June 1912. 9. the GLEN ORCHY

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164 THE Qr.~N ORCaY ANrICtX~r,. [June 1912.

9. The GLEN ORCHY ANTICLINE (ARGYLLSHIRE). By EDWARD BATTERS~r B~Ir.EY, B.A., F.G.S., and MtraRAY MACeR~e0R, M.A., B.Sc. (Read February 28th, 1912.) [PLATE X--~IAr.]

CONTENTS. Page I. Introduction ...... 164 II. History of Research ...... 166 III. Tectonics ...... 167 (h) The Beinn Udlaidh ~ Fold ...... 167 (B) The Loch Doehard Fold ...... 172 (C) Rocks overlying those of the Beinn Udlaidh and Loch Doehard Folds . 172 (D) The Beinn Doirean Inversion ...... 177 IV. Difficulties ...... t77 V. Bibliography ...... 178

I. INTRODUCTION. IN a previous paper [1, p. 586] 2 an explanation has been offered of the tectonics of the schists occupying a district of the West High- lands between Loch Tulla and Appin, in the south, and the River Spean, beyond Ben Nevis, in the north (fig. 1). In the present paper the south-eastern continuation of this district is dealt with, from the head of Loeh Awe to Beinn Aehallader, one of the summits of the Beinn Doirean range (P1. X). In the district originally described lying to the west and north of Loch TuUa the following stratigraphical sequence was established; but, whether it should be read upwards or downwards, is still unaseertained :-- 9. Eilde Flags. 8. Glen Coe Quartzite (fine-grained). 7. Leven Schists (grey phyllites, with ' Banded Series' next the Glen Coe Quartzite). 6. Ballachulish Limestone (dark grey, with a thick cream-coloured margin next the Leven Schists). 5. Ballachulish Slates (black). 4. Appin Quartzite (pebbly). 3. Appin Limestone (cream, pink, or very pale blue). 2. Appin Phyllites (with a large proportion of flaggy quartzite). 1. Cull Bay Slates (black). These rock-groups, as previously shown, are disposed in two major recumbent folds, known as the Ballachulish and Appin F~)lds respectively, to which may be added the subordinate Aonach Beag Fold, recognized as yet in the neighbourhood of Ben Nevis only. For convenience of description, the cores of these three folds have been arbitrarily defined as those portions which are made up of i Pronounced approximately B e n O od I y. References in square brackets are to the Bibliography, ~ V, p. 178. Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on May 31, 2016

:Fig. 1.--S]cetch-map and section illustrating the effect of the Glen, Cretan Syncline and Glen Orchy Anticline ~on the recumbent folds of Ballachulish and Appin, (Scale of man--1 : 506,880.) Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on May 31, 2016

]66 MR. E. B. BAILEY AND XR. M. XACGHE{]OR ON [June 1912,

rocks included in Groups 1-6 of the foregoing stratigraphicaI table. Fig. 1 (p. 165), which shows the new district in relation to the old, has been constructed on this basis, and illustrates very clearly how the Ballachulish Core has been bent into a widely-extended secondary syncline in Glen Creran and into a corresponding anticline in Glen Orchy. The presentation of the evidence which has enabled us, as we believe, to trace the outcrop of the Ballachulish Core round the rim of the denuded Glen Orchy Anticline is the main feature of the following account, incidentally, however, a description is given of a particularly convincing example of a recumbent fold in Beinn Udlaidh, and of an important inversion of the Ballachulish Core in Beinn Doirean.

II. ]~ISTORY 0F RESEARCH. A comparison between our map (P1. X) and the adjacent northern corners of Sheets 45 & 46 of the Geological Survey Map of Scot- land shows many new features of importance. The subdivision of the schists did not attract special attention when the original mapping of this particular district was carried out. Before publishing Sheet 45, however, a joint traverse of a few days' duration was,~ undertaken in 1907 by Dr. ]=[orne, Mr. Clough, and Mr. J. B. Hill, to revise the boundary-line of the Eilde Flags, or, as they were called at that time, the ' Moine Series.' For one day Mr. Kynaston was also present, as he happened to be in Scotland on furlough from the Transvaal. The party kept to the east of the fault which runs up Glen Strae, and for this part of the district the line engraved on the published map to represent the boundary of the 'Moines' has been of great service to us. During the same visit the quartzite and mica-schist of Beinn Udlaidh were differentiated from the 'Moine Series' along the bottom of Glen Orchy, where this valley enters Sheet 45. The quartzite of tile Beinn Udlaidh outcrop was considered to be quite distinct from that which occurs south-west of the ' Moine Series' farther down the glen. In the following pages we suggest, however, that these two quartzites are on one and the same horizon; but this is a matter wherein there is still room for differences in regard to interpretation. The results of the joint traverse were incorporated [2, p. 38] in a short account published in the Memoir dealing with Sheet 45. In 1908 one of the present writers accompanied Mr. Clough [3, p. 63] on a second brief visit to the district; it was hoped, and the hope was justified, that light would be thrown upon the stratigraphical succession of the schists in the complicated country north-west, of the Etive Granite. Much was learnt during the traverse, mainly as a result of Mr. Clough's experience in mapping :Highland schists. The Beinn Udlaidh quartzite and mica-schist were carried eastwards into Sheet 46, where the latter bad already been in part separated from the' Moines' by the late Mr. Grant Wilson. The folded nature of the Beinn Udlaidh rocks was Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on May 31, 2016

Vol. 68.] Trrv er.E~ ol~cHY AI,ITICrINv.. 167 suspected, but not proved. It was found, too (as was, in fact, already known at the time), that a band of rock mapped as epidiorite by Grant Wilson around the flanks of the Beinn Doirean range (Sheet 46) was in reality a tremolite-schist representing an impure limestone, frequently accompanied, it is true, by definite sills of epidiorite. The AIR Coire an Easain section was mapped, and a metamorphic limestone was found in it. This and the Beinn Doirean limestone were both taken to be on the same horizon as another bettor-known limestone already mapped by Messrs. Kynaston, ~aufe, and Clough in the region of the Windows of Etive. During the same traverse the Loch Dochard limestone was visited and its true nature recognized by )Jr. Clough, who, in Mr. Kynaston's absence, modified the account given of it in the Memoir descriptive of Sheet 45, then passing through the press. Another conclusion arrived at as a result of this traverse was the correlation of the mica-schist mapped by Grant Wilson in Beinn Doirean with that occurring in the Beinn Udlaidh, Loch Dochard, Allt Coire an Easain, and Etive sections. This correlation has subsequently been maintained, although in 1910 it was pointed out that the Loch Dochard mica-schist is probably on a much lower structuraI level than that of Allt Coire an Easain and of the region around the Windows of Etive [1, p. 616]. The results outlined above were all obtained by members of tl~e Geological Survey Staff employed in their official capacity. Our field-work ]ast year was carried on in holiday-time. As will be seen presently, it has confirmed the suggestion just mentioned in regard to the relative structural positions of the mica-schists of Loch Doehard and Allt Coire an Easain, and has shown, what was totally unexpected even in 1910, that. the mica-schist of Beinn Udlaidh is similarly situated on a much lower structural level than that of Beinn Doirean. Another unforeseen discovery is the finding of outliers on the summits of the :Beinn Doirean range in which the Beinn ])oirean limestone and its associated strata. occur in inverted sequence, as compared, that is, with the succession prevalent throughout the rest of the district now considered. This last result was obtained by Mr. Macgregor, who has re- mapped the Peinn Doirean range. Beinn Udlaidh and the countrv to the south has been revised'by Mr. Bailey. The Loeh Dochard Fold, and the sections of Allt Co're an Easain and of the Windows of Etive, have not been re-examined since the publication of the earlier paper already referred to [1]. IIl. TEcTonics. In describing the geology of the Glen Orehy district there can be no question as to where we should begin. Beinn Udlaidh affords a firm foundation whereon to build. (A) The Beinn Udlaidh Fold. Three rock-groups enter into the composition of the Beinn Udlaidh Fold--a flag group, a quartzite group, and a mica-schist group. Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on May 31, 2016

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170 ~. v,. B. BAILEY AND MR. ~. :WACGR~e0~ 0~ [-June x9x2,

The flag group consists of well-bedded quartzo-felspathic gneisses, in which there is a constant alternation of more and less micaeeous layers. In places small pebbles of quartz or felspar have been recognized, but it is fairly certain that the original impure sandy sediments, from which the gneissose flags have been derived, were essentially fine-grained. The flags occurring in Beinn Udlaidh resemble exactly the Eilde Flags of the Loch Eilde district east of Loch Leven, and we have accordingly placed them in this group. Rocks of like character occur, almost uninter- ruptedly, between Beinn Udlaidh and Loch Eilde ; although, since they are crossed by the Glen Strae Fault and also by the Glen Coe Fault with its accompanying Fault-Intrusion [4, p. 611], their outcrop cannot properly be described as continuous. Over 500 feet of flags are exposed ill the slopes of :Beinn Udlaidh, and the group is certainly very thick indeed. The succeeding division consists of fine-grained, white, highly siliceous, thinly-bedded quartzite. In Beinn Udlaidh it varies in thickness from about 50 to 100 feet; but it is much thinner south of Glen Lochy, where it averages about 15 feet--and for a short distance north of Lochan Bhe (pronounced Vay)it is entirely absent. Apart from its small thickness and its thinly- bedded character, this quartzite agrees very closely with the Glen Coe Quartzite o[ the type-locality. Since, moreover, the sediments associated with it on either side are precisely such as accompany the Glen Coe Quartzite, we feel great confidence in referring it to this well-known horizon. In certain outcrops previously described in connexion with the Glen Coe district, among them that situated near Loch Dochard, which is included in the map (Pl. X) accompanying the present paper, the Glen Coe Quartzite is very thin, and in some cases altogether absent. Such occurrences, up to the present, have a]l been explained as a result of slidingl; but we hesitate, without local evidence, to extend this interpretation to account for the thinness of the Glen Coe Quartzite in Beinn Udlaidh. It seems equally probable here that the feature may be an original sedimentary character. A banded passage-zone links together the quartzite and mica-schist groups of Beinn Udlaidh. It is very well exposed in the northern spur of the mountain crossed by Section C (figs. 2 & 3, pp. 168 & 169), and also at the side of the road which runs along Glen Orchy (Section H). In the latter locality the Banded Series, as we may call it, consists of intercalated quartzite and grey mica- schist, with thin black seams of pelitic material. An exactly similar banded series links the Glen Coe Quartzite of the Loch Leven and Glen Nevis district with the Leven Schists. The main mass of the mica-schist group of Beinn Udlaidh consists of well-crystallized, homogeneous, grey, pelitic mica- schist, often studded with conspicuous garnets. Garnets are well 1 Slide is used throughout this paper as a syaonym of fold-fault [1, p. 593]. Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on May 31, 2016

Vol. 68.] THs ,~Lz~r ORCEY X~XXCL~Z. 171 known in the Leven Schists of the type district, but are not often really, conspicuous, except in the outcrop which passes immediately west of the Kinlochleven Reservoir. Apart from this difference, depending upon metamorphism, the mica-schists of Beinn Udlaidh resemble the Leven Schists of Loch Leven and Glen l~evis very closely; and, as the rest of the sequence is so clearly comparable, we have no hesitation in regard to their correlation with this group.

The tectonics Of Beinn Udlaidh are extremely simple and diagrammatic. The Leven Schists and Glen Coe Quartzite have been folded into the heart of the Elide Flags in the form of a recumbent anticline 1 closing towards the north-west. The minimum magnitude of this Beinn Udlaidh fold is 2 miles. Owing to a variety of favourable circumstances--including the nature of the topography, the abundance of exposures, the marked lithological contrasts, the absence of slides, and also of any serious complication due to later movements,--the Beinn Udlaidh fold is clearer and more readily understood than any yet described in the Scottish High- lands; in fact, it can take its place alongside of the best examples of small-scale recumbent folds afforded by the Alps. Localities showing the close of the fold are, of course, of critical importance. They are two in number: one of them is the steep western slope of Beinn Bhreac Liath 2 lying between Sections A & B (figs. 2 & 3, pp. 168 & 169). Exposures are not continuous in the most important part, but they are fully sufficient, for at short intervals the hill is washed bare by lateral streams ; at the south- eastern end of the slope the escarpments of the various bands give rise to conspicuous crags. Half-way up the hill-face a thick mass of gently-inclined mica-schist forms a very definite belt, with quartzite and flags both above and below ; in a north-westerly direction this belt of mica-schist thins out, allowing the upper and lower bands of quartzite to come together : finally, the quartzite disappears, and the flags are left continuous. The other locality for showing the close of the fold is situated at the bottom of Glen Orehy (Sections F, G, & H, p. 169). Nothing could be much clearer, once the mapping has been completed, than the manner in which the band of Glen Coo Quartzite, underlying the Leven Schists, sweeps across the River Orchy to meet the band which overlies the same. A little west of the crucial point the overlying band is displaced by a small normal fault; but, fortu- nately, this accident does not in the least affect the matter at issue. Exposures, although not continuous, are quite adequate, and can be reached at once from the road and from a bridge which crosses the River Orchy in the most convenient position imaginable. As appears from the map and sections (figs. 2 & 3, pp. 168 & 169), t Anticline is here used in the sense of anticline of observation , p. 600]; there is no implication that the Leven Schists and Glen Coe ~ uartzite are older than the Eilde Flags. 2 Pronounced approximately Ben Vrec Le ca. Q. J. G. S. No. 270. o Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on May 31, 2016

172 MR. ]~. B. BAILEY AND MR. M. MACGREGOR ON [-June ~912, the Beinn Udlaidh Fold has been bent into a gentle secondary anticline, which in the vicinity of Beinn Udlaidh is very clearly dome- shaped. This dome is a part of the extensive anticlinal structure already spoken of as the Glen 0rchy Anticline. Readers familiar with Scottish geology will not fail to realize the essential resemblance between the :Beinn Udlaidh Fold bent into the Glen Orchy Anticline and the Carrick Castle Fold bent into the Cowal Anticline, as described several years ago by :Mr. Clough [5, fig. 47, p. 204, & pl. x]. (B) The Loch Dochard Fold. At Loch Dochard the :Eilde Flags enclose a mass of garnetiferous mica-schist, with a central core of tremolite-schist representing limestone. Thin quartzite locally occurs at the junction of the flags and the mica-schist. Upon indirect evidence the mica-schist and the limestone have been correlated with the Leven Schists and the Ballachulish Limestone respectively, and it has been suggested that they occupy an anticlinal fold rising up fl'om beneath the Eilde Flags [-1, p. 616]. Analogy with the Beinn Udlaidh Fold greatly strengthens this interpretation. At the same time, the relation of the two folds cannot be very simple, for the Leven Schists of Loch Dochard are exposed far to the north-west of the line along which the Glen Coe Quartzite and Eilde Flags close round the termination of the Leven Schists of Beinn Udlaidh.

(C) Rocks overlying those of the Beinn Udlaidh and Loch Dochard Folds. A glance at the map (Pl. X) shows that a well-defined succession of rooks follows the Eilde Flags that overlie the other groups included in the Beinn Udlaidh and Loch Dochard Folds. This succession, stated in downward structural sequence, is as follows :-- (e) Unclassified schists, consisting, in their lower part, of pebbly quartzite and black pelitio schist. (d) Thick grey pelitic schists. (c) Thin limestone. (b) Banded pelitic schists and quartzite. (a) Quartzite, locally very thick. (a) On the east side of the Glen Orehy Anticline the quartzite overlying the flags first puts in an appearance in Beinn Doirean. When followed southwards it swells out enormously, and where it crosses Glen Orehy it is fully 1000 feet thick. In most of its course the whole of the deposit is somewhat impure, and of a grey or pinkish tint. In Glen Orchy, however, especially in the south- western slopes, there is a thick mass of pure white siliceous quartzite at the top of the group. On both sides of Glen Strae, pebbles can be readily detected, and in Allt a' Ghiubhais 1 the)" are very large and abundant; but elsewhere the quartzite is ~horoughly fine-grained. 1 Pronounced approximately Alta Gyuse. It is the tributary, shown in P1. X, entering Glen Strae almost at right angles from the east. Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on May 31, 2016

u 68.] Tar. er.~r ox~ca~r x~ICr.IN~,. 173

The north-western side of the Glen Orchy Anticline is in large measure obliterated by the Etive Granite; but in Allt Coire an l~asain, within the boundary-fault of the cauldron subsidence of Glen Coe, thin quartzite follows the Eilde Flags in a perfectly regular manner. It will be noticed that in the diminished thickness of its quartzite Allt Coire an Easain agrees very closely with Beinn Doirean, which it directly faces across the anticline o~" Glen Orchy. The thinning in Allt Coire an Easain is but local, moreover, and 2 or 3 miles to the north-west the quartzite emerges in great force from beneath an unconformable covering of Glen Coe lavas. It is also exposed in the Windows of Etive half a dozen miles west of Allt Coire an Easain, and here again it is of considerable thickness. In all these north-western exposures the quartzite is fine- grained, white, and highly siliceous, and it has been proved beyond reasonable doubt to belong to the horizon of the Glen Coe Quartzite ~l, pp. 607 & 609]. Accepting this correlation, it is fair to draw the following important conclusion. The Eilde Flags, although underlain by Glen Coe Quartzite in Beinn Udlaidh, are overlain by the same group all round the rim of the denuded Glen Orchy Anticline from Allt Coire an :Easain to Beinn Doirean. This involves, of course, the existence of a large-scale recumbent fold. It may be pointed out that the rather impure character of the Glen Coe Quartzite, in the upper limb of the fold just mentioned, can be paralleled in the type region in many 6uterops north of Loch Leven. In these, however, the somewhat impure variety is as a rule closely associated with the more conspicuous white rock, which is justly regarded as typical of the Glen Coe horizon. The thinning of the Glen Coe Quartzite in Allt Coire an F.asair~ and Beinn Doirean deserves closer investigation than we have been able to afford, for the time at our disposal was strictly bruited.

(b) The Glen Coe Quartzite, everywhere in its course around the Glen Orehy Anticline, except perhaps in Allt Coire an Easain, is overlain by a Banded Series of grey mica-schist with quartzite ribs. Where it is exposed in the Windows of tgtive, this Banded Series has been shown to be that part of the Leven Schists which normally comes next to the Glen Coe Quartzite.

(c) In the Beinn Doirean range the Banded Series is overlair~ immediately by a somewhat calcareou s tremolite-schist which varies from about 5 to 30 feet in thickness. This impure highly- metamorphic limestone is more persistent in a northerly direction than the underlying Banded Series and Glen Coe Quartzite, so that for some few miles it rests directly upon the Eilde Flags. It is found again, with about the same thickness, aloug the southern rim of the Glen Orchy anticline, where it has been traced for considerable distances. It is not so highly metamorphic here as iu Beinn Doirean. On the summit south of Glen Lochy, for example, where reference to Mr. Kynaston's field-maps shows that 02 Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on May 31, 2016

174 ~R. E. B. VaILE~ ann ME. N. MaceR~OR O~ [June ~ 9 ~ z, he was the first to note its presence, it contains very little tremolite indeed, and is most fitly described as a grey, highly calcareous mica-schist, with occasional seams of cream-coloured limestone. The next good exposures of the limestone are met with in Glen Orchy, near a sheepfold close to the road and in several streams draining the ridge on the north-west side of the valley; in fact, Mr. J. B. ]:Jill shows it on the published 1-inch map, Sheet 45, near the summit of this ridge, and has also noted its presence in his field-mapsmalong a tributary stream flowing in the opposite direction into Glen Strae. In these exposures it is sometimes about 40 feet thick, and is a cream-coloured limestone, sandy and mica- ceous, and, so far as has been observed, free from tremolite. On the west of Glen Strae it has been traced in clear exposures for half a mile from the margin of the Etive Granite towards Loch Awe ; it is here 20 or 30 feet thick, and is in the condition of calc-silicate- hornfels owing to the proximity of the Etive Granite. It is similarly affected by the granite where it reappears in ALlt Coire an Easain and in the Windows of Etive. In these two last-mentioned exposures it has been proved that the limestone belongs to the Ballachulish Limestone Group; and that it is underlain by a fold-fault, known as the Ballachulish Slide, which cuts out the thick pelitic portion of the Leven Schists that one might expect to find between this limestone and the Banded Series below [1, pp. 607 & 609]. As the condition of affairs characteristic of these" north-eastern sections continues throughout the region now under consideration, it seems necessary to suppose that here also the Ballaehulish Limestone rests upon the Balla- chulish Slide ; and incidentally we are led to postulate a displace- ment of 24 miles, or more, along this slide-plane. It is possible that the marked transgression of the limestone, which brings it to rest directly on Eilde Flags north of Beinn Doirean, is due to the Ballachulish Slide; but this is a point which has not been closely investigated in the field.

(d) The grey pelitic mica-schist which follows above the limestone is a most useful group, from the map-maker's point of view, on account of its being very thick and therefore conspicuous. At the northern extremity of the Beinn Doirean range it contains a considerable proportion of quartzose material; but in Beinn Achallader, Beinn Doirean, and Beinn a' Chaisteil it is very free from such intercalations, and is a massive garnetiferous mica-schist of the type that occurs in the heart of the Beian Udlaidh Fold. In some sections this massive mica-schist starts immediately on the top of the limestone ; in others it contains a band or so of quartzite interbedded near its base. South of Glen Lochy the mica-schist is again garnetiferous, and contains quartzite bands--some of which lie a little above the limestone. In the Glen Orchy exposures and in the outcrop near Loch Awe, west of the Glen Strae Fault, bands of quartzite are found in the mica-schist near the limestone ; and above this comes massive grey pelitic schist, very seldom Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on May 31, 2016 u 68.] THS aL~.~ 0RCHY A~TlCr.Z~. 175 garnetiferous. In Allt Coire an Easain and in the Etive sections the limestone is succeeded by a great thickness of grey pelitic mica- schists, free from quartzite and locally garnetiferous; in these north-eastern sections it is quite certain that we are dealing with the pelitic portion of the Leven Schists. In one feature, the presence of thin quartzite bands between the limestone and the main mass of the pelitic schists, the sections round the southern rim of the Glen Orchy Anticline differ from many of the Beinn Doirean exposures and also from those afforded by the whole of the type-region of Ballachulish, including Allt Coire an :Easain and Glen Etive. In the type area the Balla- chulish Limestone is, it is true, sometimes extremely siliceous near its junction with the Leven Schists; but, at the same time, actual bands of quartzite are not known to occur in the mica-schist close to the limestone. We emphasize this difference because the natural way to read the southern sections, if we pay no regard to the Balla- chulish and Glen Coe evidence, is to interpret the limestone as merely marking an episode in the accumulation of the Banded Series. On account of the Ballachulish and Glen Coe evidence, however, we regard the local appearances as deceptive in this matter, and we believe that the quartzite bands overlying the limestone are quite distinct from those of the Banded Series underneath. The assumption that a few thin beds of quartzite have locally come into the Leven Schist Group near its junction with the Ba]lachulish Limestone is, in igself, by no means startling.. It is much easier to accept this than to dispute the correlation of the Beinu Doirean, Glen Orchy, Allt Coire an Easain, Glen Etive, Glen Coe, and Ballachulish Limestones.

(e) The Leven Schists overlying the Ballachulish Limestone east and south of the Glen Orchy Anticline are succeeded by a thick group of pebbly quartzites and black pelitic schists. In Beinu a' Chaisteil and near the head of Loch Awe there is a strong development of pebbly quartzite at the base of the group. In the intervening Glen Orchy district, however, this quartzite is not very recognizable. The line inserted on our map for the base of this pebbly quartzite and black schist group, indexed as ' unclassified schists,' is strictly provisional: we have as yet merely raised the question of the relation of these schists to the Leven Schists below, without having had time to carry through an investigation of the subject. The evidence so far obtained is conflicting: on the one hand, there are suggestions in some localities, especially in Glen Orchy, of a transitional zone linking the Leven Schists with the pebbly quartzites and black schists above; on the other hand, one cannot help suspecting that this appearance is deceptive, since i.n the Glen Etivo district the pelitic portion of the Leven Schists is capped by outliers of perfectly normal Banded Series and Glen Coe Quartzite. The Beinn Doirean range will probably supply the answer to our Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on May 31, 2016

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Vol. 68.] THe. GLEN ORCHr ANTICLINE. 177 riddle when it comes to be more fuUy investigated, especially in its northern continuation. Meanwhile, we take this opportunity of drawing attention to a feature in the tectonics of the range which may in the future prove to be of decisive importance in solving the problem.

(D) The Beinn Doirean Inversion. The massive garnetiferous mica-schist of the Beinn Doirean range is capped by three important outliers, in which the structural sequence, given in descending order, is as follows :m :Fine-grained quartzite. Banded mica-schist and quartzite. Thin calcareous tremolite-schist. After careful examination, it seems practically certain that we are dealing here with exactly the same sequence as that which underlies the garnetiferous mica-schist, only in inverted order. It is a very small point of difference that quartzite is found farther north in the outliers than in the basal outcrop. The broad exposures of th6 Banded Series in the outliers are due, not to any excessive thickness of the group, but merely to the form of the ground. The question immediately arises: what is the relation between these inverted outcrops and the pebbly quartzite of the ' unclassified " schists' on the east ? Two alternative explanations suggest them- selves (fig. 4, 176), and we have, as yet, not been able to decide between them. Even a preliminary examination makes it clear, however, that the pebbly quartzite rests either directly, or almost directly, upon the great garnetiferous mica-schist. Accordingly, we followed the approximate junction of the two, in a hurried traverse, in order to see whether we could identify the thin zones of the outliers in this position. In a cliff-exposure in Beinn a' Chaisteil we found what might stand for representatives of this sequence, including a thin tremolite-schist ; but we prefer for the present not to lay much stress on this coincidence, as our search at other points remained unrewarded.

IV. ]DIFFICULTIES. Our main conclusions and suggestions are sufficiently summarized in the maps and sections which accompany this paper. In the following paragraphs u few important difficulties will be dealt with in succession. (a) 1% definite zone of mylonite marks the outcrop of the Ballachulish Slide in this, any more than in the type region. In explanation it is suggested that sliding, when it occurs under con- ditions such as' lead to regional metamorphism, does not necegsarily result in the production of mylonite. 1 l Mr. Clough draws our attention to the fact that the pre-Torridonian thrusts of the North-West Highlands are not invariably accompanied by mylonites. Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on May 31, 2016

178 MR. E. B. laAILmr A~D liB. Zl. ZACOmleOR ON [June xg~x~

(b) The persistence of a thin remnant of the Ballachulish .Lime-i stone for many miles immediately above the Ballachulish Slide is not what one would expect 5 priori. In fact, all the phenomena revealed in the district here described can quite well be accounted for, without invoking a slide at the base of the limestone at'all. The need for caution in interpreting the type sections in the neighbourhood of ]Jallachulish, north and south of Loch Leven, and in Glen Creran [1, pp. 605 & 609J is correspondingly increased; but the evidence in these localities is so definite that we have not felt ourselves at liberty to abandon it.

(r In the great spread of southern schists, which we treat as ' unclassified" for the purposes of the present paper, there are many sheets of foliated basic igneous rocks. Some of these Dr. Peach [6, chap. vii] has clearly shown to represent ancient lavas, con- temporaneous with the sedimentary schists among which they occur. Many, on the other hand, have been proved to be intrusive, but until recently it has been assumed that even these intrusions are of earlier date than the folding of the schists. Since the recog- nition of great horizontal movements in the schists, this interpretation has been questioned. In the lower part of Glen Creran (fig. 1, p. 165), for instance, many small, foliated, basic intrusions of restricted distribution occur in equal abundance on the two sides of the BaUachulish Slide, as if they are of later date than all, or almost all, the movement which has taken place along the slide [7, p. 188]. In like manner, in the district here described, sheets of epidiorite occur fairly often for some distance below the base of the 'unclassified schists,' and are especially common in this position in association with the limestone of the Beinn Doirean range. If a great slide is eventually demonstrated at the base of the ' unclassified schists,' it will probably be necessary to assign these epidiorite intrusions also to a late date in the tectonic history of the district. V. :BIBLIOGRAPHY. I. E. B. BAIL~Y, ' Recumbent Folds in the Schists of the Scottish Itighlanda ' Q. J. G. S. vol. lxvi (1910) pp. 586-618. 2. H. KYNASTON, in ' The Geology of the Country near Oban & Dalmally Mere. Geol. Surv. Scotl. 1908. 3. C. T. CLOVGH& E. B. BAILey, in ' Summary of :Progress for 1907,' Mere. Geol. Surv. 1908. 4. C. T. CLOUGH,H. B. MAUFE, & E. B. BAILEY, ' The Cauldron-Subsidence of Glen Coe, and the Associated Igneous Phenomena' Q. J. G. S. vol. lxv (1909) pp. 611-76. 5. C. T. CLOVeH, ' The Geology of Cowal ' Mere. Geol. Surv. Scotl. 1897. 6. B. N. P~.AcH, in ' The Geology of Knapdale, Jura, & :North Kintyre' Mere. Geol. Surv. Seotl. 1911. 7. E. B. BAILEY, ' The Geology of the Neighbourhood of Fort William' Prec. Geol. Assoc. vol. xxii (1911) pp. 179-203.

EXPLANATION OF PLATE X. Geological map of the neighbourhood of Glen Orchy, on the scale of 2 miles to the inch, or 1 : 126,720. (By a draughtsman's error, the inch-scale has been wrongly lettered on the map.) ~*z~z,.# ,k,tu,n.. ¢wog. 8oe. l~g:. ~ Fg.X.

GEOLOGICAl. )lAP OF THE GLEN ORCHY DISTRICT, BY" X ,x ;~ X X "K,4q E.B. I~,Ana~,B.A.,F.G.S: a,,a M.31~c(;R~:t~o~ M.A_.B.Sc. 'x × /'K >~ X X : ~ X X ,x /, x" X >x/ >x/x" , X X 2\ x X ;'#~ lntdizted (~xm~ttt~t tn deqrre.¢)l . ~ ,,- ,U × x ~ ,, ~ !1 "-. [nrlint,d llq~ill lt~d(tlC~tl'tJit# ( De~Of~ "X'xX. / >C y > >'. x Xi x ~, ~ × :< :Jl / ~rtiod /'~%~/, "/d + Horq iorl&tl % lttclL, ied (ttme~x_tl'~ 9r,~ffegt,e~'a) k'olit(tiorl SCALE i:/i 2G 720 ~i~ Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological ','~ _ _z ...... ~ ...... ~, . University on May 31, 2016

" ~"~Y' t-" X"X" *"Y ~2< ~ x ~,,.~,-,j'<>xxx×,Xx×× x/"%. x > ">, x ,~',x 9 K X ~. X %" ~X "X X "x x x ~ "x / 7 , P< )4..~ × ~ X~ >, ' × / x s<' x "l ;'~ > 24__ ,~ ,i / w~ : ><..× X W >/× ~ "~, ,~ × ~ ,: Y. x C ! )4. "x'. X X ~ X \\"~ > < X "~."< x x ...' " z" / X .X .X "××"X"~'~ 't x ,< X ~ "~ ~ X / )4>*~x x;x"x:"/',-7 ,-, X, ~'. ,~, >, :,)x /~ ,:, / x "/ 'x × x × × , ~, x. x ~ > ,:~x X W X X X X"x " ~ x " / ×.~, > ~ X X ,X >~ X x x ,< "<~" ' ,~ " " .]~ 4' ~'~ 7

X xk > , ,,J/ ~~" - /" ''jt7 . , ~,,. " < , LO¢'t! >c ~ X "K ~l~ 0 o /" (

ppino >re/ + ,oo ~ / x-'>< x X X ,.~ v x. )4XXXX x'xi \ ( ;d X b< × ~<,''x ,.~ X X .'< X >C././'e "~ " ~o?t

"t 1 / z/ \ / _ 4o°i 8 / LATER \ 40 ° IGNEOUS ROCKS /oo .v/ \ 9 + \ _ ~ :~.... 7=!- ~... "..c X >C ?, X X')4 × "X K X "x ×Sx.'g~

FXK K / " Z ~ i Gr&niI3eS'~Yx*\*'~+~x~ ×'" <

I i I i , × ~ X ,-vx X "x" HIGHLAND 9 lil,/'- SOI'tlS'[~...... ~ /11 / / ( ~,,o x~( Y / / ° \ Glen Coe Quartzite. 8

/ 40 ° Leven Schists

-'~-~-"--'~" f/ff/ Ballachulish Limestone Ben Lui I / UnclassiFied \ i Epidiorite Sheets omitted Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on May 31, 2016

Vol. 68.] THE SL~N ORC~r as~laLt~E. 179

])ISCUSSION. Mr. G. BAIu~OW remarked that too much importance seemed to be attached to small variations in the succession. The Geological Survey had published numerous sections of colliery shafts ; in these the coal-seams were seen to continue for great distances, but the succession between the seams was never alike in any two of them. Similarly, along the ~orth-East Yorkshire coast tile marine beds of the Lower Oolite were singularly persistent ; but it was impossible to find a similar succession in the intervening beds, even at such small distances as 500 yards apart. With regard to the sudden thickening of the quartzite, the speaker had never seen such a phenomenon; the increase or decrease in thickness of this rock was very slow, and always in definite directions. The apparent increase was due to ' pitch' or inclination of the axis of folding, which cut the ' surface-section ' obliquely. :From experience in other districts, the speaker was inclined to adopt a different interpretation of the outcrops shown on the map, which formed a very small portion of the lines shown in the diagram. Mr. E. H. CU~NI~G~A~-CRAI~ asked what relations the structures described by the Authors bore to the main folding of the country, and whether the anticline was earlier or later than the main folding. He could not entirely accept their reading of the succession, and felt a doubt as to the conclusions that the Authors had put so clearly before the Society. Mr. E. B. BAILer, replying for the Authors, stated that the Glen Orchy Anticline was regarded as, in all probability, a later structure superinduced upon the recumbent folds of the neighbourhood. The fan-structure of :Ben Lawers (Perthshire) may also eventually prove to be a comparatively late development. Mr. Barrow's suggestion that the limestone which had been traced along the front of the Beinn Doirean range did not continue regularly underground for any considerable distance was negatived by the very evident relation between the outcrop of the limestone and the surface-irregularities of this region. In fact, although it might be admitted that certain of the more speculative conclusions arrived at were still sub judice, it was confidently claimed that the local geometrical relations of the various rock-masses were correctly represented in the maps and sections accompanying the paper.