Notes on the Microscoloic Structure of Some of the Rocks by Pro- 1. Introduction

Downloaded from http://jgslegacy.lyellcollection.org/ at University College London on June 15, 2016 META~IORI~HIC AND 0VERLYII~G ROCKS OF ROSS AI~D II~VERNESS. 14I 11. On the M~rA~oRP~IC and overlying ROCKS "in laarts of Ross and I~VER~v.SS S~rRES. By H~RY HIcxS, M.D., F.G.S. With .Notes on the Microscoloic Structure of some of the Rocks by Pro- fessor T. G. Bo~.r, M.A., F.R.S., Sec.G.S. (Renal February 7, 1883.) (PLAT~. VI.) Contents. 1. Introduction. 2. Upper part of Glen Logan. 3. Neighbourhood of Aehnasheen. 4. Ben Ray and Loeb Clare to Glen Carron. 5. Achnashellaeh, Loch Doule and Strathcarron. 6. Loch Kisborn to Loch Carron. 7. Attadale, Loch Carron. 8. Strome Ferry and Loch-Alsh Promontory. 9. Loeh Shiel to Caledonian Canal. 10. Conclusions. 1. Introduction. SIncE I communicated my paper to the Geological Society in 1878, on the rocks in the neighbourhood of Loch Maree, Ross-shire, I have revisited the north-west Highlands on two occasions, viz. in the spring of 1880, and in the spring of last year. During these visits I devoted my time to the examination, in various areas, of sections which seemed likely to furnish evidence confirmatory or otherwise of the views which I had ventured to lay before the Society. In this paper I propose to give the general results obtained from these examinations, and to treat more fully those sections which appear to offer any conclusive evidence. In each of the areas examined I made large collections of rock-specimens ; and numerous thin sections for examination with the microscope have been made from these rocks, and submitted for special examination to Prof. T. G. Bonney, and Mr. T. Davies. The former has kindly furnished the notes on these sections which are appended to this paper; and to both, on this as on so many former occasions, I am indebted for most valuable assistance in the petrological work. The notes by Mr. Davies in the paper I published in the Geological Magazine in 1880, on the Gairloch and Ben Fyn districts, have also so important a bearing on the questions discussed in this paper that I shall find it necessary to refer frequently to them. The notes in that paper on the Gairloch specimens are particularly important, as they prove clearly the presence in that western area of rocks which cannot be differentiated from many rocks in the eastern area of Ben Fyn, included by Murehison, Geikie, and others in the so-called newer gneiss of Silurian age. This group of Ben Fyn is in this paper taken as ~he type ~o which is referred the newer series of metamorphic rocks of Pro-Cambrian age in these areas as distin- guished from an older, or Loch-Shiel series, and the supposed s~ill older Loch-]Karee series. Downloaded from http://jgslegacy.lyellcollection.org/ at University College London on June 15, 2016 I42 DR. H. HICKS ON THE METAMORPHIC AND In a former paper* I placed the axis of the oldest rocks in the direction of Loch ~Iaree, with newer beds tb the south west at Gair- loch and to the north-east about Ben Fyn. In doing this I did not necessarily mean to correlate the series at Gairloch with those of Ben Fyn, but believed them to be so closely allied that they could be easily included in one great group. The more hornblendic varieties along the shores of Loch Maree and the granitoid rocks south of Poolewe were, in my opinion, of older types; but whether separated from the former by actual unconformity or not I was unable to say. Without referring again to the several views maintained in regard to the metamorphic rocks in the eastern areas, with which I have chiefly to deal, it is clear, as specially bearing upon those views, that the following points have to be kept constantly in mind in these inquiries. (1) Is the stratigraphical evidence as to a continuous upward succession, maintained by ]~urchison and Geikie, of that conclusive character that it must overthrow all petrological evidence which may seem to render this, if not impossible, yet highly improbable ? (2) Has this upward succession from Torridon Sandstone through the quartzite and limestone series anywhere been observed to gra- duate into gneiss rocks of the type such as we classify under the name Ben-Fyn series. (3) Are not the flaggy micaceous rocks, such as those which we have described from the east side of Glen Logan, which probably overlie the limestone series (though there undoubtedly separated from the latter by a fault), more intimately allied in their microsco- pical and general characters to the Torridon series than to those of the Ben-Fyn type, which they are supposed to immediately underlie? (4) Are there evidences of the disturbance of the strata by faults and inversions along these lines of such a kind as would be likely to greatly interfere with and to complicate the order of succession ? (5) Do the eastern metamorphic rocks show at different horizons and places that variability in the amount of alteration which is usually noticed in rocks subjected to local influences only ? or do they not rather everywhere show an identical state of alteration, as if the result of some wide-spread cause, rather than due to local mechanical disturbances ? Before proceeding to describe the various sections examined since my paper was read in 1878, I may here take the opportunity of referring to a point in connexion with the section published in that paper which needs some explanation (though I have to a certain extent done so in my paper in the Geological ~Iagazine, 1880). In that section the beds were accidentally placed at too high an angle in the part east of Glen Logan, and the floor made to appear to continue too uninterruptedly eastward. The published section, however, as mentioned in the discussion, was intended merely as a diagram; the views that I entertained were fully explained in the text t. Yet as it might tend to give a misleading idea as to the condition of the floor at this part, I think this explanation necessary. The presence Geol. Mag. Dec. ii. vol. vii. 1880. ? Quart. Journ. Geol. Soc. vol. xxxiv, p. 816, Downloaded from http://jgslegacy.lyellcollection.org/ at University College London on June 15, 2016 OVERLYING ROCKS OF ROSS AND INVERNESS. "1~3 of the floor in the entrance of Glen Docherty, as nearly as possible in the position indicated in that section, I clearly recognized again during my last visit. It is also seen as we leave the upper end of the glen; but between these points the ground has been so much broken up by faults passing along the glen and at right angles to ;t, that it is impossible to trace it continuously. The presence of the old floor in Glen Logan as far as the entrance into Glen Doeherty (shown also in Prof. Bonney's diagrams and recognized clearly by him 8, and referred to also by Mr. Hudleston in his paper in the Proceedings of the Geologists' Association, 1879) is a fact of so much importance in these inquiries, in consequence of its being found eastward of the quartzite and limestone series, that the question of its presence in Glen Docherty may be almost dispensed with in the discussion. If the oldest rocks can be seen to rise up anywhere eastward of the limestone series, then the probability of the eastern true metamorphic rocks being parts also of the old floor, as maintained in my former papers, is rendered still stronger, especially if, as I hope to show in this paper, there is abundant evidence tending towards that conclusion. I may here also refer briefly to the prevailing faults found in these areas. There can be no doubt, as shown by the directions of the lochs and valleys, that the main faults trend from N.E. to S.W. and from N.W. to S.E. Branching off from these, however, are many minor faults ; but though these are continually met with, they do not usually greatly interfere with the succession, though they occasion- ally cut off considerable thicknesses of strata. Comparatively a few only of the faults are indicated on the map (PI.u and those mainly which have a bearing on the questions considered. Some of the local names are taken from the recently published Ordnance map ; but the majority are from that published by Black, as they agree with those used in my former paper. At that time the one-inch Ordnance map of these areas had not been published. The numbers on the map refer to the positions of the rock-specimens described, and correspond with the notes by Prof. Bonney. All the rocks described are from points not referred to in my former papers, and they include all the varieties which appeared at all typical in the traverses made. One section only was examined to the north of the area previously described, the others being all to the south and east. The one to the north may be looked upon as a continuation to the N.E. of the former Glen-Logan sections. 2. Glen Logan (or Laggan), UTTerpart. The gneiss, note no. 1, has a well-marked strike from N.W. to S.E., and in its general appearance seems typical of the older or what we call the Loch-Maree series. About the cottages near the entrance to the glen on the west side leading to Loch Nafatt are exhibited well-rounded surfaces, upon * Quart.

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