June 1912. 9. the GLEN ORCHY

Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on May 31, 2016 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 [ 108, ] ~176 .~ I Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on May 31, 2016 [ 169 ] ! oo II litl ~ I.'/ t.tili /,'"Fif!i ~7 ','., 'tti!i'.. ,, .-,,.~,:, ~, ,,,"~ ::::.. ,%, ',~{kl{,{t~;,, ,! o~ ,r i 'V ~" I~,'::~, .~.~ .f..,.. i{:!)i I o.'$ Ib o,,,,.,~ ,& '~"'<:"\1 Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on May 31, 2016 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.

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