower part of Glen Orchy, the sills 
 belonging to this series are usually of the common schistose 
 epidiorite type, though slight variations are common and 
 porphyritic types are often characteristic. Vesicular varieties 
 do not occur in this area. On the east side of Glen Shira and 
 over the high ground about Beinn Bhuidhe a coarse type of 
 epidiorite occurs showing scarcely any signs of foliation, and 
 containing comparatively large aggregates of green hornblende. 
 
 To the north of this area, as we enter Glen Orchy or traverse 
 the high ground on the north side of the Strath of Orchy, we 
 find that the epidiorites have been mostly converted into horn- 
 blende-schists, owing to their being situated in a more highly 
 metamorphosed area. They consist usually of fresh green horn- 
 blende and plagioclase felspar, with a varying proportion of 
 biotite, add occasionally a small quantity of granular quartz 
 [5527]. Some of the sills also contain numerous small garnets. 
 About two miles to the south-west of Dalmally a large sill occurs, 
 which for the most part consists of the normal type of chloritic 
 schist, but in parts shows a considerable amount of variation. 
 It is often very felspathic, and in one place, about a mile west 
 of the Monument on the road between Dalmally and Cladich, 
 exhibits a modification which is practically an acid biotite-gneiss. 
 A section of the rock shows the following composition and 
 stinicture under the microscope [9611] : — Orthoclase, plagioclase, 
 and microcline are present in larger masses, which have some- 
 times, though not often, traces of crystalline form, and in smaller 
 grains of imperfect shape. Quartz in considerable amount 
 mingled with the smaller felspars to form a granular or inter- 
 locking mosaic. Fresh brown biotite is seen in small 
 
Contact Metamorphism. 45 
 
 interwoven plates disposed in -well-deiined curving lenticular 
 folia ; epidote, in well-shaped crystals and small rounded grains, 
 scattered through the micas and felspars ; magnetite, apatite, 
 zircon. Apparently there is no hornblende nor sphene, but brown 
 and greenish hornblende have been observed in other parts uf the 
 mass [5685]. The rock was probably originally a porphyritic 
 granite, and has been to a large extent reconstructed. The 
 structure is that of a gneiss. Muscovite and calcite are present, 
 though not abundant. Micropegmatite also sometimes occurs 
 [5684, 5519], and the eifects of strain and mechanical ruptures 
 are well shown. 
 
 In the north-east corner of the map a peculiar rock occurs to 
 the south of Clais-gobhair, referred to in the previous chapter, 
 which may be provisionally classed with the epidiorites. A similar 
 rock has been mapped in Sheet 46 to the east at about the same 
 horizon. It is a greenish schist or gneiss, and appears to consist 
 of a complex containing hornblendic, epidotic, and pyroxenic 
 gneisses, sometimes with sphene, while some portions have a 
 large proportion of calcareous matter. 
 
 The sills of the Pass of Brander area frequently show the 
 effects of the contact action of the Ben Cruachan granite, and 
 these will be described in the section dealing with that subject. 
 
 In the area to the north-' ast of Glen Salach intrusions of 
 epidiorite are not nearly so numerous as in the Loch Awe area. 
 They are very seldom of large size, and frequently only a few feet 
 in breadth. Between the head of Loch Creran and the margin 
 of the Ben Cruachan granite they become still less common, and 
 frequently appear to behave rather as dykes than sills. The 
 original rocks of some of these dyke-like intrusions were probably 
 closely allied to porphyrites. They have been altered by the 
 Ben Cruachan granite, and will be referred to again in a later 
 section. 
 
 ii. Contact Metamorphism. — -Along the margin of the sills of 
 epidiorite, chlorite-schist, &c., described above, where they are 
 in contact with the schists of sedimentary origin, the latter have 
 usually imdergone a certain amount of contact alteration, but it 
 is only in cases where the schists consist of rocks especially 
 susceptible to such alteration, such as impure limestones, that 
 this phenomenon has been remarked to any great degree. In the 
 Kilchrenan district, where the older igneous masses are often of 
 considerable size, they frequently occur in contact with bands of 
 limestone and impure calcareous rocks. The purer bands are 
 often converted into a pale highly-crystalline limestone, or 
 marble, while the less pure rocks become hard and hornstone- 
 like, assuming rather the condition of hornfels. To the east of 
 Kilchrenan and to the south-west of Achnamady, in the same 
 district, compact, cream-coloured, hornstone-like rocks are seen 
 in contact with the epidiorites. These rocks apparently belong 
 to the limestone series, but they are so siliceous that they were 
 probably originally bands of siliceous limestone, though to a 
 certain extent there may have been a replacement of carbonate 
 
46 Older Igneous Bocks. 
 
 by silica. Under the microscope the constituents of the rocks are 
 often too small for precise identification, but they appear to 
 consist mainly of colourless quartz and felspar and granular 
 pyroxene or epidote, and some carbonate still remains [7633, 
 7634]. 
 
 No contact alteration of any very marked degree has been 
 noticed in connection with the black schists or the quartzites. 
 
 H. K. 
 
CHAPTEE VI. 
 
 Folding and Progressive Eegional Metamorphism. 
 
 The metamorphic rocks that fall ■within this sheet are almost 
 entirely restricted to the eastern portion of the map, being cut 
 off to the westward by the Ben Cruachan granite and the Lower 
 Old Red volcanic roclvs of Lome and Benderloch. In the noi-th- 
 western portion of the map a smaller area of metamorphic rocks 
 lies in the district about Loch Creran and that part of the island 
 of Lismore which is included in this sheet. The eastern area 
 falls naturally into two districts, one of which, forming the basin 
 of Loch Awe, is comparatively low-lying, while the country 
 extending beyond those limits is elevated and mountainous. A 
 line following the course of the Pass of Brander, the upper part 
 of Loch Awe, Strath Orchy, and from thence curving round to 
 Grlen Shira, approximately deiines the limits of these divisions. 
 These areas, however, are not only marked by a distinct topo- 
 graphy, but the rocks which they contain represent degrees of 
 metamorphism equally divergent. While in the Loch Awe 
 basin are found some of the least-altered rocks that are known in 
 the Highlands, the mountainous region which borders it is made 
 up of crystalline schists of the normal type that enters into the 
 composition of the South-East Highlands. These extreme 
 metamorphic distinctions within such narrow geographical limits, 
 and the progressive gradations which link them, are of the 
 greatest interest in elucidating the genesis of the crystalline 
 schists. Their study has enabled us to- demonstrate* that a 
 great group of rocks which traverse the Highlands in the condi- 
 tion of typical crystalline schists pass along their strike into 
 sediments practically devoid of crystallisation, the lithological 
 characters of which are equivalent to many Palaeozoic deposits. 
 
 The sediments lying within the Loch Awe basin embrace the 
 Ardrishaig and the Loch Awe groups. Both these groups pass 
 into the metamorphic area. "The latter group enters largely into 
 the geology of the Strath of Orchy and the mountainous area 
 lying to the north of that strath. The Ardrishaig group does 
 not occur in that northern area, but its metamorphic type 
 occupies the mountainous tract between the slopes of Beinn 
 Laoigh and Glen Shira. To the east of that group the garneti- 
 ferous mica schists, the Lech Tay limestone, green beds, and 
 albite schists are all confined to the area of advanced meta- 
 morphism. 
 
 Epidiorites and associated intrusions of Dalradian age are a 
 marked feature in the less-altered area of the Loch Awe basin. 
 
 * J. B. Hill on the Progressive Metamorphism of some Dalradian Sediments 
 in the Region of Loch Awe, Quart, Jour, Geol, Soc, vol, Iv,, p, 470, 1899, 
 
48 Folding wnd Progressive Regional Metamorphism. 
 
 In tlie metamorphic districts they are more rare, but are of more 
 common distribution in the region south of Strath Orchy than 
 north of that line. 
 
 Besides these Dalradian intrusions, the metamorphic area 
 has been invaded by the granite masses of Ben Cruachan and 
 Glen Fyne. The tract between these plutonic masses in this sheet 
 constitutes the metamorphic area. Moreover, smaller bosses of 
 granite and kentallenite are seen in the intei'vening ground, and it 
 is possible that these schists are underlain by a connected belt of 
 plutonic rock. The regional metamorphism of the area is 
 associated, therefore, to some extent with thermal metamorphism, 
 and where the rocks have been subjected to both types of 
 alteration it is not always possible to differentiate their respective 
 limits. 
 
 Perhaps the least-altered rocks are those on the western side of 
 Loch Awe between Kilchrenan and the Pass of Brander. The 
 Ardrishaig phyllites of that area are practically in the condition 
 of clay slates. In Chapter IV. these rocks have been described 
 from their unaltered condition on Loch Awe, through their 
 various stages of increasing metamorphism, until they finally 
 culminate on the flanks of Beinn Bhuidhe as typical crystalline 
 schists. Moreover, the Loch Awe group with which they are 
 associated south of the Pass of Brander is in a similar unaltered 
 condition. The black schist which is here typically developed 
 occurs mostly as black slate, which has been quarried for roofing 
 pui'poses. The cleavage-dip does not usually exceed 45°, and 
 the original bedding planes are often very clear and frequently 
 lying almost horizontal. Moreover, they show a much smaller 
 amount of puckering than in the more metamorphosed beds east 
 of Loch Awe. The folding of the whole group is isoclinal and 
 shallow, so that there is a considerable reduplication of the 
 strata. The limestone is frequently very impure and often inter- 
 mixed with black slate and Ardrishaig phyllites; these 
 alternating bands bring out very clearly the plication. The 
 limestone is often of the pebbly type. The quartzite in this 
 area is mainly in the condition of a grit which has suffered 
 very little from shearing, but it is occasionally well cleaved. It 
 frequently contains inclusions of black slate, and is often divided 
 by partings of that material. The clastic grains of quartz and 
 felspar are practically uninjured even when the rock is cleaved. 
 
 Traced in a northerly direction towards the Pass of Brander, 
 both the Ardrishaig and Loch Awe groups have suffered contact 
 metamorphism by the Cruachan granite. This is mainly shown 
 in the impure limestones, not only by their induration but by the 
 development of calc-silicates, the production of epidote having 
 induced yellow and green colorations to the rock. Further, this 
 metamorphism more markedly differentiates the calcareous and 
 phyllitic alternations by which the rock is strongly banded, the 
 argillaceous material being of a purple colour in striking contrast 
 to the green and yellow calcareous bands. The alterations of 
 those rocks on the north side of the Pass of Brander will be dealt 
 with later. 
 
Folding and Progressive Regional Metamorphism. 49 
 
 The eastern extension of the Loch Awe group between 
 Loch Awe and Glen Shira shows a gradual increase in 
 metamorphism. This is most clearly seen in the grits and 
 quartzites which occupy the elevated tract between Acurragh 
 and Glen Shira. To the west of Beinn Ghlas very massive 
 quartzose beds alternate with coarse grit bands, both being 
 highly folded, while the finer grit bands are much sheared. To 
 the east of that hill the coarse grit bands have been sheared, and 
 even the larger pebbles flattened and elongated. Nowhere, how- 
 ever, in this area has deformation proceeded so far as to destroy 
 the clastic structures. This group can be followed no further to 
 the eastward on account of the compound fold previously 
 referred to. Progressive metamorphism is from thence repre- 
 sented in the Ardrishaig group. These phyllites at the head of 
 Glen Shira are already in the condition of crystalline schists, and 
 their eastward extension from thence to Beinn Bhuidhe shows 
 an advancing type of crystallisation. This increase in crystall- 
 isation, however, although an important feature, is not of the 
 same interest as the tracing of the group from Loch Awe to 
 Beinn Laoigh. The latter shows the complete passage from 
 unaltered rocks into highly crystalline schists ; the former 
 records only progressive metamorphism from one stage of 
 crystallisation to another. 
 
 This tract eastward of Glen Shira is confined to steep moun- 
 tainous slopes that culminate in the peaks of Beinn Bhuidhe and 
 Ceann Garbh. The Ardrishaig phyllites along this ridge not only 
 become more coarsely crystalline, with plates of chlorite and 
 white mica conspicuously differentiated, but this advanced state 
 of crystallisation is accompanied by marked induration, so 
 that the rocks, instead of being easily disintegrated, resist 
 weathering to such a degree as to form precipitous crags. In 
 these highly- altered beds small garnets are sometimes developed. 
 Moreover, these features of crystallisation and induration are 
 accompanied by a most intense type of plication that not only 
 produces a gnarled structure in the field, but the convolutions 
 are so rapid and irregular that the dip as regards both bedding 
 and foliation is never constant. This intense plication is well 
 brought out on the map by the irregular outlines of the epidiorite 
 sills in the neighbourhood of Ceann Garbh. 
 
 Not only are the eastern limits of the Ardrishaig group from 
 the slopes of Beinn Laoigh to the ridge of Beinn Bhuidhe in a 
 highly metamorphic condition, but the schistose groups between 
 that group and the eastern portion of the map are likewise in an 
 advanced stage of metamorphism. These groups include the 
 garnetiferous mica-schists, Loch Tay limestone, green beds, and 
 albite-schists ; they occupy the slopes of Glen Fyne, and, as 
 regards the first group, the slopes of Beinn Laoigh also. They 
 exhibit a high degree of gnarling and contortion, and have 
 been so completely deformed that clastic structures have often 
 been entirely destroyed. Moreover, in addition to mineral 
 reconstruction, certain secondary minerals, such as garnet. 
 
50 Folding and Progressive Regional Metamorphism. 
 
 actinolite, and albite, have been so extensively developed as to 
 impress distinctive characters along stratigraphical horizons. 
 The gritty sediments that are represented in these groups now 
 consist of biotite gneisses, in which grannlitic aggregates of 
 quartz and felspar are separated by folia rich in mica. Phacoids 
 of quartz and felspar are commonly broken up into lenticular 
 granulitic aggregates with tail-shaped terminations. Moreover, 
 the mosaic of the phacoids often blends with that of the flaser 
 matrix. In addition transverse structures have been induced by 
 secondary planes of schistosity, and these banded gneisses are 
 frequently puckered and gnarled. The phacoids are usually 
 the sole indication of the original clastic structure, and in the 
 absence of these grains all traces of such clastic conditions are 
 obliterated. 
 
 But, in addition to the destruction of the original constituents 
 of the rock, the authigenic minerals include garnet, actinolite, 
 and albite of a later generation. The garnets are extensively 
 developed in the garnetiferous mica-schist group, but are not 
 rigidly confined to that horizon. They are sparingly distributed 
 in that part of the Ardrishaig series which adjoins this group 
 when the former are in an advanced stage of metamorphism. 
 Moreover, they are occasionally seen as small pale grains in the 
 limestones, and they also occur at times in the bands immediately 
 below the limestone, including the green beds. In some bands 
 garnets account for a large proportion of the rock. Although 
 usually idiomorphic they are frequently fractured and foliation 
 planes tail, round them. Moreover, they are often represented 
 by pseudomorphs of chlorite, and these latter frequently form 
 lenticles that tail at their extremities parallel with the foliation 
 planes. 
 
 Actinolite is of less frequent occurrence, and is not confined 
 to the garnetiferous group, but is equally abundant in the albite- 
 bearing schists. Although this mineral has also been often 
 replaced by chlorite, it does not appear to have been deformed. 
 In some cases, however, the chloritic pseudomorphs after 
 actinolite have been penetrated by garnets, so that although the 
 production of these minerals is not restricted to identical periods, 
 it is probable that they overlap one another. 
 
 The secondary albite which occurs in tiny crystals in the 
 schists below the green beds are also found in the latter zone. 
 This mineral appears to play the same role as the garnets do on 
 another stratigraphical horizon. Unlike the garnets, however, 
 they exhibit no mechanical change. Considering them, therefore, 
 as groups, albite is of later introduction than garnet. 
 
 The high degree of metamorphism in this area is no less marked 
 in the Green Bed group. While to the south of this sheet the 
 clastic nature of these rocks is apparent, in the Grlen Fyne area 
 the allothigenic constituents have disappeared. Moreover, 
 actinolite has taken the place of chlorite, and these rocks are now 
 represented by hornblendic schists the structure of which is 
 indistinguishable from that of igneous rocks. Seeing the 
 
Folding and Progressive Regional MetamorphAsm. 51 
 
 advanced state of metamorphism that the schistose groups on the 
 south-eastern portion of the sheet exhibit, it is not to be expected 
 that progressive metamorphism should be marked. Nevertheless 
 their northern prolongation shows a coarser type of crystallisation, 
 both as regards the micaceous and chloritic minerals and the 
 accessory minerals, garnet. and actinolite, which are of a later 
 generation, 
 
 North of Loch Awe. — Having described the metamorphism of 
 the area south of Loch Awe and the Pass of Brander, the region 
 to the north will now be dealt with. This district includes a 
 narrow strip between the Pass of Brander and the Cruachan 
 granite, which expands at the head of Loch Awe into an extensive 
 region between that granite and the eastern margin of the map, 
 and extending southward as far as Strath Orchy and Glen Lochy. 
 That undulating line from the Pass of Brander to Glen Lochy 
 not only demarcates areas of distinct types of metamorphism, but 
 also marks the southern limits of the Grampians. The moun- 
 tainous region to the north, instead of showing a gradual declivity 
 towards the less elevated tract forming the Loch Awe basin, 
 terminates abruptly as a mountainous barrier. This sharp topo- 
 graphical change is the result of an equally sharp contrast in the 
 metamorphic action that marks these respective areas. It will 
 be shown that the lithological types of the mountainous region 
 to the north are the equivalents of the rock types further to the 
 south, but in an altered condition. So much is this the case that 
 it is probable that the northern area is almost restricted to the 
 Loch Awe group itself. As the Ardrishaig phyllites in their 
 passage to the east, instead of retaining their extremely soft 
 character, have built up the rugged chain defined by the peaks 
 of Beinn Bhuidhe and Ceann Garbh, so the Loch Awe group which 
 is so largely developed in the basin of Loch Awe have in their 
 northern extension given rise to the wide mountainous tract 
 extending to the Black Mount. 
 
 The marginal zone between those tracts occupies the east and 
 west basin of the lower portions of the River Orchy and of the 
 River Lochy. Along the southern side of that basin the 
 Ardrishaig phyllites have been traced, and with an increasing 
 metamorphism from the west to the east. 
 
 Strath Oi'chy Area. — In this Strath Orchy area the Ardrishaig 
 phyllites are succeeded normally to the north by the Loch Awe 
 group, in which the black-schist division is strongly represented 
 due to repeated folding. The succession is especially clearly 
 seen in the Allt Aindrea burn below Ra Ghreag, where the lime- 
 stone is interposed between the Ardrishaig phyllites and the 
 black schists. The limestone adjacent to the Ardrishaig 
 phyllites is made up of thin beds, greyish- white to brown in 
 colour, which seldom reach the thickness of a foot, separated by 
 partings of Ardrishaig phyllite. As the top of the band is 
 approached the limestone is of a cherty nature, and is dark in 
 colour, and is there associated with dark slates of Loch Awe type. 
 For upwards of a quarter of a mile along the burn and across' the 
 
52 Folding and Progressive Regional Metamorphism. 
 
 strike tlie section is made up of sharp alternations of black slate 
 and limestone too closely folded to admit of differentiation. 
 This close inter-association of dark slate and limestone is the pre- 
 dominant type in the Strath Orchy area, so that masses of pure 
 limestone are comparatively rare. This character is important 
 in its bearing on the more metamorphic region to the north. 
 The burns further west which drain into the Orchy afford similar 
 sections, but the limestone is not so well exposed. The black 
 schists are intermixed with paler argillaceous material, as is 
 usual in the Loch Awe group. Not only so, but these dark 
 schists are often more siliceous and intermediate in character 
 between quartzose and argillaceous rocks. Moreover, they are 
 closely interbanded with thin quartzites and less siliceous bands, 
 some of the latter being dark in colour. In places where the 
 quartzites are more strongly developed they are seen to be of 
 the characteristic type of the Loch Awe group, in parts being 
 made up of coarse grit. They contain also the pockets of dark 
 slate so characteristic of the Loch Awe division, and are similarly 
 divided by thin partings of dark slate. The gritty type of 
 limestone is, however, only of exceptional occurrence. The 
 Strath Orchy zone, therefore, is in reality an interf olded admixture 
 of impure limestone, dark phyllite, and quartzites, but in which 
 the dark phyllites are predominant. Moreover, all these 
 members belong to the Loch Awe group, and the succession from 
 that group to the Ardrishaig group conforms to the usual 
 sequence. 
 
 The easterly prolongation of this group in Strath Orchy 
 exhibits an increasing metamorphism corresponding to that of 
 the adjacent Ardrishaig group already described. It may be 
 stated at the outset, however, that the material of the dark slates 
 do not yield so readily to the metamorphic agents as the softer 
 Ardrishaig phyllites. This is possibly due to the graphite which 
 enters into their composition. The easterly prolongation of the 
 belt, therefore, from the Strath of Orchy is rather marked by a 
 more massive and indurated condition than a coarser type of 
 crystallisation. Moreover, the development of secondary 
 minerals in increasing force along this belt is the principal 
 characteristic in its progressive metamorphism. 
 
 The dark limestone near Dalmally is very impure. It con- 
 tains minute crystalline grains of carbonate, irregular grains of 
 quartz, very small specks of opaque matter, probably car- 
 bonaceous, and some fairly large cubes of pyrite [5202]. In the 
 neighbourhood of Socach, where more metamorphosed, the lime- 
 stone is represented by a calc-epidote-schist. It contains large 
 irregular crystalline grains and aggregates of carbonate, lying 
 in a foliated matrix. The former take on a lenticular character, 
 and appear therefore as " augen " on the sliced surface of the 
 rock. The schistose matrix is composed of quartz, epidote, and 
 biotite, with which grains of carbonate are also associated. 
 Typical calcite forms but a very small portion of the rock, and it 
 is traversed by films rich in biotite [5210, 5518]. Moreover, 
 
^ 
 
 T3 
 
Folding aiid Progressive Regional MetamorpMsm. 53 
 
 with the development of the calc-silicates the rock ceases to 
 effervesce with cold dilute acid. (Plate III.) 
 
 The dark phyllites are also often rich in carbonates, and these 
 calcareous phyllites grade into the impure limestones. About a 
 mile east of Dalmally they occur as lead-coloured calcareous 
 phyllites containing some narrow coarser bands and lenticular 
 folia rich in magnetic pyrites. Under the microscope the prin- 
 cipal constituent is seen to be a carbonate, which does not 
 effervesce with cold dilute acid. The other constituents are 
 quartz, opaque grains and particles, probably graphite, some 
 indeterminable minerals and tourmaline in scattered idiomorphic 
 prisms [5203]. Brown mica is also present in the rock. Some 
 of the graphitic schists in the same locality contain calcite 
 [5205], and in the River Lochy, 2\ miles east of Dalmally, the 
 calcareous graphitic schists are of the same type. In the same 
 river half a mile east of Inverlochy these rocks are coarser in 
 texture. A typical dark schist from the neighbourhood of Inver- 
 lochy shows under the microscope opaque grains scattered through 
 the mass of the rock, which is of very fine grain. The two 
 principal constituents are quartz and white mica as in many 
 phyllites. In addition there are two conspicuous minerals, brown 
 mica and chloritoid. The former is comparatively free from 
 inclusions, and shows a marked tendency to lie with its flat sur- 
 faces approximately parallel with the schistosity. The latter is 
 often full of inclusions of opaque particles, which run through it 
 in lines approximately parallel with the general foliation of the 
 schist ; on the other hand, the flat surfaces of the mineral lie 
 right across the general foliation, thus proving that growth took 
 place after the cleavage had been produced [5204]. 
 
 At Inverlochy and further eastward along the River Lochy 
 garnets and actinolite appear, the latter in blades sometimes 
 decomposed to chlorite. The dark schists are here more massive 
 and are often very rich in actinolite and garnet. About a 
 quarter of a mile east of Inverlochy the garnets increase in size, 
 and there are small zones in which this mineral is thickly dis- 
 tributed, and in which they exceed half an inch in size. More- 
 over, some of the garnets have been crushed and form eyes with 
 the foliation tailing round them. In this same locality some of 
 the limestones are gritty, and the coarse grits of Loch Awe type 
 are seen enclosing lenticles of dark leaden schist corresponding 
 to the clay slate pockets seen in less-altered areas. These rocks, 
 however, have been so altered that the pockets are flattened out, 
 and the clastic grits no longer project on the weathered surfaces 
 of the rock. Moreover, the rock when fractured behaves as a 
 homogeneous mass and cuts across the clastic grains, which are 
 not only partially granulitised but elongated and flattened. 
 
 Northern Area. — The mountainous slopes that rise to the north 
 from the Strath of Orchy and Glen Lochy consist of the same 
 assemblage of rocks as have been described in the latter, but the 
 siliceous members predominate. Moreover, the metamorphism 
 has advanced a stage further, so that the maximum crj'-stallisa- 
 
54 Folding and Progressive Regional Metamorjjhism. 
 
 tioa pertaining to the latter belt probably represents the 
 minimum, metamorphism of the northern area. Among the 
 quartzose rocks both black and white mica have been abundantly- 
 developed. They have become granulitised, and by recrystallisa- 
 tion pass over in their extreme phases of metamorphism into 
 gneisses. But notwithstanding this alteration the coarser grit 
 can still be detected ; although the larger pebbles are flattened, 
 pulled out, and crushed, the alternations in bedding of coarse and 
 fine bands can be seen, and the coarser conglomeratic zones 
 occurring in the less-altered beds of the Loch Awe basin are here 
 represented. The black slate partings are also still recognised, 
 but no longer in the condition of a slate, having been altered into 
 a hard dark micaceous rock. The pockets of phyllite within 
 the grit, so characteristic of the unaltered beds to the south, are 
 still seen in these highly-altered quartzose beds. Like the slate 
 partings, however, they have been converted into dull, lead- 
 coloured, crystalline, micaceous material. The rocks no longer, 
 in weathering, show the distinctive characters of hard and soft 
 alternations, but the micaceous partings and the quartzite bands 
 have, as it were, become welded into a solid homogeneous mass. 
 The dark micaceous alternations, when the rock is in this condi- 
 tion, impart a banded structure and a bluish coloration to the 
 mass. Moreover, the large quantity of secondary biotite which 
 is scattered through both the quartzose and micaceous zones still 
 further darkens them. These grits and quartzites have passed 
 into rocks which are gneissose in character; the more siliceous, 
 however, remain as quartzites and quartzite-schist. 
 
 Biotite, garnet, actinolite, tourmaline, and epidote are 
 common as secondary minerals in this highly-altered area. The 
 biotite has been one of the latest minerals to crystallise, and has 
 been generally unmoved. It often occurs in long blades with 
 somewhat ragged margins. The tourmaline may be of the same 
 age, as they occur in long pnsms, unbroken, some of which are 
 half an inch in length. The garnets and actinolites attain con- 
 siderable dimensions ; the former occur as large as half an inch, 
 and the latter are in blade-like forms sometimes an inch long. 
 Although garnets and actinolites are widely distributed over this 
 area, they are most closely concentrated in the more micaceous 
 beds. In certain zones they are especially abundant; for 
 instance, one bed was observed on the slope of Glen Orchy at AUt 
 Odhar about 6 feet in width literally studded with garnets half 
 an inch in size or even larger. In this case the foliation was 
 clearly seen to curve round the garnets, which, besides being 
 crushed, are often cleaved parallel to the foliation-planes of the 
 rock. In most cases, however, that mineral is idiomorphic and 
 shows no sign of deformation. In this instance the garnets form 
 pebble-like patches that are partially granulitised, and in which 
 there is a small amount of granulitic quartz. Moreover, the 
 breaking down of the garnet has given rise to a reticulated net- 
 work of chlorite which marks the initial stage of the pseudo- 
 morphous replacement of garnet by chlorite so commonly seen 
 
. Foldinc) and Progressive Regional Metamorphism. 55 
 
 over the garnetiferous zones generally. In this case the 
 deformed garnet blends at its edges with the gramilitic matrix 
 just as the qnartz phacoids do in the deformed grits. The 
 schistose matrix consists of grauulitic quartz and felspar, white 
 mica, brown mica, iron-ores, and a ferriferous carbonate [5530]. 
 The progressive metamorphism from the area of Strath Orchy 
 into this mountainous district is well illustrated along the River 
 Orchy between Corryghoil and Bochyle. Near Achanafaundich 
 the beds' exhibit a uniformly massive condition, and at first sight 
 appear as a continuous succession of quartz-schists and quartzites. 
 Some of these hard beds, however, are very micaceous and 
 represent altered phyllites, and others are calcareous mica-schists 
 that represent the impure limestones of Strath Orchy. The 
 river section half a mile south of Bochyle exhibits a still higher 
 degree of metamorphism, and these rocks are in the condition of 
 banded gneisses of the Struan type. The banding has been 
 brought about by the excess of biotite along alternations which 
 probably mark bedding planes. But notwithstanding the large 
 development of that mineral it occurs in tiny scales ; nevertheless 
 the dark slate alternations of Strath Orchy are here entirely 
 crystallised and represented by biotite-schist, while the more 
 sandy phyllites have been converted into biotite gneiss. At 
 Bochyle there is a greater development of micaceous beds, 
 which are in parts almost entirely made up of biotite. About 
 half a mile north of that locality the quartzose beds are inter- 
 mixed with much dark micaceous material. Moreover, these 
 bluish micaceous zones are highly charged vrith garnets and 
 actinolite, but these minerals have usually been altered into 
 chlorite. These bluish schists are so highly garnetiferous as to 
 recall the garnetiferous mica-schist group. The matrix, how- 
 ever, of the rock is not typical of the latter, and there can be 
 little doubt that these rocks are the metamorphosed representa- 
 tives of the Loch Awe phyllites. Under the microscope a 
 specimen from this locality of one of these highly schistose rocks, 
 inteimediate in character between a phyllite and a garnetiferous 
 mica-schist, and which consists of numeroiis garnets more or less 
 converted into chlorite, and a phyllitic matrix, shows the follow- 
 ing characters : — ^The principal constituents of the matrix are 
 quartz and white mica, the individuals being somewhat larger 
 than is usual in ordinary phyllites. In this matrix occur some 
 large and more or less irregular individuals of brown mica and 
 the garnets already referred to. The foliation-planes curve 
 round the garnets [5208]. Here also the alternation of bands 
 richer or poorer in biotite give the rock a striped gneissose 
 character, and when these two are sufficiently intermixed the 
 coarse banding disappears and the rock appears as a uniform 
 bluish gneiss. A little to the north a large exposure of the dark 
 biotite-schist is seen representing an infold of the metamorphosed 
 black phyllites free from quartzose intercalations. Between 
 this black schist and Larig a true quartzite is seen. This rock 
 has a highly baked appearance. It is not only intensely hard. 
 
56 Folding and Progressive Regional Metamhorphism. 
 
 but is almost white in colour, and resembles a rock that has been 
 almost completely resilicified, and so much is this the case that it 
 closely resembles the quartz segregations into which it almost 
 appears to blend. The original foliation-planes are very faint, 
 and in some cases are getting obliterated ; they are more 
 conspicuous, however, on the weathered surfaces. On the whole 
 the appearance in the field is very suggestive of a rock that has 
 been subjected to a high degree of thermo-metamorphism. A 
 little further up the burn, where this highly siliceous character 
 has disappeared, small pegmatite veins are common. In many 
 cases these segregations are made up entirely of red felspar, which 
 occurs as a coating or skin along planes of fracture and joint 
 faces. It also borders the outer edges of quartz veins. AVIiere 
 it coats a joint face, the adjacent rock for a short distance is also 
 impregnated with this red felspar, so much so as to itself take on 
 a pegmatitic appearance, but with the schistose structures still 
 remaining. 
 
 The mountainous region between Glen Orchy and Glen Strae 
 presents similar lithological types to those described in Glen 
 Orchy. As a rule, the beds have been so welded and indurated 
 that neither the intense plication of the rocks nor the original 
 bedding planes affect the general weathering. In some cases, 
 indeed, both these earlier structures have been almost entirely 
 obliterated. In the neighbourhood of Beinn Donachain, how- 
 ever, some thin micaceous schists reveal the intensity of the 
 contortion. Garnets and actinolites of large size are abundantly 
 distributed in the more micaceous and calcareous zones, but are 
 absent in those more purely siliceous. The limestone bands are 
 represented by calcareous schists that are often chloritic. An 
 extreme phase of the alteration of one of these impure limestones 
 is seen half a mile south of Beinn Donachain. Here large blades 
 of black hornblende are closely packed in a schistose matrix con- 
 sisting of white mica, brown mica, felspar, quartz, epidote, 
 rutile, and a ferriferous carbonate [5531]. At Na Cailleachan 
 a considerable tract of quartz-schist and mica-schist exhibits such 
 extreme alteration that the more siliceous portions have been con- 
 verted practically into a solid mass of quartz in which the bedding 
 planes have been preserved. The more micaceous portions have 
 a baked appearance, the mica is very coarse, and garnets and 
 actinolites are abundant. An adjoining zone is made up of 
 quartzose beds, in which red pegmatites are seen similar to those 
 in Glen Orchy. 
 
 West of Glen Strae the quartzose-schists are charged with red 
 felspar grains to such an extent as to give a pinkish hue to the 
 crags extending along that hillside. Moreover, red felspar veins 
 are common similar to those described in Glen Orchy. It not 
 only occurs as veins and strings along joint planes, but also along 
 foliation-planes, and in these latter the felspar increases in 
 amount as a joint plane is approached. While there is no doubt 
 that so far as these veins and strings are concerned the felspar 
 is secondary, in the case of the isolated grains of similar material 
 
Folding and Progressive Regional Metamorphism. 57 
 
 in their contiguity we have no criterion to decide how far they 
 may be secondary or to what extent they represent clastic grains. 
 It is probable, however, that both these conditions are repre- 
 sented. That these red felspathic strings, however, are bound 
 up with the metamorphism of the area is demonstrated by the 
 fact that they are confined to this metamorphic area, notwith- 
 standing that the same rock bands of identical composition enter 
 largely into the geology of the less metamorphosed district. 
 
 Metamorphism of the Igneous Bocks. — Having described the 
 progressive metamorphism of the sedimentary bands that radiate 
 to the north and east from the comparatively unaltered focus 
 south of the Pass of Brander, the corresponding metamorphic 
 changes which have been induced on the Dalradian intrusives 
 will now be briefly touched upon. These rocks, although 
 presenting wide variations in their composition, have been 
 grouped for convenience with the family of epidiorites. Strictly 
 speaking, however, they contain types that, as regards both com- 
 position and structure, extend beyond the petrological limits of 
 that group. They are likewise unevenly distributed over the 
 area. Although in the Loch Awe basin they are profusely 
 developed, they show a marked diminution towards the east ; 
 while in the northern region to the north of Strath Orchy they 
 are still more sparingly distributed. Owing to the isoclinal 
 folding of the district these rocks are exposed at the surface as 
 isolated masses, and consequently the progressive stages in the 
 metamorphism across single intrusions cannot be continuously 
 traced. In spite, however, of this lack of continuity, the meta- 
 morphic character of these rocks bears a definite relation to the 
 crystalline condition of the schists amongst which they occur. 
 
 Beginning with the unaltered area lying between the western 
 side of Loch Awe and the Pass of Brander, these intrusions are 
 no more altered than the greenstones of Cornwall that have been 
 intruded in the killas of that region. Many of them are doleritic 
 in structure, the original felspars are largely unaltered, and the 
 augite is now represented by uralitic hornblende, and in some 
 cases seems to be represented by pseudomorphous products. A 
 feature in these rocks of the Kilchrenan area is the occurrence of 
 biotite similar to the development of that mineral in areas of con- 
 tact metamorphism. This character still further links them with 
 the Cornish dolerites in areas of contact metamorphism. In the 
 Kilchrenan district, however, the Ben Cruachan granite is too 
 far distant to have produced contact action on these rocks. The 
 presence of this mineral, therefore, in association with the early 
 stages of the deformation of these intrusives indicates thermo- 
 metamorphism, and attention has already been drawn to the 
 regional metamorphism of the region immediately to the north- 
 east being of a thermal type. 
 
 From Loch Awe to the mountainous area of Beinn Bhuidhe 
 these intrusives are abundantly distributed, but as the latter area 
 is approached have undergone an increasing metamorphism 
 corresponding to that of the sediments with which they are 
 
58 Folding and Progressive Begional MeLamor'pMs'm. 
 
 associated. The doleritic structures have been obliterated, and, 
 except as regards the porphyritic felspars, the original minerals 
 have been completely replaced. Except the very coarsely 
 crystalline types, the rocks have been completely foliated, and 
 consist of epidiorites, hornblendic and chloritic schists. More- 
 over, the poi-phyritic felspars so abundant in the basic andesitic 
 varieties of Loch Awe have here suffered much deformation. 
 Small garnets are also an exceptional feature, while in the 
 neighbourhood of Socach they contain white mica. 
 
 In the highly metamorphosed area between Glen Strae and 
 Glen Orchy tliese Dalradian intrusives have undergone great 
 alteration. They have been considerably granulitised and their 
 recrystallisation is almost complete. The hornblende is usually 
 very dark in colour, and black actinolite is common. In this 
 area chlorite, which is so common a feature in the epidiorites of 
 the southern district, is absent, and if originally present has been 
 converted into hornblende and biotite. Moreover, the 
 porphyritic idiomorphic felspars have been considerably 
 granulitised, and are represented by irregular blotches. The 
 epidiorites of this northern area are hard and compact, and, like 
 the schists with which they are associated, decompose less readily 
 than the rocks of the area south of Glen Orchy. 
 
 The peculiar type of metamorphism of this northern area is 
 one associated with intense induration, and the rock bands have 
 the appearance of having been acted on by a regional type of 
 thermo-metamorphism. Moreover, this alteration represents the 
 last phase in their history. For not only has the final crystallisa- 
 tion of the area been unmodified, but the red felspar veins have 
 occupied the later joints, and finally the pre-existing fault 
 fractures have shared in thatj metamorphism, and their breccias 
 have been welded as a solid mass in common with the enclosing 
 rock. As stated earlier, these rocks bear a strong resemblance 
 to those of Struan, consisting of flaggy granulitic gneisses and 
 mica-schists. 
 
 The westerly extension of this northern area along the Pass of 
 Brander has not been touched upon in this chapter, as that strip 
 almost entirely falls within the contact aureole of the Cruachan 
 granite, a description of which will be given in Chapter X. 
 
 J. B. H. 
 
 In the districts of Appin, Benderloch, Lismore, and Oban the 
 rocks, though much folded and cleaved, are otherwise compara- 
 tively unaltered except near the granite masses. Evidence of 
 crushing and brecciation of the metamorphic rocks of the same 
 general age as the plication is presented at several localities, and 
 notably in Benderloch round the shores of Ardmucknish Bay. 
 Perhaps the most remarkable example is the crush conglomerate 
 of Port Selma, which has been carefully studied by Dr. Flett. 
 
 On the shore below the Beregonium, about three hundred yards 
 west of Benderloch Eailway Station, there is a very fine exposure 
 of crush conglomerate. The importance of rocks of this class in 
 
-^^-N. 
 
 
 ^*!r' 
 
Crush Conglomerate of Port Selma. 59 
 
 Argyllshire was first insisted on by Mr. J. B. Hill,* who found 
 them in the Loch Awe and Kilmartin districts. Few better 
 instances than that about to be described could be adduced in 
 any part of the county. Lying on the sea shore, where the rocks 
 are scoured by the shifting gravels of Ardmucknish Bay, it 
 exhibits very clearly the manner in which a mixed series of rocks 
 can be broken up by movement and converted into a con- 
 glomerate which closely simulates a true boulder bed. 
 
 The semicircular sweep of the east side of the bay is inter- 
 rupted at this point by a promontory composed of bare rock 
 rising through the sands and forming the west end of the small 
 hill on which the Beregonium stands. On the north side, near 
 the crofts of Selma, a shining calcareous black schist is the 
 principal rock. Followed southwards it gives place to a dark 
 graphitic limestone, which has the characters of the Easdale 
 limestone of Argyllshire. The foliation is nearly vertical and 
 strikes about north-east, almost at right angles to the shore line. 
 
 On the south edge of the limestone the conglomerate comes 
 in ; the junction is very sharp, and the limestone is free from 
 brecciation at its margin. There are no calcareous blocks in the 
 conglomerate at this point. The limestone, however, contains 
 lumps of a grey or brownish quartzite, from a few inches up to 
 twelve feet in length. The smaller blocks are well rounded, but 
 some of the larger are not more than eight inches thick, and 
 show a tendency to subdivide into smaller pillow-like masses. 
 All the quartzite is of the same character. Clearly these blocks 
 represent bands of quartzite which have been dragged apart and 
 rounded by the flow of the more plastic limestone matrix around 
 them. They resemble the fragments of epidiorite in limestone 
 described by Mr Hillt as occurring near the head of Loch Craig- 
 nish. The amount of interstitial movement which their shape 
 and relations to one another prove to have taken place in the 
 limestone is far more considerable than might have been 
 suspected from the appearance of the limestone itself. 
 
 From this point the crush conglomerate occupies the shore for 
 a distance of about forty ysirds, and finally is lost under the sand 
 and gravel of the south part of Ardmucknish Bay. Its appear- 
 ance may be inferred from the photographs (Plate IV., figs. 1 
 and 2). The blocks are of all sizes, some being six to twelve 
 feet long, but the majority are not over a foot in diameter. 
 Very commonly they are of ovoid shape with rounded surfaces, 
 and might be mistaken, if the exposure was less satisfactory, for 
 clastic boulders. Others, however, are cuboidal with squared 
 ends, while many of the softer rocks are flattened and lenticular. 
 They are principally a grey or brownish impure grit or quartzite, 
 like that above-mentioned as occurring in the limestone. The 
 
 *Hill, J. B. — "On Crush-Conglomerates from Argyllshire," Quart. 
 
 Jour. Geol. Soc, vol. Ivii., p. 313 (1901). Memoirs of the Geological Survey 
 
 "The Geology of Mid-Argyll," by J. B. Hill and others, p. 123 (1905). 
 
 + Hill, J. B. — "On the Crush-Conglomerates of Argyllshire," Quart. Jour 
 Geol. Soc., vol. Ivii., p. 321 (1901). 
 
60 Grush Conglomerate of Port Selma. 
 
 largest blocks are mostly of this nature. Limestones are few 
 except near the south edge of the outcrop, where many rounded 
 lumps of this rock, about a foot in diameter, are scattered 
 through the conglomerate. They weather with a brown crust, 
 and do not exactly resemble the massive limestone on the north 
 of the boulder-bed. 
 
 A few large pieces of black or grey shale are present, usually 
 much drawn out, but a more common type of boulder consists of 
 an alternation of thin bands of grey shale with yellowish fine 
 grits. Large fragments of this rock occur, though less frequent 
 than those of quartzite, and this would awaken suspicion as to 
 the nature of the conglomerate, seeing that they are not adapted 
 to form large boulders, but would split up if pounded by waves or 
 currents. These boulders have often an internal brecciation, 
 and can be traced passing, by various intermediate stages, 
 through contortion into brecciation, and finally into con- 
 glomerates little less perfect than that which surrounds them. 
 
 The matrix of the conglomerate is a grey, sandy, and shaly 
 rock with varying composition from point to point, but not, as 
 a rule, calcareous. It shows a foliation or banding parallel to 
 the strike of the limestone and black schist, and the blocks are 
 usually arranged with their long axes in this direction. The 
 simulation of bedding is, in consequence, often very perfect. 
 The ingredients of the matrix are by no means very thoroughly 
 mixed, but flakes and patches of grey shale, dark shale, and 
 grittj' shale lie in a slightly different ground-mass. These are 
 usually thin and lenticular, and give the conglomerate a 
 speckled appearance which is very suggestive of differential 
 internal movement. The impression produced by an examina- 
 tion of this matrix is that it might have arisen from the mashing 
 of a series of grey and black shales containing thin seams of 
 more quartzose character. Much of the arenaceous material 
 may have been derived from the attrition of the larger gritty 
 blocks, which, to judge by their rounded shape, must have been 
 considerable. 
 
 The especial merit of this exposure, however, is the clear 
 manner in which the mode of formation of the rock can be 
 established on the ground. On the north-west side of the out- 
 crop, just below high-tide mark and adjoining the limestone, 
 there is a mass of gritty and shaly beds about twenty feet in 
 length. It is partly covered by the beach sands, but around it 
 a belt of conglomerate or breccia can be traced as far as the 
 exposure continues. In this mass there are strong quartzite 
 bands, a foot or more in thickness, with alternations of grey and 
 blackish shale or phyllite. The bedding as a whole is well 
 preserved, and in the greater part of the mass there is little 
 internal brecciation, but on the east side it can be seen to pass 
 gradually into the surrounding conglomerate ; the grit bands 
 become bent and dragged apart; the soft shale flows into the 
 gaps. Movement in the plastic phyllites produces a substance 
 exactly like the conglomerate matrix; the grits yield boulders 
 
Crush Conglomerate of Port Selma. 61 
 
 which become more and more rounded as they are swept away in 
 the moving shales. All the ingredients of the conglomerate at 
 this spot can be matched in the unbroken beds. 
 
 Small inliers showing the same structures occur sprinkled 
 through the crush conglomerate, and might be taken for large 
 boulders, or, when they are more crushed, for brecciaied 
 patches of interbedded sediment. On the east side of the 
 exposure, partly covered by the grass of the bank above the 
 shore, there are larger masses of folded grits and phyllites, and 
 here the phenomena are all repeated. With the exception of 
 the brown-weathering limestone, all the rocks which are found 
 in the conglomerate occur in these masses. At the junction 
 between the conglomerate and the non-brecciated beds transition 
 stages are very frequent. In places, also, the bedded quartzite 
 and phyllite series is broken up, and resembles the less highly 
 developed portions of the crush conglomerate. 
 
 The most conclusive evidence that a rock is a crush con- 
 glomerate, and not a boulder bed due to local erosion, is the 
 presence of fragments of the overlying rocks. This cannot be 
 demonstrated here, as on the south side no massive rocks are 
 seen owing to the sand and gravel which obscure the section. 
 There is, however, no room for doubt as to the nature of this con- 
 glomerate, as the stages of its formation are so perfectly 
 exemplified and the very large size of the boulders, together with 
 the fact that many of them consist of weak and friable rocks, 
 preclude the hypothesis that they were detached from the 
 parent beds and reduced to rounded shapes by the processes of 
 sub-aerial erosion. J. S. P. 
 
CHAPTER VII. 
 
 Lower Old Red Sandstone. 
 
 i. Distribution, Order of Succession, and Field Relations. — 
 A very large area of the present sheet is covered by strata of 
 Lower Old Red Sandstone age. These consist mainly of 
 volcanic rocks, only a very small proportion of the area being 
 occupied by beds of sedimentary origin. The volcanic rocks 
 occupy almost the entire south-western quarter of the map, 
 extending from the southern shores of Loch Etive, west of 
 Bonawe, to the southern margin of the map west of Loch Awe, 
 and from the south-western margin of the map to the Pass of 
 Brander. On the north side of Loch Etive, they occupy the 
 western portion of Benderloch, situated between Loch Creran 
 and Loch Etive, and between the western slopes of Beinn Lora 
 and Gleann Salach. The entire area is probably not far short of 
 110 square miles. To the west the Lower Old Red Sandstone 
 volcanic area extends into Sheet 44, and reaches the shore of the 
 Firth of Lome to the south-west of Oban. On the south the 
 volcanic rocks occupy a portion of the north-west comer of Sheet 
 37, and south-westwards they are continued into Sheet 36 as far 
 as Loch Melfort. This extensive region is occupied by the 
 Lome volcanic series, formerly known as the Lome "traps." 
 The scenic features of this area are characteristic and distinctive, 
 the monotonous succession of scarped and terraced hills, and the 
 peat-covered moors with their numerous rounded knobs and 
 bosses of rock, unrelieved by any conspicuous peak or lofty 
 eminence, being in striking contrast to the steep slopes and 
 bold contours of the granitic and metamorphic country to the 
 east. 
 
 The rocks which occupy this area consist for the most part of 
 lava flows of basic, intermediate, and occasionally acid composi- 
 tion, with their associated agglomerates and tuffs. At or near 
 the base of this series we frequently find sedimentary rocks, con- 
 sisting mainly of conglomerates, sandstones, shales, and mud- 
 stones, and similar intercalations, though generally of no great 
 thickness, are frequently met with at varioiis horizons amongst 
 the volcanic outpourings. The sediments are apparently not 
 always continuous, and varj^ considerably in thickness from place 
 to place. They would seem rather to be of a local character, and 
 indicate aqueous conditions of no very great depth or extent. 
 
 If we traverse the Lower Old Red Sandstone area from Oban 
 in a more or less easterly direction, we should pass over the 
 sedimentary and volcanic series in a generally ascending order. 
 At the base of the series about Oban we find resting upon the 
 
Lower Old Bed Sandstone Strata. 63 
 
 upturned and deniided edges of the black schists a series of 
 coarse conglomerates, sometimes attaining a thickness of over 
 100 feet, and passing \ipwards into reddish and purple sand- 
 stones and shales. The conglomerates are well developed on the 
 island of Kerrera, in Sheet 44, and to the north of Oban, and 
 again form a fine feature beneath the andesites at Ledaig, on 
 the east side of Ardmucknish Bay. Towards the south of Oban, 
 however, they diminish in importance and decrease fairly 
 rapidly in thickness. 
 
 The composition of these coarse, massive, rudely-stratified 
 conglomerates suggests some reflections as to their source and 
 formation. Though the conglomerates lie immediately upon 
 the black schists, their component pebbles have only been to a 
 trifling extent derived from tnis underlying platform. Around 
 Oban, for example, the coarse conglomerates are to a large extent 
 composed of pieces of andesitic lava. The most remarkable 
 features of these pebbles are their frequent large size and well- 
 rounded forms. Some of them must weigh nearly half a ton, 
 and very considerable wave-action would seem to have been 
 required to wear down these lava-blocks into their present shapes. 
 
 From the preponderance of andesite boulders in these detrital 
 strata, it is obvious that there must have been large masses of 
 this rock at the surface when the conglomerates were formed, 
 and as the lavas of the Lorne plateau overlie all these basement 
 conglomerates, it is equally certain that the volcanic phenomena 
 did not commence with even the lowest lavas of that plateau. 
 Lava-flows must evidently have been erupted before any of the 
 visible conglomerates began to be formed. There may have 
 been volcanic islands in the \vater-basin wherein the Lorne sedi- 
 mentary series was deposited, and the destruction of these islands 
 by denudation, with the consequent spreading out of their 
 detritus as exceedingly coarse shingle over the rocky floor of 
 black shists, might have given rise to the conglomerates. Some 
 of the prominences of schist probably remained submerged 
 during the accumulation of the lower sedimentary series, for on 
 the south side of Loch Etive, a little to the east of Connel, Mr. 
 Symes found the older rocks immediately surmounted by the 
 lavas. The finer grits, flags, and purple shales, which intervene 
 between the conglomerate and the base of the volcanic series, 
 greatly resemble the ordinary strata of the Lower Old Red 
 Sandstone of Central Scotland. Similar sediments continue to 
 appear as occasional intercalations among the overlying lavas. 
 Thus, on the west side of Connel Station, one of these partings 
 consists of no more than eight inches of shale between two lava- 
 flows, while in the immediate neighbourhood another sedi- 
 mentary intercalation reaches a thickness of 5 feet, and is made 
 up of shales and thin sandstones. It is thus evident that the 
 detrital deposits continued during the successive outpourings of 
 lava, and that the volcanic rocks were erupted, or at least flowed, 
 iinder water. 
 
 To the east of Oban the observer traverses the lower series of 
 
64 Lower Old Red Sandstone. 
 
 lava-flows. These consist almost inTariably of basic andesites, 
 often closely allied to basalts and sometimes closely resembling 
 them in the field. When fresh these rocks are almost black in 
 colour and often compact and very close-grained in texture, and 
 without conspicuous phenocrysts. Porphyritic varieties, how- 
 ever, are not uncommon. The distinction between the different 
 flows is often very clear, the upper portions being usually in a 
 highly vesicular condition. These vesicles now constitute 
 amygdules of calcite, agate, and other minerals. Occasionally 
 the amygdules have been destroyed by weathering, and the rock 
 has reassumed something of its original vesicular aspect. The 
 upper surface of the flows is often in a slaggy and highly 
 brecciated condition, showing numerous angular and sub- 
 angular fragments of andesite in an andesitic matrix, doubtless 
 the result of the breaking up of the consolidated upper layer of 
 the flow by the still moving lower masses of lava. These lavas 
 weather with a reddish-brown crust, while decomposition of the 
 less compact varieties produces a purplish or sometimes almost 
 brick-red rock by the destruction of the f erro-magnesian minerals 
 and the oxidation of the iron-ores. Tn dividual flows are often 
 well defined, and form well-marked lines of crag and escarpment, 
 which constitute so characteristic a feature of the hills of this 
 area. Thus the slopes of Beinn Lora in Benderloch and of Deadh 
 Choimhead on the north side of Glen Lonan exhibit this character 
 in a marked degree. 
 
 Cracks filled up with fine sandy material are frequent all 
 through these andesitic sheets, and are sometimes so numerous as 
 to give the rock almost the appearance of a breccia. This pheno- 
 menon is in every respect similar to that observed so commonly 
 among the lavas of Lower Old Red Sandstone age of other areas. 
 The sandy silt has been washed into fissures and cavernous 
 hollows of the cooled lava, and where these spaces have run along 
 the middle of a single sheet they sometimes present a deceptive 
 resemblance to ordinary sedimentary intercalations that separate 
 distinct flows of lava. 
 
 Evidence of flow structure in the basic lavas may be often seen 
 in the crags of the less decomposed varieties. Fine parallel 
 ridges may be observed on the weathered surfaces, and the rock 
 tends to split up into thin slabs or plates and sharp splintery frag- 
 ments. This structure is well brought out in thin slices under 
 the microscope, and will be referred to again in the section 
 dealing with the petrology of the volcanic rocks. 
 
 Bands of fine red breccia, ash, or ashy grit are occasionally found 
 interbedded with the flows ; a good exposure may be seen among 
 the basic andesites two miles S.S.W. of Connel Ferry. Xeno- 
 liths of quartz grains were found to be fairly common in these 
 basic andesites in the western part of Benderloch near Ledaig, and 
 ■at Airds Point, north-west of Bonawe. 
 
 Basic andesites of the above type form the lower portion of the 
 series over practically the whole of the Lome volcanic area. To 
 the east of Oban one ascends over these basic sheets until one 
 
Volcanic Series. 65 
 
 reaches an liorizon of tuff. This tuff is of an acid character, 
 and is well seen 1^ miles east of Connel Ferry, and again at Airds 
 Point, but it is better developed in the western portion of Bender- 
 loch, where it can be traced from Dubh Loch Beag round the 
 western slopes of Beinn Lora, and thence in a general N.N.E. 
 direction almost to the boundary of the volcanic series. Large 
 masses of a somewhat similar tuff are again well seen in the 
 Benderloch area further east, and on the south side of Loch Etive 
 to the south of Achnacloich, in the area around the Sior Lochs, 
 and on the east side of Glen Nant ; but it is probable that these 
 latter belong to a slightly higher horizon. Again, a thin band of 
 tuff identical in character with that exposed Ij miles east of 
 Connel is found between basic andesites on the upper slopes and 
 crags of Cruach Ard-dhuine, 2^ miles S.S.W. of Taynuilt. 
 
 The acid tuff, as seen near Connel Ferry, at Airds Point, and 
 on the slopes of Beinn Lora, varies in colour from reddish purple 
 to pale yellowish. It is often compact and flinty in texture, but 
 more usually shows a strong flaky or platy structure, splitting 
 easily into thin slabs parallel to the planes of bedding. The rock 
 is frequently full of small lapilli of felsite and andesite, with here 
 and there fragments of quartzite and mica-schist, while scattered 
 throughout the matrix are small broken crystals of ^7hite, and 
 sometimes glassy, felspar and flakes of biotite. Portions of the 
 mass are often apparently entirely devoid of lapilli, and when 
 this is the case the rock bears a very strong resemblance to a 
 felsite. This tuff is well bedded, and in Benderloch and on 
 Cruach Ard-dhuine forms well-marked lines of crag. 
 
 At a slightly higher horizon a massive crystal tuff occurs, 
 which covers comparatively large areas in Benderloch to the east 
 of the area occupied by the acid tuff above described. On the 
 south side of the loch it has been traced in a S.S.W. direction 
 from the railway cutting a short distance west of Achnacloich 
 Station to the high ground overlooking Glen Lonan. Over part 
 of this ground the tuff forms a striking feature in the landscape, 
 the outcrop often terminating in an almost vertical wall of crag. 
 In the railway cutting near Achnacloich and in the grounds to the 
 south of the railway, this tuff has the appearance of a massive 
 pink porphyritic rock, which might easily be taken for a felsite 
 or felspar-porphyry. It has a dull, compact matrix, throughout 
 which are scattered abundant crystals of felspar and a smaller 
 proportion of decomposed biotite. In places, however, the true 
 pyroclastic nature of the rock is made evident by the abundant 
 lapilli. Precisely similar tuffs occur in the area around the Sior 
 Lochs, to the west and south-west of Loch Nant, and to the east 
 of Glen Nant. These have been described by Mr. Kynaston in 
 the Transactions of the Geological Society of Edinburgh, vol. 
 viii., p. 87. 
 
 Immediately below these tuffs near Achnacloich, we find the 
 normal type of basic andesite, while overlying them is a massive- 
 flow of porphyritic hypersthene-aridesite, which extends from the 
 ground south-east of Achnacloich to the high groiind on the north ; 
 
 E 
 
66 Lower Old Bed Sandstone. 
 
 side of Glen Lonan. No beds of sedimentary origin have been 
 observed associated witli the above-described type of crystal-tufE. 
 In connection, however, with the more acid type of tuff occurring 
 on Cruach Ard-dhuine, S.S.W. of Taymiilt, beds of reddish shale, 
 with sometimes sandstone and conglomerate, were frequently 
 observed both above and below the pyroclastic rock. The acid 
 tuffs are here from 30 to 40 feet in thickness. The more massive 
 crystal-tuffs of Achnacloich probably have a maximum thickness 
 of from 50 to 60 feet. 
 
 At about the same horizon as these latter tuffs, we find in the 
 area between Achnacloich and Glen Nant a series of pale felsitic 
 lavas, and these are again well developed in the area between 
 Glen Nant and the Pass of Brander. To the north-west and west 
 of Taymiilt these acid lavas are seen to underlie the massive series 
 of porphyritic hypersthene-andesites, already referred to, and 
 therefore to occupy here the same position that is occupied by the 
 tuffs near Achnacloich. 
 
 These lavas have more the appearance of a felsite or rhyolite 
 than an andesite. They are compact in texture, and vary in 
 colour from purple to a pale yellowish or cream colour. As a 
 rule, there is a well-marked banded structure due to flow, and 
 small felspar crystals and occasionally flakes of biotite are distri- 
 buted sparingly throughout the rock. The flow banding has 
 frequently a uniform dip coinciding with the general dip of the 
 series, but just as often it may lie at almost any angle from the 
 horizontal to the vertical, or again it may show a folded and con- 
 torted appearance, caused by the movements in the original 
 viscous mass of partially consolidated lava. Brecciation is also 
 sometimes well seen, as, for instance, in the mass west of 
 Gorstain, south-west of Taymiilt, which in certain parts consists 
 of numerous angular felsitic fragments embedded in a felsitic 
 base, showing fluxion-banding sweeping round the fragments. 
 Spherulitic structures are also well exhibited, and are well seen 
 in some of the exposures on the east side of Glen Nant. The 
 usual appearance of these spherulitic varieties is that of numerous 
 small nodules, of about the size of peas, crowded closely together 
 in a felsitic matrix. Occasionally these nodules are as large as 
 walnuts, and show hollow cavities in their centres. Associated 
 with these felsitic lavas we usually find mica- and hornblende- 
 andesites. These are well seen on both sides of Glen Nant in the 
 neighbourhood of Tailor's Leap, where they apparently overlie 
 the felsites. The crystal tuffs of the Glen Nant area succeed 
 these acid lavas and hornblende-andesites. 
 
 Covering large portions of the high ground to the south-west of 
 Taynuilt, about Carn Gaibhre, Beinn Ghlas, and the high ground 
 further west, we have a considerable extent of mica-andesites, 
 often more or less felsitic in texture. These possibly occupy the 
 same horizon as those of Glen Nant, but it is difficult to say so 
 with certainty. 
 
 These mica-andesites overlie a somewhat basic flow, and 
 . usually stand out in conspicuous crags, as at Creag-an-Fhithich, 
 
Volcanic Series. 67 
 
 ■^-mile south of Carn Gaibhre. JTluxion-banding is often seen, 
 but brecciation is perhaps more common, large masses of the rock 
 having a thoroughly brecciated structure throughout, and 
 showing numerous angular fragments of reddish, yellowish, and 
 greenish felsite in a rather decomposed reddish matrix. Pheno- 
 crysts of felspar are occasionally seen, and small biotite flakes are 
 very common. On Beinn Ghlas a porphyritic hypersthene- 
 andesite rests immediately upon the mica-andesite, but further 
 south-west, in the area about Lochan Dubha, other rocks appear 
 to have been locally intercalated between these two. Thus we 
 find here, resting upon the mica-andesite, a flow of very basic 
 andesite closely resembling basalt, above which is a well-marked 
 band of agglomerate and ashy material, succeeded by a purple 
 porphyritic andesite, which is probably the same as that 
 of Beinn Ghlas. The basic lava, resting upon the mica-andesite, 
 is allied to basalt, and is often highly vesicular, the vesicles being 
 represented by amygdules, often as large as hens' eggs, of reddish 
 agate with usually an inner layer of crystalline quartz. 
 
 The agglomerate is well exposed on the north and west sides of 
 Lochan Dubha, and forms the conspicuous crag of Creag Mhor, 
 south-east of Kilbride in Glenfeochan, and it has been followed 
 for some distance above the mica-andesites to the south of 
 Clachadow in Glen Lonan. It consists almost entirely of frag- 
 ments of difierent varieties of andesite in a finely fragmental 
 ground-mass. They are occasionally more or less rounded, and 
 often reach a diameter of 4 or 5 feet. Fine-grained ashy bands 
 are associated with this coarse material. This fragmental band 
 probably attains a thickness of about 50 feet, but it apparently 
 represents quite a local outburst, as it has not been met with in 
 other parts of the volcanic area. 
 
 Overlying the tufls of Achnacloich and the felsitic lavas to the 
 west of Taynuilt is the porphyritic hypersthene-andesite, already 
 referred to. This occupies a considerable area between the upper 
 part of Glen Lonan and the south side of Loch Etive. It is a 
 dark purple, sometimes almost black, andesite with porphyritic 
 crystals of plagioclase and greenish pseudomorphs after hyper- 
 sthene. Flow structure is also sometimes seen. A very similar 
 porphyritic lava occupies the upper part of Beinn Ghlas and over- 
 lies the mica-andesite, already described, and again a massive 
 porphyritic flow covers all the upper portions of the hills between 
 Kilbride, in Glenfeochan, and Loch Scamadale. Similar lavas 
 appear again to the south of Loch Scamadale, and occupy the 
 higher hills, such as Beinn Chapull and Beinn Dearg, to the east 
 and north-east. These latter lavas evidently overlie the crystal 
 tuffs, so well developed in that district, but are possibly at a 
 higher horizon than those of Loch Scamadale. In fact, these 
 upper flows of hypersthene-andesite are probably the highest 
 lavas now seen in the Lome area. 
 
 If we briefly review the general order of succession of the Lower 
 Old Eed Sandstone series of the Lome area, we find that the lower 
 portion of the series consists of conglomerates and local breccias 
 
68 Lower Old Bed Sandstone. 
 
 with shales and sandstones, followed by a series of basic andesites, 
 often allied to basalt, and with these we occasionally find flows of 
 less basic hypersthene-andesite. Ascending in the series we find 
 ■acid tuffs intercalated with the basic flows, but followed at no 
 great distance by another series of tuffs, perhaps somewhat less 
 acid, with felsitic lavas and hornblende- and iaica-ande«ites. 
 These again are overlain by thick flows of hypersthene-andesite, 
 with in some places intercalations of basic andesite and andesitic 
 agglomerate. 
 
 We thus commence with basic material and ascend to outpour- 
 ings and outbursts of acid lavas and tufis, but these latter again 
 were succeeded by more basic material, so that the volcanic 
 sequence of this area not only shows, during part of the period, a 
 marked increase in the acidity of its material, but also in its 
 later phases there was evidently a return to the more basic 
 conditions. 
 
 Over the greater portion of the area the dip of the volcanic 
 series and its associated sedimentary zones varies from E.S.E. to 
 S.S.E. It is usually at a low angle, but inclinations up to nearly 
 30" degrees have occasionally been observed. In the south-west 
 corner of the map, in the area 'about Loch Scamadale, the dip is to 
 the north-west. The volcanic series probably reaches its maxi- 
 mum thickness in the central portion of the region, in the area 
 about Beinn Ghlas and Lochan Dubha. The thickness here is 
 probably at least 2000 feet, but that of the entire series is pro- 
 bably considerably more. In the south-eastern portion of me 
 area towards Loch Awe the covering of lavas is of no great thick- 
 ness, and inliers of the older schists sitwated at various levels are 
 of not uncommpn occurrence. Outliers of the lavas and of the 
 basal conglomerates are occasionally seen in the area between 
 Loch Nant and the Pass of Brander. The situation and 
 behaviour of these inliers and outliers serves to illustrate in a 
 most instructive manner the remarkably uneven, or rather hilly, 
 nature of the ground over which the lavas were poured out. We 
 thus find inliers occasionally appearing amid the lavas on a hill- 
 side, or in the upper portions of the small glens, while the out- 
 liers are occasionally preserved as though clinging to the hill- 
 slopes of the metamorphic area. The older surface of schists was 
 evidently very rugged and uneven, and possibly exhibited moun- 
 tainous features such as we see in the same type of area at the 
 present day. The conglomerates may thus have been piled up 
 against steep slopes of the older rocks where the action of 
 breakers would have been strong, and water-worn boulders would 
 be carried down bv streams. The breccias, so commonly seen at 
 the base of the andesites in the absence of typical conglomerates, 
 would doubtless, from their appearance and the local character of 
 their materials, represent superficial accumulations, such as talus 
 or scree material, so characteristic an accompaniment of the 
 surface denudation of such an area. The presence, moreover, of 
 the zones of conglomerates, sandstones, and shales at various 
 horizons and in. different localities shows that aqueous conditions 
 
Vokanic Series. 69 
 
 of a somewhat local character prevailed during probably the 
 greater portion of the volcanic period. 
 
 With the general easterly and south-easterly inclination of the 
 series in this area we get at the same time a gradual thinning out 
 of the series in an easterly and south-easterly direction. The 
 result of this is that we have a gradual overlapping of the higher 
 flows, so that in the eastern portion of the area we find higher 
 horizons resting upon the older rocks than is the case to the west. 
 Thus, in the region near the Pass of Brander and near Loch Nant, 
 we find the massive crystal-tuffs only separated from the older 
 schists by comparatively thin intercalations of the basic flows, 
 while in the eastern part of the Benderloch area these tuffs are 
 occasionally found resting immediately upon the old platform. 
 
 H. K. 
 
 Oban and Kilmore District. — The succession of the Lower Old 
 Eed Sandstone rocks entering into the Oban and Kilmore region 
 in descending' order is as follows : — 
 
 Acid Tuffs. 
 
 Feet. 
 
 h. Porphyritic hypersthene-andesites with some 
 
 basic andesite flows, ... ... ... 300-400 
 
 g. Basic andesites and basalt lavas with 
 
 occasional basic tuffs and agglomerates, ... 500-600 
 
 f. Zone of red mudstones and sandstones with 
 a thin concretionary limestone, locally 
 developed, and underlain in places by tuffs 
 and agglomerates, ... ... ... ... 30-40 
 
 e. Basic andesite and basalt lavas and occasional 
 
 basic tuffs or agglomerates, ... ... ... 400 
 
 d. Zone of sandstone and red mudstones and 
 grey shales with plant remains (Pachijtheca 
 sp., &c.), ... 50-100 
 
 c. Basic andesite lava flow, 
 
 b. Red and grey sandstones and shales with 
 fossils, Gephalaspis lornensis, Mesacanthus 
 mitchelli* Thelodus sp., Pterygotus anglicus, 
 Kampecaris forfarensis, ostracods and plant 
 
 50-100 
 
 a. Coarse conglomerates with occasional sand- 
 
 stones and at least one lava flow. Stones 
 well rounded up to 3 or 4 feet in diameter, 
 consisting of quartzite, andesite, granite, 
 porphyrite, epidiorite, and various schists, 100-200 
 
 The rocks of Lower Old Red Sandstone age that form the Lome 
 plateau may be looked upon as being arranged in a shallow 
 compound syncline whose axis trends north-east and south-west, 
 
 * The remains of this fish, common in the Old Red Sandstone of Forfar- 
 shire, were found in the rooks of this zone at Gallanach, a little to the west 
 of Oban, in Sheet 44. 
 
70 Lower Old Red Sandstone. 
 
 truncated on the east by the fault traversing the Pass of Brander 
 and continuing northwards across Loch Btive through Glen 
 Salach. This synclinal structure is partially concealed by local 
 minor folds and by a more or less constant overlap of higher strata 
 on to the uneven floor of metamorphic rocks in a direction from 
 north-west to south-east. Moreover, the basin is cut up into 
 sections by two systems of faults nearly at right angles to each 
 other, those of the earlier system having a general north-east and 
 south-west direction, and it is further traversed by a double 
 system of intrusive dykes. The earlier of these, with a north-east 
 and south-west trend, more or less parallel with the earlier faults, 
 are of the same general age as the volcanic rocks of the region, 
 though necessarily of a slightly later date than those which they 
 actually penetrate, while the latter dykes, running in a nearly 
 north-west and south-east direction nearly parallel with the later 
 faults, are of Tertiary age. 
 
 In consequence of the above arrangement the lowest 
 members of the series, which, as already stated, are mostly sedi- 
 mentary, shown in Fig. 1, are best developed in the Oban portion 
 of the region along the north-west edge of the syncline. 
 Accordingly the lower conglomerates (a, Fig. 1) and sandstones 
 (b), which here attain their maximum thickness, and dip to the 
 south-east at about 20°, form the islands at the mouth of Loch 
 Etive, as well as a considerable stripe of country extending along 
 the coast from DunstafEnage westwards to the edge of the map. 
 To the south-east they dip beneath a basic lava-flow (c), but 
 owing to the system of step faults already mentioned, the coarse 
 basement conglomerate with its outlj'ing sandstones and shales, 
 together with a basic lava-flow very local in its development, are 
 repeatedly brought to the surface for some distance to the south- 
 wards. These sedimentary rocks, therefore, underlie the greater 
 part of the area upon which the upper part of Oban is built, and 
 also form a considerable portion of the cliff that overhangs the 
 town. They also extend up Grlen Crutten, giving rise to most of 
 the escarpments that bound the floor of the glen composed of 
 raised beaches and lake alluvia. 
 
 It is from the sandstones and shales at the top of this series that 
 the fossils were obtained which determined the exact geological 
 age of the rocks of the Lome volcanic plateau. The chief fossili- 
 ferous locality is within Oban itself, in a clifl: near Selma Cottage, 
 situated at about 200 yards N.N.B. from the North Quay, and at 
 a height of about 300 feet above sea level. The flaggy sand- 
 stones which constitute the cliff have been quarried partly for the 
 purposes of widening the roadway and partly for building 
 material, and have yielded numerous specimens of Gephalaspis 
 lomensis, Traquair,* a form very nearly allied to G. lyelli,f which 
 is characteristic of the Lower Old Red Sandstone of Forfarshire 
 and the Welsh Border. 
 
 The other locality within the present map is in the railway 
 
 * Nature, 17th June, 1897, %'ol. Ivi., p. 157. 
 
 t Mem. Geol. Sur. — Summary of Progress, 1897, pp. 82-83. 
 
Volcanic Series. 
 
 71 
 
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 ^-^ i-g 
 
 Sh 
 
 CO 
 
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 <D 
 
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 dig -S3 
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 C3 tS O 
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 Sot! 
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 ^ 3 -g n^H 
 
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 r' c3 ._ s • 
 
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 ^ 
 
'2 Lower GId Bed Sandstone. 
 
 cutting a little to the north-east of Dalintart Farm in Glen 
 Crutten, where the shales, in addition to G. lomensis, have yielded 
 Pterygotus angliciis and Kam/pecaris forfarensis, arthropods also 
 found in the equivalent rocks of Forfarshire. The underlying 
 conglomerate (a), which has here diminished to less than 50 feet 
 in thickness, is more sandy than at Oban and has smaller pebhles, 
 composed mostly of quartzite. 
 
 To the east and south of Oban and Glen Crutten the fossili- 
 ferous strata (b) dip to the south-east and pass under strata con- 
 sisting chiefly of lava-flows ; but, owing partly to reversal of dip 
 and partly to faulting, they reappear near Loch Nell, and are 
 exposed on both sides of the loch, near the foot of which they roll 
 over an anticline. The lowest beds visible consist of con- 
 glomerates (a), but there is every reason to believe that the bed 
 of the loch is carved out of metamorphic rocks which must form 
 the core of the fold. To the south of the loch where the south- 
 east dip is resumed the sediments once more disappear beneath 
 the lavas, and in consequence of the overlap already mentioned 
 must soon run out against the old floor as shown in the horizontal 
 section (Fig. 1) , for, where the metamorphic rocks once more 
 emerge in the Kilbride inlier about two miles to the south-east, 
 the h3rpersthene-andesite-lavas of group (h), about 1000 feet 
 higher up in the succession, rest directly on the black schists and 
 limestones. Eastwards from Oban the overlap is even more 
 rapid, for in the Achavaich inlier near Connel, already men- 
 tioned, the metamorphic rocks are immediately overlain by basic 
 andesites without the intervention of any sedimentary rock 
 belonging either to groups (a) or (b) . 
 
 (c). The basic andesite lava-flow (c), stepped by faults, follows 
 the outcrop of the fossiliferous band (b), and is usually found 
 capping the cliff overhanging Oban and Glen Crutten. 
 
 (d) . The lava-flow last mentioned is followed in turn by a belt of 
 sediment, consisting chiefly of red and grey mudstones, sand- 
 stones, and shales. Broken up into segments by the double 
 system of faults, its outcrop has been traced at intervals from 
 Pennyfuir, 1| miles north-east of Oban, round the cliffs bounding 
 Oban and Glen Crutten to Soroba House, where the band passes 
 into Sheet 44. It is also exposed in the railway cutting on the 
 Connel side of the watershed and at Ardchonnel, about a mile 
 from Connel Ferry. This band appears to die out rapidly to the 
 south, as it has not been recognised on the Loch Nell anticline. 
 
 (e) . The basic andesites and basalt lavas of zone (c) form the bulk 
 of the rocks entering into the ground lying to the west of a line 
 drawn from a point a little to the east of Connel Ferry, along the 
 course of Gusragan Burn, through Loch Nell to Loch Feochan 
 and the outcrop of the last-mentioned sedimentary band (d). 
 East of Loch Nell they form a stripe of over j-mile in breadth, 
 and floor the Feochan Bheag valley to a little above Dalnacabeg, 
 more than a mile east of Kilmore. The rocks are chiefly fine- 
 grained, dark bluish or purplish when tolerably fresh, but reddish 
 when decomposed. The individual flows are usually slagg^y at 
 
Votcanic Series. 73 
 
 top and bottom, and sometimes show traces of flow structure and 
 flow brecciation. One of the flows at or near the base of the 
 group, having a considerable development and making up a good 
 part of the ground to the north of Glen Crutten, differs somewhat 
 from the rest in being greyish or fawn-coloured. It shows 
 marked fluxion banding, and has its steam vesicles much drawn 
 out as if it had flowed in a highly viscid state, and altogether 
 appears to be a less basic rock than the others. This flow, how- 
 ever, derives its chief im.portance from being identical in 
 character with a rock which fills the greater part of a vent or neck 
 (to be described in the sequel), which occurs close to Glen Criitten 
 farmhouse, about one mile east from Oban. The lavas of the 
 group usually have the cracks and irregularities of their surfaces 
 filled in with more or less clastic material, marking short pauses 
 in the volcanic activity of the period, and occasionally there is an 
 intercalation of tuff or agglomerate made up of the same material 
 as the lava-flows. Such rocks occur close to Connel Ferry at the 
 roadside. Exposures of tuff are also found at Benbeg, about 
 halfway between Connel Ferry and Oban, where, owing to the 
 synclinal arrangement of the strata, they cap low hillocks, thus 
 forming the highest rocks of the neighbourhood. These may 
 even be outliers of the overlying succeeding belt (f ). 
 
 (f). The outcrop of this zone, much interrupted by faults, has been 
 traced from Torr-an-tuirc, near the head of Loch Nell, to Barna- 
 carry, and thence along the slopes overlooking Kilmore up the 
 Feochan Bheag valley to beyond Dalnacabeg, where it crosses 
 the river at a waterfall. Near Torr-an-tuirc it consists chiefly 
 of a basic tuff, which makes it highly probable that the exposures 
 already mentioned as occurring at Benbeg are outliers of this 
 band. In the slopes above Kilbride the tuff is overlain by red 
 mudstones with an occasional thin impure concretionary lime- 
 stone. Where the zone crosses the Feochan Bheag it is composed 
 chiefly of red shales, overlain by red sandstones, and succeeded 
 by a bed of greenish shale, which is surmounted by the basement 
 lava of the succeeding groups. The waterfall above referred to 
 is due to the weathering out of the shaly beds beneath the harder 
 strata, especially the lavas (g), and to the dip being upstream. 
 In fact, the erosion of this band everywhere gives rise to water- 
 falls in streams as well as to cliffs and landslips on the hill slopes. 
 On the south side of the Feochan Bheag the solid rock is much 
 concealed by moraines^and by landslipped masses further up the 
 slopes, so that the zone has not been accurately traced ; but its 
 position is fairly well indicated by the persistent crag that over- 
 hangs the valley, from which the landslipped masses have been 
 derived. Owing to a change of dip to the north-west along a line 
 drawn south from Craigentaggart, the outcrop of the band must 
 gradually descend the hill slope and pass below sea level near the 
 head of Loch Feochan. The thin belt of ash and red shale dip- 
 ping to the north-west, which crosses the valley of the River 
 Euchar below Loch Scamadale, is probably the continuation of 
 this zone beneath the overlying pile of lavas. On the south tide 
 
''* Lower Old Red Sandstone. 
 
 of the valley the ash is accompanied by red shales, and also gives 
 rise to extensive landslips. 
 
 (g). The rocks of this group of lavas are almost identical in 
 character with those of group (e). With the exception of an 
 occasional thin bed of tufE, we find a constant succession of lavas. 
 Still there is a slight difference in texture and in the thickness of 
 the individual flows, which gives rise to more conspicuous escarp- 
 ments in some cases. 
 
 _ The members of this group form the greater part of the mass of 
 high ground lying between Loch Nell and the Feochan Bheag 
 from the outcrop of the sedimentary band (f) up to a line drawn 
 southwards from Clenamacrie round the west shoulder of Ton- 
 Dhamh, and eastwards to the fault that truncates the Kilbride 
 inlier. In this block the dip is everywhere persistently to the 
 south-east. South of the Feochan Bheag these lavas constitute 
 the greater part of the high ground between that valley and 
 Scamadale west of the line of fault which truncates the Kilbride 
 inlier, and which is continued along the course of the Eas Ruadh 
 to Scamadale Farm. In this block of ground west of a line 
 drawn roughly from Craigentaggart in the Feochan Bheag valley 
 to Scamadale farmhouse there is a roll over to the north-west, and 
 there is probably a greater thickness of material and a greater 
 number of flows than in the ground to the north of the Feochan 
 Bheag. 
 
 (h). Hypersthene-andesite lavas chiefly characterise the rocks of 
 this zone and are conspicuously porphyritic. The individual 
 flows are also appreciably thicker than those of the former group, 
 and in consequence give rise to more prominent escarpments. In 
 the present area they mostly lie east of a line drawn from Kil- 
 maronag on Loch Etive through the upper Black Lochs south- 
 wards to Clenamacrie and the head of Loch Scamadale. A 
 few outliers of these porphyritic lavas, however, cap the hills to 
 the west of the line north of Scamadale. To the east of the above 
 line hypersthene-andesite lavas, alternating with basic andesite 
 flews like those of the former group, form the ground eastwards to 
 the outcrop of the acid tuff indicated by Mr. Kynaston as entering 
 this part of the plateau south of Loch Etive at Achnacloich, and 
 traceable at intervals to Glen Lonan. South of this point their 
 upper limit is bounded partly by the outcrop of that band and 
 partly by the acid lava flow already described by Mr. Kynaston. 
 
 Volcanic Vent of Lower Old Red Sandstone Age. — One of the 
 vents which probably supplied material to build up part of the 
 Lome volcanic plateau occurs m Glen Crutten, close to Glen 
 Crutten farmhouse, about J-mile east of Oban, as shown in Sheet 
 45 and in Fig. 1. The vent or neck, which is hour-glass shaped 
 and about l-mile long, is filled partly with a coarse volcanic 
 breccia made up of fragments of andesite and metamorphic rocks, 
 and partly with a greyish-coloured andesite. These materials 
 rise through the black schists on the west side and through the 
 overling Old Eed basement conglomerate and sandstones on the 
 east, the junctions where visible being more or less vertical. The 
 
Petrology of the Volcanic Series. 75 
 
 neck appears to be truncated on the north by the Glen Crutben 
 fault, one of the series of north-east and south-west faults with 
 downthrow to the north that step the structures so conspicuously 
 in the Oban region. The grey andesitic rock found in the vent 
 is in part massive, but in places shows a marked platy structure 
 with drawn-out steam vesicles, the fluxion planes being more or 
 less vertical. Where the rock exhibits these characters it exactly 
 resembles the lava-flow already mentioned as occurring at or near 
 the base of group (e), which evidently emanated from this centre. 
 
 ii. Petrology of the Volcanic Series. — During the course of the 
 mapping of the Lome volcanic area, numerous specimens of the 
 different types and varieties of lavas were collected, and a number 
 of these were selected for microscopic examination. The results 
 of this examination proved of great value in determining the dis- 
 tribution of the different types in the field and their petrological 
 relationship. It will be of interest, therefore, to give a short 
 account of the petrological characters of the principal types of lavas 
 represented in this area. 
 
 1. The Basic Andesites. — The general field characters of these 
 rocks have already been described. In hand specimens the rocks 
 are either close-grained and compact, or highly vesicular. They 
 are almost black when fresh, and frequently bear a close 
 resemblance to basalts. In fact, it would be more correct to class 
 some of the flows with the basalts than with the andesites. 
 Porphyritic varieties often occur, but perhaps the commoner type 
 is a dark dense rock without conspicuous porphyritic felspars, 
 though frequently showing numerous small reddish-brown scales, 
 resembling the mineral described under the name of iddingsite. 
 These rocks are often highly decomposed, thus rendering micro- 
 scopic examination somewhat unsatisfactory. 
 
 Basic andesites are well developed in the neighbourhood of Tay- 
 nuilt, and fairly fresh examples from that area have been studied 
 under the microscope. Specimens from Cruaoh Ard-dhuine, 
 S.S.W. of Taynuilt, are black fine-grained rocks closely 
 resembling basalts, and often very vesicular. Under the micro- 
 scope they show [9628-9631] pseudomorphs of calcite and 
 limonite, and sometimes of serpentine, after olivine, and these are 
 usually the only phenocrysts. The ground-mass is often some- 
 what coarse-grained, consisting of lath-shaped plagioclases, with 
 a fluidal arrangement, brownish augite, and ilmenite in skeleton 
 crystals and flattened scales. This ground-mass is intersertal, the 
 pyroxene being in great part moulded on the felspar (sub- 
 ophitic), and is usually quite holocrystalline. Occasionally the 
 augites are fairly idiomorphic [9628], and in some cases a dark 
 interstitial matter is present which probably represents an 
 original glassj' base [9630]. Apatite is also present, with 
 secondary calcite, limonite, and chlorite. As a general rule 
 in these rocks the felspar greatly predominates over the ferro- 
 magnesian constituents, so that they perhaps belong rather to the 
 basic andesites than to the olivine-basalts, while representing 
 intermediate types between the andesites and the basalts. 
 
^^ Lower Old Bed Sandstone. 
 
 The basic lavas of this type pass by gradations into the typical 
 hypersthene-andesite. The phenocrysts of olivine become fewer 
 and hypersthene makes its appearance, and phenocrysts of 
 plagioclase become more common. A good example of this kind 
 of lava occurs at Creag Mhic Righ, a conspicuous crag about 1| 
 miles S.S.W. of Taynuilt. The microscope shows [9627] 
 chlorite after porphyritic augite, bastite after hypersthene, black 
 heaps of granular magnetite apparently from the resorption of 
 biotite, and opaque masses of iron oxides after olivine, in an 
 abundant, rather coarse-grained, fluidal, non- vesicular ground- 
 mass, which consists entirely of lath-shaped plagioclase felspars, 
 mixed with iron oxides and secondary calcite. Much chlorite 
 interspersed among the felspars seems to indicate the former 
 presence of a second generation of pyroxene. No porphyritic 
 felspars are present in the slide. In some specimens, however, 
 taken from a similar type of rock one mile north of Baraehander, 
 north of Kilchrenan, porphyritic plagioclases are well seen 
 [7409, 9022-9024]. The microscope also shows porphyritic 
 augites, some enstatite, and a few pseudomorphs after olivine, in 
 a ground-mass consisting of small plagioclase laths, pyroxene 
 grains, and magnetite. 
 
 2. The Hypersthene- Andesites. — These cover a considerable 
 area in the more central portion of the volcanic plateau. They 
 are dark grey or purple rocks with conspicuous phenocrysts of 
 plagioclase and greenish pseudomorphs after hypersthene. 
 Vesicular varieties are also common. Typical hypersthene- 
 andesites occur on the east side of Glen Nant [7408, 7589], and 
 cover a large area to the west of Taynuilt. A specimen from the 
 roadside quarry, 1^ miles north-west of Taynuilt, shows under 
 the microscope [9632] phenocrysts of plagioclase honeycombed 
 with glass enclosures and slightly corroded. There is much 
 bastite after idiomorphic hypersthene, and pseudomorphs of 
 calcite possibly after augite. The ground-mass is of the very 
 fine-grained pilotaxitic type, consisting principally of minute 
 felspars. This matrix contains granules of quartz which are 
 probably secondary, and little irregular vesicles full of quartz, 
 chlorite, and a semi-opaque material. In some examples the 
 ground-mass seems to have been fairly glassy [9634]. The 
 rock from the quarry at Australia, north-west of Taynuilt, shows 
 good phenocrysts of plagioclase, hypersthene, and augite [9635]. 
 The hypersthene is usually altered to bastite, but the augite is 
 often fairly fresh. The felspars give extinctions indicating 
 labradorite to andesine. A specimen of the same rock, 2\ miles 
 west of Taynuilt [9636], shows a ground-mass which seems to 
 have been largely a glass with forked felspar microlites, but 
 which is now dusty and devitrified, and filled with calcite and 
 secondary quartz. Apatite is usually present as an accessory, 
 and is occasionally pleochroic [9637]. 
 
 Conspicuously porphyritic andesites again cover a large area 
 to the south of Kilbride, near the head of Glenfeochan. Under 
 the microscope [10129] these rocks appear as rather decomposed 
 
Volcanic Series. 77 
 
 hypersthene-andesites, in which the hypersthene is replaced by 
 bastite, and with the pilotaxitic type of ground-mass. 
 
 About a mile to the east of Kilbride and in the crags on the 
 north and north-east sides of Beinn Ghlas, we find porphyritic 
 andesites which appear to be transitional between the typical 
 hypersthene-andesites and the hornblende- and mica-andesites 
 [10130-10132 and 10144-10146]. These rocks appear to 
 be of a slightly more acid type, and contain biotite and 
 occasionally resorption pseudomorphs after hornblende as well as 
 hypersthene, in a pilotaxitic or hyalopilitic ground-mass. 
 
 3. Hornhlende- and Mica- Andesites. — These rocks only occupy 
 a comparatively small proportion of the volcanic area, compared 
 with the more basic types. They are well developed in Glen 
 Nant, on both sides of the glen in the neighbourhood of Tailor's 
 Leap, and mica-andesites occur in considerable masses, as already 
 pointed out, about the high ground to the south of Glen Lonan. 
 
 Hornblende-andesites occur at Tailor's Leap, Glen Nant, and 
 in the same glen about three miles south-east of Taynuilt. They 
 are usually considerably altered, and the hornblendes are pseudo- 
 morphosed and show black resorption borders [7590, 7591]. 
 Sometimes pseudomorphs of another ferromagnesian mineral are 
 also present, probably augite [7591]. In the bum to the west 
 of Tailor's Leap we find mica- and mica-hornblende-andesites. 
 
 A section of the mica-homblende-andesite shows corroded 
 brown biotite and greenish-brown hornblende, with some pale 
 green augite ; several rounded quartz grains (xenocrysts) with 
 corrosion borders of pale green pyroxene, partly replaced by 
 calcite. The ground-mass consists of rather well-formed small 
 lath-shaped felspars, irregularly scattered, and mixed with 
 magnetite and decomposition products. There are no porphy- 
 ritic felspars seen in this section, but they are usually fairly 
 common [9639]. The biotites and hornblendes frequently have 
 intense black corrosion borders [9639]. In the mica-andesite 
 [9638] the biotite is usually represented by pseudomorphs of 
 granular magnetite, though occasionally a little fresh biotite 
 remains. Other pseudomorphs are also present possibly after 
 pyroxene. The ground-mass consists of fairly large lath-shaped 
 fluidal plagioclase, rather trachytic in aspect and not glassy, con- 
 taining secondary quartz, calcite, &c. There is much magnetite 
 in small grains and irregular masses. Vesicles of irregular shape 
 are also present. 
 
 4. Felsitic Lavas. — These rocks are perhaps most typically 
 developed in Glen Nant, also in Airds Park, to the north-west of 
 Taynuilt, and again to the west and south-west of the same place. 
 They are characterised by their pale colour, compact and flinty 
 texture, and the well-developed fluxion banding. Spherulitic 
 and nodular varieties are also occasionally met with. They are 
 frequently associated with the hornblende- and mica-andesites, and 
 are certainly more acid, and represent the most acid phase among 
 the lava,9 of this area. The banded felsites of Glen Nant are seen 
 under the microscope [7593-7596] to contain phenocrysts of 
 
78 Lower Old Red Sandstone. 
 
 felspar, whict, in part at least, is plagioclase, and frequently 
 biotite as well. They may perhaps represent rhyolitic forms of 
 a somewhat acid biotite- andesite. They do not contain pheno- 
 crysts of quartz, and can therefore scarcely be referred to dacites 
 or true rhyolites, while the presence of plagioclase would seem 
 rather to connect them with the andesites. In one of the sections 
 [7596] an idiomorphic phenocryst of orthoclase is seen, con- 
 taining a core of plagioclase. This would tend to indicate some 
 affinity with the trachytes. 
 
 The felsitic rock of Airds Park [9642, 9643] shows pheno- 
 crysts of plagioclase and biotite in a felsitic ground-mass, some- 
 times of the spotted felsitic type, throughout which are scattered 
 small felspar microliths and infiltrations of quartz. A specimen 
 from Monadh Glas, about two miles south-west of Taynuilt 
 [9644], shows a few felspar phenocrysts, some of which are un- 
 twinned or simply twinned, and probably include sanidine. 
 The felsitic matrix shows good flow structure, and in places con- 
 tains imperfect spherulites. 
 
 From these descriptions it would appear that these rocks belong 
 to acid tjrpes intermediate between biotite-andesite and rhyolite. 
 
 5. The Tuffs. — The petrological characters of these rocks have 
 been described in the paper, already referred to above, on some 
 tuffs associated with the andesitic lavas of Lome. It will there- 
 fore suffice here to give a short account of the more interesting 
 features. The more acid type of tuff is exposed in the western 
 portion of Benderloch, at Airds Point, and on Cruach Ard- 
 dhuine, S.S.W. of 'Taynuilt. It is usually compact and flinty in 
 texture, and varies in colour from reddish-purple to a pale 
 yellowish or sometimes grey tint. It is often crowded with 
 small fragments of felsite and with some andesite, and shows 
 biotite flakes and crystals of an acid felspar. There is very little 
 difference between these tuffs and those which cover larger areas 
 in the more eastern portion of Benderloch, in the ground south 
 of Achnacloich, and in the area about, the Sior Lochs. These 
 latter tuffs would appear to contain more numerous felspar 
 crystals, and to have their lapilli more largely made up of 
 andesitic fragments than of felsitic. Under the microscope the 
 structural peculiarities of the two varieties are fairly similar. 
 For palfeozoic tuffs these rocks have retained their original 
 characters remarkably well. 
 
 Sections of the more typical examples [7247, 7598, 7404, 
 7706, 9646-9651] show broken and more or less corroded 
 crystals of sanidine, some plagioclase, biotite flakes, grains of 
 magnetite, and lapilli. The sanidine is often clear and very 
 fresh, and shows the normal twinning on the Carlsbad plan. 
 The amount of plagioclase (oligoclase or andesine) varies in 
 different sections, and the mineral is usually more or less turbid 
 from alteration. The biotite is of the variety characteristic of 
 the hornblende-andesites and trachytes. It is sometimes bent 
 and often bleached from alteration, and occasionally partially or 
 entirely replaced by granular iron-ore. Hornblende is also some- 
 
Petrology of the Volcanic Series. 79 
 
 times present, but is usually represented by chloritic alteration 
 products. 
 
 The general matrix seems to be almost purely volcanic. In 
 many cases it is a good deal altered, but the clastic character is 
 nearly always still evident. The characteristic feature of this 
 matrix is the beautiful ash-structure (aschen- and bogen- 
 struktur of German petrographera) which it frequently presents. 
 Some sections show this in a very perfect manner [7706, 7716, 
 7592]. The appearance is more or less similar in each section. 
 We see numerous small, more or less clear, irregular fragments 
 ■and particles, sometimes concavo-convex in outline, and some- 
 times bi-concave, packed closely together. Sometimes the frag- 
 ments contain infilled vesicles, and in some cases [7706] many 
 of the smaller lapilli consist of highly vesicular fragments with 
 ragged outlines. Frequently the fragments and finer particles 
 lie with their longer axes approximately parallel, indicating a 
 lamination of the material, or again they may alternate with a 
 still finer and more dust-like type of matrix. There seems to be 
 no doubt that these fragments were originally glassy, and were 
 derived from the shattering of a highly vesicular lava resembling 
 pumice. This structure has been well described and .illustrated 
 by Mugge in a study of some acid tuffs associated with soda- 
 rhyolites in the Devonian of the Lenne district in Westphalia.* 
 These rocks show very clearly the characteristic forms of the con- 
 stituent ash-particles, bounded by concave curves, which are so 
 suggestive of broken-up pumice. 
 
 The lapilli of these tuffs appear to be mainly andesitic, though 
 in the more acid varieties fragments of felsite are often more 
 common. The andesitic lapilli contain plagioclase phenocrysts 
 in a ground-mass rendered dark and sometimes almost opaque by 
 minute granules of magnetite. The ferromagnesian constituents 
 are usually too decomposed for identification. It is probable, 
 however, that most of the andesitic fragments represent the 
 pyroxene- and basic-andesites so common in the surrounding 
 area. Occasionally a fragment is seen [7247] of more acid, or 
 rather trachytic, character, containing phenocrysts of sanidine, 
 and probably representing a portion of the parent rock from 
 which the numerous sanidine crystals in the tuffs were derived. 
 Lapilli of hornblende-andesite are fairlj;^ common [7246, 7722], 
 and these closely resemble the hornblende-andesites which are 
 found below the tuffs in Glen Want. A section of one of these 
 fragments [7722] shows phenocrysts of idiomorphic hornblende, 
 biotite, plagioclase, and some corroded quartz grains. The 
 hornblende has the characteristic outlines of the brown 
 variety characteristic of the hornblende-andesites, but it 
 is now almost completely replaced by granules of iron-ore, as is 
 so frequently the case in rocks of this type. The biotite shows a 
 deep resorption border, within which the original substance of 
 
 *Mugge— "Unfcersuchuiigen iiber die Lennenporphyre in Westfalen und 
 den angrenzenden Gebieten," Neu. Jahrb. Beil. Bd, viii. (1893), p. 648, 
 pi. xxiv., figs. 20 and 21, 
 
^^ Lower Old Bed Sandstone. 
 
 the mineral is bleached and almost colourless. The quartz 
 grains would seem to be original ; they are more or less corroded, 
 and show characteristic inlets and inclusions of the ground-mass. 
 Their presence allies the rock to the dacites . In some of the lapilli 
 of this type [7246] the biotite is in excess of the hornblende. 
 The ground-mass is much obscured by specks of iron-ore and 
 alteration products, but is seen to consist largely of small felspar 
 microlites. 
 
 Non-yolcanic fragments derived from the older platform of 
 crystalline schists are occasionally met with, and are perhaps 
 more frequently seen in the more felsitic types of tuff. One of 
 the sections [7717] shows a lapillus consisting of a schistose rock 
 containing biotite, sericitic mica, quartz, -and felspar, with a lew 
 small garnets. Fragments of quartzite are also sometimes seen. 
 
 Tuffs of this type, with such well-preserved "ash- 
 structure," have not yet been described from any other area of 
 Lower Old Red Sandstone volcanic rocks in this country. The 
 " ash-structure " most nearly resembles that of the acid tuffs 
 described by Mugge, and already referred , to. Recently a some- 
 what similar structure has been described in a brecciated soda^ 
 felsite from Wicklow Head in Ireland,* but in this case the 
 structure is apparently due to the breaking up of a glass by 
 brecciation in situ. In the case of our Lome rocks there can be 
 little doubt that the "ash-structure" is the product of showers 
 of minute glassy particles ejected from a volcanic orifice by dis- 
 charges of a highly explosive character. The characteristic fnrm 
 of the glassy particles is doubtless due to the immense quantity 
 of gas bubbles and steam cavities present in the molten mass at 
 the time of eruption. 
 
 The petrological characters of the Lome lavas correspond 
 remarkably closely with those of the rocks of other volcanic areas 
 of the same age. Thus, the basic lavas and pyroxene-andesites 
 of the Cheviots, the Pentlands, the Ochils, the Ayrshire coast, 
 and other areas bear a close resemblance to those of Lome. 
 Biotite-andesites are known from the Cheviots, but only from the 
 lower part of the volcanic series. They occur at Camphill in the 
 South-eastern Pentlands, and near Dollar in the Ochils. Acid 
 flows are not known from the Cheviots, but they occur in the 
 Pentlands and Ochils. Some of the banded felsitic rocks found 
 in the Pentlands strongly recall those of Glen Nant, and portions 
 of the Craig Rossie rock, in Perthshire, closely resemble some of 
 the acid types of Lome. Felsitic tuffs also occur in other Lower 
 Old Red Sandstone areas, but the characteristic " ash -structure " 
 has so far only been described in the Argyllshire tuffs. 
 
 H. K. 
 Lismore. — At Sliabh nam Ban-fionn, half a mile west by south 
 of Fennacrochan, a breccia of a peculiar kind lies unconformably 
 on the limestone and black schist. It consists of fragments of 
 schistose limestone with a few of black schist embedded in a cal- 
 careous matrix. Most of the limestone fragments vary in size 
 * Summ. Prog. Geol. Survey for 1899, p. 179. 
 
Lismore Conglomerate. 8 J 
 
 horn, one to six inches, but are mixed with, larger blocks which 
 sometimes reach a length of six feet. The small peebles are 
 sometimes almost rounded, but the larger blocks are angular and 
 sub-angular. The breccia, whilst being obviously younger than 
 the great denudation which took place after the foliation of the 
 schists, has undergone considerable contact alteration by the 
 neighbouring mass of diorite, which is presumably of Old Red 
 Sandstone age. In its internal structure and in its relation to 
 the schists it bears a striking similarity to certain scree-like 
 breccias occurring beneath and around the Lower Old Red 
 volcanic rocks of Glencoe. For these reasons the breccia has 
 been classed as doubtful Lower Old Red Sandstone age (c^ ?) , but 
 it must be understood that the breccia has little in common with 
 the well-rolled conglomerates lying at the base of the Lome 
 volcanic rocks, or with another type of Old Red Sandstone, an 
 outlier of which occurs near Kilmalieu in Ardgour (Sheet 53), 
 less than eight miles to the north-north-east. 
 
 H. B. M. 
 
CHAPTER VIII 
 Newer Igneous Eocks. 
 
 Under this title will be described liere those rocks which have 
 been intruded into the metamorphic series at a later date than 
 the movements which caused their foliation. In the present 
 sheet they comprise the granite masses of Ben Cruachan, the 
 Blackmount, and Glen Fyne, aiid their more basic modifications, 
 outlying masses, and associated dykes. 
 
 i. Ben Cruac/ian Granite and Outlying Masses. — Under the 
 term "Ben Cruachan Granite" we may conveniently describe 
 the large mass of granite which extends from the north-east 
 slopes of the Pass of Brander to the northern margin of the sheet, 
 and as far eastwards as the watershed between Loch Etive and 
 Glen Strae. On the west side of Loch Etive the granite occupies 
 another broad belt reaching on the west the watershed between 
 Loch Etive and Loch Creran. The whole area covered by the 
 Ben Cruachan granite on the present map alone is probably at 
 least 100 square miles, and constitutes the wildest and most 
 mountainous portion of this part of Argyllshire (Plate V.). 
 
 What may be termed the Ben Cruachan granite proper forms 
 the great mass of Ben Cruachan itself and the line of hills on the 
 north side of Glen Noe, extending eastwards to Beinn 
 a' Chochuill and Beinn Eunaich, and thence north-east- 
 wards along the south side of Glen Kinglass to Meall 
 Garbh. To the north of this tract- the rest of the 
 granitic area consists of a slightly later and more acid type of 
 granite, sharply marked off from that of Ben Cruachan. In the 
 north-east corner of the map the Ben Cruachan type of granite 
 comes on again at the west end of Loch Dochard, and extends 
 northwards to Stob Ghabhar. The same type again forms the 
 outlying masses around Clais gobhair. On the west side of 
 Loch Etive similar relations are seen between the two types, that 
 of Ben Cruachan extending northwards from Bonawe Ferry to 
 Beinn Sguliaird, and forming, as it were, an outer border or 
 fringe to the more acid type, while the latter occupies the more 
 central portion of the granite area. The boundary line between 
 the two types is usually quite distinct, the latter or Blackmount 
 type being the coarser and more acid and conspicuously porphy- 
 ritic. Occasionally, however, the t^A'o granites appear to pass 
 gradually the one into the other. 
 
 The Blackmount type of granite forms a roughly oval mass, of 
 fairly regular outline, extending northward into Sheet 53. On 
 its outer margin it is everywhere in contact with the Ben 
 Cruachan granite, except for about two miles en the east side of 
 
Ben GruacJian Granite 83 
 
 Glen Kinglass, where the Cruachan graiiite is not seen, and the 
 more acid type is brought against the schists. 
 
 The outer margin of the Ben Cruachan granite is, on the 
 whole, sharply defined and easy to follow in the field. This is 
 more especially the case along the west side of Glen Strae and 
 on the northern slopes of the Pass of Brander; but in the area 
 between Loch Etive and Loch Creran the boundary line becomes 
 exceedingly irregular, and constitutes a marginal area of gieat 
 complexity. To the east of Loch Creran the contact zone 
 between the granite and the schists consists of a network of sills, 
 veins, bands, and tongue-like protrusions of granite, covering a 
 belt of mountainous ground sometimes over a mile in breadth. 
 The vein-like offshoots do not, as a rule, anastomose with rne 
 another, but tend to run in a roughly parallel direction, coincid- 
 ing with the planes of foliation in the schists, although veins f nd 
 irregular intrusions, having apparently no relation to any such 
 planes, are not uncommon. The complication is such that a line 
 can only with difiiculty be drawn between the schists crowded 
 with granite veins and sill-like bands and the granite crowded 
 with strips and inclusions of altered schist of every size up to a 
 mile or more in length. The contact area thus constitutes a zone 
 of vein-like injections of great complexity. Furthermore, 
 towards the inner portion of this injection zone, i.e., as we 
 approach the main mass of the granite, the schists are seen to be 
 so impregnated with granitic material that it is impossible in a 
 hand specimen to distinguish clearly the igneous portion from 
 the material of sedimentary origin. The originally finely- 
 banded character of most of these schists doubtless largely 
 assisted the impregnation, which appears to be clearly parallel 
 to the type of injection termed " lit par lit." The same struc- 
 ture has also been observed close to the main mass of the granite 
 in the banded calcareous schists of the Pass of Brander, where 
 both "injection lit par lit" and "superposition" (Michel Levy) 
 may be seen sometimes in the same specimen. 
 
 Again, in many places the schists have been broken up under 
 the process of injection, and a breccia has been formed, 
 occasionally at some distance from the main mass of the granite, 
 consisting of a confused intermingling of altered schistose frag- 
 ments in a granitic matrix. The fragments in these breccias are 
 angular and usually sharply marked off from the matrix. 
 Occasionally, however, the outline is not well defined, and 
 appears indistinct with a lens, showing, as it were, a gradual 
 transition from the included fragment to the matrix. Such 
 fragments are usually crowded with flakes of secondary biotite in 
 more or less parallel layers, and are somewhat suggestive of the 
 origin of certain ill-defined patches rich in biotite occasionally 
 seen within the granite. Sometimes the breccia is very coarse, 
 as is well seen on the west side of Meall Garbh, the fragments 
 measuring several feet in diameter. This structure was evidently 
 an accompaniment of the intrusion, and is seen to involve 
 homogeneous rocks, such as quartzite, as well as the various types 
 of horafclsod acliist. 
 
""^ Newer Igneous Bocks. 
 
 Thus in the marginal, area of the granite every kind of injec- 
 tion may be observed from sill-like protrusions of the main mass, 
 ^^iiy yards in breadth, to injections of microscopic dimensions. 
 And similarly the granite contains included portions of the 
 schists, varying from masses more than a mile in length to 
 minute fragments. Even the smallest granite veins are com- 
 paratively coarse in texture throughout, and are usually of the 
 same texture as the main mass to which they belong. 
 
 The normal type of rock which constitutes the Ben Cruachan 
 rnass is a hornblende-biotite-granite of medium, though some- 
 times of fairly coarse grain, and grey in colour. This is the 
 most persistent and widespread type. The microscope shows 
 both plagioclase and orthoclase, of which, however, the former 
 (probably oligoclase) largely predominates. A greenish horn- 
 blende is conspicuous, and is frequently associated in groups 
 with the dark brown biotite. Quartz is but sparingly distri- 
 buted in small grains and aggregates, and is interstitial. 
 Apatite, magnetite, and pyrites, with a few small grains of 
 sphene, occur as accessories. 
 
 Two other types may also be distinguished in the Cruachan 
 mass, the one a more acid and the other a more basic type. The 
 first constitutes almost the whole of the topmost portions of Ben 
 Cruachan and of many of the neighbouring hills. It is 
 occasionally seen on the shore of Loch Etive, and forms a con- 
 siderable part of the high ground from Beinn Duirinnis, north of 
 Bonawe Ferry, to Beinn Sguliaird, on the northern boundary of 
 the map. It consists of a tolerably coarse pink granite, in which 
 the dominant minerals are pink orthoclase and quartz. This 
 decidedly more acid type may sometimes be seen to pass imper- 
 ceptibly into the normal grey granite, or to occur in fairly well- 
 defined bands and patches. 
 
 The more basic type is of less common occurrence. It is a 
 dark grey medium- grained rock, consisting mainly of green 
 hornblende, biotite, and plagioclase, while the orthoclase and 
 qxiartz, if not almost entirely suppressed, occur merely as 
 accessory constituents, so that this type would more correctly be 
 termed diorite than granite. It appears in small patches near 
 Port-na-mine, about two miles north-east of Bonawe, and in 
 larger masses along the ridge of hills on the north side of Glen 
 ISToe. This rock is never sharply marked off from the normal 
 type of hornblende-granite, but passes imperceptibly into it, and 
 doubtless represents merely a slightly more basic modification of 
 the magma. 
 
 The marginal portion of the granite mass, especially along the 
 Pass of Brander and on the south slopes of Beinn-a-Bhuiridh, 
 and to the north-west of the head of Loch Awe, is usually slightly 
 more basic and finer-grained than the main mass. Where, 
 however, the boundary line is irregular and consists of a complex 
 of veins, as in the ground between Loch Etive and Loch 
 Creran, there is no marked modification either in composition or 
 texturf. The finer-grained variety is well seen in the Bonawe 
 
Bioritic Phase of Ben Cruachaii Granite. 85 
 
 quarries, where it has for raauy years been largely worked for 
 granite sets. Under the microscope plagioclase is seen to be the 
 dominant felspar in this rock, and it is usually zoned and idio- 
 morphic, while biotite, interstitial quartz, and alkali-felspar 
 (microcline in part) are also present. The coarser and typical 
 granite is well seen further to the north-east in the Craig Point 
 quarries, and it forms the greater portion of Beinn Duirinnis, 
 though here and there, especially about the top of the hill, the 
 more acid pink granite occasionally makes its appearance. 
 
 The dioritic phase is well exposed on the south-western and 
 southern slopes of Beinn-a-Bhuiridh, and on Monadh Driseag, 
 to the west of the head of Loch Awe, and coarse grey dioritic 
 varieties are largely worked in the Loch Awe quarries further 
 to the north-east. A specimen from the south-west side of 
 Beinn-a-Bhuiridh is seen, under the microscope, to be largely 
 composed of a zoned plagioclase, full of minute inclusions 
 arranged in definite planes, together with biotite, hornblende, 
 interstitial quartz,. and a little alkali-felspar. Sphene, iron-ores, 
 and apatite occur as accessories. Dr. Teall, who has examined 
 these rocks, calls it a quartz-biotite-diorite, and remarks that the 
 plagioclase is exactly similar to that of the rocks belonging to 
 the hyperite-facies of the Southern Upland granites. Other 
 portions again of the Beinn-a-Bhuiridh diorite are still more basic, 
 and contain brown hornblende and a pale greenish pyroxene 
 (augite-biotite-diorite) — another point of resemblance to the 
 Southern Upland series. The diorite on the south side of Beinn- 
 a-Bhuiridh and on Monadh Driseag is somewhat similar, con- 
 sisting of zoned plagioclase (oligoclase-andesite) , biotite, brown 
 hornblende, interstitial quartz, and micro-pegmatite, the two 
 latter constituents appearing only in very small quantity. 
 Further to the north-east, in the Loch Awe quarries, the rock is 
 coarser, but of more or less similar composition. 
 
 As we follow the Ben Cruachan granite from this area in a north- 
 easterly direction, it is found on the east side of Glen Kinglass, 
 to die out against the more acid mass of the Blackmount and 
 Glen Etive. We pick up the Cruachan type again, however, 
 about a mile south-west of Loch Dochard. Here it rapidly 
 widens from a breadth of only a few yards to nearly half a mile, 
 and assumes a fairly coarse texture. It is a pinkish hornblende- 
 granitite, becoming fine-grained and dioritic in appearance at its 
 junction with the quartzose schists to the east. To the north of 
 Loch Dochard the Cruachan granite rapidly spreads out over a 
 wider area, the boundary line passing through the ridge of Stob 
 Ghabhar, and thence into Sheet 53 to the north. To the south 
 and south-east of Stob Ghabhar, a separate mass of the same 
 general type is seen in the neighbourhood of Clais-gobhair. 
 This mass is exposed for a mile and a half in the Clais-gobhair 
 Burn, and is continued north-eastwards as a narrow strip as far 
 as the margin of the map. The rock is a medium-grained 
 greyish, and sometimes pinkish, hornblende-granite. Under 
 the microscope [8620] it is seen to consist of plagioclase-felspar, 
 
86 Newer Igneoas Mocks. 
 
 green hornblende, biotite, and a small proportion of alkali- 
 felspar and interstitial quartz, sphene, apatite, and iron-ores 
 occurring as accessories. Further to the east occurs another 
 mass of similar general type, varying from hornblende-granite tO 
 diorite. Only a portion of this mass is seen within the present 
 sheet. 
 
 We will now pass on to the more acid mass of granite, which 
 forms, as already pointed out above, a kind of central core to the 
 granite of Ben Cruachan type. This rock constitutes a well- 
 defined mass, occupying large areas on both sides of Loch Etive, 
 and the mappiug in Sheet 53 to the north has shown that it is 
 almost completely surrounded by the granite of Ben Cruachan 
 type. _ 
 
 It includes an area of approximately 60 square miles, and 
 is only covered to an insignificant degree by morainic material or 
 other superficial accumulations. It is characterised by a wild 
 and imposing type of mountain scenery, embracing no less than 
 five peaks more than 3000 feet in height. The ground occupied 
 by this more acid type of rock is more rugged and more heather- 
 covered than the Ben Cruachan area, which is characterised 
 rather by steep grassy slopes, and withoiit extensive areas of bare 
 rock, except in the higher parts of the hills and corries. 
 
 As a rule, a fairly sharp boundary line between the two 
 granites is traceable. Occasionally, however, there appears to be 
 a passage between the two — as, for instance, along the eastern 
 flanks of Ben Sguliaird and to the north of Loch Dochard. In 
 the former locality the two types of granite are scarcely dis- 
 tinguishable from one another over a belt of about 20 yards in 
 breadth. In other places, however, the line appears perfectly 
 sharp, and small veins of the acid type are seen to protrude into 
 the Ben Cruachan type. In many parts of Glen Kingla=s, 
 especially about Acharn, the Cruachan granite assumes a well- 
 marked foliated or gneissic structure close to the margin of the 
 coarser, more acid type. This structure may possibly have been 
 induced by the intrusion of the Glen Etive granite before the Ben 
 Cruachan granite had been completely consolidated. 
 
 On passing from the Ben Cruachan type to that of Glen Etive 
 and the Blackmount, we notice a fairly abrupt change from a 
 medium-grained rock, often rich in hornblende and plagioclase, 
 to a very coarse and conspicuously porphyritic type of granite, in 
 which the hornblende is greatly diminished, while quartz is fre- 
 quently conspicuous. This latter rock is evidently of more acid 
 composition than the normal Ben Cruachan granite. It is a 
 hornblende-granite, showing large white and flesh-coloured 
 crystals of felspar, often quite an inch in length, porphyritic, in a 
 coarse granitic ground-mass consisting of orthoclase, plagioclasie, 
 and quartz, with some elongated crystals of dark green hornblei de 
 and biotite plates. 
 
 The coarsely porphyritic structure which is everywhere so ccji- 
 spicuous of the margin is, however, not a uniform feature. The 
 porphyritic crystals gradually die out towards the inner portion of 
 
Qlen Mive type of Granite. 87 
 
 the mass, so that the porphyritic structure is iu this case only a 
 marginal feature. There is, in fact, a coarsely porphyritic 
 marginal zone to this later type of granite, varying from ahout a 
 quarter of a mile to a mile in breadth. Within this zone lies a 
 generally coarse, homogeneous, pinkish biotite-granite, with 
 hornblende as an occasional accessory constituent. The rock 
 often resembles the more acid pink variety of Ben Cruachan, and 
 there ai'e occasionally, as, for instance, on Ben Starav, consider- 
 able Variations in texture. 
 
 Specimens of this Glen Etive or Blackmount type of granite 
 from the Glen Kinglass area were examined under the micro- 
 scope. A coarse variety from near Doire-nan-Saor was found to 
 show zoned plagioclase, biotite, and interstitial alkali-felspar in 
 large plates, while quartz was comparatively scarce. Horn- 
 blende, sphene, and iron-ores occur as accessories. The inter- 
 stitial felspar is often developed as microperthite. Other 
 sections of the rock show a good deal more quartz and sometimes 
 rather less biotite, though on the whole only comparatively 
 slight variation in composition is to be observed in this type of 
 granite. 
 
 Fine-grained varieties are found on the upper part of Ben 
 Starav, and on the ridges and corries on its south side, but the 
 constituents are the same as in the coarser varieties. The alkali- 
 felspar occasionally shows microcline structure. A fine-grained 
 grey rock from Coire-nan-Lair, on the south side of Ben Starav, 
 shows porphyritic or pseudo-porphyritic constituents in a finer- 
 grained matrix. The larger constituents comprise zoned plagio- 
 clase, microperthite, quartz, and biotite, while the ground-mass 
 is a micro-crystalline aggregate of quartz and felspar. Small 
 veins and bands, fine-grained, and consisting mainly of quartz- 
 and felspar, are not uncommon in certain places. Quartz veins 
 with hollow centres and veins of coarse pegmatite are also met 
 with about Ben Starav and Ben Trilleachan. 
 
 A noteworthy feature in connection with these two granite 
 masses is the fact that the Ben Cruachan type of granite is nearly 
 everywhere traversed by numerous dykes of porphyrite and 
 quartz-porphyry, while these intrusions are entirely absent from 
 the inner mass of Glen Etive and the Blackmount. These dykes 
 are exceedingly numerous in the Ben Cruachan area on the south- 
 west side of the Glen Etive mass, and again are equally plentiful 
 in the Cruachan type of granite to the north-east in Sheet b3. 
 This fact alone is suggestive of a later date of intrusion for the 
 inner mass. Throughout the entire area of the Glen Etive 
 granite only one small poryhyrite dyke has been observed, and 
 this occurs near the margin of the mass in Glen Kinglass. With 
 this exception, the only dykes found cutting the Glen Etive 
 granite are some peculiar basic types occurring in the Ben 
 Starav area, and these will be described in the following chapter, 
 together with the porphyrites and quartz-porphyries. It is pos- 
 sible that the majority of the porphyrites were intruded into the 
 Ben Cruachan area before the intrusion of the Glen Etive 
 
88 Newer Igneous Bocks. 
 
 granite; or, on the other hand, they may have been later than 
 both granites, the fissuring which afforded them an escape 
 upwards being mainly confined to the area peripheral to the 
 central and more plug-like granitic mass. 
 
 The variation observed in the Glen Etive granite is mainly one 
 of texture, while mineralogical differences are far less pro- 
 nounced than in the Ben Cruachan type. The relationship, 
 however, between the two masses is so close that it is evident 
 that they have had a common magmatic origin and belong to the 
 same general period of intrusion. 
 
 A characteristic of the ground occupied by the Glen Etive 
 granite is the comiparatively extensive area of bare rock actually 
 seen at the surface and practically devoid of any covering of 
 heather or grass. Large tracts of bare rock are thus seen in the 
 Ben Starav and Ben Trilleachan areas and on the north side of 
 Stob Coir an Albannaich, further east. The finer-grained rock 
 is often much disintegrated, and, being split up by close parallel 
 joint planes, forms thick talus-slopes on the hillsides. The coarse 
 type of granite weathers freely over the upper portions of the 
 hills, so as to form a fine quartzose sand overlying the bare rock, 
 and frequently along the exposed ridges large blocks of granite 
 weather. out with a rounded form so as to resemble boulders. 
 This is well seen on Stob an Duine Ruaidh, Ben nan Aighean, 
 and Stob Coir an Albaunaich. The northern and north-eastern 
 faces of the mountains are, as a general rule, much steeper than 
 the southern and south-western slopes, and are frequently marked 
 by abrupt lines of precipitous crags. A similar feature 
 characterises the hills of the Ben Cruachan range to the south, 
 though here, owing to the harder and less easily disintegrated 
 character of the rock, the summit ridges are usually sharper and 
 narrower. The pink variety of the Cruachan granite, however, 
 resembles some parts of the Glen Etive mass, and appears to 
 yield, under the influence of atmospheric agencies, more rapidly 
 thfin the more basic grey granite. It tends to form more or less 
 T0\inded and hummocky surfaces, while the grey rock stands out 
 in bold crags. 
 
 Well-marked vertical combined with horizontal joint-planes 
 are conspicuous about the summit of Ben Cruachan, and give the 
 rocks a roughly mural or rudely castellated appearance; or, 
 frequently, when a set of strong parallel joint-planes lies at an 
 angle to the vertical, the structure of the rock mass assumes a 
 deceptive resemblance to inclined bedding. This " pseudo- 
 bedded" structure is well seen on the northern, and north-eastern 
 faces of Ben Cruachan, and in many of the small burns running 
 down the north side into Glen Noe. 
 
 Outlying Masses associated with the Ben Cruachan Granite. — 
 Two masses of granite of Ben Cruachan type are seen in the 
 extreme north-east corner of the map, in the neighbourhood of 
 Clais-gobhair. These have already been described above. ^ It 
 remains to give some account of the smaller outlying intrusions 
 which are found to the east of the large granite mass of Ben 
 
Outlying Masses associated %vith the Ben Gruaohan Granite. 89 
 
 Cruachan and Glen Etive. These consist of different varieties 
 of diorite, and the rock seen on the west side of Glen Orohy, 
 recently described under the name of kentallenite.* 
 
 In connection with the smalle. ' granite masses in the north-east 
 corner of the map, there occur two small intrusions of diorite. 
 One of these is seen to lie within the granite mass to the north- 
 west of Clais-gobhair. It is i-a oval-shaped mass of coarse 
 hornblendic diorite, and appears to represent a basic modiiication 
 of the larger mass within which it occurs. It varies from a 
 fairly coarse rock, containing about equal proportions of horn- 
 blende and plagioclase, to a coarse hornblende-rock in which 
 scarcely any felspar is visible to the naked eye. The former 
 variety is seen under the microscope [8621] to be in a great 
 m.easure made up of large plates of green hornblende, which 
 include small idiomorphic plagioclases and some small flakes of 
 altered and ragged biotite. A few small pyroxene grains are also 
 present, while hornblende and plagioclase also occur inter- 
 stitially between the large plates of hornblende. There is no 
 quartz. The other intrusion of diorite lies about a quarter of a 
 mile to the north-east, just outside the margin of the granite. 
 It is a similar type of rock to that above described, and the pro- 
 'portion of felspar to ferro-magnesian constituents is apparently 
 more uniform. 
 
 Passing further south to the area about the head of Glen Strae, 
 an elongated dyke-like intrusion of dioritic affinities is seen in 
 the flaggy quartzose schists of Meall an Laoigh. A short distance 
 to the west of this intrusion two smaller dyke-like masses of the 
 same type are seen. These three intrusions are specially of 
 interest, as they appear to be of a composite nature and show an 
 interesting type of differentiation. 
 
 The marginal portion consists of a black porphyritic rock, 
 showing phenocrysts of augite and pseudomorphs after olivine in 
 a dark compact matrix. This passes into a medium-grained 
 dioritic rock, consisting mainly of hornblende, felspar, biotite, 
 and idiomorphic augite. The central portion is a hornblende- 
 lamprophyre of a type by no means common in this district. The 
 passage between the two kinds of rock is somewhat sudden, yet 
 the lamprophyre can scarcely be regarded as a distinct intrusion, 
 as it has no fine-grained margiii, and the characteristic idio- 
 morphic augites of the diorite are still seen to occur in the 
 lamprophyric rock, though less abundantly. The lamprophyre 
 frequently veins the diorite in an intricate manner, and the two 
 types are often so intermingled tbat they can hardly be otherwise 
 regarded than as portions of one and the same mass. 
 
 The basic rock forming the marginal portion of these intru- 
 sions shows under the microscope [8944] phenocrysts of idio- 
 morphic augite and pseudomorphs after olivine in a micro- 
 crystalline ground-mass, consisting of lath-shaped plagioclase, 
 
 * See Hill and Kynaaton — "Kentallenite and its relations to other Igneous 
 Rocks in Argyllshire," Quart. Jour. Geol. Soc, vol. Ivi. (1900), pp. 531-557, 
 pis. xxix.-xxxi. 
 
90 
 
 JSIewer Igneous Bocks. 
 
 small greenish (? pyroxene) granules, and interstitial matter 
 jpossibly including some alkali-felspar and accessory magnetite. 
 The pseudomorphs after olivine illustrate the amphihole type of 
 alteration of that mineral. They consist of a fine felted aggre- 
 gate of colourless amphibole needles, sometimes associated with 
 iron-ore, and are usually surrounded by a border of green fibrous 
 amphibole, often mixed with colourless needles. A similar 
 alteration product after olivine is seen at the margin of a picrite 
 which occurs in Glen Orchy, and which will be described below. 
 It is probably similar to the " pilite " of Becke.* This marginal 
 phase has affinities with certain basic lamprophvres and with 
 certain varieties of the kentallenite, to be shortly described. 
 
 A specimen from the dioritic portion of these intrusions shows 
 under the microscope [8622] large pale greenish porphyritic 
 augites, with well-defined crystal outlines and a zonal structure 
 which is well seen between crossed nicols. Marginally the augites 
 are partially replaced by greenish hornblende. The rest of the 
 section is dioritic in appearance, and consists of plagioclase, 
 green hornblende, biotite, and some accessory magnetite. The 
 biotite appears in scattered flakes, groups of which are often in 
 optical continuity, an appearance which is due to the biotite 
 being moulded upon the felspars. The plagioclase is mostly * 
 idiomorphic, and there would appear to be also a certain amount 
 of interstitial alkali-felspar. In some sections [8945] an 
 occasional accessory pseudomorph after olivine is seen. Some 
 sphene is also present. 
 
 The central portion of each intrusion, however, is a rock, as 
 already remarked, of slightly more acid composition, consisting 
 essentially of elongated prisms of greenish-brown hornblende and 
 alkali-felspar, with accessory plagioclase [8946]. The alkali- 
 felspar consists of a granular aggregate, and forms the ground- 
 mass in which the idiomorphic hornblendes lie embedded. 
 
 A similar structure is seen in some of the Cornish minettes 
 described by Collins, t in which the matrix that encloses the other 
 constituents is a coarse-grained aggregate of unstriated felspar. 
 This variety of the Meall an Laoigh rock may be classed as a 
 vogesite, hornblende being the predominant ferro-magnesian 
 mineral. Sphene and apatite are accessory, and pseudomorphs 
 of hornblende after idiomorphic augite also occasionally occur. 
 The intimate association of a rock of this type with augite-diorite, 
 in the same intrusion, is strong evidence of the close relationships 
 which undoubtedly exist in the district between the diorites and 
 the lamprophyres. In the further descriptions of these outlying 
 intrusions these genetic connections will still be found to hold 
 good, not only for the lamprophyres and diorites, but also for the 
 kentallenites and other associated rocks. 
 
 Augite- diorites showing close affinities to that of Meall an 
 Laoigh occur on the high ground on the west side of Glen Strae, 
 
 *Teall — Brit. Petrog., p. 86, and Rosenbusch and Iddings — Microscopic 
 Physiography, p. 217, and pi. xix., fig. 2. 
 tSee Teali— Bn't. Petrog., pp. 355-357 
 
Kentallmiite of Qien Orchy. 91 
 
 and are exposed on the north side of Meall Copagach and on the 
 east side of Beinn Lurachan. The intrusion of Beinn Lurachan 
 is roiighly circular in outline, and nearly half a mile in diameter, 
 while the two intrusions on the north side of Meall Copagach are 
 more dyke-like in behaviour. These intrusions closely resemble 
 one another in composition and show very little variation. They 
 consist of an augite-diorite resembling some of the rocks 
 associated with the granites of the Southern Uplands. A hand 
 specimen shows numerous idiomorphic crystals of dark greenish 
 augite in a pale felspathic ground-mass. The augite is mostly 
 in short, stumpy prisms, and gives the rock, at first sight, a 
 spotted appearance somewhat like that of forellenstein. Biotite 
 is present in small patches and plates, but is not conspicuous. 
 Under the microscope the augite resembles that of the Meall an 
 Laoigh rock, and is frequently partially, and sometimes entirely, 
 replaced by green hornblende [7760, 7/61]. Hornblende is also 
 seen to occur as an original constituent, and to be of a variety 
 similar to that observed in the granite and diorite of Ben 
 Cruachan. The rest of the rock consists of biotite, plagioclase, 
 orthoclase, and quartz. The biotite occurs in ragged patches and 
 groups of detached flakes in optical continuity, a feature already 
 noticed as characteristic of the biotite in the Meall an Laoigh 
 rock, and also occurring in the kentallenites. It is of later 
 formation than the plagioclase, which is markedly idiomorphic. 
 The alkali-felspar and quartz are interstitial, and there is a 
 larger proportion of the former present in these rocks than in the 
 Meall an Laoigh diorite. 
 
 Somewhat similar diorites occur at no great distance from the 
 margin of the granite of Ballachulish, where they have been 
 mapped by Mr. Grant Wilson.* This tyjje of augite-diorite is 
 not known to occur anywhere within the granite mas-s, but only 
 as intrusions in the schists not far from the granite margin. It 
 evidently represents a more basic modification from the same 
 magma, and indicates a marginal type of differentiation. 
 
 The Kentallenite of Glen Orchy. — On the west side of G-len 
 Orchy, about a mile north of Catinnis and 7 miles from Dal- 
 mally, there occur an elongated mass and two smaller oval 
 patches of a basic rock of a dark, almost black, colour. A 
 similar type of rock to that here exposed was originally described 
 from Kentallen, near Ballachulish, and from the neighbourhood 
 of Ben Bhuidhe in the Annual Reports of the Geological Survey, 
 under the name olivine-monzonite ; but reasons have been given 
 in the paper by Messrs. Hill and Kynaston, already referred to, 
 for discarding this term in favour of a new one, and the term 
 Kentallenite is now generally used, the rock from Kentallen, 
 near Ballachulish, being taken as the type. The principal 
 expouure of this rock in Glen Orchy constitutes an elongated 
 mass almost a mile in length and an eighth of a mile in breadth. 
 It is a medium-grained or coarse massive rock, in which the chief 
 constituents, olivine, augite, biotite, and felspar, are easily seen 
 
 * Ann. Bep. Geol. Sur. for 1896, pp. 21, 22. 
 
92 Newer Igneous Bocks. 
 
 with the naked eye. The coarser rock forms the greater part of 
 the mass, and weathers into characteristic cup- shaped hollows. 
 Thus a weathered block or boulder may frequently be seen to be 
 pitted or honeycombed all over with hollows up to 9 or 10 inches 
 in diameter, and often resembling the smaller potholes seen in 
 the bed of a stream. These hollows will hold rain-water and 
 growths of moss and lichen, and so tend to assist the chemical 
 action which doubtless euch growths largely contributed in 
 starting. (Plate VI.) Huge rugged blocks generally mark the 
 outcrop of the intrusion. The rock has a rusty brown crust, but 
 is almost black when freshly fractured. 
 
 A section from the main mass of the Glen Orchy rock shows 
 the following features under the microscope [8614]: — The 
 olivine is seen to occur mostly in rounded grains and seldom to 
 show any well-defined crystal outlines. It is fairly fresh, is 
 traversed by numerous anastomosing veins of magnetite, and 
 often occurs as inclusions in the augite. Augite and olivine 
 appear to be present in about equal proportions, and together 
 with biotite make up perhaps the larger proportion of the rock. 
 The augite chiefly occurs as comparatively large individuals with 
 well-defined crystal outline, and frequently showing a marked 
 zonal structure between the crossed nicols. It also appears in 
 smaller grains and granular aggregates of irregular shape scat- 
 tered throughout the section. The biotite is strongly dichroic 
 and occurs in ragged plates and patches, and several detached 
 flakes are often seen to be in optical continuity, a feature which 
 has already been noticed in connection with the biotite of the 
 augite-diorites. The interspaces between the larger individuals 
 are filled up by felspar, together with some biotite and small 
 grains of augite. Both orthoclase and plagioclase are present in 
 approximately equal proportions, the plagioclase being perhaps 
 slightly in excess. The plagioclase is distinctly idiomorphic, 
 while the alkali-felspar is interstitial and plays the role of 
 ground-mass. Magnetite fills the veins in the olivine, and 
 appears also as small grains associated with both the olivine and 
 the biotite. 
 
 Such are the petrological features of the rock forming the 
 greater part of the Glen Orchy intrusion. A detailed examina- 
 tion, however, of the section exposed in the stream which cuts 
 through the mass shows that it is by no means of uniform com- 
 position throughout. 
 
 The lowermost portion of the mass is seen to consist of a coarse 
 dark green hornblende-picrite. At its lower margin this picrite 
 exhibits a marginal modification resembling a basic lamprophyre, 
 while at its upper margin it passes somewhat abruptly into the 
 kentallenite in such a way that no very sharp line of demarcation 
 can be traced between the two, nor any well-marked transitional 
 phase. The lower portion of the kentallenite is, however, rather 
 more basic than the normal type, showing very little felspar, and 
 a high proportion of augite and olivine, with a very coarse 
 texture. Followed upwards in the burn-section this type passes 
 
Kentallenite of Glen Orchy. 93 
 
 into the normal rock, which again shades into a rather less basic 
 and slightly finer-grained variety towards the upper margin of 
 the mass. The variation throughout the intrusion is thus well- 
 marked, and in a vertical direction. The upper portions of 
 the mass, as in the carboniferous picrite of Blackburn, Linlith- 
 gowshire, are the more acid, while the more basic minerals have 
 tended to concentrate in the lower part, and have consolidated as 
 a layer of ultra-basic rock. 
 
 Under the microscope [9009J the picrite is seen to consist 
 essentially of olivine, hornblende, augite, and biotite. Only a 
 very minute and insignificant quantity of felspar is occasionally 
 to be seen interstitially. Magnetite is accessory, but chiefly 
 occurs in connection with the alteration of the olivine. The 
 olivine is fairly plentiful in rounded grains. The hornblende 
 varies from brown to greenish, or is sometimes almost colourless, 
 the various gradations of colour being often visible in the same 
 individual ; it occurs in large irregular poikilitic plates, which 
 enclose both the olivine and augite. The pleochroism is not very 
 strong. Occasionally the hornblende seems to form a marginal 
 outgrowth to the augite, and may perhaps be partially formed at 
 its expense. The augite is colourless or almost so, and seldom 
 shows crystal form. The biotite is of a deep orange-brown 
 variety, and strongly dichroic. It is of later formation than the 
 hornblende. Felspar is rare, but occasionally occurs in small 
 interstitial patches, without any signs of twinning. This is 
 somewhat suggestive of orthoclase, since we know that the plagio- 
 clase of the closely-associated kentallenite crystallised out before 
 the biotite, and the orthoclase last of all interstitially. 
 
 The order of consolidation of the minerals in the picrite is — 
 olivine, augite, hornblende, biotite, and finally felspar. 
 Towards its lower margin the picrite becomes finer grained and 
 shows aflinities to the basic lamprophyres, while vein-like streaks 
 and patches rich in felspar are not uncommon. The micro- 
 scope shows [9007, 9008] this marginal portion of the mass to 
 be rich in hornblende, while the proportion of olivine and augite 
 has considerably diminished. The hornblende is green and 
 greenish-brown, and occurs in idiomorphic prisms, frequently 
 giving a lath-shaped section, as in the homblende-lamprophyres. 
 The augite is considerably altered and sometimes pseudo- 
 morphosed. The olivine is represented by pseudomorphs of 
 colourless amphibole needles and magnetite, and secondary 
 green hornblende needles often form a peripheral border to the 
 pseudomorphs. Some biotite is present in small flakes, and 
 sometimes small grains of sphene. Felspar is fairly plentiful 
 •and resembles that of the lamprophyres, occurring as small 
 plagioclase crystals with good idiomorphism. A small quantity 
 of interstitial quartz is also present. Patches rich in felspar are 
 also seen, and are somewhat coarser than the surrounding rock, 
 but graduate imperceptibly into it. They consist" of idiomorphic 
 plagioclase and micropegmatite. The lower margin of the 
 picrite thus presents a peculiar modification of more acid com- 
 position, allied to the lamprophj-res. 
 
94 Newer Igneous Hocks. 
 
 Passing to the upper part of the picrite mass, we find, as we 
 approach the kentallenite, that the rock hecomes slightly- 
 coarser, and the microscope shows [9010] that the hornblende 
 has diminished while the proportion of augite has increased, 
 the hornblende usually being confined to the marginal portion 
 of the augites. The proportion of felspar, however, does not 
 appear to have perceptibly increased. 
 
 The next phase of the mass that is exposed is a basic form of 
 the kentallenite, showing a low proportion of the felspars and 
 occasionally a small quantity of pale green hornblende [9011]. 
 In the typical kentallenite, which succeeds this phase, the horn- 
 blende has entirely disappeared, and the biotite is of a browner 
 colour, and does not show the rich orange tinge characteristic of 
 that mineral in the picrite. The upper margin of the kent- 
 allenite does not assume a particularly fine-^grained aspect, but 
 appears to become slightly more acid and to show an approach to 
 lamprophyre. 
 
 Occasionally within the normal type of kentallenite we find 
 inclusions consisting of roughly spheroidal masses, up to two 
 feet or more in diameter, consisting of a finer-grained type of 
 kentallenite. These masses show well-formed olivines and 
 augites in a ground-mass consisting of biotite, plagioclase, and 
 alkali-felspar. There is a sharp distinction between the ground- 
 mass of biotite and felspar and the relatively large and con- 
 spicuous augites and olivines, the biotite and felspar being not 
 nearly so coarse as in the normal rock. The augite and olivine 
 apparentlj' represent the first generation of crystallisation, while 
 the felspar and biotite would appear to represent a second 
 generation of more rapid crystallisation. 
 
 In one place the kentallenite is seen to be traversed by a 
 narrow vein of very coarse white felspar and large biotite plates, 
 doubtless analogous to the pegmatite veins so common in certain 
 granites, and consisting, as is also the case with granite peg- 
 matite, of these constituents of the parent rock which were the 
 last to crystallise out from the intruded magma. 
 
 Some interesting inclusions may be observed in the picrite. 
 Ignoring occasional fragments of quartzite, these are of two 
 kinds: — (1) Mushroom-shaped inclusions, 6 inches or so in 
 diameter, of fine-grained kentallenite, and (2) nodular lumps, a 
 few inches in diameter, of a fine-grained dark greenish rock 
 resembling certain lamprophyres, and showing conspicuous 
 pheuocrysts of hornblende. The rock of the mushroom-shaped 
 inclusions consists [9013] of relatively large, idiomorphic, zoned 
 augites and olivines, in a fine-grained ground-mass of felspar, 
 with elongated, irregular strips of biotite and small pyroxene 
 granules. It, thus, in many respects resembles the rock com- 
 posing the spheroidal inclusions in the kentallenite. The other 
 inclusions consist of large brown and greenish hornblendes 
 resembling the hornblende of the picrite, in a micro-crystalline 
 and rather peculiar ground-mass. This ground-mass is not 
 uniform in composition ; some parts, though rather obscured by 
 
Keniallenite of Olen Orchy. 95 
 
 decomposition, appear to consist of plagioclase and orthoclase 
 with some scattered hornblende needles, while other parts f-re 
 rich in alkali-felspar and crowded with smiill colourless grains 
 and crystals of pyroxene. Some sphene, apatite, and magnetite 
 are also present. It is not easy to explain these inclusions ; they 
 may perhaps represent magmatic concretions or consolidations of 
 an earlier phase. That they are intimately and genetically con- 
 nected with the mass in which they are found, their petrological 
 composition and affinities sufficiently show, and they would 
 appear to indicate a certain amount of intermingling of the 
 various products of the differentiation of the present magma. 
 They afford, moreover, an interesting link in the chain of 
 evidence which indicates a common origin for the picrites, kent- 
 allenites, and lamprophyres. 
 
 If we compare the kentallenite with the augite-diorites, 
 already described, associated with the marginal area of the Ben 
 Cruachan granite, it is evident that there are close petrological 
 affinities between the two. In the case, for instance, of the 
 augite-diorite of Meall an Laoigh, the addition of olivine and 
 the elimination of the hornblende would give us a rock closely 
 resembling the kentallenite of Glen Orchy. The augite is 
 similar in both rocks, and the biotite has a similar mode of 
 behaviour, while interstitial orthoclase is a characteristic feature. 
 Moreover, the evidence of the inter-relationships of all these out- 
 lying intrusions, and their relation to the granite, is perhaps 
 still more complete in the area about Glen Fyne and Ben Bhuidhe 
 to the south-east, for here we have an intrusion consisting partly 
 of kentallenite and partly of angite-diorite, and in another part 
 of the same area we find a direct prssage between augite-diorite, 
 allied to kentallenite, and biotite-granite. (The majority of 
 these intrusions occur in Sheet 37 to the south, and are described 
 in the Explanation dealing with that sheet.) We thus have a 
 series of intrusions consisting of picrite, kentallenite, augite- 
 diorite, and lamprophyre, which may be shown to be so closely 
 related to one another by common petrological characters and 
 mode of occurrence that we must conclude that they were 
 derived, by some process of differentiation, from a common 
 magma. Furthermore, the evidence, not only from the Ben 
 Cruachan area, but also from the other areas where a similar 
 assemblage of intrusions occurs, points to the fact that these out- 
 lying intrusions constitute basic moditications of the same general 
 magma from which the larger masses of granite and diorite were 
 derived. The entire evidence on this head has been discussed 
 at length in the paper already referred to, and to this the reader 
 is referred for further information on this subject. 
 
 The numerous dykes and sills which cut not only the Ben 
 Cruachan granite, but traverse the surrounding country in great 
 numbers, may similarly be shown to be closely allied to one 
 another, and to have close genetic relationships with the granite. 
 These will be described in the following chapter. 
 
96 Newer Igneous Rocks. 
 
 The Altered Bode, allied to Porphyrite, of the South-eastern 
 Spurs of Ben Gruachan. — Before leaving the Ben Oraachan 
 area, it is necessary to give some description of a remarkable 
 mass of rock whicli occupies a well-marked belt m the south- 
 eastern portion of the Ben Cruachan range. This mass has an 
 elongated form slightly over four miles in length, and measur- 
 ing half a mile at its maximum breadth. It stretches from the 
 ridge of Meall Cuanail (2200 feet), situated about half a mile due 
 south of the summit of Ben Cruachan, in an almost due easterly 
 direction to Coire Gruachan (1200 feet), and thence, still in an 
 easterly direction, to the summit of Beinn-a-Bhuiridh (2941 
 feet), where it attains its greatest breadth. On the north side 
 of Beinn-a-Bhuiridh precipitous crags of at least 800 feet in 
 height are formed entirely of this rock. From Beinn-a- 
 Bhuiridh the mass takes a more north-easterly trend, and is well 
 seen in AUt Coire Ghlas and AUt Coire Creachan, descending to 
 a level of 500 feet. Further to the north-east the rock appears 
 in Allt Mhoile at slightly over 400 feet, and then rises sharply 
 to 1930 feet on the ridge separating Beinn Eunaich from Stob 
 Maol, and thins out to a narrow strip slightly further on. "We 
 thus see that this rock-mass traverses elevated ridges and deep 
 corxies alike ; it goes up one side of a ridge and down the other 
 side in a manner strongly suggesting the behaviour of a great 
 vertical dyke-like mass. The total vertical thickness of this 
 mass exposed is not less than 2500 feet, while its maximum 
 breadth is half a mile. The mass is bounded almost 
 entirely by the granitic and dioritic rocks of the Ben 
 Cruachan range. Thus, on its south and south-east sides 
 we have the diorite and hornblende-granite of the lower slopes of 
 Beinn-a-Bhuiridh and Monadh Driseag, while on it^ north side 
 the more acid type of the Ben Cruachan granite is in 
 contact with it. It is only for a short distance at 
 its north-eastern end that the rock is in contact 
 with the older metamorphic rocks. The whole of the western 
 portion of the mass consists of a rock frequently closely 
 resembling that of some of the porphyrite dykes, and is of a dark 
 grey or dark greenish colour. In Allt Coire Ghlas, AUt Coire 
 Creachan, Allt Mhoile, and in the intervening ground, however, 
 the rock loses its porphyrite-like appearance and assumes a 
 schistose structure. But no line can be drawn between the two 
 phases, the normal structure of a fine-grained porphyritic rock, 
 resembling porphyrite or andesite, appearing gradually to pass 
 into a structure indistinguishable from that of a thoroughly 
 foliated rock. At its north-eastern extremity, however, to the 
 north of Stob Maol, no schistose structure can be detected, and 
 the rock again assumes the appearance of a porphyrite. The 
 planes of schistosity in the foliated portion are vertical or almost 
 so, and lie parallel to the general line of elongation of the mass. 
 The rock has evidently been altered by the granite of Bon 
 Cruachan and shows contact phenomena, whicli may be well seen 
 under the microscope. The presence of this alteration, combined 
 
Altered Igneous Rbclcs south-east of Ben Cruachan. 97 
 
 ■with the foliated structure which has been super-induced upon 
 a considerable portion of the mass, has rendered the investiga- 
 tion of the original nature and relationships of these rocks a 
 matter of great difficulty. The foliated rocks, with their 
 frequent plagioclase phenocrysts, suggested, at first, affinities to 
 some of the older metamorphic rocks such as the schistose 
 epidiorites ; but the fact that the microscopic features and com- 
 position of these rocks did not recall those of 'any of the older 
 epidiorites, and that the greater portion of the mass showed no 
 signs whatever of foliation, seemed to render this view 
 improbable, and to suggest affinities rather with the later series 
 of andesites, porphyxites, and diorites. 
 
 A large series of specimens has been collected from all parts 
 oE this interesting mass and examined under the microscope, and 
 the evidence of this examination is extremely sxiggestive in con- 
 nection with the relations of the rock-mass as a whole. 
 
 It will be convenient, first of all, to deal with those parts of the 
 mass which show no marked signs of any foliated structure. 
 This includes the larger proportion of the mass, though it cannot 
 be said that here a tendency to some parallel structure is 
 invariably absent. We have, as it were, occasional transitions 
 or tendencies to the schistose structure, indistinguishable from 
 the flow structure of a normal andesite or porphyrife, and it is 
 probable that such structures are in fact largely due to fluxion 
 movements in the original magma. A large portion of the rock, 
 however, shows no signs of any movement of any kind whatever. 
 
 The hand specimens of the majority of the altered rocks from 
 the unfoliated portions have a remarkable resemblance to altered 
 andesites. They are various shades of grey and sometimes 
 greenish-grey in colour, but the more highly altered rocks near 
 the granite are pale grey, while the least altered are very dark. 
 This feature has been found to be very characteristic of the 
 alteration of andesitic rocks by a large mass of granite, and is 
 exemplified both in the Cheviot area and in the alteration of the 
 andesites of the Glencoe series by the Blackmount granite.* 
 
 The microscopic examination of these rocks shows that they 
 consist for the most part of contact-altered augite and horn- 
 blendic andesites, and diorite-porphyrites. On the whole they 
 have strong affinities with porphyritic intrusive rocks of the 
 porphyrite and andesite types. A specimen of an altered augite- 
 andesite was collected from the extreme north-eastern portion of 
 the mass. It consists [9311] of phenocrysts of plagioclase and 
 augite, the latter for the most part altered into pale green uralitic 
 hornblende, in a ground-mass of rather trachytic structure 
 formed mainly of small lath-shaped plagioclases. Some small 
 flakes of pale green hornblende, possibly secondary, are scattered 
 about, but there is very little secondary biotite. In the ferro- 
 magnesian phenocrysts the form of the original augite crystal is 
 often well seen, and occasionally some epidote is associated with 
 their alteration. A little sphene is also present. It is doubtful 
 
 * See Summ. Prog. Geol. Svr. for 1900, p. 82. 
 
98 Newer Igneous Bocks. 
 
 if there was any original hornblende present. This rock shows 
 considerably less alteration than is usually observed in the mass, 
 but a specimen, collected by Mr. J. B. Hill, from the AUt Coire 
 Glas bum shows even less, and is remarkably fresh. It is 
 closely allied to the pyroxene-bearing hornblende-andesites. 
 
 Under the microscope [9267] one sees porphyritic augites and 
 a few phenocrysts of clear plagioclase. The augite is of a very 
 pale greenish colour when fresh, and is partly altered into green 
 fibrous uralite. Numerous small prisms of original, and fresh, 
 brown hornblende are also present, giving the characteristic 
 rhombic cross-sections with truncated acute angles and lath- 
 shaped vertical sections. Both larger and smaller sizes are seen. 
 These hornblendes are of the same type as those so characteristic 
 of the hornblende-andesites, but without the resorption border 
 so commonly observed in these rocks. The ground-mass consists 
 mainly of small lath-shaped plagioclases, with an arrangement 
 indicating flow structure, small hornblendes, interstitial matter, 
 and accessory magnetite. This rock shows several points of 
 resemblance to the pyroxene-nornblende-andesite of the north 
 side of the Clach Leathad in the Blackmount ; it also has very 
 much the aspect of some of the newer porphyrites. 
 
 In the less-altered types the ground-mass is usually of the 
 felspar-lath type, and shows a high though varying proportion 
 of small lath-shaped plagioclases. In one of the specimens, 
 however, there seems to have been a considerable proportion of 
 originally glassy base. This occurs near the foot of AUt Coire 
 Ghlas, and the rock has a very strong resemblance to a normal 
 andesite. Under the microscope a section of this rock [9038] 
 shows crystals and crystalline groups of plagioclase, with 
 characteristic inclusions of the ground-mass, and some altered 
 hornblendes, in a ground-mass which was possibly originally 
 glassy and now has a greyish-brown dusty appearance, with 
 paler aureoles round the ferromagnesian phenocrysts. Apatite 
 and magnetite are accessory. The plagioclase has a peculiar 
 greyish cloudy appearance, frequently seen in contact altered 
 andesites. The jiornblendes are rather decomposed, but appear 
 to be now represented almost entirely by aggregates of small 
 biotite flakes, green hornblende, and magnetite. Some of the 
 patches look as if augite had been originally present. 
 
 In the more altered specimens the ground-mass appears to 
 have been completely reconstructed, and consists of a clear 
 granular mosaic of felspar and quartz, throughout which 
 numerous flakes of secondary biotite, of the contact type, and 
 green hornblende lie scattered. Apatite is almost invariably an 
 accessory, and sometimes small grains of sphene. In no case 
 does the alteration appear to have gone so far as to have had any 
 very marked effect upon the felspar phenocrysts, though they 
 usually show the peculiar cloudy appearance already noticed. 
 They are frequently honeycombed with inclusions of the ground- 
 mass in a characteristic manner [9317, &c.]. 
 
Altered Igneous Eochs south-east of Ben Gruachan. 99 
 
 The proportion of phenocrysts to matrix varies very consider- 
 ably, some bands of the mass being so rich in phenocrysts as to 
 resemble a diorite. Indeed, it is sometimes difficult in the field 
 to distinguish these rooks from the dioritic phase of the Ben 
 Cruachan granite which is in contact with the southern margin 
 of the mass. Under the microscope these varieties, rich in pheno- 
 crysts, show strong affinities to many of the later porphyrites. 
 They occur on the west side of Coire Cruachan, on Beinn-a- 
 Bhuiridh, and in the AUt Mhoile burn. A specimen from the 
 latter locality [8865] has strongly the appearance of a porphyrite 
 which has undergone contact metamorphism. It contains 
 numerous phenocrysts of plagioclase and greenish uralitic horn- 
 blende, probably after augite, in a ground-mass consisting of a 
 micro-crystalline aggregate of felspar, quartz, and granules of 
 green hornblende. Apatite is accessory. The ground-mass is 
 crowded with patches and aggregates of small flakes of secondary 
 biotite. Some sphene is also present. In another specimen of 
 the same variety from the west side of Coire Cruachan [9313] 
 the ground-mass is rather of the felspar-lath type. These rocks 
 were probably originally augite-porphyrites, or some closely 
 allied variety. The augite is now, as a rule, almost entirely 
 replaced by a pale green, often fibrous, hornblende (uralite), 
 which often behaves as a distinct individual with the general 
 outline of the original augite crystal, but is sometimes broken up 
 into a granular aggregate. Occasionally the original unaltered 
 pyroxene is seen remaining as a core surrounded by the secondary 
 hornblende [8611]. 
 
 Original hornblende, probably of the brown variety, was 
 evidently present in many of the rocks. It has now been 
 replaced by aggregates of secondary biotite flakes, grains of green 
 hornblende, and magnetite. A similar type of alteration has 
 been described in the case of the Blackmount andesites.* 
 Occasionally the original form of the hornblende crystal is still 
 preserved [9312, &c.], but more often it has become completely 
 lost and is represented by an irregular patch of the secondary 
 minerals. Original biotite has only been observed for certain in 
 one specimen [9314]. Flow structure is very common, and 
 by becoming more and more marked appears sometimes to 
 graduate imperceptibly into the more schistose type of structure 
 which characterises the north-eastern portion of the mass. 
 
 Yesicular varieties are sometimes met -with [9315], and some- 
 times the ground-mass contains irregular patches and streaks of 
 clear crystalline quartz. Some of these patches may represent 
 vesicles, or some may be included fragments of quartzite, since 
 in certain parts of the mass, especially in the crags on the west 
 side of Coire Cruachan, fragments of pale quartzite up to 4 or 
 5 inches in diameter has been observed. 
 
 We will now pass on to the schistose or foliated varieties of the 
 mass, which occur, as already pointed out, in its north-eastern or 
 narrower portion. In dealing .with these rocks we find that they 
 
 * See Summ. Prog. Geol. 8ur. for 1900, pp. 82, 83. 
 
100 Newer Igneous Rocks. 
 
 consist of precisely the same minerals as are found in the main 
 portion of the mass, already described, and the plagioclase pheno- 
 crysts may still be frequently recognised. The majority of the 
 rocks have the appearance of fine-grained biotite-augen-gneisses 
 or hornblende-biotite-gneisses, the relative proportions of horn- 
 blende and biotite varying considerably. The phenocrysts are 
 sometimes somewhat scarce, but occasionally are very numerous, 
 as was also seen to be the case in the unmoved portion of the 
 mass. Granulitic and cataclastic structures may frequently be 
 observed, but often the schistose appearance seems to be due 
 rather to a fluxion-foliation structure. The schistose structure 
 is perhaps best developed among the rocks exposed in the AUt 
 Mhoile and AUt Coire Creachan burns and in the intervening 
 crags. It is also strongly developed along the northern margin 
 of the mass, on the north side of Beinn-a-Bhuiridh, but dies out 
 rapidly as one enters the main mass of Beinn-a-Bhuiridh. 
 Specimens from Lairig Torran, on the north side of Beinn-a- 
 Bhuiridh, shows a well-marked gueissic structure. Sections 
 from these and from a large number of other specimens were 
 examined under the microscope by Dr. J. S. Flett, and the 
 following notes are partly taken from his descriptions. 
 
 The rock exposed at Lairig Torran is gneissic in structure, n.nd 
 shows well-marked granulitic and cataclastic features under the 
 microscope [7627, 7628]. Porphyritic felspars and occasionally 
 quartzes are present and often tend to form lenticular augen. 
 Sometimes the phenocrysts show signs of bending and fracture. 
 The matrix is granulitic and streams past the phenocrysts ; it 
 consists of felspar, quartz, green hornblende, and biotite. The 
 granulitisation, however, is not of an advanced type. Mere 
 fluxion, with or without contact-action, would hardly have 'oeen 
 suiEcient to give the rock its present structure. There has 
 evidently been shearing as well. 
 
 In the AUt Mhoile burn and in the crags above it we find fine- 
 grained grey gneissose rocks. These show under the microscope 
 [8258-8260] comparatively large and more or less turbid pheno- 
 crysts of plagioclase, often with irregular borders, in a ground- 
 mass of granular quartz and felspar, hornblende and biotite, with 
 sphene and iron-ores as accessories. There are also some patches 
 of green hornblende, which are often micro-poikilitic. 
 Occasionally [8259] augite is seen as cores in the hornblende. 
 
 These examples will give a good idea of the structure and com- 
 position of the schistose portion of the mass. The rocks evidently 
 show varying degrees of granulitisation and shearing, together 
 with structures indicating fluxion foliation. These rocks cer- 
 tainly do not represent the ordinary types of the schistose 
 epidiorites — that is to say, the metamorphic igneous rocks of 
 earlier date than the general foliation of the Highlands. But 
 we know that these so-called epidiorites vary in composition and 
 metamorphism, and one might quite well suppose that these rocks 
 belonged to some acid type of schistose epidiorite in which the 
 metamorphism was of a comparatively low degree, if one were to 
 judge by the aspect and character of these particular specimens 
 
Altered Igneous Bocks south-east of Ben Cruachan. 101 
 
 alone. Neither are they usual types of the newer intrusives. 
 Some are good biotite-gneisses. There can be no doubt about the 
 cataclastic structures in some, while others show a fluxion- 
 foliation due to movement during consolidation. The effects of 
 contact-action in these gneissic types are doubtful and obscure, 
 but these are not rocks tbat would lend themselves easily to con- 
 tact alteration. 
 
 These schistose varieties, however, belong to the same series, 
 and form part of the same mass, as the porphyrite and andesite- 
 like varieties already described ; and these latter constitute the 
 greater part of the mass and show strong affinities to the later 
 diorite-porphyrites and andesites. It would appear, then, by no 
 means improbable that the whole mass belongs to the newer 
 series of intrusives. 
 
 In that case, the development of the gneissose facies is of great 
 interest. The intrusion of the mass evidently took place prior 
 to that of the Ben Cruachan granite, but probably, from the 
 evidence of its belonging to the later intrusives, during the same 
 geological period. The movements which caused the foliation 
 structures were of a local character, and probably affected a 
 portion of the mass previous to its complete consolidation, con- 
 tinuing to a certain extent after consolidation was complete, and 
 thus producing the shearing and cataclastic structures observed. 
 This mechanical action may possibly have been connected with 
 the movements which accompanied, or immediately preceded, 
 the intrusion of the enormous mass of the Ben Cruachan granite. 
 
 As regards the unfoHated portion of the mass, the petrological 
 evidence already brought forward of its close relationships to 
 the newer intrusives would seem to be sufficiently clear. Apart, 
 however, from an examination of the rocks by themselves, this 
 relationship is brought out in a remarkable manner, if we 
 compare the altered porphyrites on the south side of Beinn-a- 
 Bhuiridh, from the marginal part of the mass, with the marginal 
 portion of the Beinn-a-Bhuiridh diorite which is in contact with 
 them, and which is a modification of the Ben Cruachan granite. 
 This diorite passes at its margin into a diorite-porphyrite, and a 
 section of this shows [9613] so many characters in common with 
 the altered porphyrite adjacent to it [9614-9616] that one might 
 almost take the two facies as representing different portions of 
 the same mass. The structure and composition are the same, 
 and the majority of the constituents of a similar tjrpe, and the 
 same alteration of the pyroxene to -uralite is seen in both. The 
 high proportion, however, of secondary biotite flakes, of a contact 
 type, seen in the altered porphyrites is always characteristic and 
 distinctive. Apart from this feature the rocks are practically 
 the same. 
 
 The gneissic facies is analogous to the foliated Galloway 
 granites. Thus a portion of the Criffel granite shows a facies 
 resulting from mechanical deformation, which varies in structure 
 from gneissose to granulitic* Moreover, the field evidence shows 
 
 *See The Silurian Rocks of Great Britain, vol. i., pp. 609, 610. 
 
102 Newer Igneous Rocks. 
 
 that the moyement took place after the rock had reached its 
 present position, and after the formation of the acid veins, and 
 probably before the cooling was quite complete. The microscope 
 shows that the last-formed minerals, namely, the alkali-felspar 
 and the quartz, yielded most to the deforming stresses, and the 
 f erro-magnesian minerals form well-marked flaser. In its more 
 extreme phase the rock loses its gneissose character and passes 
 into a grey granulite ; the secondary micro-crystalline aggregate 
 forms a large proportion of the main mass, and cataclastic struc- 
 tures are strongly marked. 
 
 Well-marked granulitic structures are also occasionally pro- 
 duced by contact-action in the Silurian volcanic rocks of the 
 same region by the larger basic masses, and banded granulitic 
 rocks have been formed in tlie volcanic zone surrounding the 
 Cheviot granite close to its contact with the andesites on the 
 southern margin of the mass. But, although contact meta- 
 morphism is thus capable of producing the granulitic structure 
 by a process of reconstruction and recrystallisation of the con- 
 stituents, it is not probable that this has been the only factor in 
 the formation of the peculiar granulitic types of the Ben 
 Cruachan district. We must have had here in addition the 
 operation of considerable mechanical stresses analogous to those 
 which affected the granite of Criffel. It is of great interest, 
 therefore, to be able to note the action of such deforming forces 
 at a period later than that of the formation of the Highland 
 schists, which has produced structures almost identical with 
 those found in those older rocks. 
 
 ii. The Glen Fyne Granite and Outlying Masses. — A small 
 portion of the Glen Pyne granite is seen to enter the extreme 
 south-east comer of the map. It occupies the hill-slopes on the 
 east side of Glen Fyne and crosses the river just below Inver- 
 chorachan, but does not extend any great distance on its west 
 side. This granite constitutes a continuation of the igneous mass 
 of Garabal Hill and Meall Breac, described by Messrs. Dakvns 
 andTealL* 
 
 The character of the rock which enters the present sheet is that 
 of a homblende-biotite-granite of fairly coarse grain, and good 
 exposures may be studied in the bed of the River Fyne below 
 Inverchorachan. The granite is of an acid type and contains 
 sphene, and bears a strong resemblance to the coarse acid type of 
 homblende-granitite which constitutes so large a proportion of 
 the large granite mass of Glen Etive to the north of Ben 
 Cruachan. Vein-like intrusions of a very similar type of granite 
 are common on the steep slopes on the western side of Glen Fyne. 
 
 Further to the west, on the ridge of Beinn Bhuidhe, a small 
 mass of granite occurs between the summit of Beinn Bhuidhe 
 and Stac-a-Chuirn. This intrusion has an irregular, more or 
 less crescent-shaped form at the surface. It crosses the ridge 
 and is seen for a short distance on the steep craggy slopes on 
 either side of it. Another smaller mass of granite occurs on the 
 
 * Quart. Journ. Oeol. Soc, vol. xlviii. (1892), p. 104. 
 
Ketitailenites of AM An t-Sithein and Brannie Burn. 103 
 
 iiorth.-west slopes of Beinn Bhuidhe. These granites are of 
 medium grain and of rather acid type, resembling some of the 
 finer-grained portions of the Glen Fyne intrusion. They are 
 undoubtedly closely related to the far larger mass of Glen Fyne, 
 and constitute subordinate modifications of the same magma. 
 These granites represent the more acid intrusions of the Beinn 
 Bhuidhe complex, the basic intrusions being represented by the 
 kentallenites of AUt an t-Sithein and the Brannie Burn. A 
 transitional series of diorites also occurs, but these do not come 
 within the area of the present sheet, and so are described in the 
 Explanation for Sheet 37 to the south. We will therefore 
 proceed to give a short description of the kentallenites and com- 
 pare them with the kentallenite of Glen Orchy, described above. 
 
 The Kentallenites of AUt an i-Sithein and Brannie Burn. — 
 About 2j miles west of Beinn Bhuidhe a roughly oval-shaped 
 intrusion of kentallenite occurs in the AUt an t-Sithein Burn. 
 The longer diameter of the mass is rather over a quarter of a mile 
 in length. Its mode of occurrence and manner of weathering is 
 exactly similar to the intrusion of Glen Orchy. A similar 
 intrusion, though perhaps slightly larger in size, is exposed near 
 the head of the Brannie Burn, aljout one mile S.S.W. of Beinn 
 Bhuidhe. These two masses closely resemble one another in 
 composition, but are slightly different from the rock of Glen 
 Orchy. They are rich in pyroxene, but do not contain quite so 
 high a proportion of olivine as the Glen Orchy rock, while, on 
 the other hand, the proportion of the felspars, and especially of 
 orthoclase, to the olivine and pyroxene is decidedly higher. Of 
 the two masses that of the Brannie Burn would seem to be 
 slightly the more basic. The augite does not exhibit crystal 
 form anything like so perfectly as in the Glen Orchy rock, but 
 tends to occur rather as corroded grains and granular aggregates. 
 The characteristic ophitic structure of the biotite constitutes a 
 pronounced feature in the Brannie rock [7414]. 
 
 In a section of the An t-Sithein rock [7415] the proportion of 
 olivine is relatively small. The rock, however, is rich in 
 pyroxene, and shows frequently a slightly pleochroic hypersthene 
 in association with the augite. But the most characteristic 
 feature of both these rocks is the orthoclase. In the Brannie 
 rock it occurs in approximately equal proportions with the 
 plagioclase, but in the An t-Sithein rock it is decidedly in excess. 
 It is poikilitic in appearance, and forms a matrix of compara- 
 tively large, clear, irregular patches, in which small, lath- 
 shaped, idiomorphic plagioclases lie embedded unorientated. 
 With respect to the olivine and pyroxene the orthoclase is inter- 
 stitial. Individual patches occasionally show twinning on the 
 Carlsbad plan. We see then in these Beinn Bhuidhe rocks the 
 same peculiar features as have already been noticed in the kent- 
 allenite of Glen Orchy. The rocks are especially remarkable for 
 their high proportion of orthoclase, in association with minerals 
 such as olivine, augite, and hjrpersthene. 
 
104 Newer Igneu'ws Rocks. 
 
 The Allt an t-Sithein mass exhibits a certain amount of varia^ 
 tion in the course of the section exposed in the burn, but to a less 
 degree than is seen in the mass of Glen Orchy. Slightly below 
 the intrusion of typical kentallenite in the Allt-an t-Sithein, and 
 separated from it by a band of schist, we find an intrusion, a 
 few yards in breadth, of a medium-grained black rock resembling 
 in hand specimen a basic lamprophyre rather than kentallenite. 
 From its microscopic characters, however, it would appear to be 
 a variety of kentallenite, poor in olivine, hut closely related to 
 the normal type of the intrusion exposed a few yards from it. 
 Under the microscope this rock [9290] is seen to consist 
 essentially of augite, hypersthene, biotite, plagioclase, and 
 orthoclase. Olivine also occurs, but is scarce and forms rather 
 an accessory than an essential constituent. It is usually enclosed 
 in the augite and is considerably altered. A small quantitj' of 
 interstitial quartz is also seen. Some greenish hornblende is 
 frequently associated with the peripheral portions of the augites. 
 The hypersthene is markedly pleochroic. The orthoclase occurs 
 in relatively large micro-poikilitic patches, as in the more normal 
 kentallenite. It shows Carlsbad twinning, and is perhaps 
 slightly in excess of the plagioclase. The rock is not a typical 
 kentallenite, though closely allied to it. It might perhaps be 
 termed a hypersthene-kentallenite. 
 
 The lower margin of this intrusion represents a slightly more 
 acid modification, and resembles a micafdiorite. A section 
 [9289] shows a few decomposed idiomorphic augites, biotite, 
 idiomorphic plagioclase, orthoclase, and quartz. The two latter 
 sometimes form micro-pegmatite. There is also some sphene. 
 A pale green hornblende, chlorite, calcite, apatite, and magnetite 
 occur as accessories. The biotite is moulded on the plagioclase, 
 as in kentallenite. The rock, however, is more acid than the 
 central part of the intrusion, and shows affinities with the augite- 
 dioritea. This intrusion is separated from the main mass of the 
 kentallenite by a narrow belt of schist. The main mass shows 
 no variation in composition worth noticing. Bands of finer- 
 grained material occasionally occur, in which the idiomorphic 
 augites may be clearly seen with the naked eye. The upper 
 margin of the mass is also slightly finer-grained. Hypersthene 
 occurs throughout. 
 
 The intrusion of the Brannie Burn is only partially seen in the 
 present sheet, the lower portion being included in Sheet 37. It 
 should be pointed out, however, in this connection that the lower- 
 most portion of this intrusion, as in the case of the Glen Orchy 
 mass, consists of a coarse greenish black picrite, which is eeen 
 for ahout 15 yards in the burn section. It consists [9293] of 
 olivine, augite, and some hypersthene, resembling that of the 
 kentallenite, together with biotite and a little interstitial plagio- 
 clase. It differs, therefore, from the Glen Orchy picrite in the 
 presence of accessory hypersthene, and in containing no horn- 
 blende. A small quantity of h3rpersthene is also frequently 
 present in the Brannie kentallenite [9294, 9295]. 
 
Analysis of Kentallenite near Ballachulish. 
 
 105 
 
 In the Beinn Bhuidhe area, as already pointed out, similar 
 relationships have been found to hold good between the kent- 
 allenites, diorites, lamprophyres, and the granites as are found 
 in the Ben Cruachan area. 
 
 As regards the chemical composition of these kentallenites, an 
 analysis of the rock of Kentallen, near Ballachulish, was 
 published in the Annual Report of the Geological Survey for 
 1896, p. 22. More recently an analysis of the Allt an t-Sithein 
 rock was made by Dr. W. Pollard, of the Geological Survey, and 
 published in the paper on the kentallenites already referred to. 
 The complete analysis of this latter rock is as follows : — 
 
 SiO^ 
 
 TiO^ 
 
 AI2O3 
 
 Cr.03 
 
 Fe,03 
 
 FeO 
 
 MnO 
 
 (CoNi)O 
 
 CaO 
 
 MgO 
 
 K2O 
 
 Na^O 
 
 CO 
 
 P=0, 
 
 01 
 
 H^O (105°) 
 
 H^O (above 105°) 
 
 Per cent. 
 
 52-09~ 
 
 ■73 
 
 11-93 
 
 •10 
 
 1'84 
 
 7-11 
 
 •15 
 
 •07 
 
 7-U Y = 100-24 
 
 12-48 
 
 3-01 
 
 2-04 
 
 •16 
 
 •34 
 
 trace 
 
 •11 
 
 •24 
 
 Specific gravity = 2-94. 
 
 The result of the above analysis fully bears out the observa- 
 tions made from the microscopic examination of the rock sections. 
 The presence of chromic oxide probably indicates the occurrence 
 of a small quantity of chromite or picotite, which is known to be 
 of common occurrence in rocks rich in olivine, though it was not 
 detected with certainty under the microscope. If this analysis 
 is compared with that of the rock of Kentallen, it will be seen 
 that the Beinn Bhuidhe rock is the more acid of the two, the 
 Kentallen rock containing 48.49 per cent, of silica. Moreover, 
 the magnesia percentage of the latter rock rises to 15.26, thereby 
 showing a closer affinity to the picrites than the Beinn Bhuidhe 
 rocks. The total alkalies are slightly higher in the Kentallen 
 rock, but their relative proportion is reversed when we come to 
 the more acid Beinn Bhuidhe rock, which shows the potash in 
 excess of soda, and this one would naturally expect from the 
 relatively high proportion of orthoclase. The analysis, however, 
 fully bears out the petrological evidence that the rocks of Beinn 
 Bhuidhe, Glen Orchy, and Kentallen belong to the same group 
 — 'a group of rocks with essential olivine and augite, and smaller 
 
106 Newer Igneous Mocks. 
 
 amounts of orthoclase and plagioclase in varying relative 
 proportions. 
 
 In concluding this account of these peculiar rocks it will 
 perhaps not be out of place to summarise briefly their petrological 
 affinities and relations, a detailed account of which has already 
 appeared in the paper referred to, and from which the following 
 notes are taken. 
 
 The kentallenites of Argyllshire constitute one of the few 
 known examples in Britain of the peculiar association of ortho- 
 clase with olivine and augite. They show affinities (1) to the 
 picrites, in their high proportion of augite and olivine ; (2) to the 
 shonkinites of America (described by Messrs. Weed and 
 Pirsson*), in the peculiar association of orthoclase-felspar with 
 olivine and augite ; and (3) to the monzonites of Prof. Brogger,t 
 more especially to the basic variety of Smalingen in Sweden, in 
 the fact that orthoclase and plagioclase occasionally occur in 
 ■approximately equal proportions. Besides these examples, we 
 may cite that of the rock of Dignas, in Norway, described by 
 BrOggerlj: as a variety of olivine-gabbro-diabase, and now included 
 in his monzonite group. This rock contains accessory olivine 
 and a small proportion of orthoclase. Moreover, the occurrence 
 of a considerable proportion of a rhombic pyroxene in addition 
 to augite in the Beinn Bhuidhe rocks recalls certain hyperites and 
 norites in which orthoclase frequently occurs in addition to 
 plagioclase, but without the association of olivine. Such are 
 the hyperites, described by Dr. Teall,§ associated with the Loch 
 Dee granite of the Southern Uplands. These rocks show 
 affinities to the so-called norites of Teller and Von John in the 
 neighbourhood of Klausen in the Tyrol, and are essentially 
 composed of plagioclase, hypersthene, augite, and biotite, with 
 small quantities of quartz and orthoclase occurring as interstitial 
 matter. Among the Cortlandt series of the Hudson River the 
 norite proper, described by G. H. Williams, || consists mainly of 
 andesine and hypersthene, with accessory biotite; but an 
 interesting feature is the occurrence in it of large crystals of 
 orthoclase enclosing the other minerals in a poikilitic manner. 
 This mode of occurrence of the orthoclase is very characteristic 
 of the kentallenites of the Ben Bhuidhe area, also of the shon- 
 kinites and the olivine-monzonite of Smalingen. It also 
 behaves in the same way in the monzonites (augite-syenites) of 
 Southern Tyrol.** 
 
 * Bull. Soc. Geol. Am., vol. vi. (1895), pp. 400-422 ; also Am. Journ. Sci., 
 ser. 3, vol. 1. (1895), pp. 467-477. 
 
 t See "Die Eruptionsfolge der Triadisohen Bruptivgesteine bei Predazzo in 
 Sudtyrol," No. ii. of "Die Bruptivgesteine desKristianiagebietes," Vidensk. 
 Skrift. No. 7 (Kristiania, 1895). 
 
 J Brogger — " The Bruptive Rocks of Gran," Quart. Journ. Geol. Soc, vol. 
 L., p. 19. 
 
 § Ann. Bep. Geol. Sur. for 1896, pp. 41, 42. 
 
 \\Am. Journ. Sci., ser, 3, vol. xxxiii. (1887), pp. 135-144, 191-194. 
 
 ** Brogger — "Die Bruptionsfolge der Triadisohen Bruptivgesteine bei 
 Predazzo in Sudtyrol," pp. 56-57, figs. 1 and 2. 
 
Newer Igneous Rocks of Lismore. 107 
 
 Kentallenites are found not only in the Beinn Bkuidhe and 
 Ben Cruachan areas, but also in two other areas in Argyllshire 
 characterised by large intrusions of granitic and dioritic rocks — 
 namely, the Ballachulish area and the district about Kilmelford 
 and Loch Avich. In each of these areas kentallenite shows the 
 same associations and relationships. H. K. 
 
 Lismore. — Four small intrusive masses, which include the 
 rock-types tonalite, syenite, and diorite, occur in the portion of 
 Lismore Island within the sheet. The first forms a small pro- 
 montory on the west side of the island, nearly half a mile due 
 west of Fennacrochan. It is shown in vertical section in the 
 cliffs and has a conical shape. The rock is a medium to fine- 
 grained quartz-biotite-diorite, containing hornblende as well as 
 augite. At the northern end of the intrusion an interesting 
 melanocratic modification is found. It contains, in addition to 
 groups of large idiomorphic pyroxenes, phenocrysts of serpentine 
 after olivine. This marginal modification resembles kentallenite 
 in the occurrence of orthoclase along with olivine, in the colour 
 and perfect form of the augite, and in the late crystallisation of 
 the biotite. 
 
 A second boss, about j-mile broad, rises as a dome-shaped 
 hill to the south-east of the first one, and its boundaries are much 
 obscured by drift. The centre of the intrusion is a medium- 
 grained augite-diorite containing chiefly pink felspar. In some 
 parts of the intrusion biotite becomes very abundant, and the 
 rock shows dark angular patches consisting of biotite, orthoclase, 
 and plagioclase. These dark patches, though similar to a mica- 
 lamprophyre in composition, are unlike the lamprophyres which 
 are associated with the diorites and are later than the intrusions. 
 Perhaps they are early consolidated portions of the magma 
 caught up in a still fluid portion which solidified later. 
 
 The margin of this intrusion is seen against the contact-altered 
 schists on the shore and is exceedingly complex. The igneous 
 rock sends out strings and veins into the limestone breccia 
 (p. 81), and itself encloses numerous fragments of limestone, 
 which, when weathered out, gives the igneous rock a scoriaceous 
 appearance. Close to the margin the diorite passes into a very 
 fine-grained grey rock curiously similar in appearance to the 
 indurated limestone. Between the diorite mass described above 
 and the beach, a band of dark mottled igneous rock, which is 
 orientated east and west, crops out between the hornfelsed lime- 
 stone of the aureole of contact alteration and a gravel deposit of 
 the raised beach. It is a syenite, the peculiar character of which 
 i.5 pointed out in the following petrological notes by Dr. Flett 
 (p. 109). The joints in the rock tend to form horizontal columns, 
 and the appearance of the outcrop is suggestive of a dyke. 
 
 A third boss-like intrusion of tonalite, about 200 yards broad, 
 occupies the floor of the valley f-mile south-west of Fenna- 
 crochan, and is surrounded by high cliffs of altered limestone. 
 On account of the position of the mass, erosion has not proceeded 
 far into it, and a marginal modification is found forming a thin 
 
10& Newer Igneous Books. 
 
 skin over parts of the outcrop. This modification is very similar 
 to the peculiar syenite mentioned above. 
 
 Two small outcrops at the edge of the peat at Sron-na-Craoibhe 
 probablj' belong to an intrusion otherwise obscured by peat and 
 drift. The limestone shows contact alteration along the edge of 
 the peat. The rock is similar to that described from the boss 
 mentioned above, and it contains fragments of the limestone- 
 schist. 
 
 All the rocks are much weathered at the outcrop, but the 
 decomposition probably does not proceed beyond a few feet. Two 
 of the outcrops are situated close to a deep and sheltered natural 
 harbour, but a drawback to the working of these rocks as 
 " granite " on a commercial scale is the variable nature of the rock 
 due to rapid changes in the proportions of the mineral constituents. 
 
 H. B. M. 
 
 The igneous rocks which occur in the island of Lismore as 
 small bosses and dykes are interesting, because they furnish 
 examples of a variety of tj'pes such as usually aiise from the 
 newer granites of the West Highlands by processes of difEerentia- 
 tion. Some of them also exhibit peculiarities which are by no 
 means common in the district. 
 
 The most acid of these rocks are aplites, which form small 
 dykes intersecting the dioritic bosses. They are probably to be 
 regarded as segregations. Their only feature deserving mention 
 is their comparative richness in oligoclase. About |-mile south- 
 west of Fennacrochan there is a mass of tonalite. This rock, 
 which is of medium grain, contains biotite, hornblende, and 
 augite, with plagioclase, orthoclase, and quartz, and is too full 
 of orthoclase to pass as a normal diorite. The hornblende is 
 clear green, as is usual in tonalites ; the biotite dark brown and 
 vividly pleochroic ; the pale green augite rarely forms inde- 
 pendent crystals, but serves as a nucleus for peripheral growths 
 of biotite and hornblende. Graphic or parallel intergrowths of 
 the pjToxene and the amphibole also occur. The augite is often 
 darkened by fine dusty magnetite, and may show a fine parallel 
 lamellation like that commonly seen in diallage. Of the three 
 ferro-magnesian minerals biotite was last to crystallise, and may 
 enclose ophitically the earlier plagioclases. The felspars of this 
 rock are principally oligoclase and orthoclase, the former 
 having far the more perfect crystalline form. Decomposed, 
 cloudy centres in many of the plagioclase crystals probably indi- 
 cate the existence of basic cores, but satisfactory optical tests 
 cannot be applied to establish this. Quartz also is present, but in 
 no great amount. The accessory minerals include much brown 
 sphene, apatite, magnetite, and zircon. Some of the zircon 
 crystals are large, pink coloured, slightly pleochroic, and of 
 marked pyramidal habit. This led to a suspicion that they 
 might include xenotime, and some were isolated and tested micro- 
 chemically, but all proved to be zircon. 
 
 Syenites are by no means common in any part of the Scottish 
 Highlands, and two rocks of this class which occur in Lismore 
 
Syenites and Diorites of Lismore. 109 
 
 are deserving at least of passing notice. One is a pale grey 
 granitoid rock from the surface of the boss situated three-quarters 
 of a mile south-west of Fennacrochan, and consists of perthitic 
 orthoclase, plagioclase (mostly oligoclase), and green chloritic 
 pseudomorphs, presumably after hornblende. It is decomposed, 
 and contains much calcite, but quartz is practically absent from 
 the rock, and in its structures and the relation of its minerals to 
 one another it is a good example of the group to which it belongs. 
 The other is a rock of less normal character, and comes from a dyke- 
 like mass 3 furlongs west by south of Fennacrochan. In the sections 
 the great abundance of pale green augite in idiomorphic crystals is 
 the most striking feature. With these there is much perthitic 
 orthoclase, some plagioclase, but no quartz. Pyrite, magnetite, 
 and a little brown biotite are the principal accessories, with many 
 large and well-foi*med prisms of apatite. The absence of hornblende 
 and quartz and the scarcity of biotite are remarkable, and give 
 the rock almost a unique character among the differentiation 
 products of the Old Red granites of Scotland. 
 
 Diorites seem to be the prevalent rocks of the bosses. They 
 mostly have the hypidiomorphic or true plutonic structure, but 
 ir some of their marginal facies the minerals have crystallised 
 in two stages, and diorite porphyrites are the result. Although 
 many of the diorites contain green hornblende they are 
 characterised rather by the abundance of pale-coloured augite 
 and biotite. Of these three minerals the augite is the earliest, 
 and is frequently surrounded by hornblende in parallel growth ; 
 in some cases the amphibole may be partly secondary. Some of 
 these rocks contain many large idiomorphic augite crystals, pale 
 green and sometimes zonal, which might quite well be regarded 
 as phenocrysts. In their perfection of outline and in their 
 colour they recall the pyroxenes of the kentallenites. This 
 resemblance is heightened bv the occurrence of much biotite, 
 dark chestnut brown changing to clear yellow when the polariser 
 is rotated, and often enclosing numerous small lath-shaped 
 felspars in ophitic fashion. There are also in some of the slides 
 obscure pseudomorphs which may represent original olivine, so 
 that the afBnities of these rocks to the kentallenites is certainly 
 very close. As Messrs. Hill and Kynaston,* however, have 
 defined kentallenite as consisting of essential olivine and augite 
 with orthoclase, plagioclase, and biotite, these rocks are excluded 
 from the group. That they belong to the facies of the newer 
 granite, which are typically developed only in the Argyllshire 
 province, is at the same time perfectly clear. The amount of 
 alkali-felspar which they contain in addition to plagioclase is 
 variable, but is usually small. 
 
 In Lismore also the lamprophyric dyke rocks of the newer 
 granite series are well represented. They are composed mainly 
 of greenish-brown hornblende with orthoclase and plagioclase. 
 The hornblende is in idiomorphic crystals, and the felspar also 
 
 * Quart. Joum, Geol. Soc, vol. Ivi., p. 540, 1900, 
 
110 Newer Igneous Bocks'. 
 
 is usually more or less perfectly formed. When orthoclase pre- 
 dominates they belong to the vogesite group ; when plagioclase is 
 the principal felspar they are best described as spessartites. 
 Biotite frequently is visible in the sections, and a peculiar dyke 
 from Sron-na-craoibhe is a minette which is very rich in porphy- 
 ritic crystals of olivine and augite, but this can only be inferred 
 from the outlines of the pseudomorphs after these minerals, since, 
 as is frequently the case in these rocks, the material available for 
 study is so decomposed that minute description is precluded. 
 
 Of the later dykes which traverse the island in a west-north- 
 west direction, only one was examined microscopically. It is 
 a compact and fresh rock which might be considered a fine- 
 grained basalt, but contains no olivine, and consequently might 
 equally well be assigned to the andesites. Its essential con- 
 stituents are plagioclase felspar, brown augite, and magnetite, 
 but occasional phenocrysts of dark brown hornblende are present 
 ■also, and give the rock a somewhat abnormal character. It may 
 belong to the same group as the dykes mapped by Mr. Bailey 
 on the mainland op.posite. J. S. F. 
 
CHAPTER IX. 
 
 Sills and Dykes. 
 
 i. the series associated with the newer granite-masses 
 and lava-flows. 
 
 A glance at the map will show that an enormous number of 
 sills and dykes traverse the Lome volcanic plateau, the Ben 
 Oruachan granite, and the neighbouring area of the metamorphic 
 rocks. It will be seen that these intrusions are most numerous 
 over a belt of country, from four to five miles in breadth, 
 extending from the Ben Cruachan area, where it comprises the 
 high ground on both sides of Loch Etive, in a S.S.W. direction 
 as far as the south-west corner of the map. To the north-west of 
 this belt it appears that dykes and sills are comparatively rare, 
 while in the central portion of the large granite tract they are 
 altogether absent. To the east of Loch Awe the intrusions 
 belonging to this later series are likewise not merely so numerous 
 as further west, but appear to increase in the area about the head 
 o£ Glen Shira. To the north of the Strath of Orchy they occur 
 here and there throughout the region of the flaggy quartzose 
 schists, and become more numerous in the granite mass of Ben 
 Cruachan facies, in the north-east corner of the map. 
 
 These intrusions have a general N.N.E. and S.S.W. trend, 
 and this feature is especially well seen throughout the belt above 
 referred to, in which they attain their maximum development. 
 This trend is maintained irrespectively of the nature of the rock 
 in which the intrusion occurs, but it would appear to be closely 
 related to the trend of the planes of cleavage and schistosity in 
 the metamorphic rocks. In the eastern portion of the area, 
 however, as we go further from the granite mass, the trend of the 
 dykes and sills frequently varies considerably, an east and west 
 trend being sometimes observed, and sometimes a single intrusion 
 may have a more or less sinuous course. 
 
 By far the larger number of the dykes and sills consist of 
 porphyrites, and rocks of more or less intermediate composition 
 closely allied to them, and these types are best developed in the 
 great zone of intrusions stretching from the south-west comer of 
 the map to the Ben Cruachan granite. Together with these we 
 occasionally get quartz-porphyries, micro-granites, diorites, and 
 lamprophyres. The lamprophyres, however, are best developed 
 in the more outlying portions of the area. They are most 
 common in the region between the head of Loch Awe and Glen 
 Fyne, and are especially numerous in the area, about Glen Shira. 
 All these intrusions show close genetic relationships to one 
 
112 Sills and Dykes. 
 
 anotlier, and the various allied types are coniiected with 
 one another by intermediate varieties. They show 
 on the whole the same relationships to one another 
 as has already been shown to exist in the larger igneous 
 masses. They represent, with some few exceptions, the latest 
 phase of the intrusive activity of Lower Old Red Sandstone 
 times. 
 
 We will now give a more detailed account of these intrusions, 
 commencing with the more acid types. 
 
 (1 Quartz-Porphyries. — Dykes of quartz-porphyry are of 
 somewhat common occurrence in the Ben Cruachan granite on 
 both sides of Loch Etive, especially in the neighbourhood of 
 Bonawe. These usually consist of a matrix which is sometimes 
 granophyric and sometimes micro-crystalline or micro-granitic, 
 and which encloses phenocrysts of orthoclase, plagioclase, and 
 quartz, the latter occurring frequently in doubly-terminated 
 crystals. Occasionally phenocrysts of biotite may also be 
 detected, and in one case the alkali-felspar was found to be 
 fresh and sanidine-like [7606]. 
 
 In the metamorphic area quartz-porphyries have been observed 
 about three miles north-east of Kilchrenan, and occasionally on the 
 north-east side of the Pass of Brander. They are also of common 
 occurrence in the area of flaggy quartz- schists about the head of 
 Glen Strae. 
 
 In the volcanic area they are somewhat rare, but have been 
 observed cutting the andesitic lava flows close to Gorstain, south- 
 west of Taynuilt, and again on the south side of Glen Lonan. 
 These closely resemble the dykes found associated with the Ben 
 Cruachan granite, the dyke from near Gorstain consisting of 
 phenocrysts of quartz, felspar, and biotite, in a micro-poikilitic 
 ground-mass of felspar and quartz, which forms small halos 
 around the phenocrysts. In the Benderloch area sills 
 intermediate between quartz-felsites and porphyrites cut the 
 andesites on the south side of Beinn Lora. Occasionally an 
 intrusion has been seen to consist partly of quartz-porphyry and 
 partly of porphyrite, the two types appearing to pass gradually 
 into one another. A good example of this is seen a mile and a half 
 west of Taynuilt, where a portion of a large porphyrite dyke is 
 evidently of more acid composition, and is seen to contain 
 numerous phenocrysts of quartz, but no sharp line can be drawn 
 between the two. On the other hand, however, quartz-porphyry 
 may sometimes be seen to be intrusive in the porphyrites. Thus, 
 on the southern side of Beinn Duirinnis, a dyke of quartz- 
 porphyry lies for a considerable distance in contact with a 
 porphyrite, and in one place small tongues of the quartz- 
 porphyry are seen to be intruded into the porphyrite. There 
 can be no doubt, however, that there is no great difference in the 
 age of any of these intrusions, and that they all belong to 
 essentially the same general period of intrusion. 
 
 (2) Porphyrites. — The dykes and sills of porphyrite and 
 closely-allied varieties are best developed in the zone, .§ilready 
 
Sills and Dyhes associated with the Newer Granites and Lavas. 113 
 
 referred to, whicli stretches from the Ben Cruachan granite to 
 the south-west corner of the map. They are again seen in some 
 numbers in the north-east corner, whence they are continued 
 with a similar trend into Sheets 53 and 54. In describing these 
 rocks the term porphyrite will be used in its more recent sense, 
 as employed, for instance, by Dr. Teall in his description of the 
 dykes associated with the CrifEel and Dalbeattie granite masses. 
 We may therefore define porphyrite as a rock of intermediate 
 composition representing the dyke phase of an andesitic magma. 
 The essential porphyritic constituents are plagioclase felspar and 
 a ferro-magnesian mineral such as biotite, hornblende, and 
 occasionally augite. 
 
 These sills and dykes traverse the Ben Cruachan granite, the 
 lava-flows of Lome, and the metamorphic rocks. They may be 
 traced from the Lome plateau on the south-west side of the Pass 
 of Brander, across the River Awe, and thence through the inter- 
 vening zone of metamorphic rocks into the granite of Ben 
 Cruachan. 
 
 Throughout the whole area in which they occur these rocks 
 maintain a similar petrological facies, though frequently in the 
 granitic area they have the appearance of having consolidated at 
 a greater depth. The intrusions vary from a few feet to 30 
 yards or more in breadth, and a single dyke or sill will often 
 show very considerable variations in breadth, and may sometimes 
 be seen to die out somewhat abruptly. Two dykes may some- 
 times occupy the sam.e fissure and thus form a compound intru- 
 sion. The trend of the dykes is often remarkably straight for 
 considerable distances, but, on the other hand, it may often 
 diverge considerably from the normal, or assume a sinuous or 
 winding course. Branching dykes are common, and such 
 branches may occasionally anastomose with neighbouring intru- 
 sions. This feature is often seen among the dykes on the north 
 side of the Pass of Brander. 
 
 In the granite area the large majority of the porphyrites con- 
 stitute well-marked dykes, sills being commoner in the Lome 
 plateau and in the metamorphic area. They belong for the most 
 part to the hornblende- and biotite-porphyrites. They contain 
 phenocrysts of idiomorphic plagioclase, hornblende, and biotite, 
 while orthoclase and quartz are only rarely present as porphyritic 
 individuals, usually being only found in those rocks which 
 constitute intermediate varieties between the porphyrites and the 
 quartz-porphyries. The rocks vary in colour from dark grey to 
 almost brick-red, the latter tint, however, being probably merely 
 due to decomposition. The proportion of ground-mass to pheno- 
 crysts varies considerably, some varieties containing pheno- 
 crysts in such numbers as at first sight to resemble granites, 
 others again showing only a very small proportion of porphyritic 
 constituents. Plagioclase, usually oligoclase, is always the most 
 conspicuous phenocryst, occurring as simple crystals and groups. 
 Biotite is perhaps the commoner ferro-magnesian mineral, and 
 may occur either with or without hornblen/1^, while in some 
 
114 Sills and DyTces. 
 
 types the hornblende predominates. The ground-mass may be 
 micro- or crypto-crystalline, while in some cases a tendency to the 
 micro-graphic structure may be observed. Sometimes the 
 structure may be seen with the naked eye to be of a fine-grained 
 granitic type, so that one might almost call the rock, in such 
 cases, a granite-porphyry. These sills and dykes would probably 
 show on analysis very little difference in composition from the 
 normal type of the Ben Cruachan granite, the structural differ- 
 ences between the two being probably due merely to variation in 
 the conditions of consolidation of the original magma which was 
 their common source of supply. 
 
 The numerous porphyrite intrusions show considerable 
 variability both in texture and composition. Perhaps the com- 
 monest and most typical variety is a hornblende-biotite- 
 porphyrite, but every variety of rock may be encountered which 
 will unite this type with the quartz-porphyries on the one hand, 
 and with the diorites on the other hand. Thus, starting with 
 the biotite-bearing quartz-porphyries, an increase in the propor- 
 tion of the plagioclase felspar with a diminution of the orthoclase 
 and qtiartz gives us a quartz-biotite-porphyrite, and a slightly 
 more basic modification of this variety brings us to the normal 
 type of the biotite-porphyrites. The addition of hornblende 
 gives hornblende-biotite-porphyrites. In structure these latter, 
 especially in the case of the larger intrusions, are often inter- 
 mediate between porphyrite and diorite, and a transition from 
 these varieties to typical diorites is of common occurrence, and 
 sphene, in these as well as in the dioritic phase of the Cruachan 
 granite, is a common accessory. Similarly numerous connecting 
 links may be found between the diorites and the hornblende- 
 lamprophyres. Thus, just as in the main mass of the granitic 
 rocks every variety may be found between an acid granite and a 
 diorite, so among the numerous dykes and sills we have transi- 
 tional varieties connecting granite-porphyries and quartz- 
 porphyries on the one hand, and fine-grained diorites and horn- 
 blende-porphyrites on the other hand. In short, such strong 
 afl&nities appear to be thus indicated between the dykes and sills 
 and the granite that we may feel fairly confident in asserting that 
 they have both been derived from the same common magma, and 
 the variation in the injections shows that difierentiation had 
 already proceeded in the magma to a considerable degree previous 
 to its intrusion. 
 
 Before passing on to the other types of dyke- and sill-rock, it 
 will be of interest to describe briefly the microscopic characters 
 of some of the more interesting varieties of the porphyrites. 
 
 Some good examples of the more acid varieties are found in 
 the neighbourhood of Clais-gobhair, in the Blackmount. A 
 specimen from Meall Bhuidhe, to the east of the upper portion of 
 Glen Kinglass, shows abundant phenocrysts of felspar, biotite, 
 hornblende, and small quartz grains in a fine greyish matrix. 
 Under the microscope [8624] all the larger phenocrysts of felspar 
 are seen to consist of plagioclase, and to be in excess of ortho- 
 
Sills and Dykes associated with the Newer Granites aiid Lavas. 115 
 
 clase, which occurs here and there in smaller crystals. Quartz 
 occurs sparingly in small, rounded grains. The biotite occurs as 
 plates with ragged outline, and is also scattered throughout the 
 rock in groups and patches of small flakes. These groups are 
 frequently associated with the hornblende, and would appear to 
 be replacing it owing to some secondary action. The hornblende 
 is pale green and occurs in irregular patches and grains, without 
 any crystal outline, and the patches appear as a granular aggre- 
 gate between crossed nicols. Occasionally these patches have 
 a fairly well-defined form, but they are always associated witli 
 numerous small flakes of biotite and grains of magnetite. 
 Apparently there has been a gradual breaking down of the horn- 
 blende, and it is now partially replaced by secondary biotite and 
 magnetite. The original biotite and hornblende both 
 occasionally occur as inclusions in the felspar. The ground- 
 mass is a micro-crystalline quartzo-felspathic aggregate with 
 small disseminated grains of biotite and hornblende. Magnetite 
 is accessory. 
 
 Another variety of hornblende-porphyrite, with acid ten- 
 dencies, is common in the Ben Cruachan granite on the ridge to 
 the west of Stob Ghabhar, near Clais-gobhair. The felspar 
 phenocrysts sometimes range up to aji inch in length, while 
 elongated crystals of horjiblende and plates of biotite are easily 
 seen with the naked eye. The microscope shows [8623] that 
 some of the felspar phenocrysts are orthoclase, though the larger 
 proportion are plagioclase. 1'he hornblende is fresh and occurs 
 in well-defined elongated prisms. The biotite occurs in about 
 the same proportion as the hornblende, and frequently as plates 
 with hexagonal outlines. Quartz occurs as an accessory, but is 
 not visible to the naked eye, and only appears in occasional small 
 rounded grains. The ground-mass is micro-crystalline. It 
 shows much idiomorphic plagioclase, small greenish hornblendes, 
 and biotite, with interstitial felspar and quartz. This latter 
 variety appears to have closer affinities with the Glen Btive type 
 of granite than with that of Ben Cruachan. 
 
 Another example from the Blackmount is a hornblende- 
 porphyrite with afiinities to homblende-andesite. It shows 
 [9020] phenocrysts of clear plagioclase, hornblende, and 
 biotite. The plagioclase is often zoned, sometimes shows peri- 
 cline twinning, and occasionallv encloses the hornblende and 
 biotite. The hornblende is fresh, idiomorphic, and of the brown 
 variety. Some pale green fibrous (uralitic) hornblende also 
 occurs, which may possibly represent augite. The ground-mass 
 is micro-crystalline, and consists of small idiomorphic plagio- 
 clases, small biotite flakes, interstitial quartz, and possibly also 
 some alkali-felspar. Sphene, apatite, and magnetite are 
 accessories. 
 
 Varieties more basic than the normal have sometimes been met 
 with as intrusions in the Lome volcanic area. Thus, in the 
 Kilchrenan area, one of the sills of normal porphyrite consists in 
 part of a basic modification which shows phenocrysts of plagio- 
 
116 Sills and Bykes. 
 
 clase, brown hornblende, and malacolite, in a ground-mass of 
 lath-shaped plagioclases, chlorite, and iron-ores [7632]. 
 
 A good example of one of the basic types of porphyrite occurs 
 on the west side of Glen Nant, and may be termed a hyperstherie- 
 diorite-porphyrite. It contains [9652] numerous phenocrysts of 
 well-formed zonal plagioclase, with perhaps also some orthoclase 
 in addition, idiomorphic hypersthene weathered to bastite, and a 
 pale greenish augite, still mostly fresh. The ground-mass is 
 granular, and consists of hypidiomorphic plagioclase, with some 
 alkali-felspar and a little quartz. There are small granophyric 
 halos of micro-pegmatite around the felspars of the ground-mass. 
 Apatite and magnetite are accessory. A somewhat similar rock 
 occurs further to the south-west, on the north side of Lochan 
 Dubha. It consists of phenocrysts of idiomorphic plagioclase, 
 hypersthene, and augite, in a ground-mass of lath-shaped 
 plagioclases, granular augite, biotite, and magnetite. These 
 more basic varieties apparently have affinities with the hyper- 
 sthene-andesites on the one hand, and with the hyperite phase 
 of the diorites on the other hand. By a diminution of the more 
 basic constituents they pass gradually into the normal types of 
 porphyrite. 
 
 Other varieties 'again, by a diminution of the plagioclase pheno- 
 crysts and an increase in the proportion of the hornblende, show 
 affinities with the hornblende-lamprophyres. 
 
 (3) The Lam/prophyres and Diorites. — The lamprophyres con- 
 stitute a rather ill-defined group, which shade ofE on the one 
 hand into the porphyrites and on the other hand into the diorites. 
 They occur chiefly as dvkes, and sometimes as sills. They are 
 distributed over the entire area comprised in the present sheet, 
 but are not nearly so numerous as the porphyrites, and tend 
 rather to occupy the more outlying portions of the area in 
 respect of the granite mass of Ben Cruachan. An occasional 
 lamprophyre, it is true, cuts the Ben Cruachan granite, and a 
 peculiar basic variety occurs in the more central mass of G-len 
 Etive, but, apart from these instances, the intrusions of this 
 group occur mainly in the metamorphic area in the eastern and 
 south-eastern portion of the map. They are especially numerous 
 in the Glen Shira and Beinn Bhuidhe district, and here perhaps 
 belong rather to the intrusions associated with the Glen Fyne 
 granite than to those associated with the Ben Cruachan mass. 
 To the south-west of the Pass of Brander lamprophyres are of 
 comparatively rare occurrence in the Lome volcanic rocks and 
 in the metamorphic area on the north-west side of Loch Awe. 
 Outside the north-west margin of the Ben Cruachan granite an 
 occasional lamprophyre is seen in the area about Glen Creran. 
 
 As a rule, the lamprophyres are fairly uniform in their 
 behaviour and maintain a similar trend to that of the 
 porphyrites. They frequently, however, deviate from the normal 
 north-east and south-west trend, and in the area about the head 
 of Glen Shira are often seen to traverse the schists in almost any 
 direction. The intrusions seldom attain so great a breadth as 
 
Sills and Dykes associated with the Newer Granites and Lavas. 117 
 
 the porphyrites, nor, as a rule, can tliey be traced for so great a 
 distance, and they frequently varj' in breadth from place to 
 place. 
 
 The lamprophyres constitute a series showing a considerable 
 amount of variation. Frequently they are highly micace(;us, 
 and resemble typical mica-traps or minettes, and these pass 
 imperceptibly into other types containing a large proportion of 
 hornblende. These hornblende-lamprophyres constitute the 
 commonest type of the district, and frequently show transitions 
 to fine-grained diorites. More basic types also are of not un- 
 frequent occurrence, which show affinities to the kentallenites 
 and augite-diorites already described. 
 
 In the Ben Cruachan area intrusions which may be classed 
 with the lamprophyres only rarely occur. An occasional dyke, 
 however, allied to the diorite-porphyrites may be seen cutting 
 the granite on the south side of Beinn Dtiirinnis, above the Bon- 
 awe quarries. Specimens from these rocks show under the 
 microscope [7612] large aggregates of hornblende crystals in a 
 ground-mass consisting mainly of hornblende needles, idio- 
 morphic plagioclase, and iron-ores. In the central mass of Glen 
 Etive some peculiar basic dykes occur, with a general north- 
 west and south-east trend, which perhaps belong to basic types 
 of lamprophyre. They are narrow dykes, only a few feet in 
 breadth, of a black rock usually considerably decomposed. A 
 specimen from Coire nan Lair, on the south side of Ben Starav, 
 shows under the microscope [7762] green pseudomorphs after 
 olivine, abundant idiomorphic augite, brown basaltic hornblende, 
 biotite of the same colour as the hornblende, felspar (scarce), 
 magnetite, and some undetermined interstitial matter. The 
 hornblende in these rocks is identical with that which occurs in 
 the camptonites, and the rocks as a whole show strong affinities 
 to the camptonites and monchiquites. It is interesting to find 
 such basic rocks of this class cutting the later and more acid 
 phase of the granite, and it is probable that they represent basic 
 modifications of the underlying magma, intruded during the 
 later portion of the dyke-forming period. 
 
 Basic types of lamprophyre, though considerably less basic 
 than the above, are also met with in the metamorphic area 
 between the head of Loch Awe and Glen Fyne. A basic sill of 
 this type is well seen about a mile S.S.E. of Dalmally. It is a 
 dark grey,, almost black, medium-grained rock. Under tJie 
 microscope [8464, 8861, 8862] it is seen to show affinities to 
 some of the basic augite-diorites, allied to kentallenite. It is 
 exceedingly rich in pyroxene, while the proportions of biotite 
 and plagioclase seem to vary in different parts of the sill. The 
 augite is almost colourless and frequently shows good idio- 
 morphism. Besides the augite there are also some pseudomorphs 
 which appear to represent olivine. There is no hornblende. 
 The biotite occurs mostly in irregularly-shaped, elongated flakes, 
 which are seen to be moulded upon the plagioclase and to be 
 therefore of later formation. The plagioclase occurs occasionally 
 
118 Sills and Dykes. 
 
 as idiomorphic porphyritic individuals, which are usually rather 
 decomposed, but mostly as more or less lath-shaped individuals 
 in the ground-mass. Interstitially there is a small amount of 
 quartz, and a very small quantity of alkali-felspar may also be 
 present. Occasionally one sees an interstitial substance border- 
 ing some of the plagioclase crystals, and showing a micro- 
 structure suggestive of micro-pegmatite. 
 
 Again, two small lamprophyre intrusions of a basic type occur 
 on the slopes of Ceann Garbh, about two miles north-east of 
 Beinn Bhuidhe. These sills are similar in appearance, and 
 consist of a medium-grained dark grey rock, in which augite and 
 some plagioclase may be seen to occur porphyritically. Under 
 the microscope [7416 and 7417] a section is seen to consist of 
 fairly numerous and relatively large phenocrysts of augite, and 
 a few of plagioclase, in a ground-mass which is mainly felspathic. 
 The augite is idiomorphic, colourless, and frequently shows a 
 well-marked zonal structure. The ground-mass consists mainly 
 of small lath-shaped plagioclases, small augite grains, irregular 
 flakes of biotite, and chloritic alteration products. There is a 
 very small quantity of interstitial quartz, and magnetite is 
 accessory. These rocks also have close aifinities with the augite- 
 diorites of the same district. 
 
 Between these basic lamprophyres, rich in augite and the more 
 normal type of hornblende-lamprophyre, various intermediate 
 varieties occur. Thus in some of the intrusions the proportion 
 of augite is comparatively small, while a brownish hornblende, 
 of the type characteristic of the camptonites and spessartites, 
 makes its appearance, and the proportion of plagioclase increases. 
 A sill occurring about a mile north of Meall nan Tigearn illus- 
 strates this transition. A section shows [8834] a few chloritic 
 pseudomorphs after augite, brown hornblende in elongated 
 prisms, more or less idiomorphic plagioclase, and a small 
 quantity of interstitial quartz. The proportion of biotite varies 
 greatly in these sills, but its absence is unusual. With the 
 increase of the proportion of the hornblende the augite disappears, 
 and we have varieties of hornblende-lamprophyre which con- 
 stitute the commoner type of this class of intrusion. In many of 
 these hornblende is often porphyritic, and by the advent of 
 plagioclase phenocrysts and an increase in the felspar of the 
 ground-mass, they pass over into the hornblende-porphyrites. 
 
 It is difficult, however, to adopt any systematic elassification 
 for the lamprophyres of this district, since owing partly to their 
 inherent variety, partly to the great readiness with which they 
 decompose, and partly to their propensity to develop distinct 
 facies in different parts of their course, it is not easy to say 
 exactly to what sub-division any particular example is to be 
 assigned. Apart from the example already described which 
 occurs in the Glen Etive granite, it does not appear that any true 
 camptonites (in the sense employed by Rosenbusch) occur in this 
 district. The commoner types of lamprophyre, consisting 
 
Sills and Dykes associated ivith the Newer Granites and Lavas. 119 
 
 essentially of plagioclase and hornblende, may be termed 
 spessartite, and when biotite occurs in addition to the hornblende 
 we have biotite-spessartites, and the augite-bearing types, some 
 of which have already been noticed, may in the same way be 
 referred to as augite-spessartites and augite-biotite-spessartites. 
 At the same time, it will be understood that transitional varieties 
 between these types are common, and that any one variety may 
 sometimes pass into another even in the course of a single intru- 
 sion. The essential minerals, biotite, brown hornblende, plagio- 
 clase, and orthoclase, are often all present in the same specimen, 
 and may occur in any relative proportion. 
 
 Lamprophyres of th« plagioclase-hornblende series, or 
 spessartite type, are certainly the most typical and widespread 
 of the district. Members of the orthoclase-bearing series, such 
 as miiiettes and vogesites, are comparatively rare. The intru- 
 sions are, therefore, more representative of a granite-diorite 
 magma rather than a syenitic. 
 
 A good example of what may be termed a typical spessartite 
 occurs on the north side of Glen Orchy, N.N.W. of Inverlochy, 
 near Dalmally. It is seen under the microscope [8615] to con- 
 tain abundant idiomorphic brown hornblendes in elongated 
 crjj^stals, idiomorphic plagioclase, with some interstitial ortho- 
 clase and quartz, and accessory magnetite. Sills and dykes of 
 this type are common in Glen Shira, Glen Orchy, and on Beinn 
 na Sroine, between Glen Orchy and Glen Lochy. These rocks 
 have been previously described in some of the Annual Summaries 
 of the Geological Survey as camptonites, but, for the reasons 
 already given, it is considered here more advisable to refer to 
 them as spessartites. A varying proportion of biotite usually 
 occurs in addition to the brown hornblende, and occasionally the 
 former mineral may be in excess of the latter, in which case the 
 rocks may be classed rather with the kersantites. A dyke of 
 this latter type is seen to cut the andesitic lava-flows of Lome, 
 one and a half miles N.N.E. of the head of Loch Scamadale. 
 It contains [10134] a high proportion of biotite; the felspars 
 are decomposed, but consist largely of plagioclase, with some 
 interstitial quartz and micro-pegmatite. There are also some 
 pseudomorphs of chlorite, possibly after pyroxene. 
 
 A characteristic peculiarity of some of the lamprophyres is the 
 occasional occurrence in them of xenocrysts and xenoliths. We 
 may thus sometimes find rounded crystals of felspar or quartz, 
 and occasionally rounded fragments of the rock which the 
 lamprophyre has traversed. A small lamprophyre dyke, occui- 
 ring on the west side of Glen Creran, shows this peculiarity in a 
 marked degree. The weathered face is seen to be thickly 
 studded with quartzite fragments, and in places the rock is so 
 full of these fragments as to resemble a breccia. The fragments 
 are sometimes rounded, but often angular. The marginal 
 portion of the dyke is a breccia consisting of lamprophyre and 
 shattered quartzite. A similar phenomenon has been noted by 
 Mr. Grant Wilson in some of the intrusions in Glen Orchy and 
 
120 Sills and Dykes. 
 
 in Grien Suileag in tlie Locli Eil district.* The dyke in Glen 
 Creran only appears at the surface for about 100 yards, and dis- 
 appears helow the quartzite at either end. Possibly the 
 abundance of fragments may be explained by supposing them to 
 represent portions torn from the walls of the fissure, and carried 
 upwards by the lamprophyre magma, so that they would tend to 
 accumulate towards the limit of its intrusion. 
 
 As regards the age of the lamprophyres, it is evident from 
 their genetic relationships that they belong to the same period 
 as the porphyrites and quartz-porphyries, and are also closely 
 associated with the larger intrusions of diorite and kentallenite. 
 The majority of the intrusions of porphyrite, &c., are clearly 
 later than the large masses of granite, and the same may also be 
 said of some of the lamprophyres, as we sometimes find them 
 also cutting the granite. It is probable, however, that some, on 
 the other hand, are of earlier date than the granite, as in the 
 Beinn Bhuidhe district we find a lamprophyre cut by a vein of 
 granite close to the main mass of the Beinn Bhuidhe granite. 
 The microscope shows that distinct contact alteration has taken 
 place in the lamprophyre. Thus, a section taken close to the 
 contact with the granite vein shows that the lamprophyre now 
 consists essentially of small scales of contact brown mica and 
 water-clear plagioclase [8467]. Slightly further from the vein 
 there are still signs of contact action, though the original 
 crystals and crystalline aggregates of hornblende may be well 
 seen [8468]. Dr. Teall, who has examined these sections, 
 remarks that these specimens seem to remove all doubt as to the 
 contact-metamorphism of the lamprophyres. It is probable, 
 therefore, that the lamprophyres of this district represent 
 different phases of intrusion acting at different periods during 
 the general intrusive activity of the area. 
 
 In addition to the different types of lamprophyre already 
 described, we also find occasional dykes that should rather be 
 described as diorites than lamprophyres. These occur prin- 
 cipally in the Glen Orchy area, and in the region between Glen 
 Orchy and the great mass of granite to the west. Although 
 referred to as diorites, these rocks are frequently of decidedly 
 lamprophyric type, and in many cases are of the same composi- 
 tion as the spessartites, the only difference being that they are 
 coarser in texture. They thus consist essentially of plagioclase 
 and brown or greenish-brown hornblende, with varying propor- 
 tions of biotite and interstitial quartz and alkali-felspar. 
 Occasionally plagioclase, and sometimes also hornblende in 
 addition, may occur as conspicuous phenocrysts. 
 
 A few dyke-like intrusions of augite-diorite have also been 
 obse'^ed, as, for instance, on the northern slopes of Meall 
 Copagach, between Glen Strae and Glen Kinglass, which are 
 identical in every respect with the mass of augite-diorite already 
 described above, and occurring in the same part of the area, 
 
 * Summ. Prog. Qeol. Survey for 1 897, p. 66. 
 
Sills and Dykes — Hast and Vr'esi Dolerite Dykes. 121 
 
 slightly further to the east. But all these intrusions are so 
 closely associated genetically with the true lamprophyres that it 
 is evident that they merely represent slightly different phases of 
 the same magma. 
 
 Attention has already been drawn to the close connection 
 between the quartz-porphyries and porphyrites and the large 
 granitic masses. The close association, therefore, of the 
 lamprophyres with the diorites, augite-diorites, and kent- 
 allenites falls naturally and consecutively into the same series 
 of phenomena, and serves still further to bind together all these 
 varying intrusion's into one connected series. H. K. 
 
 Lismore. — The lamprophyre dykes on Lismore, though 
 usually having a north-east to south-west trend, are not 
 uniformly orientated like the majority of the dykes connected 
 with the newer granites of Northern Argyllshire. They are 
 irregular, frequently branching, or appearing as a series of 
 lenticular masses placed end to end, and can seldom be traced 
 for any distance. Some small amount of earth-movement seems 
 to have taken place since their injection, as they are frequently 
 broken up. The most frequently occurring type of rock is a dull 
 greenish or brownish fine-grained vogesite, shovsdng to the eye 
 more or less biotite and decomposing hornblende or augite. 
 
 A rather different type of vogesite containing long idiomorphic 
 pointed prisms of hornblende is found chiefly on Eilean Ramsay 
 and the neighbouring islets. It forms dykes ten to twelve feet 
 broad, which trend in various directions. The spessartites are 
 represented by a biotite-spessartite dyke intruded into the 
 quartzite at the north end of Eilean Dubh, whilst the mica 
 lamprophyres are represented by the miuette from Sron-na- 
 Craoibhe. 
 
 The lamprophyre dykes traverse the hornf els zone surrounding 
 the diorite intrusions without being themselves altered, and 
 small veins of lamprophyre were noticed in the diorite itself. 
 Without doubt they belong to the numerous class of dykes 
 which followed the intrusion of the newer granites in Ar,(<yll- 
 shire. None of the ordinary porphyrite dykes have been found 
 on Lismore, but it has been noticed in other districts that the 
 lamprophyre dykes become relatively abundant as the 
 porphyrites diminish in number at a distance from the large granite 
 masses. H. B. M. 
 
 ii. EAST AND WEST DOLERITES. 
 
 This series of intrusions can hardly be said to be very well 
 represented upon the present map. Dykes of the east and west 
 type are perhaps best seen in the neighbourhood of Dalmally. 
 They enter the eastern portion of the sheet in Glen Lochy, and 
 are continued westwards as far as the southern flanks of Ben 
 Oruachan and the Pass of Brander. They occur as far south as 
 
122 East and West Dolerite Dykes — Tertiary Dykes. 
 
 the head of Glen Shira, but do not extend further north than 
 Beinn na Sroine, between Glen Lochy and Glen Orchy. 
 
 They constitute broad and well-marked dykes, which may 
 often be followed for considerable distances with a general east 
 and west trend. They occasionally form a depressed feature in 
 the rocks which they traverse, as is seen in the case of the dyke 
 on the southern slopes of Beinn a Bhuiridh, but more usually 
 they constitute conspicuous craggy features, so well seen on the 
 hill sides on the north side of the Strath of Orchy. In the Ben 
 Cruachan area these dykes sometimes apparently coincide with 
 lines of east and west dislocation in the granite, and occupy 
 some of the depressions or cols between the more elevated portions 
 of the mountain-ridges. 
 
 Petrologically these dykes consist of medium-grained to coarse 
 dolerites. They are never vesicular, as is so often the case with 
 the dolerite dykes of Tertiary age. They are relatively rich in 
 plagioclase, and belong rather to the typical augite dolerites than 
 to the more basic olivine-bearing group. 
 
 iii. TERTIARY DYKES. 
 
 The dykes of Tertiary age are entirely confined to the western 
 portion of the map. They are found in Lismore and in the area 
 to the south of Oban. The normal trend of these dykes, unlike 
 those of earlier age, is north-west and south-east, though 
 occasional deviations from the normal direction are not uncom- 
 mon. In the south-western comer of the map there is a zone of 
 Tertiary intrusions of great complexity. In this zone the dykes 
 are exceedingly numerous and vary greatly in size. They range 
 from basic to acid, and compound intrusions are very common. 
 
 a. Dolerites and Basalts. — By far the greater proportion of the 
 Tertiarj' dykes consist of dolerites and basalts. In the district 
 extending from Oban to the area about Loch Scamadale they 
 are exceedingly numerous. They are subject to local augmenta- 
 tions of number, and in some places become so plentiful that it 
 is often difficult to represent them all on the map. The dykes 
 vary in size from two or three feet in breadth, and sometimes 
 even less, up to 50 feet, or sometimes more; the smaller intru- 
 sions, however, are by far the more numerous. 
 
 In the area about Loch Scamadale and about the head of liOch 
 Peochan, the dykes are often of a more yielding nature than the 
 rocks which they traverse. The material of the dyke 
 decomposes and weathers away more rapidly than the surround- 
 ing rocks, so that the site of the dyke forms a narrow cleft or 
 gorge. Such gorges and ravines, due mainly to the disintegra- 
 tion of the basic dykes, are very common on both aides of Loch 
 Scramadale, and are usually occupied by streams flowing down 
 from the higher portions of the hills. The general trend of 
 these dykes is north-west and south-east, but local variations of 
 trend are common, and some of the dykes have often a move or 
 
Tertiary Dykes. 123 
 
 less sinuous course. The normal trend, again, may someiimes 
 be diverted into a completely different direction along pre- 
 existing lines of faulting. This is well seen in the Eas Ruadh, 
 on the north side of Loch Scamadale, where the Tertiary dykes 
 have been diverted into a north-easterly direction along an older 
 line of fault in the Lome volcanic rocks. 
 
 The larger dykes frequently send off smaller branches into the 
 surrounding rock, or, again, several small neighbouring dykes 
 may sometimes be followed until they unite and form a single 
 intrusion. Compound dykes are of common occurrence, and 
 may sometimes consist of as many as five, six, or even seven 
 separate intrusions running side by side in the same fissure. 
 Many of these compound dykes consist of intrusions differing 
 markedly from one another in composition, and varying from 
 olivine-basalts to acid dykes allied to rhyolite. The more ac^d 
 intrusions are invariably the younger, and these are frequently 
 seen to split up the more basic. So also in the compound dykes, 
 consisting of thoroughly basic rocks, the more acid varieties 
 almost invariably split up the more basic. These composite 
 dykes will be referred to again in connection with the acid 
 intrusions. 
 
 The basic dykes consist of basalts and dolerites of slightly 
 varying composition. They usually contain olivine, but the 
 proportion present varies considerably, some varieties containing 
 a much larger amount than others. The dolerites, in addition 
 to the olivine, contain grains and ophitic plates of augite, the 
 ophitic structure being frequently well developed. Vesicular 
 varieties are also common. 
 
 Many of the finer-grained dykes have a spherulitic structure 
 developed along their outer margins, the spherules being fre- 
 quently as large as peas and often grouped in rows, like those 
 observed by Mr. Clough in Skye. These spherulitic margins 
 are frequently two or three inches in breadth. 
 
 Porphyritic varieties are not common in the area described, 
 but have occasionally been met with. H. K. 
 
 Lismore. — A series of basaltic dykes trend across the island 
 from W.N.W. to E.S.E. or occasionally from west to east, and 
 traverse both the diorite bosses and the lamprophyre dykes. 
 Usually between two and three, but never more than six feet 
 broad, they are both the most numerous and most regular of the 
 dykes. They may often be traced as low grass-covered ridges 
 across the island. They vary from dark bluish compact rocks 
 with a conchoidal or platy fracture to fine-grained greenish- 
 black rocks, which are probably all olivine-basalts. 
 
 Dykes of andesitic basalt containing corroded prisms of horn- 
 blende, which sometimes reach a length of 1\ inches, Avere 
 noticed in the cliff 150 yards north-east of Port Maluag and on 
 the shore a quarter of a mile north-west of Sron-an-Craoibhe. 
 The Port Maluag dyke cuts a fine-grained olivine-basalt. 
 
 H. B. M. 
 
124 Tertiary Dykes. 
 
 Ardmucknish Area. — The Tertiary dykes of the promontory of 
 Ardmucknish, on the west side of the Benderloch, present some 
 interesting peculiarities which do not appear to be repeated 
 among the numerous intrusions of the same age that occur in 
 the south-west portion of the map. Two of these dykes have 
 been described in detail by Mr. Bailey in a paper which was read 
 to the Edinburgh Geological Society.* Both are fine-grained 
 rocks with a somewhat vitreous lustre, and under the microscope 
 show variolitic structure in great perfection. Stellate groups of 
 long narrow felspars and plumose growths of augite lie in a small 
 quantity of clear glassy base (PI. VIII., lig. 1). These rocks are 
 free from olivine, and contain numerous small vesicles filled with 
 calcite. One of the varifelitic dykes occurs near Ardentiny, at the 
 north end of Ardmucknish ; the other cuts a much larger dyke of 
 decomposed dolerite in the north-east corner of Camas an Fhais. 
 Both rocks are remarkably fresh, and Mr. Bailey concludes that 
 there can be little doubt of their Tertiary age. 
 
 The other Tertiary dykes of this area which are represented in 
 the Survey's collection are dolerites, camptonites, and monchi- 
 quites. 
 
 Several dykes of olivine dolerite accompany the variolite above 
 mentioned as occurring at the north-east coi-ner of Camas an Phais. 
 They are rather fine-grained and non-porphyritic ; olivine is present 
 only in minute granules. In most of them there are interstitial 
 patches of brown glass. The augite has a purplish colour and 
 forms little wedge-shaped areas between the long rectangular 
 felspars. Iron ores and apatite are present in some amount, but 
 neither analcite nor biotite. Other rocks having these characters 
 occur in difierent parts of Ardmucknish. They represent a normal 
 type of Tertiary olivine dolerite, and differ in several respects from 
 those containing biotite, hornblende and analcite, which occur along 
 with them. 
 
 A dyke of well-preserved dolerite is found on the shore just 
 south of Port a Bhuiltin. It contains many fresh olivines, 
 highly idiomorphic, and often a millimetre in diameter, along 
 with phenocrysts of augite in a ground-mass of augite, plagio- 
 clase, and magnetite. The pyroxene is usually zonal, and often 
 shows hour-glass structure. Its colour is purplish-brown, but, 
 especially towards its margins, a strongly-m^arked violet tint is 
 noticeable, together with a feeble dichroism. The phenocrysts 
 are idiomorphic and often composite (glomero-porphyritic) ; 
 the smaller crystals of the second generation are less perfectly 
 shaped, though not unfrequently exhibiting traces of crystalline 
 form. The structure, however, does not approach the ophitic. 
 The colour of the augites in the ground-mass resembles that oi 
 the outer zones of the phenocrysts. The felspars are all small 
 and lath-shaped ; decomposition is more advanced in them than in 
 
 * E. B. Bailey — "On the Occurrence of Two Spherulitic ("Variolitic") 
 Dykes in Ardmucknish, Argyll," Trans. Edin. Q-eol. Soc, vol. viii., p. 363, 
 1905. 
 
Camptonites and Monchiquites. 125 
 
 any of the other minerals; they become cloudy with secondary 
 mica and zeolitic minerals. Turbid, nearly isotropic areas 
 appear around them ; these may be a devitrified pale glassy base, 
 but more probably are granular analcite. This mineral may be 
 regarded as secondary, as it is usually mixed with calcite and 
 other alteration products, and sometimes forms pseudomorpLs 
 after the felspars. As the rest of the rock is very fresh, it is 
 perhaps due to pneumatolytic changes immediately following 
 consolidation. The magnetite which forms small octahedra 
 in the ground-mass has often narrow rims of dark brown biotite. 
 The appearance of interstitial analcite in this rock indicates 
 affinities to certain varieties of teschenite. It has also close 
 relationships to the camptonites and monchiquites of this area. 
 
 Several dykes of caniptonitic character are known from 
 Ardmucknish (PI. VIII., fig. 2). One is found a half-mile south 
 of Ardentiny ; another occurs on the north-west corner of Oamas 
 an Fhais, and there are others in the peninsula. These 
 are fine-grained rocks with small phenocrysts of augite and 
 olivine. The latter mineral is usually decomposed ; the porphy- 
 ritic augite has pale brown and greenish-brown centres and 
 darker violet margins, resembling the augite of the dolerite 
 above described. The ground-mass of these rocks contains 
 plagioclase, augite, and magnetite. The felspar forms more or 
 less perfect laths, and is partly labradorite, but has often zonal 
 structure with extinction angles diminishing towards the outer 
 borders. The pyroxene of the second generation also is remark- 
 ably idiomorphic, its short stout prisms yielding good cross 
 sections. Magnetite in octahedra and apatite in very narrow 
 needles abound in the ground-mass. Analcite appears also, 
 partly with chlorite aiid calcite in small cavities, partly lying 
 between the felspars and other minerals like a ground-mass. 
 
 The affinities of these rocks with the camptonites are best 
 shown by the abundance of dark brown, strongly pleochroic 
 hornblende, and biotite of very similar colour and still more 
 intense pleochroism. Both minerals occur principally in very 
 good crystals, the biotite as thin plates with hexagonal cross 
 sections, the hornblende in prisms with the typical six-sided 
 transverse sections and elongated parallel to the principal axis. 
 Biotite on the whole preponderates over hornblende. The 
 amphibole often surrounds crystals of augite in parallel growth, 
 being always later in crystallisation. The peculiar " ocellar " 
 patches, consisting of subradiate felspar and hornblende, so 
 common in this group of rocks, occur in the dyke from Camas 
 an Fhais. Their centres are occupied by analcite and calcite, 
 and may originally have had a miarolitic cavity. 
 
 Another camptonite dyke occurs on the north-east corner of 
 Sailean Sligenach. It is more coarsely crystalline and less dis- 
 tinctly porphyritic than the two others. Olivine has been present 
 but not in great quantity, and is now weathered to calcite and 
 serpentine. This rock contains abundance of dark brown horn- 
 
126 Tertiary Dykes. 
 
 blende and violet-brown augite in perfectly idiomorphic crystals. 
 Parallel growths of these minerals are very frequent, the 
 amphibole being peripheral. Plagioclase, iron oxides, and 
 apatite form the rest of the rock, and calcite, chlorite, and 
 analcite occupy spaces probably of miarolitic character. The 
 abundant analcite suggests close affinities to the teschenites. The 
 felspar includes labradorite and probably also andesine. Ocellar 
 structure occurs also in this rock. 
 
 The analysis of the camptonite from Sailean Sligenach, 
 Ardmucknish, is given below with several others for comparison. 
 
 I. II. III. 
 
 SiO^ 42-22 39-13 40-60 
 
 TiOj - 2-49 4-02 4-20 
 
 Al^Og - 10-62 11-38 12-55 
 
 PejOg 4-74 7-33 5-47 
 
 Or^Og - -10 
 
 V^Og -05 
 
 FeO' 6-18 8-13 9-52 
 
 MnO -50 -42 
 
 (CoNi)O - -05 
 
 GaO - 14-80 11-77 10-80 
 
 BaO - - -04 
 
 SrO - nt.fd. 
 
 MgO - - 8-68 8-64 896 
 
 K2O 1-41 1-93 1-19 
 
 Na^O 2-46 2-47 2-54 
 
 LigO ?tr. 
 
 H2O(atl05°) - -50 2-871 
 
 H^O (above 105°) 1-16 ... / 
 
 2-28 
 CO/ 3-57 2-41 2-68 
 
 P,0, - -73 
 
 •12 
 
 100-42 100-50 100-79 
 
 I. — Sailean Sligenach, Argyllshire. (Anal., E. G. Radley.) 
 II. — Hoxa, Orkney. (Flett, Trans. Boy. Soc. Edin., vol. xxxix., p. 387.) 
 III. — Gran, Norway. (Brogger, Quart. Jour. Geol. Soc, vol. 1., p. 15.) 
 
 More basic dykes than the above, containing principally augite 
 and olivine, occur also in the peninsula. One of these is seen 
 about 400 yards north of Fionn-aird and crosses also Garbh- 
 aird, running in an east-south-east direction. It is about five 
 feet broad, but splits up into three dykes where it reaches the west 
 shore of Camas an Fhais. Through most of its course it is fine 
 grained and much decomposed, but specimens taken from the 
 dykes at the west side of Camas an Fhais are in a good state 
 of preservation. The slides show small perfectly-fresh olivines, 
 somewhat rounded by corrosion, and minute prisms of brownish 
 augite, lying in a turbid, almost completely isotropic ground- 
 
Gamptonites and Monchiquites . 127 
 
 mass, which has the appearance of analcite. In this translucent, 
 finely-granular matrix, minute subradiate fibres are seen with 
 high powers ; some of these have a weak birefringence, and may 
 be weathered felspar. Iron oxides are abundant, and are mostly 
 surrounded by narrow borders of dark brown biotite. A few 
 minute prisms of brown hornblende may also be detected in the 
 slide. In some parts the groundmass is a pale brown glass, which 
 can be distinguished from analcite by its higher refractive index. 
 Rounded patches of analcite occur in many specimens of this rock. 
 They are clearly secondary, as they often contain idiomorphic 
 crystals of calcite or dolomite. It is possible, however, that they 
 are pseudomorphs after leucite, a mineral occasionally found in 
 rocks of this group. 
 
 The special feature of this rock which is most interesting is the 
 presence of large, rounded, corroded crystals of anorthoclase or 
 soda microoline. These show no crystalline faces, and have a 
 reddish-pink colour with two excellent cleavages. Under the 
 microscope the microcline twinning is very prominent. In parts 
 the sections of this mineral are entirely transparent, and show 
 the microcline structure only very faintly ; other parts have a 
 turbid or granular character with less transparency, but in these 
 the twinning is much more conspicuous and the double refraction 
 stronger (PI. VIII., fig. 3). With careful examination, however, 
 the twin banding can be traced continuously from the turbid into 
 the transparent parts of the mineral. The origin of these differ- 
 ences in optical character is not easily explained. They are known 
 to occur also in the anorthoclase of the rhomben-porphyr which 
 has been found on Kilimandjaro in Africa.* 
 
 Small cavities in these felspars are filled with calcite and 
 chlorite. Larger crystals of dark brown biotite are also included 
 in the anorthoclase. They are much corroded and surrounded 
 with dark borders full of finely granular magnetite. Their out- 
 lines also are rounded, though traces of the original perfectly 
 idiomorphic outlines are very clearly discernible. 
 
 Monchiquite dykes with large anorthoclases are known to me 
 only from one locality, the east of Fife, where they occur in 
 some number around Elie. They have been described by Sir 
 Archibald Geikie,t and the analysis prepared by Dr. Heddlet 
 shows that the felspars belong to anorthoclase or a variety of 
 orthoclase very rich in soda. Their optical properties are the same 
 as those of the crystals from Camas and Fhais, and the rock in 
 which they occur also is a monchiquite like that of Ardmucknish. 
 
 * Finck, L. — "Die Rhombenporphyre des Kilimandsharo " Festschrift, 
 H. Bosenbusch, p. 379 (1906). 
 
 f Memoirs of the Geological Survey of Scotland — "The Geology of East 
 Fife," by Sir Archibald Geikie, p. 240 (1902). 
 
 I ileMle— Mineralogy of Scotland, vol. ii., p. 2, anal. 8 (1901). 
 
128 
 
 
 Tertiary Byhes 
 
 
 
 Aiialyses 
 
 of Moiichiquite Dyhe-roalcs. 
 
 
 
 I. 
 
 II. 
 
 III. 
 
 SiO, 
 
 
 43-55 
 
 46^48 
 
 43-74 
 
 TiO^ 
 
 
 2-52 
 
 0-99 
 
 2-80 
 
 AI2O3 
 
 
 14-60 
 
 16-16 
 
 14-82 
 
 Fe.Og 
 
 
 5-34 
 
 6-17 
 
 2-40 
 
 FeO- 
 
 
 7-46 
 
 6-09 
 
 7^52 
 
 MnO 
 
 
 ■55 
 
 
 ... 
 
 Co(Ni)0 
 
 
 •05 
 
 
 
 BaO- 
 
 
 •27 
 
 ... 
 
 
 SrO 
 
 
 nt. fd. 
 
 
 
 CaO- 
 
 - 
 
 7^94 
 
 7-'35 
 
 id^'si 
 
 MgO 
 
 
 4-77 
 
 4-02 
 
 6^98 
 
 K^O 
 
 - 
 
 1-40 
 
 3-08 
 
 2-90 
 
 Na,0 - 
 
 
 5-01 
 
 5-85 
 
 3-08 
 
 Li^O 
 
 
 Tr. 
 
 
 
 HgO (at 105°) 
 H2O (above 10£ 
 
 n 
 
 -63 \ 
 4-58/ 
 
 4-27 
 
 2-94 
 
 P2O5 
 
 
 •88 
 
 t • • 
 
 0-64 
 
 FeS, 
 
 
 •31 
 
 
 S 0^10 
 
 CO3 
 
 
 . -56 
 
 6-45 
 
 1^50 
 
 
 100^42 
 
 100-91 
 
 100-23 
 
 side of Camas an 
 
 Fhais, 
 
 , Argyllshire 
 
 !. (Anal.. 
 
 , E. G. Radley.) 
 
 II. Sta Cruz Bahn, Rio de Janeiro. 
 III. Rio do Ouro, Serra de Tingua, Rio de Janeiro. 
 
 A large anorthoclase phenocryst was broken out of this rock, 
 pulverised, and washed with very dilute hydrochloric acid to free it 
 from adherent calcite. It was then passed through a solution of 
 borotungstate of cadmium. The specific gravity of the material 
 selected for analysis was 2-608. The impurities present were a 
 small amount of chlorite and iron oxide. 
 
 I. II. III. 
 
 SiO„ 65-83 1-0898 63^07 
 
 AI2O3 19-97 -1954 18-69 
 
 FooO., -75 -0047 2-47 
 
 BaO- - - -53 -00345 MnO -06 
 
 SrO -29 -0028 
 
 OaO - -22 -0039 2-20 
 
 MgO -05 -0012 
 
 K„0 3-10 -03287 662 
 
 Na^O 8-70 -1401 5'50 
 
 H„0 (at 105") ^07 
 
 HgO (above 105°) ^18 ^0099 1-39 
 
 99-69 ... 100-00 
 
 I. Anorthoclase felspar in raonchiquite dyke, west side of Camas an Fhais, 
 Ardmucknish, Argyllshire. (Anal., Dr. W. Pollard.) 
 II. Molecular ratios of above. 
 III. Sanidine (anorthoclase), Kinkell, Fife (Heddle, Mineralogy of Scotland,, 
 ii., p. 2, 1901). 
 
Contact Alteration by the Tertiary Dykes. 129 
 
 The analysis shows that the Ardmucknish felspar contains 18'38 
 per cent, of potash felspar, 73'80 per cent, of soda felspar, and 3"31 
 per cent, of lime (with barium and strontium) felspar. There is 
 present also a small amount of kaolin (about 1-^ per cent.). 
 
 The occurrence of dykes of olivine dolerite, camptonite, and 
 monchiquite in this part of Argyllshire is of great interest. There 
 can be little doubt that these rocks are of Tertiary age ; they very 
 closely resemble the similar dyke rocks of the Orkneys, which also 
 are believed to be Tertiary.* They show also that though no 
 nepheline syenites of this period are known to occur in the West 
 of Scotland, camptonites and monchiquites are fairly widespread, 
 but not common. A similar assemblage is found also in the island 
 of Scarba, about twenty-five miles to the south-west of Ardmuck- 
 nish. The teschenitic character of some of those dykes {e.g., 
 Sailean Sligenach) recalls the Tertiary teschenites of Arran,t 
 described by Mr. Harker, and the nepheline-bearing teschenite of 
 (Jrawfordjohnl (which also is believed to belong to this epoch of 
 intrusion). 
 
 The dyke of camptonite at Sailean Sligenach produces striking 
 contact alteration in the rocks into which it is intruded. These 
 are silky greyish or greeuish-grey phyllites, consisting of chlorite 
 and muscovite in very small scales arranged in parallel fashion 
 among lenticular grains of quartz. The foliation is very pro- 
 nounced, and is intersected by a slight crumpling. The rock con- 
 tains also minute grains of zircon and iron oxide (Fig. 5, PI. VIII.). 
 At the immediate contact with the dyke this phjdlite is changed 
 to a black vitreous substance (buchite, basal t-jaspis), which has 
 much of the appearance of a pitchstone, but is minutely speckled 
 with small white or grey spots. Under the microscope it is seen 
 that the whole rock has been fused, and from the resulting glass, 
 which is of a pale brown colour in thin section, crystals of 
 cordierite, augite, magnetite, and spinel have separated. The 
 magnetite is in exceedingly small and perfectly-formed 
 octahedra, and is often grouped in clusters with transparent 
 dark green crystals of similar shape and equally perfect. These 
 latter have a very high refractive index, and must be assigned to 
 spinel. The augite is in very long and narrow prisms, the ends 
 of which are pointed, frayed, or forked. There is often a funnel- 
 shaped glass cavity passing inward from the ends of the crystal, 
 and when this is present at both terminations the crystal in 
 longitudinal section has some resemblance to a capital letter H. 
 Longitudinal and cross fractures are very common in these augite 
 prisms. Their colour is dark green, and they have a distinct 
 dichroism, best seen in the transverse sections. Referred to the 
 direction of their elongation the sections are optically positive, 
 and the scheme of pleochroism is Z — dark green, Y — paler green, 
 
 * Flett, loc. cit. 
 
 i Memoirs of the Geological Survey of Scotland — "The Geology of North 
 Arran," p. 112 (1903). 
 \ A. Lacroix, "Comptes Rendus," oxxx., p. 1271 (1900). 
 I 
 
130 Tertiary Dykes. 
 
 X — brownish and pinkish-green. Extinction angles as high as 
 40 degrees were measured. Transverse sections have nearly, 
 square or rectangular outlines, and extinguish parallel to their 
 sides ; hence the prism faces are absent, and the bounding forms 
 are the pinakoids. A few, however, show the typical octagonal 
 outlines of pyroxene. Very often the corners of the crystals 
 have grown outwards so rapidly as to form projecting spikes, 
 while depressions correspond to the centres of the pinakoidal 
 faces. The average diameter of these microliths is one or two 
 hundredths of a millimetre ; their length may be three-tenths of a 
 millimetre. 
 
 The cordierite is the most conspicuous mineral in the slides. 
 It is colourless and transparent except for crowds of enclosures of 
 spinel, &c. I have not been able to detect any dichroism in this 
 mineral, even after subjecting it to a red heat for some minutes. 
 The crystals are rarely simple, but are mostly penetration twins 
 on the plan usual in cordierite, and have the outlines of hexagonal 
 prisms (Fig. 6, pi. VIII.) Cross sections in polarised light are 
 resolved into a number of sectors (often three or six), which are 
 beautifully clear when a selenite plate is inserted. Longitudinal 
 sections are often characterised by a faint striping parallel to the 
 long axes of the crystals, but all the bands have a straight extinc- 
 tion. A fairly good vertical cleavage can often be remarked in these 
 pseudo-hexagonal prisms. They exhibit a strong tendency to 
 aggregate in clusters which are often parallel, or at times sub- 
 parallel growths. Their outlines are sometimes perfectly hexa- 
 gonal, but very frequently the edges of the prisms show depres- 
 sions which indicate the complex structure of the growths. 
 
 The other elements of this rock, on account of their small size, 
 are mostly indeterminable. They include colourless prisms 
 (perhaps felspar) , which occur in felted masses among the clusters 
 of cordierite ; verji- minute globulites and microliths, which some- 
 times render the brown glass quite turbid ; and small curved and 
 branching transparent trichites (Fig. 5, pi. VIII.) There are also 
 rounded spots, often with brown margins and composed of crypto- 
 crystalline or spherulitic aggregates. These are, perhaps, steam 
 cavities filled with siliceous infiltration products. 
 
 The specimens from contact with the dyke rarely show any 
 grains of quartz, and these are invariably in a highly corroded 
 condition, being merely residues of original grains with con- 
 cavities on their surfaces where they are being attacked by the 
 surrounding glass. The vitreous base is often flecked with spots 
 and streaks of darker brown. Swarms of dark grains of 
 magnetite and spinel sometimes render it nearly opaque ; they 
 often follow curved lines recalling the original foliation of tne 
 rock. The glass surrounding the partly-dissolved quartz grains 
 is usually paler than elsewhere. 
 
 Specimens taken six inches from the edge of the dyke contain 
 much clear glass with well-crystallised cordierites and dark 
 swarms of magnetite and spinel, but contain a considerable 
 
Contact Alteration by the Tertiary Dyhes. 131 
 
 amount of undissolved quartz. At one foot from the dyke the 
 quartz is almost as abiindant as in the unaltered rock, but around 
 it lies a turbid brownish substance, semi-opaque, and iilled with 
 innumerable small grains which seem to be magnetite and spinel ; 
 at any rate some are transparent, while others are black and 
 opaque. There is no cordierite, and the chlorite and muscovite 
 of the original rock are no longer traceable. At two feet from 
 the contact, glass, spinel, magnetite, and probably augite micro- 
 liths are visible, but there is a large amount of quartz partly 
 corroded. In fact, this specimen is rather more altered than the 
 previous one. Three feet from the contact the rock is a phyllite 
 with muscovite, quartz, chlorite, and iron oxides, and shows little 
 evidence of alteration under the microscope, but in the hand 
 specimen has a darkened and indurated appearance. At five 
 feet the same changes are visible, but in a less degree, and at six 
 feet from the igneous rock the phyllite has quite its normal 
 character. 
 
 Changes of a similar nature have been induced by basic sills 
 in some of the Torridonian grits of the Isle of Soay. They have 
 been described by Mr. Harker,* who found that the sediments 
 were vitrified and contained crystals of magnetite, cordierite, and 
 a pyroxenic mineral. The cordierite formed small six-sided 
 prisms, but did not show multiple penetration twinning. The 
 same phenomena are widely known in connection with the 
 Tertiary igneous rocks of the Continent,t and have been described 
 also in shales which had been fused by contact with ignited seams 
 of lignite. + 
 
 In order to determine whether there had been any transfusion of 
 material from the igneous dyke into the surrounding phyllites, 
 which might be held to explain the ease and completeness with 
 which fusion took place, chemical analyses were prepared by 
 Dr. W. Pollard of the fused phyllite in contact with the camptonite 
 and of the unmodified phyllite at a distance of six feet from the 
 margin of the dyke. These are given below, and difier very little 
 from one another. The vitrified phyllite is slightly richer in soda 
 (0'76 per cent.), but is poorer in potash (1-42 per cent.). Hence 
 there seems no reason to believe that there has been any transfer 
 of alkalis. It is curious to note that the "buchite" is richer in 
 combined water than the other, and this must be explained by 
 supposing that the glassy base, so abundant in the former, contains 
 a certain amount of water. 
 
 * Memoirs of the Geological Sv/rvey of Scotland — "The Tertiary Igneous 
 Rocks of Skye," by A. Harker, p. 245 and pi. xxi. (1904V 
 
 t A. Lacroix — " Les Enclaves des Roches Volcaniques " (1893). F. Zirkel 
 - "Lehrbuch der Petrographie," B. III. (1894). 
 
 IE. L. Berstin — "Note on Baked Clays and Natural Slags in Eastern 
 Wyoming," Journal of Geology, vol. xiii., p. 408 (1905). 
 
132 
 
 
 Tertiary 
 
 By1f.es. 
 
 
 
 
 I. 
 
 II. 
 
 SiO^ 
 
 
 62-01 
 
 58^63 
 
 TiO, 
 
 
 1-02 
 
 1-01 
 
 Al.O,, 
 
 
 17-78 
 
 18-59 
 
 Fe^Og 
 
 
 1-25 
 
 1-93 
 
 FeO 
 
 
 6-63 
 
 6-46 
 
 MnO- 
 
 
 •49 
 
 •88 
 
 (OoNi)O 
 
 
 •04 
 
 ■03 
 
 OaO 
 
 
 •42 
 
 -56 
 
 BaO - 
 
 
 •05 
 
 ■06 
 
 SrO - 
 
 
 tr. 
 
 tr. 
 
 MgO- 
 
 
 2-04 
 
 '2-62 
 
 K^O- 
 
 
 3-43 
 
 2-01 
 
 Na„0 
 
 . 
 
 1-84 
 
 2^58 
 
 Li,0 
 
 
 tr. 
 
 ?tr. 
 
 H^O (at 105 
 
 °) - 
 
 •08 
 
 1-38 
 
 HgO (above 
 
 105°) 
 
 3-02 
 
 4^45 
 
 P2O5 
 
 . 
 
 •11 
 
 ■08 
 
 CO, - - 
 
 
 nt. fd. 
 
 nt. fd, 
 
 S. - 
 
 - 
 
 nt. fd. 
 
 nt. fd. 
 
 100^21 
 
 100^77 
 
 I. Phyllite, Sailean Sligenaoh, Ardnmcknish, Argyllshire, six feet from 
 
 margin of camptonite dyke (unaltered). 
 II. Buchite or fused phyllite, with cordierite, etc., in contact with 
 
 camptonite dyke, Sailean Sligenach, Ardraucknish, Argyllshire. 
 
 J. S. P. 
 b. Acid Dykes. — Although by far the larger proportion of the 
 Tertiary dykes are of basic composition, a series of considerably 
 more acid composition is found in the south-western corner of the 
 map. This acid series occupies a zone or belt, of about a mile in 
 breadth, stretching from the head of Loch Feochan in a general 
 south-easterly direction to the southern margin of the map. 
 
 In many cases the more acid intrusions constitute simple dykes 
 traversing the andesitic lavas of Lower Old Red age, but fre- 
 quently they are seen to converge with basic dykes and unite to 
 form compound or conjugate intrusions. Or these compound 
 dykes may consist of an alternating series of basic and acid rocks, 
 formed by the splitting up of the basic rocks by the more acid. 
 These compound dykes may be well seen on the south-east side of 
 Loch Feochan, and again in the area both to the north and south 
 of the eastern end of Loch Scamadale. A well-marked line of 
 these compound intrusions runs along the course of the Braglen- 
 more Burn, south-east of the head of Loch Scamadale, and is 
 continued to the north-west in the direction of the head of Loch 
 Feochan. 
 
 The dykes of more acid composition than the dolerites and 
 basalts consist of two types, the one being rather of a trachytic 
 facies, the other being more acid and more nearly allied to the 
 rhyolites. The trachytic type occasionally occurs as simple 
 dykes, but is perhaps more usually found taking part in the 
 
Tertiary Dykes. 133 
 
 formation of the compound series. It forms a grey or greenish- 
 grey rock of fine-grained texture, weathering with a rougher 
 surface than the rhyolitic type, and breaking up into irregularly- 
 shaped blocks, which sometimes show a rudely conchoidal 
 fracture, while the basic dykes usually break up into more 
 rectangular masses. These trachytic rocks are highly felspathic 
 in composition, containing only a very small proportion of ferro- 
 magnesian constituents, and some show affinities to the 
 bostonites [10137]. Several examples of this trachytic type of 
 dyke may be seen on the south side of the eastern end of Loch 
 Scamadale; and again further south-east in the Eas nam Damh. 
 Specimens from near the head of Eas nam Damh show under the 
 microscope [10280, 10281] that the rock consists mainly of 
 small lath-shaped plagioclase crystals, with ragged edges, and 
 these often have a centric arrangement, and may thus give the 
 appearance of imperfect spherulitic structures. There is a dark 
 greenish interstitial substance, probably an alteration product 
 after a ferro-magnesian mineral. There also appears to be some 
 interstitial alkali-felspar present. Amygdules of calcite are not 
 uncommon. Another specimen from the south side of Loch 
 Scamadale shows [10282] much lath-shaped plagioclase with 
 interstitial felspar and quartz, with chloritic patches, calcareous 
 matter, and magnetite. 
 
 The more acid or rhyolitic types are also well developed in the 
 Loch Scamadale area. They are usually various shades of grey 
 in colour, and sometimes greenish and purplish, weathering to 
 shades of grey, green, purplish, or yellowish. They are usually 
 much shattered and decomposed, often too much so for satis- 
 factory examination. They exhibit a well-marked flow-structure, 
 and often break up on weathering, like a mass of slate, into thin 
 plates parallel to the margin of the dyke. The course of these 
 dykes is often sinuous and twisting, and branch-like offshoots are 
 occasionallj'- given off. They are usually compact and flinty at 
 the margins and become holo-crystalline towards the more cen tral 
 portions. Phenocrysts of bi-pyramidal quartz crystals are 
 occasionally seen in them, but the usual phenocrysts are nmall 
 felspar crystals, or, on the other hand, porphyritic structures may 
 be entirely absent. 
 
 A specimen of one of the acid dykes to the north of Loch 
 Scamadale was examined under the microscope by Mr. Harker, 
 and the following note, submitted by him, was published in the 
 Summary of Progress of the Geological Survey for 1898 : — " A 
 dull, white, compact-looking rock with a few little glistening 
 specks, which are crystals of quartz and felspar. The slice shows 
 scattered porphyritic crystals, usually less than gij^-inch of 
 quartz in good pyramidal crystals, and felspars (albite and 
 sanidine) sometimes intergrown in microperthite fashion. The 
 ground-mass is essentially of felspar and quartz, the latter 
 evidently in great part of secondary origin. The felspar is in 
 bunches of little prisms, but the original structure of the rock is 
 much obscured. It seems to belong rather to the rhyolitic tnan 
 
134 Tertiary Dykes. 
 
 to the trachytic family. The secondary quartz of the ground- 
 mass might be derived from the rock itself, or from an extraneous 
 source, but the porphyritic quartz seems to mark the rock rather 
 as one of acid composition, since the felspars are doubtless all 
 alkali-felspars. Leaving out of account rocks of the dacite 
 family (which is not in question here) , porphyritic quartz crystals 
 may be taken as characteristic of rhyolitic rocks of intrusive 
 type ; in the rhyolitic lavas they are often wanting." 
 
 Another dyke of the same class, seen on the shore near the head 
 of Loch Feochan, shows under the microscope [10138] pheno- 
 crysts of quartz, orthoclase, plagioclase, biotite, and micro- 
 pegmatitCj in a very felspathic ground-mass which is partly micro- 
 poikilitic, and contains small grains of biotite and hornblende. 
 Several of these dykes are well exposed in the Allt Braglenmore 
 to the south-east of Loch Scamadale, and the following remarks 
 are taken from Dr. Flett's notes on the microscopic sections 
 [10139-10142]. They are fine-grained, greenish-grey vesi- 
 cular, felspathic rocks, with small phenocrysts of felspar, which 
 appears to belong to anorthoclase or varieties of plagioclase rich 
 in alkalies. Some contain quartz as phenocrj-sts. In others the 
 quartz appears to be secondary. The ground-mass is highly 
 felspathic, and mostly perfectly micro-poikilitic, and stained with 
 small spots of limonite, chlorite, &c. These rocks are not 
 ordinary quartz-porphyries, trachytes, or ^bostonites. The 
 abundance of an alkali-plagioclase, the felspathic ground-mass 
 with chlorite, the sparing amount of quartz, are suggestive of the 
 quartz-keratophyres and keratophyres. The keratophyres, 
 however, do not usually show a micro-poikilitic ground-mass. It 
 is possible, however, that the rocks may belong to this class. 
 Similar rocks occur in Eas nam Damh and show [10278] small 
 phenocrysts of plagioclase and alkali-felspar in a felsitic base, in 
 part micro-poikilitic. 
 
 The occurrence of these rocks and also of the trachytic type in 
 compound dykes, with the dolerites and basalts, has already been 
 referred to above. These compound intrusions may consist of 
 from two up to as many as seven distinct members. A good 
 example of a compound dyke consisting of five members is seen 
 in a small bum on the south side of Loch Scamadale. Thus at 
 one margin we find an acid rhyolitic dyke with well-developed 
 flow-structiire ; this is followed by a basalt, and the basalt by a 
 trachytic dyke. This is succeeded by another basalt, which is 
 again followed by a trachyte. If we follow up this intrusion we 
 find that the rhyolitic dyke gradually diverges from the others 
 and follows an independent course. Another compound dyke in 
 Eas nam Damh consists of seven members, thus — ^basalt, rhyolite, 
 basalt, trachyte, basalt, trachyte, basalt, in consecutive order 
 from margin to margin. When traced, however, further to the 
 south-east, we find the same compound intrusion consisting of 
 only four members, thus — ^basalt, trachyte, basalt, rhyolite, the 
 marginal basalts and one of the rhyolites having disappeared. 
 
Tertiary Bylies — Vents. 
 
 135 
 
 Several of the fissures occupied by these composite dykes can 
 be shown to have been used as an escape for gases, after they had 
 been filled by one or more of the earlier molten intrusions, as they 
 are partly filled with agglomerate and tuff, made up of small 
 fragments of these rocks as well as of the walls of the fissure, and 
 through this agglomerate some of the more acid rhyolitic rocks 
 can be seen to have risen. A very good example of such a dyke is 
 exposed within a few yards of the road along the south shore of 
 Loch Feochan, about one-third of a mile from its head. (Kg- 
 2.) The oldest rock filling the fissure is an olivine-basalt. (I., 
 Fig. 2.) This is pierced by a massive dolerite dyke (II.), which 
 
 ,lVC£^''''^s^^^'' ^""^'^ 
 
 36 FEET ■ 
 
 ^CH DEPOSITS 
 
 Fig. 2. — Composite Dyke exposed on south shore of Loch Feochan, near 
 road, 4 miles south from Oban. 
 
 Po. Andesite lavas. I. Olivine basalt dyke. I'. Chilled edges of dyke. 
 II. Dolerite dyke. 11'. Chilled edges of dyke. III. Agglomerate 
 filling cracks. IV. Rhyolite with bipyramidal quartz. IV. Glassy 
 margin showing fluxion structure. 
 
 exhibits chilled edges against the basalt. The dolerite and the 
 basalt are traversed by fissures filled in with agglomerate (III.) 
 formed by their fragments, and lastly a dyke of rhyolite (IV.) 
 with bi-pyramidal quartz grains and showing line fluxion 
 structure along its margins cuts both the basalt and the 
 agglomerate. The association of a similar type of agglomerate 
 with these composite dykes is again a common feature in the Loch 
 Scamadale area, and will be again referred to in connection with 
 the vents. 
 
 c. Vents. — In connection with the Tertiary intrusions above 
 described, a feature of special interest is the occurrence of masses 
 of tuff, ash, and agglomerate, which apparently behave as 
 volcanic vents. These vents are arranged along lines of fissure, 
 occupied partly by simple and partly by composite dykes, which 
 stretch from Balinoe, near the head of Loch Feochan, in a south- 
 easterly direction past the head of Loch Scamadale, and thence 
 extend along the course of the AUt Braglenmore to the southern 
 
136 Volcanic Vents. 
 
 margin of the map. The vents are usually oval or elliptical m 
 outline, or occasionally constitute elongated masses from 20 to 
 30 feet in breadth, associated with composite dykes. They are 
 well seen in the small burn flowing down into Glen Feochan at 
 Balinoe, and again there are excellent exposures in the bed of the 
 Allt Braglenmore. Near the head of the former burn there is a 
 good example of one of these vents, which was first noticed by 
 Mr. E,. G. Symes, and was described by him in the Annual 
 Summary of the Geological Survey for 1898, page 154. 
 
 This vent is more or less pear-shaped in plan, narrow at the 
 north end and widening southward, its maximum length being 
 about 120 feet. It is filled with coarse agglomerate, some of the 
 blocks of which measure two or three feet in diameter. The 
 ragments consist largely of greenish and greyish rhyolitic rocks 
 with well-developed flow-structure, but they also include pieces of 
 the surrounding andesitic lavas of the pink biotite-porphyrite 
 which occurs in the dykes and sills of the Lome volcanic plateau, 
 and of some basic rocks like the material of many of the Tertiary 
 dykes. While the vent pierces the andesitic lavas and their 
 associated porphyrite dykes, it appears also to truncate one of the 
 numerous rhyolitic dykes of this district, already described. 
 Thus, immediately to the south of it, appears a rhyolitic dyke 
 which is exactly similar to the peculiar rock of which the blocks 
 in the neck so largely consist. 
 
 A rather larger vent is very clearly exposed in the Allt Braglen- 
 more, at its junction with the Allt Dearg, the burn which flows 
 into it from Fineglen on its north-east side. This vent is roughly 
 oval in outline, and is well seen for 50 yards or more up either 
 bum above the junction. It pierces the andesitic lavas of Lower 
 Old Red age. The included fragments, as in the case of the vent 
 already described, consist mainly of rhyolitic rocks, basalt, and of 
 the surrounding andesites. Some fragments also of epidiorite 
 from the underlying schists were also observed in one of the 
 neighbouring exposures of agglomerate. These fragments are 
 cemented together in a finely fragmental matrix. The vent is 
 seen in one place to have truncated and broken up one of ihe 
 Tertiarj?^ dykes, while in another place it is cut by a rhyolitic 
 intrusion. 
 
 Between this vent and the head of Loch Scaraadale, and again 
 occasionally in the Eas nam Damh to the east, smaller masses of a 
 similar agglomerate have frequently been met with. Sometimes, 
 as already observed, the agglomerate is seen to behave as elon- 
 gated masses and to occupy the same fissure as one of the 
 composite dykes. A good example of this is seen on the north- 
 east bank of the bum, and the following section is exposed in 
 crossing the ravine from south-west to north-east: — (1) A dyke 
 of olivine-basalt from 2 to 3 feet in breadth ; (2) lying against 
 this dyke, agglomerate of about 3 feet in breadth, and consisting 
 of fragments of basalt, rhyolite, and andesite ; (3) an acid dyke 
 of 4 or 5 feet in breadth ; (4) agglomerate with a breadth of 
 
Volcanic Vents. 137 
 
 about 20 feet, with angular fragments mainly of rEyolite ; (5) 
 resting against this agglomerate another acid dyke 18 to 25 feet 
 broad. These five members occupy one and the same fissure, and 
 each dyke appears to be distinct in itself and shows chilled edges 
 against the agglomerate. 
 
 The proportion of rhyolitic to basaltic fragments in the 
 different exposures of agglomerate observed frequently varies 
 considerably, some masses being almost entirely made up of 
 rhyolitic rocks, while others are mainly due to the breaking up of 
 basalts. The material seen in the vents and fissures varies from 
 a very coarse agglomerate to a moderately fine-grained frag- 
 mental rock resembling tuff. 
 
 It does not appear likely that these vents ever gave rise to any 
 outpourings of lava ; they rather mark the site of small local 
 eruptions of an explosive nature, and their intimate association 
 with a particular zone of dyke-formation further brings out their 
 analogy to the small blow-holes and cinder-cones frequently 
 observed in modern volcanic areas, as, for instance, in Iceland, 
 formed by escaping high-pressure steam and gases along lines of 
 fissure. They belong essentially to the period of the formation 
 of the outlying dykes of Tertiarj' volcanic times, connected Avith 
 some larger focus such as that of Mull. Though later than the 
 formation of some of these rhyolitic and basaltic dykes, the 
 material of the vents was itself again pierced by others of these 
 dykes, so it is probable that the eruptions which they indicate 
 had ceased before the final dying out of the dyke-forming phase. 
 
 H. K. 
 
 Ohan and Kilmore Area.- — Two distinct vents occur along the 
 course of the composite dyke north of that first described by Mr. 
 Symes.* The nearest to the latter is about a quarter of a mile 
 to the N.N.W., and floors the bottom of the deep gorge which 
 opens on the Feochan valley at BalKnoe for about half a mile 
 from its head, having an average breadth of only about 200 feet, 
 and is, therefore, a typical fissure-vent. It has followed the 
 same fissure which has been taken advantage of by the earlier 
 basic dykes, but the explosion has disriipted the dykes and a good 
 portion of the andesites and their older porphyrite intrusions, 
 the broken ends of several of which are seen on each side of the 
 vent. The neck is chiefly filled by a comparatively fine-grained 
 agglomerate or tuff composed of fragments of the rocks through 
 which the vent has been driven, but with the addition of rhyolitic 
 and felsitic material ; the whole being much decomposed. In 
 this neck there is evidence that the explosive conditions were 
 followed by flows of rhyolite, which are seen cutting the agglo- 
 merate, and isolated masses of agglomerate are found included 
 within platy rhyolite in such a manner as to show that they were 
 rolled along and wrapped up within the highly viscid lava in its 
 passage up the vent. 
 
 * Summary of Progress for 1898, p. 153-154. 
 
138 Volcanic Vents. 
 
 A second' vent also occurs in the gorge near its foot just above 
 Ballinoe. It is almost circular, with a diameter of about 300 
 yards, and pierces the andesitic lavas and the basic dykes. It 
 is partly filled with coarse agglomerate on its west side, which 
 contains a large proportion of felsitic debris. Through this has 
 welled up a felsitic rock with platy fluxion structure, the planes 
 being much involved. The rock gives off apophyses, and appears 
 to be merely an expansion of the rhyolitic dykes which connect 
 this vent with the one to the south, and which can be seen 
 splitting up the basic dykes both to the north and south of the 
 vent. 13. N. P. 
 
CHAPTER X. 
 
 Contact Metamorphism Produced by the Newer 
 Igneous Masses. 
 
 The huge granitic mass of Ben Oruachan affords special facilities 
 for the study of the phenomena of contact metamorphism, for not 
 only does it comprise an enormous body of eruptive material, but 
 the sedimentary formations around it had undergone comparatively 
 so little alteration before it invaded them, that its effects upon them 
 can be satisfactorily studied over a large area. It is not possible, 
 however, to define a definite aureole of metamorphism round the 
 granite, on account of the exceedingly variable nature of the rocks 
 affected by it. Thus, along the greater portion of the granite 
 margin comprised within the present map, the older sediments 
 consist of alternations of different material which have been affected 
 in different degrees. Hence, in some areas, such as that of the Pass 
 of Brander, where we have mixtures of quartzose and argillaceous 
 materials and impure calcareous beds, we have a high degree of 
 contact alteration ; while in other areas, such as that bordering part 
 of the eastern margin of the granite, where highly quartzose and 
 granulitic rocks occur which underwent considerable metamorphism 
 previous to the granite intrusion, the later metamorphism of the 
 contact type is only comparatively slight. Nor is it always possible, 
 along the southern margin of the granite, to trace the highly contact- 
 altered rocks into their least-altered representatives, owing to various 
 causes, such as the folding of the rocks, and the overlapping of rocks 
 of a different type such as the volcanic series of Lome. More- 
 over, the extent to which the country around Ben Oruachan is cut 
 up by lochs and rivers prevents to a great extent the possibility of 
 tracing any particular rock from its least-altered state up to its 
 contact with the granite. However, in spite of these obstacles, it 
 has been found that a well-marked zone of contact alteration can be 
 followed along the margin of the granite from the head of Loch 
 Awe along the Pass of Brander to Bonawe, and to the west of 
 Loch Etive this alteration zone may be traced continuously along 
 the entire western margin of the granite. We thus have a belt of 
 contact-altered rocks extending continuously for a distance of about 
 17 miles. The maximum breadth of this belt is about a mile and a 
 half on the western side of the granite and about a mile in the Pass 
 of Brander area. 
 
 We will deal first of all with the district near the head of Loch 
 Awe and the Pass of Brander. 
 
140 Contact Metamorphism prodtceed hy the Newer Igneous Masses. 
 
 1. LOCH AWE DISTRICT AND THE PASS OF BKANDER. 
 
 In this area the members of the schistose rocks, on being traced up 
 to their contact with the granite, are found to have undergone very 
 considerable contact-alteration. The degree of alteration increases 
 towards the granite in such a manner as to leave no doubt that it 
 has been caused by the direct action of the granitic intrusion. The 
 sedimentary rocks here exposed may be conveniently divided into 
 the following groups : — 
 
 (1) A banded calcareous series of the Pass of Brander. These 
 
 rocks are especialljr well seen in the neighbourhood of the 
 Falls of Oruachan, and occasionally good bands of pure 
 limestone are intercalated among them. 
 
 (2) The black schists of the south and south-eastern slopes of 
 
 Beinn-a-Bhuiridh . 
 
 (3) Quartzose schists, frequently containing some proportion 
 
 of argillaceous material, and quartzites of the eastern 
 portion of Beinn-a-Bhuiridh and Monadh Driseag. 
 
 Calcareous beds occasionally occur with the black schists, and 
 black schists are frequently intercalated with the more quartzose 
 beds. 
 
 During the mapping of this area a large number of specimens 
 were collected at various distances from the granite, and these were 
 submitted for microscopic examination to Dr. Teall. The follow- 
 ing account of the contact-alteration of these rocks is mainly taken 
 from Dr. Teall's report on the microscopic sections and specimens. 
 
 (1) The Banded Calcareous Series. — "The rocks of the banded 
 calcareous series were evidently fine-grained sedimentary rocks, in 
 which the relative proportions of argillaceous, arenaceous, and 
 calcareous material were liable to repeated variations. Subsequent 
 to their formation they must have been folded and cleaved, so that the 
 original laminae of arenaceous and calcareous material were more or 
 less broken up into lenticular folia. Sometimes the different types 
 of sediment were, as it were, kneaded together without the develop- 
 ment of any well-defined parallel structure. This, however, only 
 applies to small portions of the specimens. Strain-slip cleavage is 
 often recognisable in the dark bands. The contact action appears 
 to have been subsequent to the movement, and to have destroyed to 
 a great extent the schistosity produced by it. 
 
 "The rocks are now fine-grained or compact, and consist of thin 
 alternating folia of a dark purplish and pale grey, greenish, or 
 yellowish colour. The dark folia represent the more argillaceous 
 laminae of the original sediment. They are now largely composed 
 of a confused aggregate of micaceous and chloritic minerals, amongst 
 which the characteristic brown mica may be observed. In one or 
 two specimens small albites occur, as they do in some of the albite- 
 schists ; but it may be doubted whether this mineral is due to 
 contact-action. 
 
Contact Metamorphism produced hy the Newer Igneous Masses. 141 
 
 " The light-coloured folia represent in most cases the more cal- 
 careous laminse of the original sediments. They are rich in minerals 
 of the type usually occurring in impure limestones which have been 
 subjected to contact action. A colourless or pale green pyroxene 
 and a colourless garnet are the two most characteristic minerals, 
 but epidote and hornblende also occur. The pyroxene is usually 
 present in small grains, and only occasionally attains dimensions 
 large enough to show characteristic cleavages and optic properties. 
 Under these circumstances, it is not always easy, in the absence of 
 colour, to distinguish the pyroxene from a mineral of the epidote- 
 zoisite group. 
 
 " Where carbonates have been preserved, which, however, is 
 rarely the case, a narrow zone of granular pyroxene may be 
 frequently seen separating them from quartz. The garnet is 
 colourless, and often shows the anomalous double-refraction of 
 grossularite. Like the pyroxene, it forms narrow zones round a 
 carbonate, with respect to which it is idiomorphic. 
 
 " In some rocks [e.g., 8238] epidote takes the place of pyroxene, 
 and the pale yellow folia are then formed of epidosite — a rock 
 composed of epidote and quartz. One or two specimens contain 
 large crystals of apatite, which appear to have been developed in 
 situ, for they are too large to be regarded as original clastic 
 constituents. Sphene is sometimes an important accessory, and 
 is often deeply coloured and idiomorphic. The other minerals, of 
 which quartz and hornblende aie the most important, do not call 
 for special notice. • 
 
 " One specimen belonging to this group [8251] is of exceptional 
 interest.* It is a rock of compound origin, consisting of igneous 
 and sedimentary material most intimately interblended. Thin 
 folia, largely composed of granitic material, may be seen traversing 
 the banded hornfels of the normal type, parallel to the banding. 
 But there is no sharp line of demarcation between the two, 
 although the granitic folia are coarser in grain than those which 
 consist of sedimentary material. Under the microscope the inter- 
 blending of the two kinds of material is very obvious. To the 
 granite must be referred orthoclase, oligoclase, biotite in large 
 plates, and some of the quartz ; to the sedimentary rock, biotite 
 in minute scales and aggregates, granular pyroxene, and the 
 remainder of the quartz. The sphene and hornblende probably 
 also belong mainly to the sedimentary rock. It is especially 
 worthy of note that, although the granitic folia are only two or 
 three millimetres in breadth, the individuals are as large as those 
 occurring in ordinary granite (1 or 2 mms.). This clearly shows 
 that the whole rock must have been subjected to the physical 
 conditions under which the granite consolidated. The granitic 
 invasion has clearly followed by preference the calcareous folia, 
 for the granular pyroxene occurs abundantly as inclusions in all 
 the granitic minerals except the biotite, whereas the dark purplish 
 
 * This specimen was obtained at the granite contact from the ridge due 
 east of the Cruachan burn. 
 
142 Contact Metamorphism produced by the Newer Igneous Masses. 
 
 bands of biotite-hornfels retain their normal character, and 
 do not show the same tendency to blend with the granitic 
 constituents. 
 
 " That a granitic magma should possess the power of eating up 
 limestone we can well understand, but it is doubtful whether 
 that power can be appealed to in the present case as an explana- 
 tion of the facts above referred to, for the granitic magma 
 does not appear to have changed its composition. It is now 
 represented by the same minerals as those of normal granite, and 
 the pyroxene of the calc-silicate-hornfels merely occurs as inclu- 
 sions in these granitic minerals. There is, therefore, direct evi- 
 dence that the lime has never been incorporated in the magma. 
 Seeing, however, that the granite of the district is associated 
 with quartz-diorites and basic diorites, it becomes important to 
 ascertain whether these rocks may be due to the absorption of 
 sedimentary rocks. 
 
 " In any case, we are here dealing with a rock of compound 
 origin. It is neither an igneous rock nor a sedimentary rock, but 
 a mixture of the two. 
 
 " Attention was directed to the existence of such rocks many 
 years ago by M. Michel Levy,* who recognised two types. In the 
 one the granitic material has been, so to speak, superposed upon 
 the sedimentary rock, so that the contact materials float in a 
 granitic matrix (superposition). In the other type the injection 
 has taken place between the folia (injection " lit par lit "). Both 
 these types are represented in this specimen. The granitic 
 material has penetrated the sedimentary rock between the thin 
 bands of biotite-hornfels, and has, moreover, superposed itself to 
 some extent upon the bands of calc-silicate-hornfels, so that the 
 contact-pyroxene occurs as inclusions in the orthoclase, oligoclase, 
 and quartz of the granite." 
 
 Professor Lacroix, in his observations on the granite of the 
 Pyrenees, has also described cases in association with contact 
 phenomena very similar to that now under consideration. f He 
 describes passages between schists impregnated with granite, and 
 granite impregnated with schist, and says that felspathisation of 
 the schists has been caused (a) by imbibition, (b) by injection — 
 i.e., by granitic material injected between the laminae (injection 
 " lit par lit "). 
 
 In the banded calcareous series of the Pass of Brander bands are 
 frequently met with, sometimes a few feet in thickness, of more or 
 less pure limestone. This material is found to be converted into a 
 finely crystalline, nearly white or greyish marble (Oipolin). A 
 specimen, one-sixth of a mile from the granite, from the ridge east 
 of Oruachan Burn, is a crystalline granular aggregate of calcite, 
 with some irregular grains of diopside, and a few flakes of mica. 
 
 * Sur les Schistes Micac^s des Environs de Saint-L4on (Allier). Bull. Soc. 
 Geol. France, 1881, pp. 181-196. 
 
 + Lacroix. "Le Granite des Pyrfen^es et ses phenomfenes de contact." — 
 Bull, des Services de la Garte geologique de la France, No. 64, p. 8, 1898. 
 
Contact Metamorphism ^produced by the Newer Igneous Masses. 143 
 
 (2) The Black Schists are found near the granite to be altered 
 into dark compact hornfels, in which quartzose lenticles are often 
 present. Wliere arenaceous material has been interlaminated 
 with argillaceous, a more or less compact hornfels has resulted 
 with strongly-marked dark and pale, or sometimes reddish bands. 
 A specimen of the altered black schist from the south side of Ben- 
 a-Bhuiridh is a dark compact hornfels with quartzose lenticles. 
 Dr. Teall reports that under the microscope " the dark hornfels 
 is seen to be thickly crowded with black opaque particles and 
 patches. The substratum in which these particles lie is crystal- 
 line, but the minerals are not easy to identify. Amongst them 
 are two colourless minerals which differ markedly in their 
 refractive indices, and one of which may be felspar. The quartzose 
 lenticles are aggregates of quartz with films and patches of a pale 
 coloured biotite often occurring in small scales. The distribution 
 of these films shows that the rock was puckered, and in some 
 cases the films may be traced as inclusions throiigh comparatively 
 large individuals of quartz, thus proving that to some extent at 
 least the crystallization was subsequent to the movement." 
 
 A dark, compact, banded hornfels, from the south side of the 
 Pass of Brander, shows strain-slip cleavage, and was doubtless 
 originally composed of alternating gritty and argillaceous layers. 
 The former are now composed of quartz grains and minute scales 
 of chloritic and micaceous minerals. The drgillaceous layers are 
 mainly composed of chloritic and micaceous minerals, amongst 
 which brown mica may be recognised. The effect of the contact 
 action has been to destroy to a great extent the previous schistosity 
 of the rocks. Some reddish bands which occur in the hornfels of 
 this type consist of a granulitio aggregate of quartz and turbid 
 felspar, while epidote, zircon, and sphene occur as accessories. 
 These bands probably represent laminae of a fine-grained felspathic 
 sandstone. 
 
 (3) The Cordierite-hearing Hocks which occur among the altered 
 sediments (quartzose schists, biotite-hornfels, &c.) of the eastern 
 and northern portions of Monadh Driseag, at the back of Loch 
 Awe Hotel, have been carefully studied by Dr. Teall, who 
 furnishes the following report regarding them* : — 
 
 " Another very interesting type of metamorphic rock is charac- 
 terised by the presence of cordierite. This type is represented by 
 medium-grained, often bluish-grey, fairly massive rocks, of which a 
 specimen from the north-east side of Monadh Driseag [8254] may 
 be selected as a type. Under the microscope one sees an 
 aggregate of colourless minerals, in which lie detached scales and 
 nests of biotite, and patches crowded with small grains and 
 imperfect crystals of a green spinelle. The colourless minerals 
 are cordierite, alkali-felspar, and oligoclase. The individuals vary 
 in size, often measuring one or two mms. across, but rarely show 
 any trace of crystalline form. The cordierite contains biotite, 
 
 *8u'mm. Prog. Geolog. Survey for 1898, pp. 86-91, 
 
144 Contact Metamorphism produced by the Newer Igneous Masses. 
 
 often in minute rounded scales, spinelle, and zircon, as inclusions. 
 It is difficult in an ordinary slide to distinguish in all cases the 
 felspar from the cordierite, as both have approximately the same 
 refractive and double-refractive power. The characteristic yellovs^ 
 pleochroic halos surrounding the zircons in the cordierite are 
 then the most useful means of distinguishing between the two 
 minerals. But if a thin slice be heated on platinum foil, a change 
 is produced in the cordierite. The pleochroic halos disappear, 
 and a faint yellow colour seen best in one position of the polariser 
 is developed all over the mineral. This faint general pleochroism, 
 produced by heating, is not so marked as that of the halos, but 
 is sufficient to distinguish the cordierite from the felspar, which 
 remains absolutely colourless. In this way it can be shown that 
 half, Or possibly even more than half of the entire rock is 
 composed of cordierite. Of the felspars, orthoclase is the most 
 abundant. Pyrite occurs in grains, and as irregular masses 
 wedged in between the other constituents. Quartz has not been 
 observed in the specimen selected as a type. 
 
 " The occurrence in a slide [8254] of this rock of one or two 
 colourless grains with very high refractive and moderate double- 
 refractive power, led to a search for corundum, which proved 
 successful. The coarsely-powdered rock was treated for about 
 forty-eight hours with hydrofluoric acid, and the residue boiled 
 with nitric acid to rernove the pyrite. As the microscope disclosed 
 the presence of some corroded fragments of andalusite, the powder 
 was placed in methylene-iodide of sp. gr. 3'315. The final residue 
 was then found to consist of corundum, green spinelle, and one or 
 two crystals of rutile. The corundum occurs in irregular grains 
 and crystals up to '3 mm. in diameter. The crystals show 
 a tendency to a tabular form, with hexagonal outlines. 
 Those which are lying flat on the slide, and which show this 
 outline, appear dark under crossed nicols, and give as a rule a 
 negative uniaxial figure. Parallel striations in three directions at 
 right angles to the sides of the hexagon may frequently be observed. 
 These are in most, if not in all cases, due to repeated alternations 
 of the rhombohedral and basal planes, in which the latter usually 
 predominate. In this way a stair-like termination to the hexagonal 
 prism is often produced." 
 
 " The forms observed are those of a hexagonal prism [1120], a 
 rhombohedron [1011], and the basal pinacoid [0001] — a charac- 
 teristic combination for corundum, and one commonly seen in the 
 Burma rubies and Montana sapphires. The steep pyramidal 
 planes, so characteristic of the Ceylon and Kashmir sapphires and 
 the common corundums of Madras, have not been observed. 
 Inclusions of green spinelle and cavities, usually irregular, but 
 sometimes in the form of negative crystals, are not infrequent. 
 
 " Other specimens of the cordierite-hornfels differ frorii the type 
 in the relative proportions of the difierent constituents, in the 
 presence of quartz, sometimes considerably abundant, and to a 
 certain extent also in structure. One specimen [7618] is especially 
 
Contact Metamorphism produced by the Newer Igneous Masses. 145 
 
 interesting as furnishing conclusive evidence as to the relative age 
 of the crystallisation of the cordierite and the movements which 
 have so povf erfully affected the sedimentaiy rocks of the district. 
 Apart from a few large patches of pyrite and flakes of biotite 
 (■4 ram.), this rock is composed of extremely minute opaque grains 
 and plates (ilmenite •01-'02 mm.) scattered through a granulitic 
 aggregate of cordierite, quartz, andalusite, and felspar. Sometimes 
 the grains of cordierite attain a considerable size, and then the 
 section loses its granulitic character. Sericitic mica, zircon, and 
 rutile occur as accessory constituents. 
 
 " The distribution of the minute opaque particles clearly shows 
 that the rock was puckered with the occasional development of 
 strain-slip cleavage, but this puckering has had no effect whatever 
 on the cordierite, or on the other colourless constitutents. All these 
 in their present form have been clearly developed since the move- 
 ment took place. 
 
 " This rock is interesting from another point of view. Puckering 
 of the type here revealed is only known in phyllites and slates — 
 that is, in rocks which were originally fine-grained sediments rich 
 in argillaceous material. Out of such sediments the cordierite- 
 bearing rocks have doubtless been formed, the alkali of the 
 micaceous minerals going to form felspar, while some, or all, of 
 the magnesia present combined with alumina and silica to form 
 cordierite. The poverty of this rock in micaceous minerals, so 
 common in the phyllites, is thus explained, and the simultaneous 
 formation of cordierite and alkali-felspar is accounted for. 
 
 " In the Explanation of Sheet 75 of the Geological Survey of 
 Scotland (1896) a general account of the characters and distribu- 
 tion of the cordierite-bearing rocks of Scotland, so far as these were 
 known at the time, was given. Since that date Mr. Barrow has 
 sent up another series from Sheet 65 (Aberdeen and Forfar).* 
 They present many points of resemblance to the rocks above 
 described. There is the same association of cordierite, andalusite, 
 biotite, and a green spinelle, with a variable amount of quartz and 
 felspar ; but the rocks are coarser in grain and always contain 
 sillimanite, sometimes also garnet. One of these rocks [7085], 
 containing cordierite, sillimanite, andalusite, felspar, biotite, 
 ilmenite, and the green spinelle, was analysed, and the analysis is 
 quoted as thowing light on the composition of the original sedi- 
 ment. It agrees remarkably with the analysis of a cordierite 
 contact-rock from Monte Aviolo, described by Salomon, which is 
 quoted for comparison. 
 
 * See also Dr. Flett in Memoir on The Geology of Lower Strathspey 
 (Explanation of Sheet 85), p. 55, and in the Memoir on the Land's End 
 District (Explanation of Sheets 351 and 358). 
 
146 Contact Metamorphism produced hy the Newer Igneous Masses. 
 
 
 
 I. 
 
 II. 
 
 SiO„ 
 
 
 42-08 
 
 44-62 
 
 Tio; 
 
 
 216 
 
 2-04 
 
 A1.03 
 
 
 32-40 
 
 33-33 
 
 Fe,0, 
 FeO 
 
 
 13-44 
 
 7-32 
 5-45 
 
 MgO 
 
 
 3-30 
 
 5-46 
 
 CaO 
 
 
 1-42 
 
 0-12 
 
 K„0 
 
 
 2-20 
 
 0-68 
 
 Na,0 
 
 
 1-60 
 
 0-53 
 
 Ignition 
 
 
 1-60 
 100 20 
 
 1-47 
 
 
 101-02 
 
 I. Oordierite-gneiss, from the north end of Black Hill, Aberdeenshire. 
 [Teall.l 
 
 II. Cordierite-contactfels, Monte Aviolo. "Geologisohe und Petrograph- 
 ische Studien am Monte Aviolo, " von ' W. Salomon. Zeit. Deutsch. Oeol. 
 Gesell, 1890, p. 502. 
 
 "The principal difference between the two analyses lies in the 
 greater amount of alkali in the Scottish rock. But Salomon states 
 that his specimen was specially selected on account of its containing 
 very little felspar, and less biotite than the average rocks of the 
 group ; so that, if an average rock had been selected, the agreement 
 might still have been closer. 
 
 " The close correspondence between the cordierite-bearing rocks 
 of the Ben Cruachan area and those of Monte Aviolo is especially 
 interesting, because in both cases the granitic rock is of the same 
 type. It is a tonalite or quartz-mica-diorite, rather than a true 
 granite. 
 
 " There is another point of some interest which deserves mention 
 in this connection. In the Explanation to Sheet 75 (Scotland) special 
 attention was directed to a rock of compound origin belonging to this 
 group [3693]. ' The igneous portion is represented by more or less 
 idiomorphicoligoclase, biotite, micro-poikiliticorthoclase, and quartz; 
 the metamorphic portion by cordierite, quartz, biotite, sillimanite, 
 and a green spinelle. The rock into which the granitic magma was 
 intruded is now represented by somewhat ill-defined shreds, patches, 
 and streaks, in a paste of igneous origin ' (Z.c, p. 37). It is not 
 possible to distinguish igneous from sedimeniary material in the 
 Ben Cruachan specimens, nevertheless something of the same kind 
 as that above referred to may have taken place, especially as there 
 is direct evidence that this has been the case with the bands of 
 pyroxene-hornfels in the ' banded series.' We thus see that the 
 cordierite-bearing contact-rocks, which always belong to the inner 
 zones of contact metamorphism, and often occur at the actual junc- 
 tion, are peculiarly liable to be invaded by igneous material and 
 more or less broken up. Salomon describes inclusions of the 
 cordierite rocks in the tonalite of Monte Aviolo, and concludes from 
 the occurrence of garnets and large crystals of biotite in a zone 
 surrounding these inclusions, that the composition of the tonalite- 
 magma was modified by the absorption of some of the contact 
 material. 
 
Contact Metamorphism produced by the Newer Igneous Masses. 147 
 
 " Under these circumstances it is particularly interesting to note 
 that cordierite, in association with the same minerals — sillimanite, 
 green spinelle, orthoclase, plagioclase, etc. — occurs frequently in 
 the ejected blocks of the Laacher See,* and as inclusions in the 
 andesites and less frequently in the trachytes of the Eifel and 
 Siebengebirge.t Mineral aggregates of cordierite, andalusite, 
 sillimanite, felspar, biotite, pleonaste, corundum, rutile, quartz, 
 garnet, zircon, and magnetite, bearing the closest resemblance to the 
 contact-rocks of Monte Aviolo and Ben Oruachan, occur as fragments 
 and ' schlieren ' in the hornblende-andesite of Bocksberg and 
 Eeugensfeld in the Eifel (Vogelsang). The fragments are usually 
 rounded, but sometimes angular. They may attain a diameter of 
 6cms., while the ' schlieren' may be 16cms. in length. The larger 
 fragments are often easily separable from the matrix, but the 
 ' schlieren ' may be most intimately blended with it. These inclu- 
 sions vary in size, and are often so small as to be only recognisable 
 under the microscope. It seems impossible to avoid the conclusion 
 that we see in the process which goes on at the contact of plutonic 
 rocks, the agency by which inclusions of the kind described by 
 Vogelsang are introduced into igneous magmas, and it is certainly 
 remarkable, though possibly only an accidental coincidence, that 
 the hornblende-andesites of the Eifel should be the volcanic repre- 
 sentatives of a tonalite magma similar to that which formed the 
 masses of Monte Aviolo and Ben Oruachan. 
 
 " This is not the place to enter upon a general discussion of the 
 natural history of the cordierite-bearing rocks, but one or two 
 remarks may perhaps be admitted. The mineral occurs in many 
 igneous rocks such as granite, quartz-felsite (Cornwall), liparite, 
 kersantite ; and although in some of these it is clearly not the result 
 of crystallisation from the normal igneous magma, in others it 
 possesses all the characters of a mineral so formed. This, for 
 example, is the case of the cordierite of the kersantite of Michael- 
 stein in the Hartz, where it occurs in idiomorphic crystals. 
 Nevertheless, a doubt is suggested as to whether, even in this case, 
 it can be regarded as a mineral belonging to the igneous magma, 
 for the rock in which it occurs contains numerous inclusions, made 
 up of sillimanite, andalusite, kyanite, spinelle, and other minerals 
 usually associated with cordierite in contact rocks and crystalline 
 schists. Moreover, the mineral itself contains spinelle as inclusions. 
 It seems probable, therefore, that in some way or other the presence 
 of cordierite in the kersantite of Michaelstein is due to the influence 
 ofthe contact rocks upon the igneous magma, in which case it must 
 be regarded as an endomorphic contact mineral. 
 
 " Apart from the somewhat rare and sporadic occurrence of 
 cordierite in ejected blocks and igneous rocks, we find the mineral 
 
 * " Mikroskopische Untersuchung der aus krystallinischen Gesteinen 
 hen'iihrenden Auswiirflinge des Laacher Sees " ; von Carl Dittmar. Verh. d. 
 Naturh. Vereines. Bonn. 1887, p. 499. 
 
 + Beitrage zur Kenntniss der Trachyfc und Basaltgesteine des hohen Eifel. 
 Carl Voge sang. Zeit. deutsch. geol. Oesell. 1890, p. 1. 
 
148 Contact Metamorphism produced by the Newer Igneous Masses. 
 
 in crystalline schists of doubtful origin, and in contact-rocks. Its 
 mode of origin in the latter case is fairly clear. It has been produced 
 by the metamorphosis of sedimentary rocks, containing argillaceous 
 material. The presence of 2 or 3 per cent, of magnesia is common 
 in slates and shales, and as 3 per cent, of magnesia is sufBcient to 
 make about 30 per cent, of cordierite, when combined with silica, 
 alumina, and iron, it is clear that rocks of this type turnish the 
 necessary chemical conditions. Do these facts throw light on the 
 origin of the crystalline schists containing cordierite — on the origin 
 of the cordierite-gneisses ? It is probable that they do, especially 
 when taken in connection with the evidence of the tendency of the 
 igneous magmas to invade, and even impregnate, the rocks of the 
 inner contact-zone. Suppose the phenomena of metamorphism, 
 injection, and impregnation, which must have taken place round 
 the Oruachan mass, to be intensified and accompanied by plastic 
 deformation — have we not then a rational explanation of the origin 
 of many of the complex groups of crystalline schists which cannot 
 be definitely classed as the metamorphosed representatives of 
 either igneous or sedimentary rocks ?" 
 
 2. AREA ABOUT BONAWE. 
 
 Passing now to the area on both sides of Loch Btive, in the 
 neighbourhood of Bonawe, we find that along the western slopes of 
 Ben Duirinnis the altered rocks seem to belong mostly to a series 
 of medium and fine-grained felspathic grits alternating with dark 
 slates, which often contain quartzose bands. Close to the granite 
 these strata have been altered into compact and medium-grained 
 hornfels of a dark grey colour, biotite-hornfels (representing the 
 more slaty portions), and occasionally a finely-banded hornfels. A 
 grey fine-grained hornfels, 10 yards from the granite, shows under 
 the microscope [7617], alkali-felspar, quartz, biotite, sericitic-mica, 
 pyrites, verj^ pale green hornblende (?), and rutile. The felspar 
 usually forms a mosaic, or occurs as grains in large patches of 
 micro-poikilitic quartz. More rarely it occurs as large regular grains 
 with inclusions of quartz. There is no trace of the original clastic 
 structure — all the constituents are authigenic and authimorphic. 
 Several other specimens, however, in addition to the above con- 
 stituents, appear undoubtedly to contain cordierite in the form of 
 shimmer-aggregates [7618, 7619, 7620, and 7623]. The occurrence 
 of cordierite-hornfels as a contact rock connected with the Ben 
 Oruachan mass is very interesting. The specimens containing 
 cordierite were taken mostly from exposures a few inches from the 
 granite, or from small included patches of altered rock within the 
 granite mass. • 
 
 In the banded hornfels some layers consist of biotite and a 
 micro-crystalline mosaic ; others contain small grains of malacolite 
 or epidote, and iron-ores. Possibly the original rock was a slate 
 interlaminated with impure calcareous bands — a common feature 
 in the black slate series of Loch Awe side. 
 
Contact Meiamorphism produced by the Newor Igneous Masses. 149 
 
 Patches of altered sedimentary rocks are exceedingly numerous 
 on Ben Duirinnis as inclusions within the granite. In some 
 places, indeed, the granite and sedimentary rocks are inextricably 
 intermingled ; the latter may be abundantly veined by the former, 
 or the granite may be crowded with inclusions of the sedimentary 
 material of all sizes. These inclusions closely resemble the altered 
 rocks along the granite margin, and the larger masses maintain 
 as a rule the normal strike of the folds, as shown by the beds 
 outside the granite mass. The material of these inclusions does 
 not appear to have been incorporated into or assimilated by the 
 granite. The outlines of even very small fragments remain per- 
 fectly sharp, and the granite immediately surrounding them does 
 not exhibit the least departure from its normal facies. The 
 intrusion of the granite would appear in this area to have behaved 
 like a gigantic complex sill, running in along lines of weakness in 
 the folds, and injecting itself along the cleavage-planes of the 
 slates and grits, frequently detaching and enveloping portions of 
 them in its course, and thus forming a marginal area (rather than 
 a boundary-line) of exceeding complexity. 
 
 About a mile to the north-east of Bonawe several included 
 patches of altered sedimentary rock occur, which probably represent 
 the impure limestone so common throughout the series about Loch 
 Awe side. They consist for the most part of a highly contorted 
 greenish and dark grey rock, the grey and green portions being 
 closely interlaminated. Sometimes the banding is parallel, but 
 more frequently much crumpled. These bands doubtless represent 
 two fairly distinct varieties of rock, the green portion being the 
 product of an impure limestone, while the grey part represents 
 shaly or slaty bands. Occasionally, distinct banding is lost, and 
 the rock consists of an intimate intermixture of the two varieties. 
 Even under the microscope the lamination is often clearly seen. 
 The greenish variety is composed of ragged micro-poikilitic plates 
 of biotite, granular malacolite, hornblende (scarce), and quartz in 
 large micro-poikilitic patches, and forming, as it were, in places 
 the substratum of the entire rock. Felspar is also sometimes 
 present. The green colour of the rock is certainly due to the 
 pyroxene. No carbonates are present. The malacolite is the most 
 important constituent, and the rock may therefore be termed 
 malacolite-hornfels. The micro-poikilitic structure is very perfectly 
 seen in the biotite and quartz. 
 
 The darker variety of the rock contains little or no malacolite. 
 It is composed of micro-poikilitic pale green hornblende, biotite, 
 and a colourless mosaic (quartz and felspar). In some slides 
 [7614] nests of a colourless granular sphene occur. Thus, a 
 banded rock formed of calc-silicate is interlaminated with biotite- 
 hornfels. It is unfortunate that the occurrence of this rock as 
 isolated patches prevents it from being traced into its unaltered 
 representative. 
 
 From the Bonawe area the hornfels zone may be followed, as 
 already remarked, along the entire western margin of the granite 
 
150 Contact Metamorphism produced hy the Newer Igneous Masses. 
 
 to the northern edge of the map. The rocks which have here been 
 altered by the granite are essentially the same as those occurring in 
 the Pass of Brander and about the head of Loch Awe, and show 
 similar petrological characters, and the detailed description which 
 has already been given of these rocks will apply here equally well. 
 
 As is well known, the older sedimentary rocks of the Loch Awe 
 and Pass of Brander districts are intersected by numerous sills of 
 hornblendic and chloritic schists, epidiorite, &c. It is difficult, 
 however, to work out the exact nature and extent of the alteration 
 which the granite has produced upon these rocks, on account of 
 their varying character, and the differences which occur even in one 
 and the same sill. Specimens from one of the small sills on the 
 south side of Beinn-a-Bhuiridh, near the head of Loch Awe, were 
 examined under the microscope. About a mile from the contact 
 with the granite, the rock is a normal foliated epidiorite, with 
 epidote and chlorite, and shows no signs of contact-action. At a 
 third of a mile from the granite, brown mica of the contact type is 
 very characteristic, and the rock is extremely hard and compact, 
 but in other respects it does not differ markedly from many of the 
 epidiorites of the same neighbourhood. It was not, however, 
 found possible to procure actual contact specimens of any of these 
 smaller sills. A similar alteration of the small dyke-like intrusions 
 of epidiorite was observed near the granite margin to the east of the 
 head of Loch Creran. 
 
 There is thus conclusive evidence of very decided contact-action, 
 wherever the ground immediately surrounding the Ben Cruachan 
 type of granite has been examined. In tie Pass of Brander and 
 the area bounding the eastern spurs of Ben Cruachan, this action 
 is shown by Dr. Teall's detailed researches to have been exceedingly 
 intense. The sedimentary rocks may be followed from their com- 
 paratively unaltered condition, as seen in the neighbourhood of 
 Kilchrenan and other, parts of Loch Awe side, into the different 
 varieties of hornfels, some containing cordierite, of the Pass of 
 Brander and the slopes of Beinn-a-Bhuiridh. The hornfels-altera- 
 tion of similar rocks, with the production of cordierite, is again 
 found to have taken place along the granite margin in the neigh- 
 bourhood of Bonawe, and along the western slopes of Beinn 
 Duirinnis, on the north side of Loch Etive, and it may here be 
 stated that a cordierite-hornfels has also been observed near the 
 Ben Cruachan type of granite in the Glen Etive area, in Sheet 53. 
 In this latter area, however, the contact zone is not So well marked 
 as it is further to the south-west, since evidently here the sediments 
 had been affected by the forces of dynamic or regional metamor- 
 phism to a far greater degree, and were already in the condition of 
 highly crystalline schists previous to the intrusion of the granite. 
 There can be no doubt, however, that the different varieties of 
 hornfels above described owe their peculiar and characteristic 
 features directly to the contact-action which took place during the 
 intrusion of the Ben Cruachan granite. 
 
Contact Metamorphism •produced by the Newer Igneous Masses. 151 
 
 The Effect of the Granite upon the Andesific Lavas of Lome and 
 their Associated Sediments. — The andesitic lavas of the Lorne plateau 
 are seen to terminate abruptly along the south-western margin of 
 the north-western half of the Pass of Brander, at a distance of about 
 three quarters of a mile from the Ben Oruachan granite. In the 
 Blackmount district it is known that volcanic rocks of the same age 
 as those of Lorne are invaded and altered by a continuation of the 
 Oruachan granite. It would be interesting, therefore, if a similar 
 alteration could be found to have taken place in the Lorne area also. 
 It does not appear, however, that the lavas in this area, as at present 
 exposed, have been affected to any great degree by the intrusion ot 
 the granite, and this is doubtless in great part due to the distance 
 by which they are separated from the granite margin. 
 
 Beneath the lavas in this area there lies sometimes a conglo- 
 merate and sometimes a breccia, consisting of fragments of the 
 underlying schists, such as black slate, limestone, phyllite, and 
 epidiorite. As this basement bed is traced towards the Pass of 
 Brander, it seems to undergo a gradual and perceptible change, as 
 if it had suffered a general induration. This alteration is especially 
 noticeable at a distance of about half a mile from the Pass. 3?he 
 rock is not exposed, however, near enough to the granite to afford 
 the means of deciding whether this induration is due to contact- 
 alteration or not. Above the conglomerate, at from a half to a 
 quarter of a mile from the Pass, there are some banded and laminated 
 beds resembling ash or tuff, which show this induration in 
 a marked degree, and they are frequently full of greenish streaks 
 and patches from the presence of epidotic material. 
 
 Above these banded beds come the normal pyroxene-andesites. 
 They, too, are much indurated, and are very refractory under the 
 hammer. The microscope shows that they are augite-andesites of 
 a basic variety, and that if they have undergone any contact-altera- 
 tion, it can only be slight. The abundance, however, of small 
 granules of pyroxene seen in the ground-mass [9031 and 9032] seems 
 unusual. Probably these rocks are too far from the granite margin 
 to show contact-alteration to any great degree. In the case of the 
 Blackmount area, contact phenomena are not perceptible in the 
 andesites at a distance greater than about a quarter of a mile from 
 the granite, so that it would not be surprising if such phenomena 
 were entirely absent at a distance of three quarters of a mile, such 
 as we have in the present instance. But, although any very 
 decided contact-alteration cannot be detected with the microscope 
 in these andesites, it is very significant to note that these rocks, 
 up to a distance of about a quarter of a mile from the Pass of 
 Brander, are frequently seen to be traversed by small veins and 
 bands, a few inches in breadth, of a rock allied to porphyrite, but 
 approaching rather to microgranite in texture, a phenomena which 
 strongly suggests that the surrounding rocks were at a high tem- 
 perature at the time of the intrusion of these microgranitic veins. 
 
 In the railway cutting close to the Awe viaduct, we find inter- 
 calated in the andesites a band of laminated shales and grits, and 
 
152 Contact Metamorjjhism prodiKed by the Neiuer Igneous Masses. 
 
 in these sedimentary beds the induration is very marked. The 
 shaly beds have been hardened and the different laminse have be- 
 come more or less cemented together, so that the rock does not tend 
 to split up along them so readily as in its unaltered state. The 
 coarser and more siliceous beds have the appearance and fracture of 
 quartzite rather than grit. The granite is not more than three 
 quarters of a mile from these beds, a distance at which it may well 
 have indurated the sediments without appreciably affecting the 
 andesitic lavas, since we know that sedimentary rocks are usually 
 affected by thermo-metamorphic agencies to a greater distance 
 than rocks of igneous origin. H. K. 
 
 Contact Action of the Olen Fyne Granite. — The schists bordering 
 this granite along the slopes of Glen Fyne were evidently in an 
 advanced stage of crystallization before the granite intrusion. It 
 is probable, therefore, that beyond induration contact action has 
 been slight. While actinolite, biotite, and garnet are abundant tn 
 the neighbourhood of the granite, the persistence of these minerals 
 in tracts far removed from that mass precludes their being assigned 
 to contact phenomena. It is possible, however, that some of 
 thase minerals may have been so produced, but if so there is no 
 means of deciding to what extent. 
 
 Contact Action of the Granites of Beinn Bhuidhe. — On the ridge 
 of Beinn Bhuidhe occur two small irregularly-shaped masses of a 
 medium-grained and somewhat acid type of granite. This ridge, 
 however, and its northern slopes show considerable hornfelsing, 
 altogether incomparable to that likely to have been effected by 
 these small graniles. It is probable, therefore, that they indicate 
 the existence of a much larger mass lying close below the surface. 
 
 One of these granite masses on the northern flank of Beinn 
 Bhuidhe has veined a lamprophyre dyke upon which it has effected 
 marked contact alteration. A section taken close to the contact 
 with the granite vein shows that the lamprophyre now consists 
 essentially of small scales of contact brown mica and water-clear 
 plagioclase [8467]. Slightly farther from the vein [8468] there 
 are still signs of contact action, while the original crystals and 
 crystalline aggregates of the hornblende of the lamprophyre are 
 seen. 
 
 Contact Action of the Kentallenites of the Beinn Bhuidhe District.— 
 In this area occur two bosses of kentallenite, one situated at An 
 Sithein and the other in the Brannie burn. The former, which is 
 little more than a quarter of a mile in length and about one- sixth 
 of a mile in width, has been intruded into the Ardrishaig series — 
 a zone of calcareous sericite schists. Here the regional type of 
 metamorphism is less intense than that bordering the small granite 
 intrusions of Beinn Bhuidhe, and the kentallenite mass, notwith- 
 standing its small size, has produced a well-marked aureole ot 
 contact metamorphism. For about 20 yards from the limits of the 
 kentallenite the phyllites have been conspicuously hornfelsed, but 
 no visible contact minerals have been produced. 
 
Contact Metamorphism produoed by the Newer Igneous Masses. 153 
 
 The kentallenite of the Brannie burn is slightly larger and has 
 been intruded into the garnetiferous mica-schist band. This 
 intrusion has produced an aureole of hornfels with a width of from 
 15 to 20 yards. The original garnets, however, of this strati- 
 graphical zone are included within the hornfels, which is therefore 
 garnetiferous. The garnets, however, are brighter in colour and 
 fresher within the aureole, and have either been partially recon- 
 structed or else the contact action has enabled them to resist 
 disintegration. A rock slice [8470] shows a knotted hornfels. 
 It is a fine-grained dark plate, which has developed a knotty 
 structure by the formation of clusters of scales of deep brown 
 biotite. The slate itself consists of pale colourless mica mingled 
 with a brownish biotite and crossed by parallel lines, grains of iron 
 oxide, and in some bands it is more quartzose. J. B. H. 
 
 Lismore Island. — The small igneous masses on Lismore are 
 surrounded by aureoles of metamorphism which vary in breadth 
 with the character of the altered rock. Narrowest in the purer 
 limestone, the aureole broadens out in the impure limestone and 
 appears to be widest in the black schist. The last-named rock is 
 altered to a dark compact hornfels. Impure limestone becomes a 
 banded calc-silicate-hornfels, whilst purer limestone is metamor- 
 phosed to a greenish or greyish granular marble, sometimes 
 containing numerous small pinkish garnets on approaching the 
 igneous rock. H. B. M. 
 
CHAPTEE XI. 
 CARBONIFBKOUS. 
 
 A small patch of sedimentary rocks, noticed long ago by 
 Macculloch, is found resting upon the Lome andesitic lavas at the 
 Bridge of Awe, at the north-west end of the Pass of Brander. It 
 consists of thin bands of conglomerates, overlain by from 40 to 50 
 feet of pinkish and variegated marls, purple shales, and pale 
 sandstones. The beds are well exposed on the right bank of the 
 river Awe below the bridge, and may be followed beneath the 
 bridge along the left bank as far as a small island which is seen in 
 the river immediately above the bridge. At their lower end, about 
 half-way between the Bridge of Awe and the railway viad^ict, 
 these beds appeap to have been faulted down against the andesitic 
 lavas. 
 
 The conglomerates which form the lower part of the series con- 
 sist of rounded pebbles of quartzite, vein-quartz, and andesite, the 
 latter evidently derived from the eroded floor of the underlying 
 andesitic lavas. There is also a considerable proportion of derived 
 volcanic material in the lower beds of sandstone. , It is evident 
 that there is a considerable unconformity between these sediments 
 and the underlying lavas of Lower Old Red Sandstone age. In 
 fact, there must be either a great stratigraphical break at their 
 base, or an upward passage from the volcanic into the non-volcanic 
 rocks, in which case there would* be evidence of a second and 
 younger volcanic series, like that of the Campsie Fells, and an 
 unconformability would probably be found to exist between this 
 series and the older lavas of Lome. There is no evidence, however, 
 of any break in the continuous succession of the Lome lavas, 
 which can only be regarded as representing one series, belonging 
 to the Lower Old Red Sandstone period. The unconformity is 
 doubtless at the base of the conglomerates which rest upon the 
 denuded surface of the lavas, and we evidently have here a small 
 outlier of Carboniferous sedimentary rocks. 
 
 The stratigraphical evidence of the age of these beds, however, 
 would hardly be decisive without some palseontological evidence in 
 addition. These strata were formerly regarded as belonging to the 
 Old Red Sandstone, though no fossils had ever been obtained from 
 them. During the progress of the mapping of this district, Mr. 
 D. Tait was instructed to make a careful search for fossils in the 
 more shaly beds, and his labours were rewarded by the discovery 
 of a number of organisms, siifficient to show that the strata in 
 question do not belong to the Old Red Sandstone, but to the 
 
Garboniferous Boclcs. 155 
 
 Carboniferous system — a fact of great interest in regard to the 
 geological history of the West Highlands. 
 
 The specimens obtained included a small number of fossil plants, 
 which, however, were unfortunately in a fragmentary state. These 
 were submitted to Mr. Kidston, who found that they consisted of 
 stems, rootlets, a fern rachis, and a calamite stem, which last he 
 referred to Asteroccdamites scrobiculatus, Schl. While of opinion that 
 the plants have a Oalciferous Sandstone (Lower Carboniferous) 
 facies, Mr. Kidston is certain that they cannot belong to the 
 Lower Old Eed Sandstone. A further point in favour of the 
 Carboniferous age of these strata is that a small patch of plant- 
 bearing Carboniferous rocks was detected by Professor Judd * on 
 the east shore of the Sound of Mull near Ardtornish, a distance of 
 only 20 miles to the W.K.W. of the strata at Bridge of Awe. 
 
 Another piece of evidence in favour of the Lower Carboniferous 
 age of the Bridge of Awe beds is afforded by the occurrence of the 
 cast of a small modioliform shell on one of the specimens with 
 rootlets [T. 3257], which was determined by Dr. B. N. Peach. This 
 form, though badly preserved, is almost identical with some varieties 
 of Modiola macadami, Portl., one of the commonest shells of the 
 Cement-stone group of the Lower Carboniferous series in Fife, 
 Liddesdale, and the North of Ireland. Three varieties of the shell 
 are figured in Portlock's " Geology of Londonderry and Tyrone." 
 
 H. K. 
 * Quart. Journ. Geol. Soc, vol. xxxiv., pp. 684-685. 
 
CHAPTEE XII. 
 
 FAULTS. 
 
 The central and eastern portions of the area embraced by the 
 present sheet is traversed by two main lines of faulting. The one 
 coincides with the Pass of Brander and Glen Salach, and the other 
 coincides with Glen Strae and the trend of the north-eastern end of 
 Loch Awe. 
 
 That the remarkable gorge of the Pass of Brander coincided with 
 a line of fracture has long been thought highly probable, and an 
 examination of the ground shows at once that this is the case. 
 Signs of faulting are evident in many places along the side of the 
 high road, and in the railway cutting between the Falls of 
 Cruachan and the Bridge of Awe. The rocks are seen to be highly 
 shattered and discoloured, and fault-breccia is seen in many places. 
 The same line of fault is continued on the opposite side of Loch 
 Btive, past Blarcreen, up the Esragan burn, and over the watershed 
 into Glen Salach. It is lost sight of beneath the superficial 
 deposits at the foot of Glen Salach, but it may possibly be con- 
 tinued on the north side of Loch Creran along the Strath of Appin. 
 The pass at the head of Glen Salach shows precisely similar features 
 to those of the Pass of Brander. Thus, on its west side we have 
 the crags of the Lome andesites of Benderloch, and on the east side 
 the older schists which skirt the western boundary of the Ben 
 Cruachan granite. 
 
 Along this line of fracture the andesites of Lower Old Red Sand- 
 stone age have been faulted down against the much older schists 
 which cling to the southern and western slopes of the granite hills. 
 There is no means of estimating the extent of the downthrow, but 
 it is probably very considerable. The faulting is of later date than 
 the period of the porphyrite dykes, as many of these are seen to 
 be shattered and brecciated on both sides of the Pass. 
 
 Another well-marked line of fracture follows a straight course 
 running due north and following the margin of the granite about 
 half a mile east of Blarcreen, on the north side of Loch Etive. This 
 line is rather a line of smashing than of actual dislocation, as it 
 smashes the granite and its associated veins, but does not appear to 
 displace the rocks to any great extent relatively to one another. 
 It follows a due northerly course as far as the eastern slopes of 
 Beinn Bhreac, and then runs in a north-easterly direction as far as 
 the northern boundary of the map. Deep gullies lined with 
 shattered rock have been formed along its course, and narrow cols 
 in the elevated ridges through which it passes. 
 
Faults. 157 
 
 The Glen Strae fault follows the course of the glen and is well 
 seen in several places in the river Strae. It does not follow the 
 entire course of the river, but crosses the col on the west side of 
 Beinn Charn, and thence continues in a straight north-easterly 
 course to the eastern margin of the map. The garnetiferous mica- 
 schists and hornblende-gneiss of the Blackmount are seen in the 
 north-east corner of the map to be cut off abruptly along this line 
 and to be faulted against the granulitic schists of Moine type. 
 When traced further to the north-east, outside the limits of the 
 present sheet, this fault traverses the Moor of Eannoch, where it 
 faults the granite, and is found to be continuous with the fault 
 which follows the line of Loch Laidon and is continued as far as 
 Loch Ericht. 
 
 In the area on the west side of Glen Strae there are numerous 
 small but probably insignificant faults, mostly with a general 
 north-west and south-easterly trend, and deep gullies and chasms 
 have been formed in the hillsides along their course. The extent 
 of the dislocation which they cause, however, is not seen, as they 
 pass through homogeneous rocks. 
 
 In the granite area lines of smashing and crushing are fairly 
 common, though they are not usually traceable for any great dis- 
 tance. They usually form deep chasms and clefts, which are 
 occupied by small streams. The walls of some of these clefts often 
 show crushed and slickensided rock-surfaces, but probably some of 
 the smaller clefts may owe their origin merely to the widening of 
 joint-features. Several of these chasms may be well seen on the 
 precipitous eastern face of Beinn Trilleachan, on the west side of 
 the upper end of Loch Etive. 
 
 In the Ben Cruachan area there are several east and west lines 
 of faulting or smashing, which are often occupied by dolerite dykes. 
 Many of the narrow cols between conspicuous heights along the 
 southern spurs of Ben Cruachan coincide with these lines of weak- 
 ness. Thus we have Lairig Torran separating Beinn-a-Bhuiridh 
 from the higher points to the north, and Bealach-an-Lochan 
 separating Meall Cuanoil from the highest point of Ben Cruachan. 
 These features, due to lines of weakness, are not seen on the north 
 side of the main ridge of Ben Cruachan, where the northern ridges 
 or spurs run out from the main ridge with a uniform slope into 
 Glen Noe. 
 
 In the Lome volcanic plateau small faults and lines of smash 
 in the andesitic lavas are very common, but none of these appear to 
 displace the rocks to any great extent. There are two principal 
 sets, one with a north-east and south-westerly trend, and the other 
 with a north-west and south-easterly course, coinciding with the 
 direction of the Tertiary dykes, and of these the latter set would 
 appear to be the younger of the two. H. K. 
 
 Oban and Kilmore Bistrict. — In the Oban and Kilmore district 
 the two systems of faults referred to above are well displayed, as 
 they have the eflfect of cutting up the rocks of the volcanic plateau 
 into a series of rectangular blocks. The earlier of the two sets, 
 
158 Faults. 
 
 with a north-east and south-west trend, occur at almost regular 
 intervals, and although their displacement seldom exceeds three 
 hundred feet, they extend laterally for great distances. In the 
 vicinity of Oban their downthrow is persistently to the north-west, 
 so that, notwithstanding the constant south-easterly dip of the 
 strata, the basement beds of the Lower Old Red Sandstone series 
 are repeatedly brought to the surface, as may be seen from the map 
 (Sheet 45) and the horizontal section (fig. 1). B. N. P. 
 
 In the area lying to the west of Loch Awe and south of the 
 Strath of Orchy, the most important dislocation is the great north- 
 north-east fault of Glen Pyne, which, beyond the limits of this sheet, 
 has been traced far into the Central Highlands in the direction of 
 Tyndrum and Loch Lyon. This fault, which runs along the bed of 
 Glen Fyne as far south as Inverchorachan, skirts the western margin 
 of the Glen Fyne granite, but has ruptured a felsite intrusion 
 which is associated with the latter. A fault of similar trend but of 
 smaller extent occupies the bed of Glen Shira. There are minor 
 dislocations of the same trend scattered over the area. Another 
 system of faults, and probably of later date, trend between north 
 and west with an average strike of about north-north-west. These 
 are perhaps most numerous along the eastern slopes of Glen Fyne. 
 To the south of Meall-nan-Tigearn the boundary of the garnet- 
 iferous mica schist group has been thrown out of its course for over 
 a quarter of a mile by a fault of this system. In the area between 
 Dalmally and Beinn Bhalgairean some minor faults occur with a 
 trend approximately north and south. J. B. H. 
 
CHAPTEE XIII. 
 
 GLACIAL AND RECENT DEPOSITS. 
 
 The present map may be said to be characterised by the relatively 
 small amount of superficial deposits belonging to glacial and recent 
 times, compared with the large extent of area over which bare rock 
 is either at the surface or very close to it. The glens, as a rule, are 
 narrow and the hills bold and steep, so that surface accumulations 
 would be apt to be rapidly carried away in a climate characterised 
 by a high rainfall. 
 
 i. GLACIAL DEPOSITS. 
 
 There is indeed overwhelming evidence throughout this area 
 of the great extent to which the whole country has been subjected 
 to glaciation. Everywhere along the sides of the glens, along the 
 hill slopes, and along the shores of the lochs, the smooth, polished, 
 and striated surfaces, and roohes moutonnees, bear striking testimony 
 to the work of the ice, while the morainic mounds filling the glens 
 indicate in an unmistakable manner the action of the huge valley- 
 glaciers which formerly occupied the country. By far the greater 
 part of the glacial phenomena of this area are entirely to be 
 attributed to the period of the later glaciation — a glaciation which 
 in this part of the country was so thorough as to leave little trace 
 behind it of the more extensive glacial phenomena which preceded 
 it. 
 
 1. Direction of Ice-flow. — Throughout the more mountainous 
 portion of the area to the north-east of the Pass of Brander fault, 
 the direction of the ice-flow usually coincides with tlie direction of 
 the major glens, and conforms generally to the contour of the 
 ground. The smaller glaciers belonging to the declining phase of 
 glacial times were evidently of a strictly local character and radiated 
 from the corries of the higher hills. 
 
 In this area the observations upon striated surfaces, erratic blocks, 
 and the distribution of the morainic material, show that the largest 
 glaciers had their sources of supply in the high ground to the north- 
 east, and thence followed a general south-westerly course. Thus, 
 from the elevated north-eastern area two main glaciers flowed down 
 towards the lower and more open area to the south-west. One of 
 these followed the course of Glen Etive and Loch Etive as far as 
 Bonawe, whence it tended to spread out over the less elevated 
 ground. The numerous erratics and ice-scratchings seen along the 
 shore of Loch Etive gives ample evidence of the direction followed. 
 
160 Glacial Deposits. 
 
 In the area about the western portion of the loch, where the ground 
 is less elevated, the direction of the striae is frequently slightly north 
 of west. Numerous blocks, however, of the characteristic Bonawe 
 granite are found about Oban, and these boulders have travelled in 
 a direction slightly south of west, Bonawe being on the north shore 
 of Loch Btive; 13 miles B.N.E. of Oban. The slight northerly 
 deflection to the west of Bonawe was evidently brought about by 
 the Ben Cruachan ice pressing upon the Loch Etive glacier from 
 the south-east. In the upper portion of Loch Etive the Glen Etive 
 glacier was evidently squeezed into narrow limits in the confined 
 space between Ben Starav and Beinn Trilleachan, and slightly 
 further along its course this glacier was joined by the smaller ice- 
 streams from Glenkinglass and the small glens on the west side of 
 the loch ; consequently one would expect that in this part of its 
 course the ice would have exerted its maximum power of erosion, 
 owing to the enormous lateral pressure to which it was subjected. 
 And we find in fact that this portion of Loch Etive is considerably 
 deeper than it is further south-west at Bonawe, or at its exit at 
 Connel Perry, and the evidence certainly tends to show that this 
 deepening of the upper portion of the loch was due to the eroding 
 action of the ice filling the loch-basin. 
 
 Further to the east a much larger glacier flowed down Glen 
 Orchy from the high ground about the Blackmount and Moor of 
 Rannoch, and received in its course a large tributary glacier coming 
 down Glen Lochy on the east, and further on was joined by the 
 Glen Strae ice on the north. The confluence of these three glaciers 
 evidently formed a large and powerful mass of ice, which swept down 
 to the south-west along the line of Loch Awe, spreading out over 
 the comparatively low ground on either side to a considerable extent. 
 At one time the entire area to the east of Loch Awe was evidently 
 over-ridden by ice from the north-east, as we find morainic drifts, 
 with boulders of Glen Orchy types of rock, in the hollows of the 
 higher parts of many of the hills. At a later period it appears that 
 a portion of the Beinn Bhuidhe ice travelled westward across Glen 
 Shira and over the low col separating that glen from the drainage 
 basin of the Oladdich river, and thence turned southward, by the 
 pressure of the Loch Awe ice, into Glen Aray. The larger portion 
 of the Beinn Bhuidhe ice doubtless followed the line of Glen Shira 
 on the one hand and of Glen Fyne on the other hand, while the 
 upper portions of these glens received glaciers from the high ground 
 about Meall nan Tigearn and Ben Laoigh, further to the north-east. 
 
 To the west of Loch Awe, the ice formed by the confluence of the 
 various glaciers from the more mountainous ground over-rode the 
 entire area between Loch Awe and the western part of Loch Etive, 
 with a general direction slightly south of west. Local variations in 
 the direction of flow are often met with, but are mainly due to the 
 configuration of the ground. 
 
 The Glen Orchy and Glen Strae ice evidently spread out in the 
 Loch Awe area and forced its way some distance to the west, as 
 we find boulders of the Glen Orchy kentallenite and the Glen Strae 
 
Glacial Deposits. 161 
 
 augite-diorite on the hills to the south of Loch Scamadale. Granite 
 boulders are also occasionally met with here, but increase enormously 
 in quantity in the ground further north about Loch Nell and Oban. 
 
 2. Boulder Clay. — Only a very small quantity of boulder clay is 
 known to occur in the present map, and it does not appear that any 
 part of this belongs to the earlier period of glaciation, but rather to the 
 later period of the valley-glaciers. In the Kilchrenan area boulder 
 clay is often met with in the depressions and hollows, but is of no 
 great thickness. It is seen to the west of Kilchrenan village and 
 again further to the south-west between Kilchrenan and Ooillaig. 
 It is a stiff, dark, and sometimes yellowish clay, and contains polished 
 and striated boulders. At Braglenbeg, near the head of Loch 
 Scamadale, a gravelly clay is seen containing well-scratched stones. 
 Again, to the east of Dalmally, in the morainic area on the south 
 side of the Strath of Orchy, some of the small streams have cut 
 their way down into a stiff clay with polished and scratched stones, 
 probably representing the fine detritus deposited by the glacial 
 streams connected with the Glen Orchy and Glen Lochy glaciers. 
 
 H. K. 
 
 In the Oban and Kilmore districts a sparse deposit of boulder'clay 
 is found in the hollows between the rocky knolls which characterise 
 this region, but whether the boulder clay represents the ground 
 moraine of the great ice sheet or that of the later confluent glaciers 
 is difficult to determine. The presumption is that it belongs to the 
 later period. The deposit, which is always thin, is normally a greyish 
 or reddish stiff clay with well-striated boulders of quartz-schist, 
 garnetiferous mica-schist, granites and porphyrites from the High- 
 land area to the east, and of the volcanic and other igneous rocks 
 of the Lome plateau, which outnumber all the others put together. 
 
 B. N. P. 
 
 Lismore Island. — The drift is represented by a stiff boulder clay 
 which spreads over the floors of the hollows between the limestone 
 ridges and extends up the gentler slopes. It rises into hummocks 
 in the depression near Park, and again in the valley above Fennacro- 
 chan. The matrix of the clay is, at the north end of the island, greyish 
 in colour, whilst further south it becomes nearly black. The change 
 appears to be gradual, and is doubtless due to the grinding up of the 
 limestone and its incorporation in the clay. Limestone pebbles are 
 also more abundant in the clay towards the south-west. Amongst the 
 boulders in the clay, most of which must have come from the main- 
 land, the following were noted : — Quartzite, schists of the Moine 
 type, porphyrite (from the dykes of Old Eed Sandstone age), 
 arkose (similar to that of Old Red Sandstone age in the Great 
 Glen), many varieties of granite and diorite, and an agglomerate 
 similar to one found amongst the volcanic rocks of Glencoe and 
 Ben Nevis. Large boulders of granite and quartzite occur 
 scattered over the hills at all altitudes. 
 
 The direction taken by the striae is very variable, but on the whole 
 they appear to indicate a general south-westerly movement of the 
 ice. On a small island west of Bilean Ramsay, a set of striae running 
 
162 Ghoial Deposits. 
 
 N. 5° B.-S. 5° W. are crossed by a later set running from N. 70' E. 
 Most of the striss are found on the dykes and are preserved almost 
 always between tide-marks. H. B. M. 
 
 3. Moraines. — The essential and characteristic glacial deposits 
 of this area are certainly formed by the extensive accumulations 
 of morainic materials brought down into the glens and straths by 
 the later valley-glaciers, and left after their retreat as indis- 
 putable evidences of their former presence. Nowhere perhaps are 
 these deposits better seen than in the glens and hollows of the 
 mountainous country in the north-eastern portion of the map. 
 It is perhaps along the lower portions of the glens and on the 
 lower slopes of the hill-sides that the morainic drift has mostly 
 been accumulated, and it is here that the characteristic mound-like 
 forms of the deposits are best developed. When traced up to the 
 heads of the glens this mound-like form is less pronounced and 
 often gives place to a more even and uniform rubble-like accumu- 
 lation, mingled with scree material from the upper parts of the 
 corries. There is frequently also a more high-level and flat type 
 of. moraine, consisting of loose angular gravelly material, found 
 strewn over the higher slopes and corries. The mound type of 
 moraine, however, frequently passes uninterruptedly from the head 
 of one glen to another across the intervening divide, where it is of no 
 great height in comparison with the average level of the glen. 
 Thus there is a continuous series of moraines from the head of Glen 
 Strae across the col on the east side of Beinn Cham down into 
 Allt Coire Bhicar into Glen Orchy on the east, and into 
 Gleann Fuar on the north-east. The moraines, also, of the upper 
 portion of Glen Kinglass are carried eastward into the area about 
 Loch Dochard and Glais-gobhair, and so are continuous with the 
 train of mounds running down to Loch Tulla at the head of Glen 
 Orchy. Doubtless this represents a local easterly extension of the 
 confluent glaciers from the higher hills of the Biackmount. A 
 similar continuity of moraine material is seen further south 
 between Glen Shira and the head of Glen Fyne, and from the west 
 side of Glen Shira over the low col between that glen and the 
 upper part of the Cladich River. The pressure of the Beinn 
 Bhuidhe glacier apparently pushed the upper Glen Shira ice over 
 this divide, thus causing it to flow down westward towards Loch 
 Awe. 
 
 A fine series of moraines was accumulated at the confluence of 
 the Glen Strae, Glen Orchy, and Glen Lochy glaciers, and cover a 
 considerable area of the more low-lying ground on the south side 
 of the Strath of Orchy, whence they are continued up into Glen 
 Orchy and Glen Lochy. The characteristic mound-forms are here 
 most beautifully developed, and sections of them may be studied 
 along the side of the railway in Glen Lochy. They usually consist 
 of coarse or sometimes fine gravels, indiscriminately mixed with 
 boulders of all sizes. 
 
 In the north-western portion of the map the slopes about the 
 upper end of fjoch Creran are singularly free from morainic 
 
Glacial Deposits. 163 
 
 material, the sides of the hills being very steep and flanked by 
 smoothed and striated rock-snrfaces. 
 
 In the Lome volcanic plateau morainic drift is found throughout 
 the entire area in the hollows and depressions among the hills, 
 mostly as small patches, and especially in the glens near the 
 western seaboard. Morainic mounds are well seen in Glen Lonan 
 and in the two small glens which unite just above the head of Loch 
 Scamadale. The contents of these moraines indicate that they 
 were evidently derived from the extension of the large glaciers 
 which flowed from the mountainous country to the north-east. 
 
 H. K. 
 
 Oban and Kilmore District. — The evidence supplied by the 
 roches moutonnees, striated surfaces, erratics, and the disposition of 
 the moraines and fluvio-glacial deposits points plainly to the great 
 confluent glacier from the Highland glens having passed 
 over the Oban and Kilmore region in a westerly direction during 
 the period of the later glaciation. 
 
 The strisB to be seen on the schistose rocks exposed between 
 tidemarks at Oban vary from 10° to 25° north of west, while the 
 direction of those found on the plateau to the east of that place are 
 from 10° to 30° south of west. To the south of Loch Nell the 
 striae observed on the high plateau point nearly due west, but in 
 the valleys which intersect the plateau the striee conform more or 
 less to their trend. 
 
 The roches moutonnees have their smooth faces directed towards 
 the east. 
 
 Terminal moraines marking the retreat of the ice are observable 
 both on the high plateau and in the intersecting valleys. Those 
 found on the high plateau stretch mostly in a north and south 
 direction across the block of high ground into which it is cut up 
 by the valleys. Those found flooring the valleys occur in such a 
 way as to show that lobes of ice passed down the valleys after it 
 had disappeared from the intervening portions of the plateau. 
 These phenomena are well exhibited in the Lonan, Loch Nell, and 
 Feochan valleys, in the Scamadale and Euchar valley, and the 
 intervening high plateau, the moraine stufi" being in great part 
 composed of schists and granites from the Highlands. B. N. P. 
 
 4. Erratic Blocks.— In the more mountainous portions of the area 
 erratics and perched blocks are not nearly so numerous as they are 
 over the lower ground to the south-west. Very few have been 
 observed at an altitude much over 2000 feet. They consist mainly 
 of granite, and have been derived evidently from the continua- 
 tion of the granite masses to the north-east. Thus, over the tract 
 characterised by the Ben Cruachan type of granite, erratics of the 
 coarse Glen Btive type are fairly common. In the district near 
 the head of Loch Creran large boulders of a coarse kentallenite and 
 coarse augite-diorite occur, which most probably were brought down 
 by the later valley ice from the upper part of the glen, but at present 
 the parent-rock of these boulders has not been identifled with 
 certainty, 
 
164 Glacial Deposits. 
 
 It is when we come to the less elevated ground to the south- 
 west that one is struck by the enormous quantity of erratics strewn 
 freely over the hills and glens, and distributed in a striking 
 manner according to the direction of flow and distribution of the 
 glaciers which transported them from their homes. 
 
 In the Benderloch area, situated between Loch Oreran and Loch 
 Btive, the entire surface of the ground is thickly strewn with 
 granite boulders of all sizes, and probably almost every variety of 
 the Ben Oruachan and Glen Btive mass is here represented. A 
 large boulder of calcareous schist — sufficiently imposing to have 
 been deemed worthy of the local name of Clach Dhonuill — may be 
 seen in this area a^short distance to the east of Dubh Loch M6r, 
 and was probably derived from the calcareous schists known to 
 occur in considerable quantity in Glen Creran. 
 
 Boulders of the Ben Cruachan and Glen Etive granites are 
 found over the entire area of the Lome volcanic rocks on the south 
 side of Loch Etive as far as Lochaweside, but they diminish 
 considerably in numbers as one approaches the Loch Awe basin 
 and give place to the metamorphic and other types of igneous rocks 
 of the Glen Orchy area. About Taynuilt and Bonawe the granite 
 erratics are very numerous and often of large size. Thus an enor- 
 mous boulder of the Glen Etive granite — locally named Glachma- 
 nessa — forms a conspicuous object on the shore of Loch Etive in 
 Airds Bay, while a boulder, not much inferior in size and consisting 
 of Ben Oruachan granite, is seen about a mile east of Taynuilt, and 
 is locally pointed out as " Rob Roy's Putting Stone." 
 
 Everywhere between Taynuilt and Oban similar boulders are met 
 with, and are again exceedingly numerous around Oban and 
 between Oban and Loch Peochan. In the Oban district they are 
 often of large size and sometimes must weigh at least 10 tons. 
 Further south, in the area about Loch Scamadale, the granite 
 erratics become comparatively scarce, as that tract lies in the 
 province of the boulders which were brought down from Glen Strae 
 and Glen Orchy. A certain proportion, however, of granitic 
 erratics doubtless found their way down to the lower ground by 
 way of these glens from the eastern portion of the granitic area, 
 and these boulders are certainly numerous on the shores of the 
 north-eastern part of Loch Awe. The granite boulders, however, 
 of the Loch Awe area are invariably of the Ben Cruachan type, the 
 boulders of the Glen Btive type of granite being confined to the area 
 of distribution of the Glen Btive ice. 
 
 The distribution of some of the igneous boulders from Glen 
 Orchy and Glen Strae is rendered comparatively easy to follow by 
 their distinctive and easily recognisable characters. Thus the 
 kentallenite which occurs on the west side of Glen Orchy, near the 
 eastern margin of the sheet, has been the parent of numerous 
 erratics which may be traced down Glen Orchy to Dalmally, and 
 thence for many miles in a west-south-west direction. Large 
 boulders of kentallenite occur in the Strath of Orchy and over the 
 morainic area to the south. They are again common in the 
 
Glacial Deposits. 165 
 
 Kilfihrenan area, and have been occasionally found among the hills 
 to the south of Loch Scamadale. With the exception, however, of 
 the erratics occurring a mile or so to the south of the Strath of 
 Orchy, kentallenite boulders of Glen Orchy type have not been 
 found to the east of Loch Awe. A large isolated boulder of ken- 
 tallenite lying near the head of Glen Aray has probably been 
 derived from one of the exposures of that rock in the Ben Bhuidhe 
 district. 
 
 Another characteristic igneous rock from the north-east, namely, 
 the augite-diorite of the west side of Glen Strae, is frequently met 
 with among the erratics in the Kilchrenan area, and has been 
 traced as far as the south-west comer of the sheet. In the more 
 immediate neighbourhood of the parent rock boulders of this 
 characteristic augite-diorite are exceedingly numerous, both as 
 large erratics and smaller blocks. Erratics are often found at a 
 greater height than that at which the parent mass is situated. 
 Thus, close to the Glen Orchy mass of kentallenite we find a well- 
 marked train of boulders, which may be followed in a south- 
 westerly direction up a branch of the burn in which the rock 
 occurs, and up the hill slopes between that burn and Glen Orchy to 
 a height of nearly 1500 feet, or about 600 feet above the exposed 
 parent rock. 
 
 In the Loch Awe area we also find occasional erratics of dolerite 
 derived from the east and west dykes to the north and east of 
 Dalmally. Large boulders of garnet-schist are also encountered 
 occasionally on the hills to the south of Glen Orchy and Glen 
 Lochy, and closely resemble the garnetiferous schists which occur 
 on the high ground to the east. H. K. 
 
 Oban and Kilmore District. — The most marked erratics are 
 granites from the Cruachan massif, trains of which cross the area 
 in such numbers that, in spite of their having supplied material for 
 the ashlar work of most of the buildings of the region, they still 
 form conspicuous objects in the landscape, especially in the high 
 ground to the north of Loch Nell. South of that loch they decrease 
 in number and become scarce south of Scamadale. In perhaps 
 fewer number and of smaller size occur fragments of highly altered 
 quartz-schist and silvery garnetiferous mica-schist, that had their 
 origin from still further within the Highlands than most of the 
 granite. One of the most striking evidences of the westerly move- 
 ment is supplied by an exposed portion of a large porphyrite dyke 
 occurring within the area on the hill top to the south of Olenamacrie 
 in Glen Lonan, from which a conspicuous train of boulders can be 
 traced to Torr-an Tuirc near the head of Loch Nell. B. N. P. 
 
 5. Muvio-glacial Gravels. — Stratified gravels and sands forming 
 well-marked terraces may be well seen along the northern shores of 
 Loch Scamadale, about nine miles S.S.B. of Oban. These deposits 
 rise up to 30 feet above the loch, which is 221 feet above sea-level. 
 They continue along the glen below the loch, along the sides ofthe 
 Euchar Kiver, which they follow until they finally merge into the 
 100-feet beach on the south side of Loch Feochan, in Sheet 44. 
 
166 Glacial Deposits — Raised Beaclies. 
 
 Associated with these terraces are more recent gravel deposits formed 
 out of them by the action of the stream. The terraces of stratified 
 materials were very probably of fluvio-glacial origin, the site of Loch 
 Nell being occupied by a lobe of ice during their deposition. Other 
 deposits, possibly of similar origin, may be seen in the valley between 
 Oban Cemetery and Dunstaffnage Oastle, where large accumulations 
 of sand and gravel rise to over 100 feet above sea-level, and nearly 
 all the material is obliquely laminated. Again some thick deposits 
 of fine sands and gravels, forming a terrace above the 100-feet 
 contour-line, occur along the Lusragan Burn about a mile and a 
 half south of Oonnel Perry. The terrace here indicated is probably 
 due to the overflow water from Glen Lonan at a period when the 
 Loch Btive glacier, during its decrease, sent a lobe down the Lonan 
 valley into Loch Nell, for these fluvio-glacial deposits of sand and 
 gravel, heaped up into mounds, extend from the Dig Bharrain, 
 the upper part of the Lusragan vallej^, across the col to Strontailler, 
 at the foot of Grlen Lonan, in such a manner as to indicate that they 
 were laid down along the margin of the ice during its decrease. 
 
 The " serpent " mound at the foot of Loch Nell, supposed to be 
 artificial, is the remains of the gravelly moraine formed at the end 
 of the Lonan and Loch Nell glacier during its retreat in 100- 
 feet beach times, while the fluvio-glacial gravels described as 
 occurring on the col to the north of the head of the loch point to a 
 time when the melting water from a lobe of that glacier escaped 
 across the col into the valley of the Lusragan. The fluvio-glacial 
 gravels seen along the roadside between Pennyfuar and Oban 
 were in all probability formed when the ice had retreated so far 
 that the water from a lobe of the Loch Btive ice filled the lower 
 part of this valley and escaped across into Oban at the time when 
 the land stood a hundred feet lower than at present, for on the 
 retreat of the ice the sea evidently had access to the valley, as the 
 100-feet beach is here partly cut out of these deposits. 
 
 B. N. P. and H. K. 
 
 ii. RAISED BEACHES. 
 
 The remarkable series of raised beaches for which the west 
 coast of Scotland is so well known, and some of which date back to 
 the glacial period, are well developed in the western portion of the 
 sheet. These terraces extend along both shores of Loch Etive. 
 They are well marked on the north side, where the Moss of Achnacree, 
 an expanse of peat about two square miles in area, lies upon the 
 sands and gravels belonging to the 50-feet beach. On the same 
 side of the fjord, between Achnaba and Ardchattan, the 100-feet 
 beach is well marked, though on the opposite side it is not so con- 
 spicuous. It is, however, well seen along the railway line to the 
 south and south-west of Achnacloich. This beach is again well 
 developed at Taynuilt, the railway station being situated upon it. 
 
 The more recent beaches are seen on both sides of Loch Etive 
 from the mouth of the loch up to Bonawe, and again further to the 
 
liaised Beaches. 167 
 
 north-east at the mouth of Glen Kinglass and Glen Liver. Thus 
 the 50-feet beach is well seen between the rivers Nant and Awe, 
 near Taynuilt. In the Benderloch area the same beach, besides 
 \inderlying the moss of Achnacree, covers a considerable area to the 
 north of Ledaig. In the Loch Creran area these later beaches are 
 seen at the foot of Glen Salach and on the south side of Creagan 
 Ferry. H. K. 
 
 Oban and Kilmore Region. — Near Kilmore the 100-feet raised 
 beach occupies a well-marked terrace at the foot of Loch Nell and 
 in the angle between the Feochan Mor and Feachan Bheag rivers. 
 The terrace, partly as a rock-notch and partly covered by beach 
 materials, can be traced to the south-east side of Loch Feochan for 
 some distance from its head, while corresponding high cones of 
 alluvium are thrown on to it by the side streams. 
 
 Eemains of this terrace also occur in the upper part of Oban, 
 near the Bowling Green, and at the foot of Glen Crutten. It also 
 occurs to the north-east of Oban, near the head of the valleys that 
 end in Dunstaffnage Bay and the Camas Bruaich Ruaidhe. It 
 forms a considerable breadth of country in the valley of the Lus- 
 ragan, and remains of the beach are found round the chain of the 
 Black Lochs, and conspicuous fragments of the terrace are found 
 near Achnacloich, further up the shore of Loch Etive. 
 
 Evidence has been given to show that the 100-feet beach 
 found near the foot of Loch Nell was laid down while a lobe of the 
 Highland ice still filled the site of Loch Nell. The valley of the 
 Feochan Bheag, above Craigentaggart, supplies evidence of having 
 been invaded by a similar lobe, for while below Craigentaggart there 
 is a fine terrace of the 100-feet beach, moraine mounds form the 
 floor of the valley beneath that level further up the valley. The 
 present area affords evidence of the long duration of the 100-feet 
 beach period, for it must have taken a considerable time for the ice 
 to retreat from the Kilmore area so as to allow the 100-feet 
 terrace to form as far up Loch Etive as Achnacloich. 
 
 B. N. P. 
 Benderloch and Ardmucknish Bistrict. — The most interesting 
 raised beach deposit in this region consists of remnants of the 
 100-feet beach in the form of mounds of circumdenudation, 
 locally known as the " Seabank," standing above the level of the 
 lower beaches. The deposits constituting the mounds consist 
 chiefly of laminated clays and sands with interstratified beds of fine 
 gravel. At Shian Ferry these mounds reach an elevation of a little 
 over one hundred feet. The deposit has suffered greatly from 
 denudation, and at one time the row of hills must have been sur- 
 rounded by the waters of the sea. There is no evidence that land 
 ice shared in the work of demolition. 
 
 The following is a list of shells that have been collected from 
 these deposits at high-water mark where the clays are well 
 exposed. These have been determined by Mr. E. T. Newton, 
 F.R.S. :— 
 
168 
 
 Raised Beaches. 
 
 Echinus sp. [spine of]. 
 Serpula triquetra, Mart. 
 Balanus porcatus (?), Da Gosta. 
 
 „ tintinabulum, Linn. 
 Cancer pagurus, Linne.. 
 Astarte borealis, Ohem. 
 
 ,, compressa, Mont. 
 
 ,, elliptica, Brown. 
 Cardium nodosum, Turt. 
 Cyprina islandica, Linn. 
 Mytilus edulis, Linn. 
 Nucula nucleus, Linn. 
 Nuculana pernula, Miill. 
 ,, pygmsea, Mimst. 
 
 Pecten islandicus (?), Miill. 
 Tellina sp. 
 
 Hydrobia ulvse, Penn 
 Lacuna vincta, F. & H. 
 Littorina rudis, Maton. 
 Natipa alderi (?) Forbes. 
 ,, [operculum of]. 
 Phasianella puUa, Linn. 
 Rissoia membranacea. Ad., var. 
 
 venusta. 
 Thracia sp. 
 
 Trochus ziziphinus (?), Linn. 
 Chalk flint [not worked], with 
 micro-organisms. 
 
 E. B. B. 
 
 Oban arid Kilmore Dist7-ict. — The 50-feet raised beach, the upper 
 limit of which seldom reaches a height of 50 feet in the present 
 region, is well developed at the head of Loch Feochan and up both 
 branches of the valley, where its broad terraces are mostly covered 
 by peat. Remnants of the beach are also found in the floor of Glen 
 Crutten. The terrace of this beach, partly as a rock-notch with 
 marginal sea caves, and partly covered by beach material, extends 
 from end to end of Oban. The cayes, some of which are still open 
 to the light of day, bear evidence from their contents of having 
 been inhabited at intervals from before historic times down to the 
 present. This beach occurs on the north side of the area between 
 Oban and Dunstaffnage, and fills part of the valleys opening on to 
 Loch Etive bebween Dunstaffnage Castle and a point above Gonnel 
 Ferry. It also forms a terrace of varying breadth along the south 
 shore of Loch Etive from Gonnel to Rudha nan Cam and even 
 further up the loch. Near Connel Ferry its storm beaches rise 
 above the 60-feet contour line. Good sections of the coarse gravels 
 were exposed at Connel Ferry Railway Station when the platforms 
 were being made in connection with the construction of the Connel 
 and Ballachulish Railway. 
 
 Fragments of terraces of raised beaches at a lower level than the 
 above are found in the same localities where the fifty-feet beach is 
 developed, marking shorter pauses in the general movement of 
 elevation of the region. B. N. P. 
 
 Benderloch and Ardrmtcknish District. — The raised beach indicated 
 by the well-marked rock-notch in Ardmucknish rises to the 
 approximate level of 50 feet O.D. There are great accumulations 
 of storm beach gravels in connection with it, especially along the 
 base of the cliffs that bound it to the east. 
 
 A second beach some fifteen feet lower has been cut in the 
 platform of gravel belonging to the 50-feet beach, and still 
 another but little above present high-water mark may be detected 
 in the northern part of the peninsula. E. B. B. 
 
 Zoch Oreran District. — On the north side of Loch Creran the 50- 
 feet beach forms a well-marked feature, for during the period of its 
 deposition the waters of the loch filled numerous narrow fjords in 
 
Raised Beaches — Alluvial Terraces. 169 
 
 the vicinity of North Shian and Rudha Garbh, where one is a mile 
 long with narrow flat bottom and steep sides. 
 
 The whole of the level ground around Port Appin is formed by 
 this deposit and extends up the Strath of Appin to Auchnacone 
 House. On both sides of Airds Bay this platform is a rock shelf, 
 and at Barriemore there is a well-defined beach which reaches up 
 to 25 feet. On Briska and at Balure both the 25 and 50-feet beaches 
 are represented. J. S. G. W. 
 
 Lismore Island. — The whole coast of Lismore within the sheet 
 bears a well-marked terrace at an altitude corresponding to the 
 50-feet beach of neighbouring areas. The inner edge of the rock- 
 shelf under the clifis commonly varies in altitude from 30ft. to 
 36ft. above O.D. Though usually a bare rock-ledge it is sometimes 
 covered by shingle, which contains pebbles of rocks similar to 
 those found in the boulder-clay. No shells were found in the 
 deposit. Unrearranged boulder-clay reaches down to this beach, 
 and no evidence of any higher one was obtained. The cliffs bound- 
 ing the beach are frequently hollowed out into numerous small 
 caves, whose walls are covered by stalactite. At Sron-na-Craoibhe, 
 where the cliffs are absent for a short distance, there is a curved 
 vidge of gravel, concave towards the sea, and formed of false-bedded 
 sand and gravel. The gravel is essentially of the same composition 
 as that found on the beach elsewhere. In cross-section the ridge 
 has a long gentle slope seawards and a short steep slope on the 
 landward side, where the drainage is obstructed and a peat-moss 
 has formed. Similar, though smaller, ridges are found in analogous 
 positions to the north and to the south of Port Ramsay, They 
 appear to be storm-beaches of the " 50-foot beach" period. A lower 
 beach, which is distinguishable with difficulty from a more modern 
 storm-beach, appears to be preserved only near Port Ramsay, 
 where the coast is protected by small islands. H. B. M. 
 
 iii. ALLUVIAL TERRACES. 
 
 Alluvial terraces are usually well developed in connection with 
 all the larger streams. Three main terraces can usually be 
 mapped out, and occasionally minor terraces have been cut 
 out from these main ones. Perhaps the best series of older 
 river terraces are those developed in connection with the river Awe. 
 These are well seen between the Bridge of Awe and the Pass of 
 Brander, and at Inverawe House. On the west side of the river 
 below Inverawe the second terrace merges into the 25-feet beach. 
 A series of three terraces have also been mapped in the lower parts 
 of Glen Creran, Glen Etive, and Glen Kinglass. 
 
 In the more mountainous portion of the area alluvial fans are 
 well developed about the mouths of the larger burns. The large 
 size of many of these detrital fans, e.g., along the shore of the upper 
 part of Loch Etive, and the coarseness of the material of which they 
 consist, is very striking. But this is perhaps not surprising when 
 we consider the great capacity for the transportation of material 
 
170 Alluvial Terraces. 
 
 possessed by the mountain-torrents during flood-time, and the 
 enormous quantity of more or less loose material, in the form of 
 talus debris and moraine matter, at their disposal. Thus, many of 
 the higher terraces and more recent alluvial deposits consist largely 
 of rearranged morainic drift. The work of the mountain-streams, 
 moreover, is largely assisted by small tributaries which run over 
 the bare granite, pitching at a high angle. Down these and down 
 the numerous scree-gullies and stone-shoots lining the mountain- 
 sides the water is poured in torrential cascades during times of 
 heavy rainfall, pushing onwards and piling up enormous boulders 
 as though they were small pebbles. 
 
 In this connection we may perhaps conveniently refer here to the 
 effects of lateral erosion in the lower parts of the course of such 
 rivers as the Etive, the Kinglass, and the Orchy. In the lower and 
 wider portions of the valleys slighb alterations in the courses of 
 these rivers appear to be constantly in progress from year to year. 
 The amount of this alteration may be appreciated by comparing 
 the course of the rivers, as represented on the six-inch Ordnance 
 maps, published more than 30 years ago, with the present courses 
 of the rivers and the distribution of the alluvial plains. The 
 terraces are seen to have been largely cut away in some places, 
 while fresh alluvium is being laid down in others. The destruction 
 of the terraces is naturally greatest at the convex side of a bend in 
 the stream, and is doubtless due to the increased eroding and trans- 
 porting power possessed by the. river during flood-time, when this 
 action may actually be observed. Such rivers as these rise rapidly, 
 owing to the rapidity with which their tributaries bring down the 
 water from the surrounding high ground, and also to the large 
 areas which are now drainedartificially into these streams by means 
 of trenches. Hence they become exceedingly powerful agents of 
 denudation during times of heavy rainfall. In many districts, as 
 for instance in the Strath of Orchy, where the winter floods are 
 often very heavy, the inhabitants are now obliged to construct 
 stout palisades to protect the alluvial terraces, so valuable for 
 agricultural purposes, from destruction. Near the mouth of the 
 river Etive it was ascertained that the bank of the river along the 
 outer side of a bend was being carried away at the rate of a foot or 
 more per annum. H. K. 
 
 Ohan and Kilmore District. — In the Lome volcanic plateau the 
 principal streams are bordered by alluvial terraces for the most 
 part cut out of the glacial and raised beach deposits. Thus, in the 
 Euchar valley below Loch Scamadale the river terraces are carved 
 out of the fluvio-glacial beds. Those of the Lonan and Feochan 
 rivers are eroded partly out of the fluvio-glacial beds of Glen Lonan 
 and the raised beaches below Loch Nell. The Dig Bharrain, in 
 the upper part of the Lusragan, has drained what has been a loch 
 resting on 100-feet beach deposits and now represented by a wide 
 spread of alluvium . B. N. P. 
 
Peat. 171 
 
 iv. PEAT. 
 
 The only considerable peat-mosses which occur within the 
 limits of the present sheet are those to the south of Connel Ferry 
 and in the western portion of the Benderloch area. Immediately 
 to the south of Oonnel Perry a considerable thickness of peat is seen 
 resting upon the 50-feet beach, and again, to the north of Oonnel 
 Ferry, in Benderloch, in the Moss of Achnacree (said to be Ossian's 
 " Plains of Lora"), we have nearly two square miles of peat resting 
 upon the same beach. In fact, the spread of peat over the raised 
 beaches of this area is one of the most characteristic features of these 
 platforms. It is again well seen in the flat ground to the north-west 
 of Beinn Lora, in western Benderloch. These peat-mosses probably 
 began to grow not long after the sea retired from the terraces which 
 they have overspread. The peat here reaches a maximum depth of 
 rather over 10 feet, and immediately beneath it are found the sands 
 and gravels of the raised beach. It is of a good qxiality for fuel, for 
 which it is largely used by the poorer classes of the inhabitants of 
 the surrounding districts. The surface of these tracts is now mostly 
 covered by heather, while the peat itself contains numerous stumps, 
 roots, and fragments of birches. 
 
 A comparatively thin covering of peat is frequently found over 
 many of the alluvial flats. Thus we find peat covering some of the 
 older river terraces, as in Glen Btive, Glen Kinglass, and Glen Orchy, 
 and also the more recent alluvial flats at the junction of the Orchy 
 and the Strae, near the head of Loch Awe. 
 
 Thick deposits of peat often occur in association with the silted- 
 up portions of the lochs and lochans, and forming more or less level 
 tracts among the hills, representing the sites of lochs which have 
 become entirely silted up, partly by the importation of sediment 
 and partly by the excessive growth and decay of various species of 
 water-plants. Several examples of this type of peat-moss occur in 
 the area about Kilchrenan. The flat peaty tract on the west side 
 of Loch Tromlee, about a mile and a half north of Kilchrenan, is a 
 good example of the extent to which this loch has become silted up 
 within comparatively recent times ; while about a mile to the east 
 of Loch Tromlee we flnd another larger and older peat-moss, which 
 may probably indicate the site of a former loch. 
 
 The formation of peat is again often largely associated with the 
 drift-covered areas occurring in the broader depressions among the 
 hills and in the wider parts of many of the glens. The peat thus 
 often forms considerable tracts covering the morainic gravels and 
 occupying the hollows and depressions between the numerous 
 mounds or occasionally protruding knobs of rock. A considerable 
 thickness of peat in this way occupies the hollow between Kil- 
 chrenan village and the farmhouse of Achnamady, and again 
 similar deposits are seen in the morainic area to the west of Loch 
 Dochard in the north-eastern corner of the map. In the former 
 locality the peat is of good quality and is largely used in the district 
 for fuel. In the mountainous portion of the area the peat covering 
 
172 Peat. 
 
 themorainic drift is frequently crowded with the roots and stumps 
 of Scotch fir, and fragments of birch are also common. 
 
 In the more mountainous ground there is very little peat covering 
 the upper parts of the hills, as these usually consist of sharp, rocky 
 ridges, where only bare rock is exposed, or is covered merely by a 
 thin coating of sandy soil supporting scanty heather and grass. 
 But wherever the mountain-ridges broaden and spread out to form 
 plateau-like areas, they are invariably peat-covered. We thus find 
 thick deposits of peat, resting either upon bare rock or loose rubbly 
 detritus, along the broad ridge extending from Creag Mhor, on the 
 north side of the Strath of Orchy, to Beinn Donachan. To the east 
 of Loch Awe, where the hills are of a more undulating character, 
 and consist essentially of broad elongated ridges, there is much hill- 
 peat covering all the more gentle slopes and plateau-like summits. 
 It is essentially a heather-peat, and weathers on the summits into the 
 characteristic peat-bogs, and is cut into small gullies and hollows 
 by small streams formed by the heavy rains. This hill-peat is only 
 used for fuel when it occurs within easy distance of a dwelling, 
 though doubtless it would be more largely used if more easy of access 
 and transport. 
 
 Throughout the area to the west of Loch Awe peat is everywhere 
 found covering the fiatter portions of the hill-tops, and occupying 
 the hollows, and clinging to the gentler slopes. Its average thick- 
 ness would probably be about three or four feet, but there is no 
 doubt that on the hill-tops it frequently attains a much greater 
 depth. H. K. 
 
 Oban and Kilmore District. — In this region the largest areas of 
 peat are to be found on the raised beach terraces between the head 
 of Loch Feochan and Loch Nell, and in the upper part of the Lusragan 
 valley on each side of Dig Bharrain. Considerable areas of peat 
 occupy the hollows on the plateau of volcanic rocks between Oban 
 and Loch Nell, and on the higher plateau to the south of Loch 
 Nell and Glen Feochan. B. N. P. 
 
 Benderloch and Ardmucknish District. — The 50-feet beach deposits 
 are mostly hidden under widespread peat-mosses which cover 
 a large part of the Ardmucknish peninsula. West of the escarp- 
 ment of the Old Red volcanic rocks and underlying conglomerates, 
 the volcanic plateau holds numerous small peat-mosses in its 
 hollows. B. B. B. 
 
 Loch Grera7i District. — The deepest and best peat-mosses in the 
 north-west corner of Benderloch lie around the Lochan Dubh, with 
 an average depth of 20 feet. At Ardentiny oak stools are plentiful 
 below the moss, while to the south of this there are several extensive 
 peat flats. All the peat-mosses in this portion of Benderloch are 
 situated upon the 50-feet beach, and are more or less locally used 
 for fuel. In Strath Appin and along the west side of Loch Oreran 
 there are a few scattered mosses. J. S. G. W. 
 

 
 (y 
 
 
CHAPTER XIV. 
 Economic Geology. 
 
 Building-stone, (&e.— The Ben Cruachan granite has for many 
 years been quarried at Bonawe. The rock here is of the fine- 
 grained Ben Cruachan type, and is of excellent quality for building 
 purposes. Perhaps, however, the principal work carried on is that 
 of making granite setts for road-paving, and these are shipped oS 
 to Glasgow and other places. A large quantity of the stone is also 
 crushed by machinery, and three grades of the crushed material are 
 turned oiit, the two finer grades being used in the manufacture of 
 cement. There is no great extent, however, of the particular fine- 
 grained type of stone which is found in these quarries ; it extends 
 merely for a short distance from the large quarry. Nearer the 
 margin of the granite the rock would not be worth working owing 
 to the presence of numerous inclusions of hornfelsed schist, and 
 further into the mass the rock becomes coarse and of poorer 
 quality. 
 
 At the Oraig quarries, about two miles further up the loch, a 
 coarser granite is quarried, which is of excellent quality for building 
 purposes. It is the typical hornblende-biotite-granite of the Ben 
 Cruachan mass. It can be extracted in large blocks, and is largely 
 employed for harbour work on the Clyde, for ornamental purposes, 
 and for building stone. There were formerly quarries at Craig 
 Point, but these have for some time been abandoned on account of 
 the large quantity of pink acid granite which occurs interbanded 
 with the dioritic type, and which proves to be of too crumbling a 
 nature to be of any commercial value. On the south shore of Loch 
 Btive some quarries have recently been opened between Bonawe 
 and Glennoe. The rock here resembles that of the Craig quarries 
 and is of good quality. 
 
 Near the head of Loch Awe, about three-quarters of a mile west of 
 Castles, two large quarries have been worked in the Ben Cruachan 
 granite for many years. More recently two more quarries have been 
 opened in a fairly coarse quartz-diorite traversed by aplite veins. 
 (Plate VII.) The stone is used for building and ornamental purposes, 
 and recently has been employed by the Caledonian Eailway Com- 
 pany in building the stone supports for the new bridge across 
 Loch Btive, on the branch line to Ballachulish, at Connel Ferry. 
 
 In parts of the. area where granite boulders are common, these 
 have been largely used, when of good quality, for building 
 purposes. 
 
174 Economic Qeohgy. 
 
 The dolerite of the east and west dykes is also an excellent 
 building stone of very durable quality. , It has been quarried at 
 Inverlochy, near Dalmally, and several of the houses in that district 
 are built of it. 
 
 The andesite of the Lome plateau is usually a poor building 
 stone, being so much split up by small joints. Occasionally, how- 
 ever, some of the more massive flows have been quarried success- 
 fully lor building purposes. The red porphyrite, when not too far 
 decomposed or shattered, is, however, a building-stone of very fair 
 quality, and the quality is found to improve with the depth from 
 the surface at which the rock can be quarried. 
 
 Slate. — The black slates were formerly quarried at Kilchrenan 
 for roofing purposes, and slate pencils were also manufactured 
 from them. Eoofing slates were also manufactured from the 
 black slates at Drumvargie, Oban. These quarries have, however, 
 for some time been abandoned. 
 
 Limestone. — The largest area of limestone is to be found in Lis- 
 more. At Port Ramsay it is burnt in ordinary open draw -kilns, and 
 produces a yellowish " thin " lime, in which the impurities are 
 mainly sand derived from the quartz grains in the limestone. 
 It is used chiefly for lime-manuring by the farmers of the neigh- 
 bouring districts, and to a less extent as a building lime. The 
 demand is insufficient to give constant employment throughout 
 the whole year. 
 
 The limestone of the Kilchrenan area was also formerly burnt in 
 many localities in that district. 
 
 Boad Metal. — Road metal is everywhere easily procurable, and 
 usually the local rock of any particular district is used for this 
 purpose. The best quality of stone, however, is undoubtedly the 
 granite of Ben Cruachan and the dolerite from the east and west 
 intrusions and the Tertiary dykes. Porphyrite, andesite, epi- 
 diorite, limestone, and quartzite are also largely employed for many 
 of the roads. H. K. 
 
 Pottery. — The quartzite at Port Appin is quarried and conveyed 
 to Glasgow, where it is ground down and used in the manufacture 
 of " delft ware." J; S. G. W. 
 
APPENDIX. 
 
 BIBLIOGRAPHICAL. 
 
 List op Woeks reperring to the GisoLOGr op the District 
 
 INCLUDED IN ShEET 45. 
 
 1799. Saint-Fond, B. Faujas. — Travels in England and Scotland and 
 
 the Hebrides. Translated from the Fi-ench. London, 1799. 
 Vol. I. (Bun-Awe), p. 305 and (Natural History of the 
 Environs of Oban), p. 330. Vol. II. (The Isle of Kerrera), p. 144. 
 
 1800. Jameson, R. — Mineralogy of the Scottish Isles, Vol. I. (Sail, 
 
 Easdale, and Oban), p. 192-199. 
 
 1810. Imrie, Lieut.-Ool. — Some Remarks upon the Pudding or Con- 
 glomerate Rock which stretches along the whole of the South 
 Front of the Grampian Mountains in Scotland, from where 
 they commence in the West to where they finish their course 
 towards the East in the German Ocean. Mems. Wern. Soc, 
 Vol. I., p. 453. 
 
 1814. MacCulloch. — Remarks on the Vitrified Forts of Scotland (Con- 
 glomerate of Lome). Trans. Geol. Soc, Vol. II., p. 264. 
 
 1817. MacCulloch, J. — Observations on the Mountain Cruachan in 
 Argyllshire, with some remarks on the Surrounding Country. 
 Trans. Geol. Soc, Vol. IV., p. 117, and Thomson's Annals of 
 PM.,Vol.V.,p. 151. 
 
 1821. Macknight, T. — Mineralogical Notices and Observations. Mem. 
 Wern. Soc, Vol. 3, p. 104. 
 
 1840. MacCulloch, J. — A Geological Map of Scotland, with Memoir on 
 do. (published several years after MacCulloch's death). 
 
 1844. Nicol, J.— Guide to the Geology of Scotland. 8vo. Edin. 
 
 1851. Hopkins, W. — The Dispersion of Granite Blocks from Ben 
 Cruachan. Edin. New Phil. Jour., Vol. XLIX., p. 334, and 
 Vol. LIII. (1852), p. 362. Brit. Assoc Rep., Vol. XX., p. 59. 
 
 1851. Hopkins, W. — On the Granitic Blocks of the South Highlands of 
 Scotland. Quart. Jour. Geol. Soc, Vol. VIII., p. 20. 
 
176 Appendix. 
 
 1853. Argyll, Duke of. — Inveraray Granite District. Quart. Jour. Geol. 
 Soc, Vol. IX., pp. 134-360. 
 
 1855. Bedford, E. J.— Notice of some Raised Beaches in Argyllshire. 
 Quart. Jour. Geol. Soc, Vol. XI., p. 549, and Vol. XII. (1856), 
 p. 167. 
 
 1859. Nicol, J. — On the Slate Rocks and Trap Veins of Easdale and 
 Oban. Quart. Jour. Geol. Soc, Vol. XV., p. 110. 
 
 1862. Murchison, Sir R., and A. Geikie. — First Sketch of a New 
 
 Geological Map of Scotland, with Explanatory Notes. 
 
 1863. Nicol, J. — On the Geological Structure of the Southern Grampians. 
 
 Qiiart. Jour. Geol. Soc, Vol. XIX., p. 180. 
 
 1865. Geikie, (Sir) A. — The Scenery of Scotland viewed in connection 
 with its Physical Geology, p. 174, etc. 
 
 1898. Argyll, Duke of. — On the Physical Geography of Argyllshire in 
 connection with its Geological Structure. Quart. Jour. Geol. 
 Soc, Vol. XXIV., p. 255. 
 
 1871. Turner, (Sir) W. — On Human and Animal Bones and Flint from 
 a Cave at Oban, Argyllshire. Brit. Assoc Rep., p. 160. 
 
 1874. Geikie, J. — The Great Ice Age (Loch Etive, from the Admiralty 
 Chart), p. 300, etc. 
 
 1874. Judd, J. W. — The Secondary Rocks of Scotland. Second Paper 
 on the Ancient Volcanoes of the Highlands. Quart. Jour. Geol. 
 Soc, Vol. XXX., p. 220. 
 
 1886. Bell, D. — Notes on the Geology of Oban. Trans. Geol. Soc Glasg., 
 Vol. VIII, p. 116. 
 
 1888. Bell, W. H. — Geognosy of the Cruachan District. Trans. Geol. 
 Soc Edit}., Vol. v., p. 513. 
 
 1892. Dakyns, J. R., and J. J. H. TealL— On the Plutonic Rocks of 
 Garabal Hill and Meall Breac. Quart. Jour. Geol. Soc, Vol. 
 XLVIII., p. 104. 
 
 1 892. Geikie, Sir A. — Geological Map of Scotland, reduced chiefly from 
 the Ordinance and Geological Surveys, with Explanatory Notes. 
 Edin., Bartholomew & Co. 
 
 1895. Argyll, Duke of. — Two Glens and the Agency of Glaciation. 
 Trans. Roy. Soc Edin., Vol. XXXVIII., p. 193. 
 
 1895. Anderson, J. — Notice of a Cave recently discovered at Oban con- 
 taining Human Remains, and a refuse-heap of Shells and Bones 
 of Animals and Stone and Bone Implements. Proc Soc. 
 Antiq. Scot., Vol. XXIX., p. 211. 
 
Appendw. 177 
 
 1895. Turner, Sir W. — On Human and Animal Remains found in Caves 
 at Oban, Argyllshire. Proc. Soo. Antiq. Scot., Vol. XXIX., 
 p. 410. 
 
 1897. Geikie, Sir A. — Ancient Volcanoes of Great Britain. Vol. I. (The 
 Lake of Lome), p. 341. 
 
 1897. Geikie, Sir A.— The Old Red Sandstone of Lome. Nature, Vol. 
 
 LVL, p. 157. 
 
 1898. Anderson, J. — -Notes on the contents of a small Cave or Rock 
 
 Shelter at Druimvargie, Oban. Proc. Sac. Antiq. Scot., Vol. 
 XXXII., p. 298. 
 
 1898. Traquair, R. H. — On a New Species of Gephalaspis, discovered 
 by the Geological Survey of Scotland, in the Old Red Sand- 
 stone of Oban. Trcms. Roy. Soc. Edin., Vol. XXXIX., 
 p. 591. 
 
 1898. Symes, R. G., and H. Kynaston. — Loch Awe Schistose Rocks, pp. 
 
 60-62., Fossils, 82-83. Volcanic Rocks, 84-85. Granitic and 
 Volcanic Rocks of Ben Cruachan and Loch Etive, 85-93. 
 Glaciation, p. 144. Summwry of Progress of the Geological Survey 
 for the Year 1897. 
 
 1899. Geikie, Sir A., R. G. Symes, and H. Kynaston.- — Schists, pp. 
 
 38-39. Lower Old Red Sandstone Rocks and Fossils, pp. 71-75. 
 Ben Cruachan and Glen Etive Granites and Contact Metamor- 
 phism, pp. 76-93. . Carboniferous Rocks, pp. 76 and 129-130. 
 Volcanic Vent near Loch Scamadale, pp. 153-155. Glacial 
 Notes, pp. 169-170. Ibid, for 1898. 
 
 1899. Peach, B. N. — On some New Myriapods from the Palaeozoic 
 Rocks of Scotland. Proc. Roy. Phys. Soc. Edin., Vol, XIV., 
 p. 113. 
 
 1899. Hill, J. B. — On the Progressive Metamorphism of some Dalradian 
 
 Sediments in the region of Loch Awe. Quart. Journ. Oeol. Soc, 
 Vol. LV., p. 470. 
 
 1900. Peach, B. N., J. B. Hill, and H. Kynaston.— Schistose Rocks. 
 
 pp. 42-47. Intrusive Rocks later than the Foliation, pp. 47-60. 
 Glaciation, pp. 162-163. Old Red Sandstone Ostracoda, 
 p. 186, Summary of Progress of the Geological Survey for 
 1899. 
 
 1900. Hill, J. B., and H. Kynaston. — On Kentallenite and its relation 
 
 to other Ingeous Rocks in Argyllshire. Quart. Journ. Geol. 
 Soc, Vol. LVL, pp. 531-557. 
 
 1901. Hill, J. B.— On the Crush Conglomerates of Argyllshire. Quart. 
 
 Journ. Geol. Soc, Vol. LVIIL, p. 313 
 
 M 
 
178 Appendix. 
 
 1901. Kynaston, H., and W. Pollard.— Lodh Awe Boulder Bed, Schists, 
 and Kentallenite, pp. 30-40. Analysis of Kentallenite, p. 160, 
 Old Red Sandstone Lavas, pp. 71-73. Ben Criiachan Granite, 
 pp. 73-75, Summary of Progress of the iieologioal Survey for 
 the Year 1900. 
 
 1901. Kynaston, H. — On some Tuifs associated with the Andesitic 
 Lavas of Lome. Trans. Edin. Geol. Sac, Vol. VIII., p. 87. 
 
 1901. Geikie, Sir A. — The Scenery of Scotland viewed in connection 
 vnth its Physical Geology. 3rd edit. Loch Etive, p. 205. Loch 
 Awe, pp. 154 and 260. 
 
 1901.— Heddle, M.F.— The Mineralogy of Scotland. 2 Vols. Edin. 
 
 1902.— Peach, B. N., H. Kynaston, and H. B. Muff.— Loch Awe Dis- 
 trict, pp. 123-126. Old Red Sandstone Volcanic Series, pp. 
 134-136. Unconformable Junction between Schists and Old 
 Red Sandstone, p. 136. Lismore, pp. 140-141. Summciry 
 of Progress of the Geological Survey for the Year 1901. 
 
 1903. — Macnair, P. — The Building of the Grampians. Proc. Roy. 
 Philos. Soc. Glasgow, Vol. XXXIV., p. 147. 
 
INDEX. 
 
 ACHAHAFAUNDICH, 55. 
 
 Achavaich, 37, 72. 
 
 Achnacloich, 65, 66, 67, 76, 78, 166, 
 
 167. 
 Achnamady, 29, 30, 32, 45, 171. 
 ActinoHte, 10, 11, 12, 14, 16, 17, 18, 
 
 50, 53, 54, 55, 56. 
 Acurragh, 27, 49. 
 Agate, 67. 
 Agglomerates, 6, 7, 8, 62, 67, 68, 69, 
 
 135. 
 Agriculture, 4. 
 Airds Bay, 164, 169. 
 Airds Park, 77, 78. 
 Airds Point, 64, 65, 78. 
 Albite, 11, 49, 50. 
 Albite schists, 6, 8, 11, 13, 47. 
 AUt an t-Sithein, 103, 104. 
 
 Coire Creachan 96, 100. 
 
 Coire Ghlas, 96, 97. 
 
 Mhoile, 96, 99, 100. 
 
 na Faing, 14, 15, 18. 
 
 Odhar, 54. 
 
 Alluvium, 70, 169. 
 
 Amygdaloidal structures, 42, 43, 44, 
 
 64, 67. 
 Analcite, 125, 126, 127. 
 Analyses, chemical, 105, 126, 128, 129, 
 
 131, 146. 
 An Caorachan, 11, 12, 18. 
 Andalusite, 10, 11, 145. 
 Andesites, 64-69, 72, 74, 75, 77, 78, 
 
 99, 110, 151 . 
 Anorthoclase, 127. 
 An Sithein, 152. 
 Aplites, 108. 
 Appin, 1, 4, 58. 
 Ardconnel, 72. 
 Ardentiny, 124, 125, 172. 
 Ardmucknish, 35, 124, 125, 126, 167, 
 
 168, 172. 
 
 Bay, 36, 58, 59, 63. 
 
 Ardrishaig, 1. 
 
 series, 6, 8, 17, 19, 20, 21, 22, 
 
 23, 27, 33, 35, 48, 49. 
 Ardtomish, 155. 
 Area and physical features, 5. 
 Aschen-structur, 79. 
 Ash-structure, 79, 80. 
 Asterocalamites scrobiculatus, Schl., 
 
 155. 
 Augite-diorite, 120. 
 Awe River, 1, 3, 113, 167, 169. 
 
 B 
 
 Bailey, E. B., 110, 124. 
 
 Balinoe, 135, 137, 138. 
 
 Ballaohulish, 34, 36, 91. 
 
 Balhmore, 42. 
 
 Barbreck, 43. 
 
 Barnacarry, 73. 
 
 Barriemore, 34, 169. 
 
 Babrow, G., 145. 
 
 Basalts, 7, 17, 20, 69, 75, 122, 123. 
 
 Basalt-jasper, 129. 
 
 Bealach nan Cabrach, 19, 23. 
 
 Beinn-a-Bhuiridh, 84, 85, 96, 99, 100, 
 
 122, 140, 143, 150, 157. 
 Beinn Bhalgairean, 2, 20, 158. 
 Bhuidhe, 2, 16, 17, 19, 20, 23, 
 
 44, 48, 49, 57, 91, 95, 102, 103, 105, 
 
 116, 120, 152, 160, 162. 
 
 Cham, 157, 162. 
 
 Chas, 15, 18. 
 
 Churalain, 33, 34. 
 
 Donachain, 56, 172. 
 
 Duirinnis, 84, 85, 112, 117, 148, 
 
 149. 
 
 Eunaich, 82, 96. 
 
 Ghlas, 2, 27, 28, 66, 67, 68, 77. 
 
 Laoigh, 2, 16, 17, 19, 21, 22, 47, 
 
 49, 160. 
 
 Lora, 62, 64, 65, 112, 171. 
 
 Lurachan, 91. 
 
 na Sroi, 39, 119, 122. 
 
 Sguliaird, 2, 82, 84, 86. 
 
 Starav, 3, 87, 88, 117, 160. 
 
 Trilleachan, 87, 88, 157, 160. 
 
 Belnahua, 36. 
 
 Ben Bheula group, 10-12. 
 
 Bhreac, 33, 156. 
 
 Cruachan, 1, 2, 8, 82, 96, 157. 
 
 Benderlooh, 2, 4, 35, 58, 62, 64, 65, 
 
 69, 78, 112, 124, 164, 167, 168, 171, 
 
 172. 
 Beregonium, 36, 58, 59. 
 Biotite schists, 40. 
 Black Lochs, 3, 167. 
 Blackmount, 1, 2, 3, 39, 51, 86, 87, 98, 
 
 114, 115, 151, 160. 
 Black schists, 2, 24, 25, 33, 37, 48, 
 
 143. 
 Bochyle, 55. 
 Bonawe, 1, 3, 4, 7, 62, 64, 82, 84, 112, 
 
 117, 139, 148, 149, 159, 164, 166. 
 Boulder bed, 6, 8, 26, 27, 28, 30, 
 
 31, 32, 59. 
 Boulder clay, 6. 1 61 
 
180 
 
 Iiidex. 
 
 Bovuy, 42, 43. 
 
 Braglenmore Bum, 132, 134, 135. 
 Brannie Burn, 17, 103, 104, 152. 
 Breccias, 67, 75, 80, 81, 83. 
 Bridge of Awe, 6, 154, 156, 169. 
 BBoaoER, Prof., 106. 
 Buchite, 129. 
 Building stone, 173. 
 
 C 
 
 Calc-bpidotb-schist, 52. 
 Calc-silicate hornfels, 30, 33, 48, 142, 
 
 153. 
 Camas an Fhais, 35, 124, 125. 
 Caraptonites, 124, 125, 129. 
 Carboniferous rocks, 154, 155. 
 Carn Gaibhre, 66, 67. 
 Castles, 173. 
 Catiunis, 91. 
 Ceann Garbh, 17, 18, 19, 23, 49, 
 
 118. 
 Cephalanpis lornensis, Traquair, 69, 70, 
 
 72. 
 
 sp., 70. 
 
 Chert, 30. 
 
 Chloritic schists, 12, 14, 20, 41, 
 
 44. 
 Chromite, 105. 
 Cipolin, 142. 
 Cladich, 1, 27, 28, 42, 44. 
 
 River, 3, 160, 162. 
 
 Clae's-gobhair, 82, 85, 88, 89, 114, 115, 
 
 162. 
 Cleavage planes, 22. 
 Clenamacrie, 74, 165. 
 Clough, C. T., 123. 
 Coire Cruachan, 96, 99. 
 Collins, J. H., 90. 
 Colour banding, 22. 
 Conglomerates, 6, 62, 63, 66, 67, 69, 
 
 70, 72, 154. 
 Connel Ferry, 1, 3, 9, 38, 63, 64, 65, 
 
 72, 73, 160, 166, 168, 171. 
 Contact-metamorphism, 140-143. 
 
 by epidiorite, 28, 30, 45. 
 
 by kentallenite, 152. 
 
 by newer granites, etc., 
 
 11, 15, 21, 29, 32, 45, 48, 120, 139- 
 
 153. 
 by Tertiary camptonites, 
 
 129. 
 
 of epidiorites, 150. 
 
 of lamprophyres, 152. 
 
 of Lome andesites, 151. 
 
 of Sedimentary series, 140- 
 
 152 
 Cordierite, 130, 143, 146, ]47. 
 Corryghoil, 55. 
 Corundum, 144. 
 Cowal, 10, 13, 16, 17, 22. 
 
 Oraigentaggart, 73, 74, 167. 
 
 Craig Point, 85, 173. 
 
 Creag Mhor, 67, 172. 
 
 Creagan Ferry, 32, 167. 
 
 Cruach Ard-dhuine, 65, 66, 75, 78. 
 
 Crush conglomerates, 58-61. 
 
 D 
 
 Dakyns, J. R., 102. 
 
 Dalintart, 72. 
 
 Dalmally, 1, 3, 8, 19, 22, 24, 34, 44, 
 
 52, 53, 91, 117, 119, 121, 158, 161, 
 
 164, 165, 174. 
 Dalnacabeg, 72, 73. 
 Diabase, 42. 
 
 Dig Bharrain, 166, 170, 172. 
 Diorite, 7, 81, 85, 89, 90, 91, 95, 107, 
 
 109, 111, 116, 120. 
 
 DiTTMAB, VON CaBL, 147. 
 
 Dolerite, 7, 42, 44, 121-124, 129. 
 Dolomitic limestone, 33, 35. 
 
 schist, 34. 
 
 Drumvargie, Oban, 174. 
 Dunstaflfnage, 70, 168. 
 Dunstaffnage Castle, 166, 168. 
 Dykes and sills, associated with newer 
 
 granite iind lava flows, 111-121. 
 Dykes, composite, 89, 90, 112, 113, 
 
 123, 132, 134, 135. 
 
 E 
 
 Eas am Damh, 133, 134. 
 Easdale, 36. 
 
 slate group, 35, 36, 38. 
 
 Economics, 173, 174. 
 Eilean Dubh, 37, 121. 
 
 Ramsay, 37, 121. 
 
 Epidiorites, 2, 7, 8, 17, 20, 23, 27, 28, 
 
 29, 30, 41, 45, 47, 57. 
 Epidosite, 141. 
 Epidotic schists, 12, 14, 20. 
 Eriska Island, 33, 34, 169. 
 Erratic blocks, 6, 160, 163. 
 Etive River, 170. 
 Euchar River, 73, 165. 
 valley, 163, 170. 
 
 Falls of Cruachan, 140, 156. 
 Faults, 3, 7, 12, 13, 15, 70, 75, 156, 
 
 157, 158. 
 Feliiitic lavas, 66, 67, 68, 77, 78. 
 Fennacrochan, 37, 80, 107, 108, 109, 
 
 161. 
 Feochan Bheag, 72, 73, 74, 167. 
 Firth of Lome, 62. 
 
Index. 
 
 181 
 
 Fissure vents, Tertiary, 135, 137. 
 Flbtt, Dr., 34, 58, 100, 107, 129, 134, 
 
 145. 
 Flow brecciation in lavas, 73. 
 structure in lavas, 64, 66, 67, 73, 
 
 77,^ 78, 97. 
 Fluvio-glaoial gravels, 165. 
 Fluxion-foliation, 100. 
 Folding and progressive regional 
 
 metamorphism, 47-61. 
 Ford, 3. 
 
 Formations and rock groups, 6-9. 
 Fossils from Old Red Sandstone, 69-72. 
 
 Carboniferous, 155. 
 
 Raised Beach, 168. 
 
 Fraoch Eilean, 22. 
 Fyne River, 12, 15, 102. 
 
 group, 
 
 G 
 
 Garnetiferous mica-schist 
 
 15-18. 
 mica-schists, 6, 8, 14, 15, 16, 17, 
 
 18, 38, 39, 40, 47. 
 Garnets, 15, 16, 17, 18, 20, 44, 49, 50, 
 
 53, 54, 55, 56, 58. 
 Gbikie, Sir A., 127. 
 Glacial deposits, 159-166. 
 Glen Aray, 160, 165. 
 
 Creran, 1, 8, 33, 41, 116, 119, 
 
 120, 164, 169. 
 
 Crutten, 37, 38, 70, 71, 72, 73, 
 
 75, 167, 168. 
 
 Etive, 1, 159, 164, 169, 171. 
 
 Glenfeochan, 67, 76, 163. 
 
 Glen Fyne, 1, 2, 3, 4, 7, 8, 11, 12, 13, 
 
 14, 16, 17, 18, 19, 40, 48, 49, 95, 
 
 102, 111, 117, 158, 160, 162. 
 Kinglass, 2, 82, 83, 85, 86, 
 
 87, 114, 120, 160, 162, 167, 169, 
 
 171. 
 
 Liver, 167. 
 
 Lochy, 1, 17, 19, 28, 39, 40, 51, 
 
 119, 121, 122, 160, 161, 162, 165. 
 
 Lonan, 64, 65, 66, 67, 75, 77, 
 
 112, 163, 165, 166, 170. 
 
 Nant, 4, 42, 65, 66, 76, 77, 116. 
 
 Noe, 82, 84, 88, 157,' 173. 
 
 Orchy, 39, 44, 53, 54, 56, 58, 89, 
 
 90, 91, 119, 120, 122, 160, 161, 162, 
 
 164, 165, 171. 
 Salach, 1, 3, 7, 33, 45, 62, 70, 
 
 156, 167. 
 Shira, 2, 3, 4, 8, 19, 20, 23, 27, 
 
 44, 47, 48, 111, 116, 119, 122, 158, 
 
 160, 162. 
 Strae, 1, 39, 56, 58, 82, 83, 89, 
 
 90, 112, 120, 156, 160, 162, 164, 
 
 165. 
 Gneissose grits, 10, 15. 
 Granite, 7, 8, 11, 20, 23, 
 
 Granite Beinn Bhuidhe, 102. 
 
 Ben Cruachan, 7, 21, 29, 32, 45, 
 
 48, 82-91, 101, 102, 111, 112, 114, 
 117, 164, 173. 
 
 basic and acid modifi- 
 cations, 84, 85. 
 
 contact zone, 83, 96, 
 
 139. 
 
 dykes cutting, 113. 
 
 gneissic structure, 86. 
 
 marginal differentia- 
 tion, 95. 
 
 biotite, 87. 
 
 Blackmount type, 82-87, 88, 
 
 164. 
 
 Glen Fyne, 11, 13, 48, 102, 
 
 116. 
 
 hornblende-biotite, 84. 
 
 Luclusions in, 149. 
 
 micro. 111. 
 
 porphyritic, 86, 87. 
 
 Granitite hornblende, 85, 86. 
 
 Granulitic gneisses, 10, 15. 
 
 schists of Moine type, 38-40. 
 
 Graphitic schist, 17, 28, 29, 36, 37, 
 53. 
 
 Gravels, 6, 165. 
 
 " Green Beds," 12, 14, 17, 47, 50. 
 
 Grits, 63. 
 
 H 
 
 Haekee, a., 129, 131, 133. 
 
 Hayfield, 21. 
 
 Heddle, Dr., 127, 128. 
 
 Hill, J. B., 24, 47, 59, 89, 91, 98, 109, 
 
 177. 
 Hornblende andesites, 66, 68. 
 
 lamprophyres, 89, 109. 
 
 rock, 89. 
 
 schist, 7, 8, 12, 14, 22, 40, 41-46, 
 
 50. 
 Hornfels, 8, 21, 23, 29, 30, 32, 33, 45, 
 
 48, 142, 143, 153. 
 
 I 
 
 Igneous rocks, 7, 8, 11, 41. 
 
 Inistrynich, 19, 23. 
 
 Intrusive rocks, 7, 8. 
 
 Inveraray, 1. 
 
 Inverawe, 22, 169. 
 
 Inverchorachan, 11, 12, 14, 15, 18, 102, 
 
 158. 
 Inverinan, 8. 
 Inverloohy, 53, 119. 
 Isoclinal folding, 29. 
 
 Joint-planes in granite, 
 JuDD, Prof., 155. 
 
182 
 
 Index. 
 
 K 
 
 Kampecaris forfarensis, 69, 72. 
 
 Kentallen, 91, 105. 
 
 Kentallenite, 7, 17, 20, 48, 89, 91-95, 
 
 103-107, 121, 152. 
 Allt an t-Sithein and Brannie 
 
 Burn, 103. 
 
 Glen Orchy, 91-95. 
 
 chemical comparisons, 105. 
 
 composition, 105. 
 
 contact alteration by, 152. 
 
 distribution, 107. 
 
 mineral associations of, 106. 
 
 modifications of, 92, 93, 104, 107. 
 
 petrological affinities of, 93, 95- 
 
 104, 106, 121. 
 Kerrera, 63. 
 Kersantites, 119. 
 KiDSTON, R., 155. 
 Kilbride, 38, 67, 71, 72, 76, 77. 
 Kilchrenan, 1, 3, 4, 8, 29, 30, 31, 42, 
 
 43, 45, 48, 57, 76, 112, 115, 150, 
 
 161, 164, 171, 174. 
 Kilchrenan Burn, 4. 
 Kilmartin, 3, 59. 
 Kilmelfort, 1. 
 Kilmore, 1, 69, 72, 73, 137, 157, 161, 
 
 163, 165, 167, 168, 170, 172. 
 Kinglass River, 3, 170. 
 Kynaston, H., 28, 65, 75, 89, 91, 
 
 109. 
 
 L 
 
 Lacboix, Prof., 142. 
 Lag na Liunge, 22. 
 Lairig Torran, 100, 157. 
 Lamprophyres, 7, 8, 17, 20, 89, 92, 
 
 109, 111, 116, 119, 120, 121, 152. 
 Landslips, 73, 74. 
 Lapilli, 65, 79, 80. 
 Larig, 55. 
 Lava flows, 62, 63, 72. 
 
 brecciation of, 66, 67. 
 
 Lavas, acid, 62, 68, 77, 78. 
 
 basic, 62, 64, 68. 
 
 intermediate, 62, 68. 
 
 vesicles in, 64, 67, 75, 77. 
 
 Ledaig, 63, 64, 167. 
 Lime, manufacture of, 174. 
 Limestone, 2, 6, 8, 13-15, 20, 21, 22, 
 
 23, 24, 25, 27, 28, 29, 33, 34, 37, 47, 
 
 69, 174. 
 Lismore, 1, 4, 8, 35, 37, 47, 58, 80, 
 
 107, 109, 121, 122, 123, 153, 161, 
 
 169, 174. 
 "Lit. par Lit." injection, 83, 142. 
 Loch Awe, 1, 2, 3, 4, 8, 19, 20, 21, 22, 
 
 23, 24, 27, 30, 41, 42, 43, 47, 48, 49, 
 
 51, 57, 59, 62, 68, 84, 85, 111, 116, 
 
 117, 139, 140, 143, 150, 156, 164, 
 
 171, 172, 173. 
 
 Loch Awe grits and quartzite, 24, 26, 
 
 28-32 
 
 1_ group, 23-37, 47, 48, 52. 
 
 Craignish, 69. 
 
 Creran, 1, 2, 4, 8, 32, 33, 34, 35, 
 
 45, 47, 62, 82, 83, 84, 166, 162, 163 
 
 164, 167, 168, 172. 
 Dochard, 39, 40, 82, 85, 86, 162^ 
 
 171. 
 
 Ericht, 157. 
 
 Etive, 1, 2, 3, 4, 7, 62, 63, 65, 
 
 67, 70, 74, 82, 83, 84, 86, 111, 112, 
 
 139, 148, 150, 156, 157, 159, 160, 
 
 164, 166, 167, 168, 173. 
 Feochan, 72, 122, 132, 134, 135, 
 
 164, 165, 167, 168, 172. 
 
 Fyne, 12. 
 
 Laidon, 157. 
 
 Linnhe, 4. 
 
 MeKort, 62: 
 
 Nant, 4, 29, 66, 68, 69. 
 
 Nell, 3, 71, 72, 73, 74, 163, 165, 
 
 166, 167, 170, 172. 
 Scamadale, 1, 3, 8, 67, 68, 73, 74, 
 
 119, 122, 132, 133, 134, 136, 161, 
 
 164, 166, 170. 
 
 Tromlee, 171. 
 
 TuUa, 162. 
 
 Lochan Dubh, 36, 172. 
 
 Dubha, 67, 68. 
 
 Loohy River, 3, 51, 53. 
 
 Lome " traps," 62. 
 
 Lusragan Bum, 166, 167, 170, 172. 
 
 M 
 
 Magnetite veins, 92. 
 Malacolite-homfeis, 149. 
 Marble, 33, 45, 142, 163. 
 Meall an Laoigh, 89, 90, 91. 
 
 Copagach, 91, 120. 
 
 Cuanail, 96, 157. 
 
 Garbh, 82, 83. 
 
 nan Tigeam, 17, 19, 118, 158, 
 
 ■ 160. 
 
 Reamhar, 17, 18. 
 
 Mesaca/nthus mitchelli, 69. 
 Metamorphism, 10, 21, 22. 
 
 regional, 47-68. 
 
 contact. See Contact meta- 
 morphism. 
 Mica andesites, 66, 67, 68. 
 
 schists, 15, 22, 40. 
 
 Michel L6vy, 83, 142. 
 Micro-granite, 111. 
 Minettes, 110, 119, 121. 
 Modiola macadmni, Portl., 156. 
 Moine schists, 6, 8, 38. 
 Monadh Driseag, 85, 96, 140, 143. 
 Monchiquites, 124, 127. 
 Moor of Rannoch, 3, 157, 160: 
 Moraines, 6, 162. 
 
Index. 
 
 183 
 
 Moss of Achmoree, 166, 167, 171. 
 Mudstones, 62, 73. 
 
 N 
 
 Na Cailleachan, 56. 
 Nant River, 167. 
 New Inverawe, 29. 
 Newer igneous rooks, 82-110. 
 Newton, E. T., 167. 
 North Shian, 169. 
 
 O 
 
 Oban, 1, 2, 4, 8, 37, 38, 58, 62, 63, 64, 
 69, 70, 72, 73, 122, 137, 157, 160, 
 161, 163, 164, 165, 166, 167, 168, 
 170, 172, 174. 
 
 Old Red Sandstone, 6, 7, 29, 62-81. 
 
 Older igneous rocks, 41-46. 
 
 Olivine monzonite, 91. 
 
 Orchy River, 3, 19, 51, 52, 55, 170, 
 171. 
 
 Ostraoods in Old Red Sandstone, 69. 
 
 Pass of Beandeb, 1, 3, 7, 8, 19, 21, 
 29, 32, 42, 45, 47, 48, 51, 57, 62, 66, 
 68, 69, 70, 82, 83, 84, 112, 113, 116, 
 121, 139, 140, 143, 150, 151, 154, 
 156, 169. 
 
 Peach, Dr. B. N., 155. 
 
 Peat, 6, 9, 171. 
 
 Pegmatite, 39. 
 
 Pennyfuir, 72, 166. 
 
 Phyllites, 2, 6, 8, 20, 22, 53. 
 
 Piootite, 105. 
 
 Picrite, 90, 92, 93, 94, 95. 
 
 PiRssoN and Weed, Messrs., 106. 
 
 Plant remains in Old Red Sandstone, 
 69. 
 
 Pollard, Dr. W., 105, 128, 131. 
 
 Population, 4, 6. 
 
 Porphyrite, 2, 20, 45, 87, 96-101, 111- 
 114. 
 
 Port Appin, 33, 34, 169, 174. 
 
 Ramsay, 169, 174. 
 
 Sonachan, 31, 43, 44. 
 
 Pottery, 174. 
 
 Progressive metamorphism. See Meta- 
 morphism, regional. 
 
 Pterygotus anglicus, 69, 72. 
 
 Pyrites, 36, 37, 52. 
 
 magnetic, 53. 
 
 Q 
 
 Quarries, 76, 85, 173. 
 
 Quartzites, 2, 6, 8, 20, 21, 22, 23, 25, 
 
 26, 27, 28, 32, 33, 34, 35, .36, 37, 38, 
 
 39, 40. 
 
 Quartzose schi.sts, 21, 22, 33, 38. 
 Quartz-porphyry, 7, 8, 87, 111, 112. 
 segregations, 18, 23. 
 
 R 
 
 Ra Craw, 19, 24, 51. 
 Raised beaches, 6, 9, 35 70, 166. 
 Recent deposits, 6, 159. 
 Regional metamorphism. See Meta- 
 morphism. 
 Rhyolites, 7, 132, 133. 
 Rholitic felsites, 7. 
 River erosion, 170. 
 
 systems, 2. 
 
 Road metal, 29, 174. 
 Roche moutonnees, 159, 163. 
 Roofing slate, 25, 29, 48. 
 RosENBUscH and Iddings, 90. 
 Ruadh Garbh, 34, 35, 169. 
 
 S 
 
 Sailean Sligenach, 125, 126, 129. 
 
 Salomon, von W., 146, 147. 
 
 Sand dykes, 64, 73. 
 
 Sands, 6. 
 
 Sandstones, 6, 62, 63, 66, 68, 69, 70, 
 
 73, 154. 
 Sanidine, 78. 
 Soamadale, 74. 
 Scarba, 129. 
 Scenery, 2, 9, 11, 18, 20, 41, 49, 51, 
 
 62, 65, 86, 88, 122, 123, 156. 
 Sohiehallion, 8, 34. 
 Selma, 36, 59, 70. 
 Sericite schists, 20, 22. 
 Shales, 6, 62, 63, 66, 68, 69, 70, 72, 
 
 73, 74, 154. 
 Shells from raised beach, 168. 
 Shian Bay, 33, 34. 
 Shira, 27. 
 Sior Lochs, 65, 78. 
 
 Slates, 22, 24, 25, 29, 35, 36, 48, 174. 
 Socach, 17, 19, 22, 28, 52, 58. 
 SoU, 4. 
 
 Spessartites, 110, 119, 120, 121. 
 Spherulitic structures, 66, 77, 78, 123, 
 
 124. 
 Spinelle, 145. 
 Sron Mhor, 27. 
 Sron na Craoibhe, 37, 108, 110, 121, 
 
 123, 169. 
 Stac a Chuirn, 20, 23, 102. 
 Stob Ghabhair, 82, 85, 115. 
 Strae River, 3, 157, 171. 
 Strath of Orchy, 4, 8, 19, 28, 38, 44, 
 
 47, 48, 51, 52, 53, 57, 111, 165, 170, 
 
 172. 
 Strathappin, 33, 34, 156, 169, 172. 
 Syenite, 107, 108. 
 Symes, R. G., 63, 136, 137. 
 
184 
 
 Index. 
 
 Tailor's Leap, 66, 77. 
 
 Tait, D., 154. 
 
 Taycreggan, 43. 
 
 TaynuUt, 1, 4, 65, 66, 67, 75, 76, 77. 
 
 78, 112, 164, 166, 167. 
 Teall, Dr. J. J. H., 85, 90, 102, 106, 
 
 113, 120, 140, 143, 150. 
 Teatle water, 3, 19, 20. 
 Tertiary dykes, 8, 38, 70, 122. 
 Tervin, 21, 22, 29. 
 Teschinites, 125, 129. 
 Thelodus sp., 69. 
 Timber, 4. 
 Tom Bharra, 28. 
 Tonalite, 107, 108. 
 Torr-an-Tuiro, 73, 165. 
 Torr Dhamh, 71, 74. 
 Tourmaline, 13, 54. 
 Trachytes, 7, 132, 133. 
 Traquaie, Dr., 70. 
 Tuffs, 7, 62, 64, 65, 66, 67, 68, 69, 78. 
 acid, 65, 66, 68, 69, 78. 
 
 U 
 
 UNCONrOBMITY, 28, 30. 
 Upper Sonachan, 28, 31. 
 Uralite, 99. 
 
 V 
 
 Variolitic structxjre, 124. 
 Veins, pegmatite, 11, 39, 56, 94. 
 
 quartz, 11, 23, 39. 
 
 Vogesite, 90, 110, 119, 121. 
 Vogelsang, Carl, 147. 
 Volcanic plateau, 2, 71 , 74. 
 rocks of Lower Old Bed Sand- 
 stone, 62-81. 
 • vents, 7, 74, 135, 137, 138. 
 
 W 
 
 Weed and Pirsson, Messrs., 106. 
 Williams, G. H., 106. 
 Williams, Gilbert, 144. 
 Wilson, J. S. Grant, 91, 119. 
 
Plate VIII. 
 
 Fig. 1 
 
 Fig. 2. 
 
 Fig. 3, 
 
 Fig. 4. 
 
 Fig. 5. 
 
 Fig. 5. 
 
 PHOTOMICROGRAPHS. 
 
 PhotomiciTj. T.O.N. 
 
DBSORrPTION OF PHOTOMICROGRAPHS. 
 
 Plate VITI. 
 
 Fig. 1. — Variolite, nortli-easfc corner of Camas an Fhais, Ardmucknish 
 [magnified 35 diameters, slide 10950 B] ; showing the 
 branching and plumose habit of the skeleton crystals of 
 pyroxene in this rock. In other parts of the slide the 
 felspars form large, coarse, radiate spherulites. 
 
 Fig. 2. — -Oamptonite, west shore of Fionn Aird, about j-mile north of 
 Rudha Fionn Aird, Ardmucknish [magnified 60 diameters, 
 slide 13197]. A. large crystal of decomposed olivine is visible 
 in one corner of the field. The rest is occupied by brown 
 hornblende, purplish-brown augite, and plagioclase felspar. 
 The hornblende is rendered in dark shades, but shows its 
 characteristic outlines in transverse sections. 
 
 Fig. .3. — Anorthoclase or Soda Microoline, from monchiquite dyke, 
 west shore of Camas an Fhais, Ardmucknish [magnified 25 
 diameters, slide 10946]. The cross-hatching is visible in the 
 photograph (which was taken with crossed nicols) and also 
 the variable double refraction in this mineral. 
 
 Fig. i. — Buchite or Vitrified Phyllite, north-east corner of Sailean 
 Sligenach, Ardmucknish, Argyllshire [magnified 250 dia- 
 meters, slide 11821]. Curved trichitic growths of green 
 augite surround a small crystal of magnetite and lie in a 
 matrix of clear brown glass. 
 
 Fig. 5. — Phyllite, north-east corner of Sailean Sligenach, Ardmucknish 
 [magnified 21 diameters, slide 11833]. This shows the 
 unaltered rock at a distance of six feet from the margin of 
 the dyke. It is well foliated and consists of chlorite, 
 muscovite, quartz, and iron-oxides. 
 
 Fig. 6. — Buchite or Vitrified Phyllite (in contact with camptonite 
 dyke), same locality as above [magnified 37 diameters, 
 ordinary light]. Crystals of cordierite, with glass enclosures, 
 lie in a clear brown glassy matrix which is dusty-looking from 
 the abundance of microliths of augite and grains of magnetite, 
 etc. The cordierite prisms have hexagonal cross-sections. 
 In polarised light they break up into sectors, showing that 
 they consist of interpenetrating twins. 
 
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