419

ORDINARY MEETING.

FRIDAY, JULY 4TH, 1884. , ESQ., M.D., F.G.S., President, in the Chair. The list of donations to the library since the last meeting was read, and the thanks of the Association were accorded to the donors. The following were elected members of the Association :­ Miss Mary Forster and F. A. Harrison. The following papers were read :- 'On the North-west Highlands and their teachings/ by Pro­ fessor J. F. Blake, M.A., F.G.S. 'On the Stratigraphy and Metamorphism of the Rocks of the Durness-Eriboll district,' by Professor Charles Lapworth, LL.D., F.G.S., a letter to J. H. Teall, Esq., M.A., F.G.S., by whom it was read on Professor Lapworth's behalf. , On the Geology of South Devon, with special reference to the Long Excursion,' by W. A. E. Ussher, F.G.S.

THE NORTH-WEST HIGHLANDS AND THEIR TEAOHINGS. By REV. J. F. BLAKE, M.A., F.G.S., Professor of Natural Science, University College, Nottingham. Those who have watched by the advancing tide will have seen that ever and anon a larger wave than usual comes rushing round some unrecovered spot and secures it for the sea j and so the tide comes in. Such waves occur in science, and if, as in geology, there be several fields, one after another thus absorbs the attention of the advancing army. So Palooontology has had its day, and Petrology is in its dawn. As once all hands were eager for the fossil, now all eyes are anxious for the slice. This has led to a deeper interest both in volcanic rocks and in those vast store­ houses of petrological treasure-the Archsean schists and gneisses, and thus he who once found fossils where no one else could find them, now finds Archsean system after system where no one else can follow him. In the study of the Archsean rocks of Britain, the foremost question to be asked has reference to the Highlands of Scotland. Are these wildwastes and loftymountain-tops carved 32 420 J. F. BL AKE ON THE NORTH-WEST HIGHT.ANDS.

out of the most ancient of deposits, or have the materials under­ gone so great a change as to turn the verdured hills of into the moorland wilds of the North? In a word, is the great mass of the Highlands Archeean, or is it not? The state of this question, up to a recent date, has been already explained to you by your late President (Mr. Hudleston), and his first impressions of Assynt have also been communicated. In II another place" further con­ tributions have been made. Your President has parcelled out the land. Oallaway and Lapworth have improved the stratigraphy, and last summer the whole forceof the Geological Survey of Scot­ land laid siege to the Highland fortr ess, though they have not yet carried it by assault.e I come before you to-night to tell you what I have seen there and what it has taught me. You will remember that according to Murchison there were many places where a "conformable upward succession" could be traced from the Limestones into the main Gneisses of the East i and hence that these Gneisses must be of Silurian age. Nicol, on the contrary, maintained that in all these places (except Assynt) there was an ordinary fault or an intrusion of igneous rock separating one series from the other. With regard to these contentions it may be stated plainly that some of Murchison's most crucial sections, e.q., Stroncrubie, are found to be absolutely wrong, when examined with the most ordinary care j and, on the other hand, many of the faults introduced by Nicol lire entirely imaginary, and do not explain the sections in question. On the other hand, it must be admitted that there are several localities, some of them quoted by Murchison, where the eye, not specially instructed, does see a clear conformable upward snc­ cession from the limestone to a gneiss, and it requires the teaching of other sections to inform us that the eye is deceived i and even then the only other interpretation of the appearances is so strange that one doubts if it can be true. I presume the special instruction required may be found in various districts, but we found it in Eriboll. By H we" I mean Mr. Teall and myself, who started last summer (1883), via Lairg, to Durness. There we were fortunate in finding Prof. Lapworth, * Since this pap er was r ead the main results arrived at by the Geological Survey have been announced by the Director General in I Nature,' Vol. xxxi, p. 29. These are quite confirmatory of th e statements of Lapworth, of Callaway, and of the present paper, lIB to the gen er al question , though differing in details, and entirely subversive of the views previously held. J. F. BLAKE ON THE NORTH-WEST HIGHLANDS. 421 who, having made himself acquainted with the district in previous seasons, was our guide to the instructive sections round the Lake of Eriboll. In this district we need take no account of the sections of Murchison or Nicol, though the latter was nearest the mark. In comparison with what may be seen, they both saw nothing. Whether we should have been in an equally bad case, had it not been for Prof. Lapworth, cannot now be known. Under his guid­ ance we did not miss the points, but became gradually convincedof an amount of overturning and churning up of the rocks that I could not have believed possiblein so slightly elevated an area. In the Alps we expect strange things, but here we are scarcely pre­ pared for the stupendous earth-movements which have taken place. The relations between the two series of gneisses being intimately connected with the stratified Assynt series that is caught up be­ tween them, it is necessary in the first place to obtain a clear and undisturbed section of the latter. Such is found at Ant Sron, as shownby Prof. Lapworth in the I Geological Magazine: Here we find about 25 separable portions of the series which in a more minute survey might be found useful, but they must at present be associatedinto groups. At the base are (a) 80ft. of quartzites in about 17 varieties, almost all of which contain round marks re­ ferred to annelids, which are more numerous or conspicuous in certain beds than in others, and, when seen along joints, appear as pipes. Other beds are more crystalline, and others like vein quartz. These varieties are important as they show that difference of structure alone will not prove a bed to be an upper quartzite. Next come (b) earthy, irregularly laminated rocks, weathering brown,and marked on the surface with horizontal branching cords. These are knownas the fucoidbeds, and are about 40ft. in thickness, including a band of green shales at the top. The succeeding rockis avery important one (c), whichhas been calledthe " Salterella Grit," because of the abundance in it of little triangular bodies, once thought to be Orthocerata, but now considered to be tubicolar annelids, and named Salterella, The rock which contains them has a peculiar bluish aspect, by which it can be recognized, even when the annelids are obliterated. Associated with this at the top is a quartzitic rock, with great holes, as of irregular weathering, called by Lapworth,from its resemblance to cinders, the ecoriacecus bed, making together about 25ft. j and, finally, is (d) the long series 422 J. F. BLAKE ON THE NORTH-WEST HIGHLANDS. of limestones,with flaggy bedsat the base, about 200ft. thick. This section is, therefore, imperfect, both above and below. We cannot tell how thick the lower quartzite may be, nor can we predicate anything as to what succeeded the limestone. But we learn that there is a natural upward succession of "anneHdan quartzite," "fucoid beds," " Salterella grit with scoriaceous bed," and" limestones." This being determined, we ascend the neigh­ bouring hill. Starting from limestone at the base, and omitting minor disturbances, we notice that the beds are becoming nearly vertical, and that we pass downwards in the series till we come to the ennelldan quartzite, more of which is here apparently seen. When wereach the first hill-crest the quartzite rolls over to become nearly horizontal, and is lost beneath a narrow bog.* On the other side of this are found the rocks which are usually referred to the Eastern Gneiss. They consist, however, of hardened slates and felspathic quartzites, and though foliated in appearance, have not the genuine crystalline structure of the true schists. Further back on the hill are some very slaty rocks with weathered holes, in which I should not despair of finding fossils. Now what comes in the bog? Certainly not the whole series of fucoid beds and lime­ stones. eo there is no upward succession. Traced indeed along the strike, the upper beds are seen to be unconformable, for to the south they overlie or abut against the limestones. To the north there soon comes in a tiny knob of rock, as it were the little-finger of a mighty giant-the so-called "Arnoboll Rock." What is this? Nicol called it syenite and granulite, and igneous rock. Heddle calls it Logan rock, and considers it intrusive. Murchison and Callaway refer it to the Newer Gneiss. Bonneyby implication would call it Hebridean. Coming direct from the study of the Hebridean on the south of Dnrness, I had no hesitation in referring it to that old gneiss, though in a modified form; but the extraordinary antics it performs, the trouble it causes, and the deception it has practised on all geologists, unite to make the abbreviation O. G., by which it was first entered in my note-book, stand rather for "old gentleman" than for old gneiss. Going north from our section, the Arnoboll Rock widens out into a larger mass, with here and there a hornblende nest. Above it are seen small patches of quartzite. These are the

• According to the Survey figure loco cit. the section is still more compli­ cated by innumerable breaks and thrusts. J. J1'. BLAKE ON 'rHlll NORTH-WEST HIGHLANDS. 423

outliers of Dr. Callaway, which lose their significance if the Arnoboll is not the newest gneiss.

Fig. I.-INVERSIoN IN ARNOBOLL.

The section to be next described is, perhaps, the most im­ portant in the district. Starting from the Limestone, on the south slope of the Loch H ope road, though we miss the Salterella Grit, the fine scarp shows at the base the fucoid beds, above which comes the Annelidan Quartzite, some 100ft. in thickness, all in regular, but inverted order, and above comes the Arnoboll Rock. This section, though the actual dips are comparatively slight, we are forced to explain by a complete inversion, from which we conclude that the Arnoboll Rock is stratigraphically below the Quartzite. Yet this is one of Murchison's upward successions, for th e ex­ planation of which he had to invent an Upper Quartzite I Going over the hill we find the Quartzite again - now lying above the Arnoboll. This we should suppose is a synclinal with the axis horizontal j but is it simply a synclinal ? At the junction between the Arnoboll and the underlying Quartzite are a few inches of very remarkable rock having a very compact and almost greasy fractur e, with streaks lying parallel to the junc­ tion, caused by the drawing out of its crystalline constituents. It is just s l~ c h a rock as might be formed at the commonsurface when one rock mass was sliding or rolling over another. Such a band at such a spot suggests, therefore, as remarked to us by Prof. "Lapworth, a sliding of the Arnoboll Rock over the Quartzite rath er than a fold­ ing; so that we should have here an almost horizontal fault, and we should designate this material" sliding-fault rock." But such a sliding can scarcely take place without having some effect upon the sliding rock, some mixture of ingredients, some breaking of its crystals, some twisting of its bands, some formation of secondary minerals. It is thus that the Arnoboll and its aliases differ from th e unaltered Hebridean and put on the appearance of igneous rocks. Now that we have seen its semi-intrusive behaviour it is essential that we should determine whether or not it is Hebrldean. It is true that we do not here, nor can we in Eriboll that I am 424 J. F. BLA.KE ON THE NORTH-WEST HIGHLANDS.

aware, trace it unbroken into the great Hebridean mass where it comes up from below. We can do so elsewhere, however, as will be seen, lind even as near as the Gualin some proof may be obtained. Thus about three miles north of Rhiconich, after travelling thence on undoubted Hebridean, We came to the Conglomerate, or northern form of the Torridon sandstone, on the left hand side of the road j and on examining the junction the Hebridean is seen to be pushed up over it, and the rock here and at Arnoboll is almost identical. So also about threomiles from Durness on tho right hand side, ascending tho slope from the limestone at the base we meet with the Hebridean with all appearance of an overlap. True igneous rocks are seen, moreover, in the Hebridean in the form of dykes. If, then, this point be determined, the stratigraphy of Eriboll, thongh marvellous, is clear, and we should not be surprised at any vagaries that this Arnoboll rock may play. These, however, I must leave for Prof. Lapworth to describe and pass to other localities for other lessons, remarking only that our observations in Eriboll have as yet taught us nothing as to the upward succession since nothing has for certain been seen there above the limestone. We will go next to Loch Glen-coul. Here we shall find the same over-riding of the old gneiss, unaccompanied by invers ion. On the north side of the loch the section can be seen from II distance and examined on the shore. To the west lies the Hebridean of rather more quartzose character than usual, and with a westerly dip j while overlying it with a moderate dip to the east is the quartzite, and above itthe fucoid beds, &c., forming low escarpments facing west; but over the irregular surface so formed rolls the never-to-be-lost old gneiss burying all beneath it. Further on to the east at the head of the loch is the inevitable crush which such a disposition must produce. Here, however, the undisturbed Hebridean and the dis­ turbed are close together, and We can see that the latter is just such as would he produced by the disturbance of the former.

Fig. 2.-SEOTION SOUTH BIDE OF LOCH GLEN-COUL. J. F. BLAKE ON THE NORTH-WEST HIGHLANDS. 425

Still more instructive, however, is the section on the south side of the loch, for here we can trace again all the details of the series, as at Ant Bron, At the base are about 100ft. of Quartzites, the lowest so coarse and red as to suggest the Torridon sandstone, but the npper part white. Next comes a varying series which may be referred to the Fucoid beds, some parts having that character while others are black slate or bluish quartzite beds, the total being about 100ft. Over this lies the Salterella grit about 10ft. thick, and then yellow dolomites, vacuous beds for 20ft., and finally grey limestone for 30ft. or more. So far the section, though showing characteristic changes, is the usual one; but the top of the limestone is exceedingly straight, and the rocks above form an overhanging roof. These consist of soft dark slates, broken and squeezed, but not producing fault-rock. In an upward direction these gradually change into an old gneiss without it being possible to draw a line between them. Now, though this section is not described by Murchison, I could scarcely conceive a better one to prove his point, were he to take the overlying gneiss to be the Newer Gneiss. Yet I quote it to teach us exactly the opposite and to counsel ex­ treme caution. For though there is no fault-rock where the slates are seen, yet when the old gneiss and the hard limestone come in contact it is produced, the lines of foliation turn up and become vertical as though we had reached the Rose, and further up the hill it is seen to come up against a quartzite, which tnrns out to be the basal one. Hence there is no conformity but an overriding fault pushing the gneiss over the surface. What the slaty rocks are doing there is hard to tell; but as we find something similar above the limestone elsewhere, this is perhaps not far from their true place, and they have yielded to the riding and escaped crushing. Further np the glen, according to Dr. Callaway's description, the Assynt series occurs all over again above the intruding gneiss, just as it does in Arnoboll. We are now in the neighbourhood of Assynt, and come to well known sections which have been held not only to prove a conform­ able upward succession, but to show most clearly the" Upper Quart­ zite." Both Nicol and Hudleston had thrown doubts on its exist­ ence; but nevertheless, until I had examined the section, I had still a lingering feeling that there might be such a quartzite. Even after Eriboll ami Glen-couI, it is somewhat staggering to rest by the side of a lake on horizontal quartzite crowded with annelids, and look 426 J. F. BLAKE ON THE NORTH-WEST HIGHLANDS. upwards to the towering mountain of GIasven with its white and glittering summit and believe thatsummit to be composed of the very quartzite on which one is sitting; yet such is the case. Here, in­ deed, we are to learn a third lesson: it is no longer the vagaries of the old gneiss, nor the local overturns of series, but stratigraphy set free. For in Assynt, as in Durness, the Silurian series is in great force and the moving earth could not simply double it up or ride over it, but threw it into a contorted and dislocated synclinal in its struggle for the mastery.

Fig. B.-SECTION OF OHOC-AN-DREIN.

To trace the climbing quartzite I worked with care over the well-worn section of Cnoc-an-drein. We start in limestone at the base, we miss any Fucoid beds, but come almost immediately to nearly horizontal quartzite with a peculiar porphyry at its base, forming a scarp and apparently overlying the limestone. This actual rock is the" Upper Quartzite" of Murchison, so we know exactly where we are, and truly appearances at first are in its favour. Further examination, however, leads to a different result. I cannot pretend to have searched out accurately the structure of this hill in which the rocks are so bent about that almost every line of section is different, but the one I followed gave the follow­ ing facts. The Quartzite, at first dipping slightly soon became vertical, and then dipped at a high angle the other way, making a sharp synclinal fold. We soon came upon white annelids in a pink rock exactly as on the lake in the basal quartzite. Then comes a band of diabase: a little above occurs another: then vertical quartzite with red porphyry: then a fold over into a dome, a green porphyry weathering red occupying the lowest visible part: then a rise brin.ging in the red porphyry in the next scarp. At the top a remarkable diabase formed a dome, and, further on, the strata are passed over again in an ascending sequence. Thus the main structure of the hill is anticlinal with the strata inverted at its base. Thus the limestone would come to lie above the quartzite in its natural position. It is this Same quartzite which J. F. BLAKE ON tHE NORTH-WEST HIGHLANDS. 427 traced in a line perpendicular to the section, i.e., to the north-west, is continuous on to the slope and summit of Glasven, and to the east into the sides of Conieveall and Ben More. In both of these mountains may be seen the cause of all the trouble, the moving gneiss extruding here and there. Here, then, any proof of an Upper Quartzite fails, and the rock in question seems by all its peculiar features and by its stratigraphy to be the ordinary lower Fig. 4.- GENERAL SECTION ABOVE STRONCRUBIE.

one, but the unanswerable demonstration is yet to come. Starting fairly on this quartzite one can skirt along the flanks of the hills, as far as the southern slope of Conieveall, and trace it across the gorge of the Traligill into the masses which form the summit of the Meal-loch. This hill, as seen from below, appears as a gentle anti­ clinal above the great mass of limestone at Stroncrubie, and is so drawn by Murchison as an Upper Quartzite, but alas! when reached, this anticlinal is a dome with a far greater dip to the west towards Stroncrubie than transversely: the beds become highly inclined, and we find overlying it in succession the Fucoid beds Salterella grit and Limestone all in the regular order. After this I gave up searching for Upper Quartzite here. Such, then, are the teachings of the Highlands on mountain stratigraphy which we must apply to the interpretation of the more difficult localities. So far as we have gone the evidence for the Silurian age of the Eastern Gneiss has all broken down, and in the last locality described Nicol was entirely in the right; but there is more to come, in which the question assumes a somewhat more doubtful aspect; for after all we have seen little or nothing as yet of the Newer Gneiss, nor of any succession above the Limestone. When we do they may tell a different tale. I shall now, therefore, draw attention to such sections as seem to show an upward succes­ sion. Beginning at the north we find one of this kind. Fig. 5.-SECTION IN SANGO BAY. 428 .T. F. RL..lXE ON THE NORTH-WEST HIGHLANDS.

At Bango Bay, in Dnmess, the eastern side is bounded by a fault to the west of which the Old Gneiss has intruded and worked a wildconfusioni limestones and smashed felspathic rocks which are quite unnameable occupy the ground in hummocks and crags, but DO order is ascertainable. Further west are isolated masses of a peculiar kind of rock of dark colour and slaty appearance, but with lenticular decomposition, and forming skins like tho coats of an onion. These shales are most like some of those on the top of Ben Arno­ boU, but here their peculiar character is specially manifest, and we may therefore call them Onion Shales; associated with them are quartzose bands or veins. The dips of these are anticlinal in the two masses i but we have no reason to consider them in place. On the western shore, however, is a mass of limestono rather broken in appearance and not wellstratified; but with some schistose bands at the top; and abovethis is a band of pure quartzite from 4ft. to 10ft. thick, also cracked and squeezedi and above again are 10ft. or 12ft. of dark shales-not like the Onion Shales, but with their lamina! contorted, or very much compressed. It is plain that all three de­ posits, limestone, quartzite, shales, IIBve been very hardly treated; but, notwithstanding, a very careful examination of the junction lines led me to the conclusion that they were natural deposits in succession and had suffered their troubles in common. Here, then, we seem really to have a succession above th e limestones which owes its preservation to faults. It is a great pity that the relation of the Onion Shales to the others cannot here be made out satisfac­ torily. They can scarcelybelong to the limestones or anything below , and to imagine they are part of the newer gneiss makes it difficult to account for their presence unbroken where they are. From petro­ logical considerations, combined with the evidence of other sections, they most probably lie above the other shales and belong to the Silurian series. If so, here comes the puzzle. The summit of Fairhead-the great promontory to the west-is almost entirely made of typical eastern gneiss, very different to anything I have spoken of before and of the same character as that which stretches from Eriboll to Tongue over the inhospitable waste of the Moin, and which we have therefore called the "Moin Series." In this vast cliff the whole series is exposed in a magnificent face, accessible at several points on its sloping side. They pass down through sericitic to more quartzose and hornblendic schists, while the lower part, continuous at first sight with the upper, J. F. BLAKE ON THE NORTII·WEST HIGHLANDS. 429

is composed of the dark onion shales, with bands of quartzose vein-rock. In this cliff, then, ifwe can find no flaw, and if we can fix the Onion Shales as Silurian, we should have a conformable

Fig. 6.-FAIRHEAD.

upward succession in spite of all we have seen elsewhere. If, however, the Onion Shales be Silurian, and the overlying schists Arohsean, We ought to find at the junction some sliding-fault rock, with fragments of the limestone, or other intervening rocks, caught up in the motion. Fortunately, though not unnaturally, there is a slight nick in the slope just at the line of junction, and we are ablc to examine it with care. The result of this crucial test was favourable to the ideas already obtained from other sections, inas­ much as contorted and sliding-fault rock is admira bly seen, and below are fragments of quartzite and limestone intermingled pro­ miscuously, whilst th ere are some signs of unconformity between the series above and below this line, which is the most marked one in th e cliff-face. This verification of prediction is a strong confirma­ tion of the truth of th e views I am enunciating j but there is another test to be tried. Along the western shore the rocks are all exposed, and about the hornbl endic series to the south, and the Onion Shales to the north, there can be no doubt. But what lies between 1 First, from the south, after some silky and micaceous schists, come about 12ft. of a kind of Adinole, below which the rocks are much disturbed, and gradually change into Onion Shales. The section is continuous, and if examined without previous pre­ paration would scarcely suggest a sliding-fault. The line of dis­ turbance, however, above the Onion Shales is seen, on climbing the cliff, to coincide with the line leading to the fault-plane in the northern cliff. The sections th erefore at Fairhead are not inex­ plicable, even if we take the Onion Shales as Silurian. At the same time, I must admit that the nature and age of these Onion Shales, both here and elsewhere, cannot be said to be satisfactorily 430 J. F. BLAKE ON THE NORTH-WEST HIGHLANDS. determined since sliding-faults might occur also in the mass of the Newer Gneisses. The other sections in which I have examined the relations of the Newer Gneiss to the Silurian series lie to the south of Assynt.

Fig. 7.-lNTERPRETATION SECTION, LOCH AILSH.

The first of these is to the east of Loch Ailsh, not far from Altna­ galagach. The rocks on the western side are schistose, and are apparently overlaid by igneous, intermingled with calcareous, bands. A careful examination of this side might yield valuable results, but we had no time to disentangle it. Crossing the stream we see some sparkling bosses of white granular-crystalline limestone, with curious concretionary patterns of green serpentinous matter. This beautiful rock is something quite different to anything we have seen before, though it has some relations to the Ledbeg marble, which occurs near here. The serpentinous patterns in the latter were taken at one time to be Eozoon, but if they are at all like what is seen in the field at Loch Ailsh, no geologist should think of them as organic for a moment. Near this limestone are igneous masses, which have been thought to cause the crystallization. This may be so, or it may be a special development of the Assynt limestone not seen elsewhere; but it is totally different from the schistose limestones of the Newer Gneiss series. It does not ex­ hibit much stratification here i but the whole dip of the associated series is towards the east, and thus it comes to be overlaid by yellow dolomite, and then by ordinary Assynt limestone, with high easterly dips. Beyond this, on rising to the hill called Alt-Ellag, we come to a small scarp composed of slaty rocks, like those of Sango Bay, more nearly horizontal, some at the top being wonderfully contorted on a tiny scale. The ground between this and the nearest limestone is covered, and we cannot demonstrate an inter­ vening quartzite i but several fragments of such a rock lie about, and we may suspect its being there in block. Now, above these shales, as we gently climb the grassy slopes, there is no sign of a dislocation i but the scattered stones reveal some fragments of quartzite and limestone, and then come the quartzose and micaceous J.• F. BLAKE ON THE NORTH-WEST HIGHLANDS. 431 schists of the undoubted Upper Gneiss. Here then, though we do not actually see it, we should judge there to be a conformable up­ ward succession, as represented by Murohison, and there is no fault such as was imagined by Nicol. If we are to overthrow the evidence of this section, we must seek for some sliding-fault rock, and indi­ cations of special disturbance. The fragments of limestone and quartzite above the shales point in this direction; but we must go behind the hill to the north to find what we require. Here the surface is not so smooth, and tracing downwards from the perfectly undisturbed gneiss, and upwards from the shales-here of rather peculiar porphyroidal character-we limit the possible line of fault to a single narrow glade. In this a careful search revealed what we require-a shallow scarp of true fault-rock associated with local crumplings of the most remarkable kind. Here then, again, wecan believe that the gneiss has been forced up, over, and on to the Silurian series. The view of these rocks from Strath Oikel suggests a difference of dip and strike of the two series, but this we had no time to specially verify. Nevertheless, at Kinloch Ailsh, on the north of the loch, there is an instructive section from this point of view. The floor of the valley north of the houses is composed of nearly vertical limestones, some portions of which are crystalline, like the mass already described. These limestones are seen in im­ mediate contact with a little cliff of quartzose gneisses, as one may call them. These, however,are more nearly horizontal, and both are so crumpled, flaked, and squeezed that there must have been con­ siderable motion between them, though the result scarcely reaches the perfection of the sliding-fault rock.

Fig. S.-STRIKE SECTION, CRAIG-AN-KNOCKAN.

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We will now go to Craig-an-Knockan. Were I saying this to you on the day I myself went, when the wind was blowing furiously and driving the never-ceasing rain into every crevice in one's clothes, so that it was difficult to keep it even off one's map, I should probably be left to go alone, as I actually was. These are not the circumstances in which to make very full observations; 482 J. F. BLAKE ON THE NORTH-WEST HIGHLANDS. but I hope they did not impair one's accuracy. Starting from Altnagalagach by the Ullapool road, you reach at Elphin, about five or six miles distance, a large mass of limestone. It descends beneath the road on which you walk, and rises in well-stratified beds throughout the hills above you. It rivals Stroncrubie. Passing the village of Knockan, the strata rise, and along the road there crop out the Salterel1a Grit, the Fucoid beds, and the Quartzite, all in regular sequence, the latter with the same red porphyry as at Dnoc-en-drein. As the road rises, these rise more, and form the whole visible portion of the cliff. It is just here that the line between Sutherland and Ross is indicated by a brook, on the day of my visit a torrent. Grown reckless by this time, in spite of the storm, I clomb the cliff, to find at the top, on the slope above, some dozen feet of limestone, fragments of porphyry and quartzite, and horizontal beds of quartzose gneiss. The limestone is brec­ ciated, the porphyry and quartzite in lumps, and the gneiss, to call it such, worked up into almost sliding-fault rock. It is the same story over again. This is no thinning out of the limestone, from hundreds of feet in a neighbouring hill: we can see the uncon­ formity, bnt it is accompanied by sliding. If we now take stock of our position, we seem to have grown very clever. We can actually see a rock lying unconformably on the edges of a lower series, and declare, without invoking an in­ version, that the upper rock is the older of the two. Too clever by half, some no doubt will say; but there is one more locality to deal with, where we shall require all the cleverness we can muster, and that is Glen Laggan, at the upper end of Loch Msree, This lovely vale has been the scene of much bloodless contention. The war was first carried into this quarter by Nicol, and, being an acces­ sible spot near a comfortable inn, it has again and again been attacked. Sir , the late, and Mr. A. Geikie, the present head of the Geological Survey, saw here a conformable upward succession, and asserted there was no better locality for observing the order of snperposition of the ancient crystalline rocks of the Highlands, a remark with which,in a different sense, I agree. Next went your President there, hammerin hand, and gave a view of the circumstances, that involved at least one element of truth not Been before. He first recognised the possibility of some of the so­ called gneisses belonging to the Silurian group, which others did not, thus drawing the line in a differentplace. Then came Prof. Bonney, J. F. BJ,AKE ON THE NORTH-WEST HIGHLANDS. 438 and pointed out that the so-called syenite was a rock produced by crushing, and many of the supposed Silurian flags true gneisses; and finally Prof. Heddle names the moving gneiss the Logan Rock, and identifies it with the over-riding rock of Arnoboll, whose vagaries we have traced, but denies its connection with the Hebri­ dean gneiss; and yet-and yet-that I should have the hardihood to say it-none of these have learnt one-half what Glen Laggan is prepared to teach, nor probably have 1. To rightly understand that glen, after the writings of Dr. Hicks, it was necessary to begin with Ben Fin. The main mass of this is garnetiferous mica-schist; but its western off-shoot, Meal Ohaoruinn, corresponding, as far as can be judged, to the lower part of Ben Fin (for a great mass of peg­ matite somewhat disturbs the sequence), is a very flaggy mica­ schist, referable to the Moin series. This is a rock very prevalent about here, and there can be no doubt that it belongs to the Newer Gneiss. Meal Chaoruinn is separated from the great mass above Glen Docherty by a gorge called Allt Duchairich, Along this valley, according to Dr. Hicks, there is a fault separating the mass from the east, which he refers to the older gneiss, from that to the west which he refers to the Silurian-rocks. I don't know whether Dr. Hicks went up that valley, but I did. Its section is clear, and a

Fig. 9.-INTERPRETATION SECTION, GLEN LAGGAN.

complete succession is visible from the base to the summit on the eastern side. Below are flaggy beds, though many are quite gneisses. Above are the micaceous flags of Meal Chaoruinn. I am not prepared to say that some portion of the series may not have been pushed over another, in a minor degree; but there is no fault-rock or large disturbance, and the lower portions are certainly gneisses. Now, the continuations of these lower portions are seen in the cliff on the opposite side of the glen; the more micaceous schists are continuous round the head, and continue to hold the ground some way across the upland moor; and finally the more fiaggy and less micaceous portions reach the surface, and form the headlands overlooking the Glen Laggan, In a word, there is no 434 J. F. BLAKE ON THE NORTH -WEST HIGHLANDS.

fault at all, and it is only as we climb down the actual cliff thatthe basal beds become so little gneissose that we doubt their nature. I made the descent Dear the head of the undivided valley, and kept along the eastern side of the stream. Now, what immediately underlies these Baggy beds? The Old Gneiss, I say. Logan Rock it may be called, but it forms the whole base for quite a mile, and it is only at a spot half.way down the valley that the limestone comes in. We have learnt, I hope, by this time some caution in dealing with these masses. Here we shall need it. The Old Gneiss at first overlies the Limestone, but where the river rushes in gurgling tor­ rents through the defiles of the gneiss the two banks are treated differently. On the western side, just at th e branching of the roads, the beds are bent into a cone, with the apex pointing up the stream. Thus the same bed is seen coming down the hill with an easterly dip and then going up again with a north-westerly dip, turning the corner at the stream ; and over the edges of the rising beds, on the north-east sides, lies the Old Gneiss-a new mode of intrusion . On the eastern side the beds are less disturbed, and we can trace the Fucoid beds and the Scoriaceous bed leading up to a considerable mass of limestone, whose relations to any higher beds I could not make out. Further down the valley, at the corner of Glen Docherty, there is material called Torridon Sandstone, sur­ rounded, except above, by the Old Gneiss. It is of no use account­ ing for these features by straight vertical faults or intrusions of ordinary igneous rocks. We have seen by this time that such is not the order of stratigraphy in these districts. I say " material called Torridon Sandstone" because,although it may seem absurd, I did not find it easy to say where the Old Gneiss left off and the Sandstone began. Distinct enough rocks one would think j but if some sandstone were churned up with plastic gneiss what sort of rock would be the result? Such a rock it is, I think, that we see here. The Sandstone has gone with the Gneiss in its motion, and has been mixed with it. Above the sandstone come immediately the Bags. The main portion of these is continuous along the glen, and whether overlying or not,it bas the samerelation of positionfirst to the Old Gneiss, then to the Limestone, and finally to the Torridon Sand­ stone. How, then, can we look upon them as succeeding the lime­ stones more than the others ? I speak of them as a whole, but the small portion where they are most easily examined, along the gorge J. F. BLAKE ON THE NORTH-WEST HIGHLANDS. 435 of a little torrent nearest to Glen Docherty, might at the base be­ long to the limestone series, and be overriden by the flaggy gneiss. I am even yet uncertain whether it is so. The only place where there was any sign of disturbance was at the base. No fault-rock, or even suggestion of it, could be found above, and yet the beds are very little gneissose, though unlike most of the slates of the North, and some are even calcareous. From microscopical examination Professor Bonney has pronounced them true Gneisses. From my point of view, however, this point, though interesting, is compara­ tively immaterial, for the true teaching of the district is to be found at the head of the glen. Here, we have seen, the Newer Gneiss rests on the Old. If we still think this basal rock an intrusive one, all doubt ought now to be expelled, for past the conical fold where the Gneiss creeps over Limestone we find no more of the latter. The whole hillside is Old Gneiss, except for patches of the Newer: no Quartzite, no Torridon Sandstone. Indeed, in the valleys, especially the western one, we bid good-bye to these disturbing elements and the two Gneisses are left alone. The area occupied by the lower, its hornblendic veins, and its relations to the upper decide its character. The line of junction between the two is obscure, but it ascends towards the north and west, so that the summit of the pass is in the older beds. Even here there has been motion between the two formations, if we are to judge by the contortions of the folise and the pressed nature of the rocks near the line of junction j but I saw absolutely no intrnsion, nor anything like it. At the summit th ere are also some masses of mixed Torridon Sand­ stone, but whether lying wholly on the Old or also on the Newer Gneiss is hard to say. On the other slope of the pass we find again our old friends the flaggy beds of Glen Laggan, the rocks at the base of AlIt Duchairich, and finally the micaceous beds of Meal Chaoruinn, gradually.sbnding the one into the other. This, then, in the district is the normal sequence of the Newer Gneiss. My goal in this excursion was Chailleach, a beautiful mountain in the fastnesses of Fannich Forest, 3,276ft. above the sea. I had been attracted to it by its white and glistening summit, which mocked me with the hope it might be Quartzite overlying Newer Gneiss. That hope died away as soon as I found the two Gneisses were alone in the district, but I was well rewarded by the climb. The wild and rugged path lay wholly on the micaceous flaggy gneiss, with very little dip. In one corry a beautiful cleavage gave 33 486 J. F. BLAKE ON THE NORTH~WEST HIGHLANDS.

it a false-bedded appearance. On the upper slopes it became coarser, and pnt on the characters of the rocks of Ben Fyn, and at last it formed a jewelled hill, with masses of ruddy garnets set in silvery mica, sparkling like a rocky diadem in the golden sun. The summit too had charms of its own. For the geologist there was an S-like fold, flattened from above, and showing that this crowning height was once beneath the press of overlying rocks, deep enough to bend the materials like paper ; for the botanist, a lawn-like surface of the mossy Cherleria, decked with the stars of Silene acaulis, and soft, like a couch of down; for the tourist, a panorama of fleecy clouds below, the dark surging hills in the south, with relics of their winter's snow, on the west the neighbouring Minch, and on the east a glimpse of the Cromarty Firth-a view from sea to sea. But to return. This traverse was to me decisive. After all, one might say: If the eastern gneiss is really of Archrean age, if it naturally succeeds the western, is there no place where these rela­ tions are undisturbed? Do the Silurian rocks always intervene? So long as these queries are unsatisfied there must be a linger­ ing doubt in the mind; but when these parvenues, treated always with scant courtesy by the ancient lords, turned upside down, rolled upon or bent into narrow folds, are absent, the natural rels­ tions appear. Such, at least, is my view, and I wait to hear any more reasonable conclusion from the facts adduced. StilI, one deci­ sive proof is wanting. No portion of the Silurian has yet been found lying isolated on undoubted Newer Gneiss-say, a piece of Annelid Quartzite on the micaceous schists. Such, I think, might be looked for south of Craig-an-Knockan, where it is the Newer Gneiss that overrides. But, on the contrary, no conglomerate of Silurian pebbles has been found within the Newer Gneiss at any locality undoubtedly removed from all effects of overriding, and we have quite as much right to demand the one as the other. In the meantime, stratigraphy having cleared the way, petrology has a word to say. To say it all would take too long a time. But there are two facts which weigh particularly with me. Snch schists as those of the Newer Gneiss can be matched among undoubted Archsean rocks, but they cannot be matched among any un­ doubtedly newer rocks. No doubt the white mica schists and foliated limestones cannot be matched in the Hebridean nor the granitoid gneisses of the latter out of it; but the hornblende and black mica schists are almost identical, and the whole are of J. F. BLAKE ON TIlE NORTH-WEST HIGHLANDS. 437 the same general type. Again, if one crosses the district of the Newer Gneiss from east to west at different latitudes, one is im­ pressed with the enormous development of the rocks, their high angles of dip, and their variety; and though we may believe in several overfolds, indicated here and there by contortions on a small scale, yet the whole must be a mighty formation. Through every part of this, except where bent in subsequent accidents or distorted by the development of local minerals, the lines are drawn with mathematical accuracy-often some hundreds to the inch, and hence some millions to the mile. In the vision of the Prophet Ezekiel the scattered skeletons came bone to bone; but this was poetry. Where in Nature is the mighty selective force that shall bring hornblende to hornblende, mica to mica, quartz to quartz, or the materials to form these minerals from the ingre­ dients of an ordinary sediment, along such mathematical surfaces? Regional metamorphism? He who can see nothing in the regions we have traversed but a " conformable upward succession" may be forced to hold this theory as well; but, after our experience of the true stratigraphy extraordinary, the necessity for this strange hypothesis is abolished for the British Isles at least. One other "regionally metamorphosed" area is cited by the author.of our latest text-book, the Green Mountains of Vermont. These I shall start next week to see. I am sure you will wish me good luck on the voyage and in the distant conntry, and if I have that luck, I will bring you word again whether or not, as far as I can see, we are bound to believe in that country what has been disproved in this.