Downloaded from http://jgslegacy.lyellcollection.org/ at University College London on June 15, 2016 NEWER 6NEISSIC ROOKS 01~ THE NORTHERN KIQHLANDS. ~55 22. The AGF~ of the NEW:ER GNv,ISSIC ROCKS of the NOIt~tIEI~N HIGH- LANDS. By C. CxL~Awx~, Esq., D.Sc., ]~.A., F.G.S. With NOT]~S on the LIT~O]~OGY, by Prof. T. G. BONNEY, M.A., F.R.S., Sec. G.S. (Read May 9,1883.) OONTENTS. Introduction. I. The Formations concerned in the result (p. 357). a. Hebridean. b. Caledonian. c. Assynt series. 1. Probable age. 2. Subdivisions. c r Torridon Sandstone and Ben More Grit. c2. Quartzite. c21. Seamy. c~ u. Annelidian. c3. :Brown flags. c 4. Sal~crella-grit and quartzite. ca. Dolomite. c51 Dark. ca u. White. II. The Relations between these formations (p. 363). Zoch Broom (p. 363). 1. Section along the shore of the loch, from Ullapool to the S.S.E. 2. Section along the road, from Ullapool to the S.E. 3. Section from Ullapool to the E.S.E. 4. Section from Ullapool over the high ground to the E.N.E. 5. Section along the south side of the Ullapool river, W. to E. 6. Section along the north side of the same, W. to E. Assyn~ (p. 867). A. The "Upper" Quartzite non-existent (p. 367). Section on the Burn of Calda. Scarp between Calda Burn and Poulan-drein. Section on Poulan-drein. Quartzite of Cnoc-an-drein. " Logan Rock" of Glasven. Section from the west end of Ben Uarran to the high road south of Inehnadamff. Section in the :Balloch under Coniveall. Section up Glen Coul. B. The "Logan Rock" not separable from the Hebridean (p. 374). (a) Objections to the Hebridean age of the " Logan Rock considered. (1) Absence of accessory minerals in the " Logan Rock." (2) Absence of bedding in the same, (3) Foliation in the "Logan Rock" not coincident with bedding. (4) Difference of weathering. (b) Author's objections to the contemporaneity of the "Logan Rock" with the Assynt series. (1) The "Logan Rock" not conformable to the rocks below it. (2) Or to the rocks above it. (3) The strata below it not metamorphosed. (4) l~o beds of passage between it and the rocks above and below. (5) Alteration and crushing at its junctions with other rocks. (6) Strikes of the "Logan Rock" and Hebridean often concordant. Downloaded from http://jgslegacy.lyellcollection.org/ at University College London on June 15, 2016 356 C. CALLAWAY 0N THE NEWER GNEISSIC c. Direct proof of the overthrow of the ttebridean (p. 379). Section up the cascade from Dhuloch More. Section up the cascade from Dhuloch Beg. Section at the north end of Scounan More. Section across Coniveall. Section up Glen Coul. Ground between Coniveall and Glen Coul. Section on Camaloeh. 1). The " Upper Limestone" non-existent (p. 386). (a) The dolomite repeated. (b) Or a part of the Caledonian. E. The Caledonian brought over the Assynt series by a reversed fault (p. 389). The Knoekan section. Sections on the north and south sides of Glen Coal. v. Structure of Assynt (p. 392). Scounan, Brebag, Ben More, Ben Uarran, Cnoc-an-drein, Glasven, :Ben Uie, Ben-na-Creisag, Ben-an-Uarran. The Stronchrubie basin. Loch Erriboll (p. 396). A. The " Granulite" of Nicol a lower division of Ihe C~dedoninn (p. 396). s. Tile Assynt ser~es folded back upon itself (p. 397). Structure of Druim-an-tenigh. Ground between l)ruim-an-tenigb and the sections on Camas-an-duin. Sections (three) on Camas-an-duin. Section in the ravine ;~bove the Free Church. Section above Erriboll IIouse. Section at Craig-n:~-faolin. Ground between Ben Arnaboll and Ben tteilem. Grour:d between Hope Ferry and Whitten Head. c. The Caledonian Gneiss brought over the inverted Assynt rocks by earth- movements (p. 403). Structure of Ben Arnaboll. Junction south of the Arnaboll valley. Junction north of H~)pc Ferry. Ground between Druim-an-tenigh and Ben Arnaboi1. D. Outliers of the Assynt series on the Caledonian (p. 407). v.. Granite not intrusive in the Assynt series (p. 408). III. Igneous rocks (p. 408). 1. Loch Ailsh group. 2. Igneous rock of the Quartzite. 3. Granite of Loch Erriboll and Durness. IV. Summary of results (p. 410). V. General considerations (p. 411). VI. Reply to objections (p. 412). Appendix. By Prof. T. @. Bonney (p. 414). ISTR01)UCTZOS. IS the summer of 1880 I commenced an investigation into the relat, ions between the fossiliferous limestone of Darness and the great gneissie series which was alleged to overlie it. A portion of my first results I communicated to the Society in a paper* in which * Quart. Journ. Geol. Soc. 1881, p 239. Downloaded from http://jgslegacy.lyellcollection.org/ at University College London on June 15, 2016 ROCKS OF TILE NORT~EI~N HIGHLANDS~ 357 I endeavoured to show that the sections at Durness and in Assynt did not display a true ascending series, the limestone in both loca- lities being separated by a fault from the rock which was sup- posed to conformably succeed it. On the relation of the limestone to the quartzite I formed no opinion ; but in the two sections de- scribed I found them brought together by faults. As the quartzite passed with apparent conformity below the eastern gneiss on the east of Loch Erriboll, I hinted, in accordance with the views of Murchison, which I had not then seen reason to abandon, that the quartzite might belong to the metamorphic series. From these limited observations I carefully abstained from drawing a general conclusion ; and I was convinced that no satisfactory result could be achieved without a detailed survey. Accordingly I devoted a fortnight in 1881, and two months in 1882, to the districts which appeare~l most promising ; and I have now the honour to submit the conclusions to which I have been led. The published theories on the relation between the Durness lime- stone and the eastern gneiss are the following :-- 1. That the eastern gneiss conformably overlies the limestone, and is of " Lower Silurian " age (Murchison, Geikie, and most authors). 2. That the eastern gneiss is merely the Hebridean brought up east of the limestone along a line of dislocation and inversion (Nieol). 3. That the identity of the limestone of Assynt and Erriboll with that of Durness has not been demonstrated, and therefore that the eastern gneiss, though it conformably overlies the former, has not been proved to be of " Lower Silurian" age (Heddle). The view which I have here to submit differs from all of the above, but approximates most nearly to that of Nicol. I maintain, with that author, that the junction of the limestone with the eastern gneiss is a line of faulting and inversion; but I shall attempt to prove that this gneiss is a distinct series, newer than, and resting unconformably on, the ]=Iebridean, that Nicol's "igneous rock " overlying the limestone is usually a true gneiss, and that both the older and younger gneissie systems have been brought up over the limestone by great earth-movements. The eastern gneiss I propose provisionally to name the "Caledonian." I. THE FORMATIONS CONCERNED IN T~E .~ESUL~. The groups whose relations are to be discussed are the Hebridean or Lewisian, the Caledonian, and the quartzo-dolomitic group which, in view of the doubt which has been cast upon its age by the re- searches of Prof. tteddle, I propose to call the Assynt series. a. ttebridean. In addition to the masses usually designated by this name, I in- clude under it a large proportion of the "igneous rocks" of authors. l~ecently the latter have been described by Dr. Heddle as a new Q. J. G. S. No. ]55. 2]) Downloaded from http://jgslegacy.lyellcollection.org/ at University College London on June 15, 2016 358 C~ CALLAWAY ON THE NEWER G~ElSSlO group, conformably overlying the Dolomite; and he names them "Logan Rock," from Glen Logan or Laggan, where they have been identified by Prof. Bonney as Hebrideau. I shall submit evidence to the effect that by faulting, accompanied or followed by powerful lateral thrust, this rock has been thrown over on to the Assynt series to a maximum breadth of about a mile, and along an outcrop of over 15 miles in Sutherland alone. This overlie often produces the appearance of conformity between the hssynt series and the Hebridean. b. Oaledonian. This system is divisible into two well-marked groups. The lower I have recognized only in the Erriboll area; and from its clear de- velopment in Ben Arnabotl, I have called it the ArnaboZ~ series (b). It consists of grey, granitoid, very felspathic gneiss, overlain by dark, striped hornblende and mica gneisses, passing up through beds of an intermediate character into the ordinary flaggy gneiss which, from its exposure on Loch Hope and in the lofty peak of Ben Hope (3040 feet), I have designated the Hope series (ha). The latter is very uniform in its lithology wherever I have seen it between Loch Erriboll in the north, and Loch Broom, in Cromartyshire, in the south, and from this line eastwards to Lairg, in Suthorland, and Ben Wyvis, i1~ Ross. It is normally a thin-bedded, highly quartzose gneiss, passing i~lto a felspathic w~riety on the one hand, and into quartz schist on the other. Within the extensive area described, I have not been able to detect any evidence of the reappearance of the ttebridcan. Near Lairg, it is true, the Shiness limestone, which is highly crystalline, is associated with bands of very massive horn- blonde, together with bands and nests of mica, stcatite, and several other minerals, all highly crystalline.