North of Coigeach, it occupies most of the west coast on the mainland up to Cape Wrath; and southwards, there is a patch of it at the Narrows of Loch Torridon. From recent researches, it will probably be found widely extended over the rest of the Highlands.
It is more or less vertical in dip on the west coast, and has there a general persistent strike from north-west to south-east. The special character of its scenery is very well seen round Loch a.s.synt, and is well presented in the parish of Gairloch. As shewn on the map, it forms the splendid peak of Alligin, above three thousand feet, which towers above Loch Torridon, from which it pa.s.ses to the head of the Gairloch, where it is admirably exhibited in structure, dip, and strike on the sh.o.r.e near the Free church and along the picturesque road to Poolewe. It contains some limestone on Loch Maree in a line parallel to the loch, for some miles on both sides of Letterewe. A vigorous attempt has recently been made by Dr Hicks to discriminate this gneiss into certain series or epochs, which he has named, and by which he seeks to interpret the rest of the Highlands.[3] In America, the Laurentian system contains the celebrated _Eozoon Canadense_, that is, the Canadian Dawnlife, the lowest organic form yet known. It has as yet proved absolutely barren in Europe, though a flutter was raised by its supposed discovery by Dr Heddle of St Andrews, Dr Carpenter a.s.serting the fact, but its discoverer, on further examination, disclaiming the honour.
II. THE TORRIDON SANDSTONE.--This is the chocolate-coloured sandstone so splendidly exhibited round Loch Torridon, where it towers into the mural dignity of Liathgach. It is well presented in the mountains of Applecross, as seen from Loch Carron, and from Loch Kishorn which lies at their southern base. This sandstone occurs only at one spot in the Outer Hebrides, round the harbour of Stornoway; it is found in Rum, its southermost position, in Sleat, Scalpa, Raasay, and neighbouring islets; it occurs continuously, except where the Hebridean gneiss appears, from Loch Carron to Coulmore near Loch Inver, and thence in isolated patches, north to the Kyle of Durness. The scenery it presents is uncommonly striking, ma.s.sive and grand, its mural character, which arises from its horizontal strata, being a special feature, and nothing in style can anywhere surpa.s.s the splendid spear-headed crest of Slioch, the monarch of the mountains, worthy though his compeers are, that stand round Loch Maree. The denudation to which this ancient sandstone has been subjected has been extraordinary. This is well seen round our loch, when we consider that Slioch is above three thousand feet in height; but still more impressively, from the sea off Loch Inver, in the sugar-loaf cone of Suilven and its brethren, all isolated stacks of Torridon sandstone,--so remarkable that Murchison selected this scene as the most striking example of denudation he knew, to ill.u.s.trate the subject in his famous "Siluria."
Round Loch Maree, it forms its southern sh.o.r.es east of Talladale, where its character can be well examined in the delightful drive from Kenlochewe. On the north side, it touches the loch only at its two extremities, at the one end, near Inveran and along the Ewe, and at the other, in Slioch, stopping short of the head of the lake, as can easily be seen from the south side. It is more or less horizontal, or dips slightly to the south-east, being deposited in thick, well-marked beds, as everywhere exhibited, and thus forms a remarkable contrast to the vertical strata on which it rests. An excellent junction of the two, easily reached and examined, occurs on the sh.o.r.es of Gairloch, at the end of the rocky peninsula on which the Free church stands. There the two are seen, the more or less horizontal Torridon superposed on the vertical Hebridean, in the most striking style, which is rendered all the clearer by the washing of the restless tides. This sandstone about Loch Maree is about four thousand feet in thickness.
It was correlated by Murchison with the Cambrian system, the second in the geologic series, and was so named by him,--a name now recognised by the chief authorities. It is well, however, to designate it by a neutral geographical term, and to retain the t.i.tle given it by Professor Nicol, that of Torridon Sandstone, or Torridon Red. In Scotland, it has as yet yielded no organic remains, though these are abundant and good in Wales, after whose ancient name of Cambria it is called, and also in Scandinavia, which remained united to Scotland till post-glacial times.
It was long thought to be a western representative of the Old Red Sandstone of the east coast, Hugh Miller, among others, looking on it as a worthy example of his pet rocks; but in his day, the geology of the Highlands was but dimly and imperfectly known, and their great problems were not even surmised.
Like the Old Red, a fact that tended to mislead early observers, its lowest bed is a thick ma.s.sy conglomerate or breccia, which is very well seen at the junction at Gairloch, and which is generally persistent throughout the system on the west coast. It consists of varied pieces, sometimes rounded, often angular, and some of them large, of the under-lying Hebridean rocks, enclosed in a finer matrix of the same materials. Portions of the "Eastern rocks" have, it seems, also been detected in it,--a fact which, if established, indicates the true age and succession of these "Eastern rocks."
III. THE QUARTZITE.--Above the Torridon Red, lies a thick-bedded whitish rock, called from its general appearance Quartzite. This French word is, however, a partly misleading term, as the rock is not quartz, though much made up of quartz grains; but it is a highly metamorphosed fine sandstone. It is here sometimes coincident in dip with the underlying Red, but it is generally non-conformable. It can be easily seen, looking from the south side of Loch Maree, at a point east of Slioch, on the right side of a glen watered by a stream called the Fasagh, which separates Slioch from the ridge to the east. In Glen Fasagh, the Torridon Red is clearly observed to rest horizontally on the Hebridean gneiss below, on both sides of the glen; the Torridon forming the most of the western side of the valley up to the summit of Slioch, but rising, on the eastern side, only half-way up, being then surmounted by the strongly contrasted Quartzite to the top of the ridge. The Quartzite continues eastwards to the wide glen beyond, generally but erroneously called "Glen Laggan," or "Glen Logan," though its real name is Glen Cruaidh Choillie.[4]
A vertical fault exists in the middle of this Quartzite ridge, situated halfway between the two glens, and is easily distinguished by the eye from the other side of Loch Maree. It has thrown down the rocks on the eastern half of the ridge some distance, and affects both the Quartzite and the Torridon Red below.
This Quartzite is devoid of mica. It pa.s.ses from pale pinkish to pure white in colour, and occurs in thick, uncommonly regular beds, with rectangular joints. It is well developed at the head of Loch Maree, and rises into the white, glistening, barren peaks and ridges of Beinn Eay.
It forms some admirable scenery, not only here but wherever it occurs, for it is widely distributed over the Highlands.
Its capabilities in this way are also well exhibited on the west coast round Loch a.s.synt, rising there into the summits of Beinn More and Queenaig, above three thousand feet; and also near Loch Carron to the south, and between a.s.synt and Eriboll to the north. On Loch Torridon, its prevailing tendency to whiteness gives rise to the name Grey Heads, very descriptive of certain contorted peaks near Coulin Lodge.
The Quartzite is interesting as exhibiting the earliest indications of organic life yet discovered in Scotland:--
1. _Annelid Borings._--The lower beds next the Torridon contain, on their surface, as described by Murchison, "large round k.n.o.bs on the top of cylindrical bodies, which pa.s.s through several layers," their number being often astonishing. These are, it is safely concluded, "infillings of excavations" made by certain worms called Annelids, and are known as _Annelid Borings_. They are noteworthy as "the oldest vestiges of life which can be detected in the North Highlands." They are often very clearly seen, as the filling in has generally been done by a different coloured sand from that in which they had been bored. They sometimes project above the surface like "pipes," and are so numerous as to cause these beds to be called "pipe-rock." Examples are abundant round Kenlochewe, and on the roadside at the entrance to Glen Cruaidh Choillie, where they are unusually good. They should be secured by the intelligent visitor from their extraordinary interest.
2. _Fucoid Remains._--Interstratified with the Quartzite, are certain brown, mottled, shaly and flaggy bands, with curious impressions of what seem leaves, which have been thought to be fucoids or seaweeds. The recent Survey explorations would seem to point to their being simply very much squeezed annelid "pipes." The shales in which they occur are thus generally known as the Fucoid Beds, and, when found, are very good evidence of the horizon of the rocks. They are often very distinct and easily seen, and are most interesting. They occur on Loch Maree near the top of the east side of Glen Fasagh, imbedded in the Quartzite, and run through the Quartzite to Glen Cruaidh Choillie.
Other organisms have been found in it elsewhere, such as orthocerat.i.te in a.s.synt, and certain small conical bodies called serpulites.
This Quartzite, with its annelid borings and fucoid beds, is placed by Murchison in the Silurian series, the third in the geological record. By others, such as Dr Hicks, it is considered possibly Cambrian.
IV. THE LIMESTONE.--On the western side of Glen Cruaidh Choillie, resting on the Quartzite, and generally conformable with it, is found a limestone. By examining the map, it will be seen that this limestone runs more or less continuously from Loch Carron to Loch Eriboll. It receives its greatest development at Inchnadamph, at the east end of Loch a.s.synt, where it forms splendid cliffs. It is of commercial value, and has been worked at various places along its outcrop. It will also be observed that there is a wide isolated patch of limestone at Durness, between Loch Eriboll and Cape Wrath.
In this Durness limestone, which was long considered unfossiliferous, like the other rocks of the North-West Highlands, sh.e.l.ls were discovered in 1854 by Mr Peach, the eminent geologist, and friend of d.i.c.k of Thurso. These were determined to be Silurian by Mr Salter, a great specialist in such matters, and were described and figured in a paper by Sir Roderick Murchison in 1858.[5] Since then finer specimens have been discovered. Their likeness to British Silurian fossils is very remote, and they are more related to American forms; but they are generally now accepted as of Silurian or Ordovician age. This discovery of fossils gave a great impetus to the study of these rocks, and formed the basis of the theory propounded by Sir Roderick Murchison.
The Durness limestone turns out, however, to be, as a whole, of a different type from the great strike of limestone which goes through Glen Cruaidh Choillie and terminates at Loch Eriboll. This Durness limestone is held by Dr Heddle, who first ascertained the fact, and by other competent authorities, to be non-dolomitic, while that of the great strike to the east is dolomitic; dolomite (so called from the French geologist Dolomieu) being a variety of limestone, which, in addition to the carbonate of lime of which common limestone mainly consists, contains more or less carbonate of magnesia,--in this dolomite, forty-eight per cent. Dolomitic beds have, however, lately been discovered in the Durness basin by the Survey. For long, no fossils were obtained from the great dolomitic strike, except an orthocerat.i.te at a.s.synt by Mr Peach, and a possible organic ma.s.s by myself at the same place; but recently a varied and important suite of fossils has been gathered by the Survey, which has clearly decided the age of the Dolomite to be Silurian. Of its position above the Quartzite there is no doubt.
It is pretty well exhibited in Glen Cruaidh Choillie, where it has been worked at various places, and where its superposed junction with the Quartzite can be seen.
V. THE "LOGAN ROCK."--Immediately to the east of the Limestone, and in contact with it, is found a remarkable rock, which appears at various parts in the middle of Glen Cruaidh Choillie, and which has caused great discussion in regard to its character, relative position, and age. By Professor Nicol, it was held to be igneous, serpentinous, felspathic, porphyritic, and intrusive, and was named by him "Igneous rock;" by Murchison, to be here a "syenite," and elsewhere a "greenstone," and "serpentinous and felspathic," interbedded with and resting directly upon the limestone; by Dr Hicks and others, to be a "syenite," or a "granitic" and "quartz diorite," and igneous, faulted, and intrusive, like Nicol; and by Dr Callaway, as the Hebridean gneiss "brought up by a fault." It is well here, as in all other cases, to designate it geographically, and call it the "Logan Rock," as first suggested by Heddle, and now generally used.
In "Glen Logan," it is best exposed in the bed of the river about two miles above the school. At a point about halfway up the glen it runs up the hill on the west side, and is seen to overlie the limestone.
This rock appears, as maintained by Nicol, more or less continuously a.s.sociated with the limestone strike, and a.s.sumes a great variety of forms, as shewn by the different characterisations it has received. It has played an important part in the history of the theories of the succession of these Highland rocks. In Sutherland, it sometimes receives a broad development.
VI. THE "EASTERN GNEISS."--Immediately to the east of this rock, rising in Glen Cruaidh Choillie at once from contact with the "Logan Rock," and forming the whole of the eastern wall of the glen, there stretches a long series of shales, schists, gneiss, and other rocks. These appear on both sides of Glen Dochartie, and thence on eastwards through Ross and the main body of the Highlands, till they are overlaid by the Old Red Sandstone of the east coast. The position and interpretation of these rocks have caused extraordinary investigation and discussion, which is still being carried on. They are variously known as the "Eastern gneiss," "Eastern schists," by Murchison and others; the term "Caledonian" has also been proposed by Dr Callaway,--all to distinguish them from the Hebridean of the west coast.
B.--THE CONTROVERSY REGARDING THE SUCCESSION OF THESE ROCKS.
Up to the Limestone, the order of succession of the rocks may be regarded as settled, all parties agreeing as to their relations though differing as to their cla.s.sification under the early geological systems.
It is held that the order is,--lowest, the Hebridean gneiss; above that, very unconformably, the Torridon Red; above that, less unconformably, the Quartzite, with its embedded organic remains; and above that, more or less conformably, the Limestone, with its numerous fossils. At this point, begins the controversy which has so long been waged regarding the nature and succession of the rocks in the North-West Highlands, and which has pa.s.sed through many phases of opinion, and even disturbed the long-tried friendship between the chief combatants, Murchison and Nicol.
The "Logan Rock" Murchison considered to rest on the Limestone, and not to be intrusive and igneous as thought by Nicol. He also maintained that the "Eastern gneisses and schists" lay more or less conformably above the limestone or interbedded syenite, and were therefore more recent,--in fact, were a continuation of the Silurian system, of which the limestone was the representative example.
Professor Nicol held to the last,[6] that the Limestone is the highest rock in the whole series of the North-West Highlands; that faulting or igneous action exists along the line of the "Igneous rocks," a.s.sociated with the Limestone; that these "Eastern gneisses and schists" do not overlie the Limestone; that where they seem to do this, the appearance is caused by an overlapping of these "Eastern rocks" through pressure from the east; and that these rocks are probably the Hebridean, or, as he called it, the "Fundamental," gneiss reappearing. Latterly, he did not condescend to identify any of these rocks of the North-West with the received geological epochs, leaving this to be settled by subsequent investigation; but he held strenuously that the succession was as he declared,--Fundamental gneiss, Torridon Red, Quartzite, and Limestone, the rocks east of this point being metamorphic forms of the western gneiss reappearing.
Murchison, at last a.s.sociated with Dr Archibald Geikie, who in 1858 wrote a joint memoir with him on the subject of great value,[7] held, on the other hand, that there exists an unbroken series from the Fundamental or Laurentian gneiss to these "Eastern gneisses and schists," and that they succeed each other in superposition and age.
They, moreover, cla.s.sified them as Laurentian, Cambrian, and Silurian; the Silurian beginning with the Quartzite, and continuing eastwards in various folds and reduplications till overlaid by the Old Red Sandstone.
Other points of difference existed between these eminent geologists, particularly as to the existence of two Quartzites, and two Limestones, as apparently exhibited at a.s.synt and elsewhere; but as these do not occur in our district, they need not be further described.
For twenty years Murchison"s theory dominated over Nicol"s, with scarcely a dissentient voice. The brave old professor maintained to the end, against the geological world, opinions to which, while seemingly less probable, he had been led both by years of unusually careful examination of the whole field, which he knew better than any, and by general considerations regarding metamorphism and other matters affecting these ancient rocks; while his opponents were so confident of their position, that Geikie, in his "Life of Murchison," headed one of his chapters "The Geological Conquest of the Highlands." But in 1878, Murchison"s conclusions began to be vigorously a.s.sailed, the attack being led by Dr Henry Hicks,[8] and has been strenuously maintained by him and other eminent geologists, of London, such as Bonney, Huddleston, Callaway, Heddle, Lapworth, Etheridge, Judd, and many others. These have written numerous papers advocating conclusions more or less adverse to those of Murchison, and agreeing in the main with those of Nicol.
Even Geikie has had to abandon his early position, and declare against the theory of his former chief. In a remarkable declaration, published in _Nature_ of November 13, 1884, prefacing a paper on "The Geology of North-West Sutherland," by the two Survey geologists Peach and Horne, Geikie made a brave and honourable retractation of these opinions, which he had so long and so ably advocated with Murchison. He there declares: "With every desire to follow the interpretation of my late chief, I criticised minutely each detail of the work upon the ground, but I found the evidence altogether overwhelming against the upward succession which Murchison believed to exist in Eriboll from the base of the Silurian strata into an upper conformable series of schists and gneisses." He found the same true all along the strike of these controverted rocks.
"The clear coast sections of Eriboll have now taught me that the parallelism between the Silurian strata and the overlying schists is not due to conformable deposition." He traced the same kind of evidence southwards for more than ninety miles, and found it "as well marked above Loch Carron as it is at Loch Eriboll."
These "Eastern gneisses" not only frequently appear to be superposed upon the rocks beneath, but, as Geikie says, the parallelism of dip and strike between them and the rocks below them is so complete in some of the Ross-shire sections, that he a.s.serts "had these sections been planned for the purposes of deception, they could not have been more skilfully devised." These Survey geologists explain these extraordinary phenomena by a system of "reversed faults" and "pushes from the east,"
by which the "Eastern rocks" have been driven westwards, in some cases ten miles, and are thus made to overlie the older rocks, through "prodigious terrestrial displacements, to which there is certainly no parallel in Britain,"--displacements which Nicol, against the evidence of his eyes, had insisted on as factors, nearly thirty years before.
Evidences of these dislocations are not so apparent round Loch Maree as elsewhere, especially near Loch Eriboll, but they are sufficiently marked round Kenlochewe as to appeal even to a non-scientific visitor.
In Glen Cruaidh Choillie, at a point already noted, the "Logan Rock" is seen superposed right upon the Limestone up to the crest of the west side of the glen; according to Heddle, it also lies over it, with a slight hiatus, as far as Glen Fasagh. It is to be remembered, following recent conclusions, that this rock did not naturally have this position, but has been pushed violently into it by unparalleled "terrestrial displacements;" and that both this and the long series that form the eastern side of the glen are portions of the Hebridean again coming to the surface, and appearing in such ma.s.s and extent up Glen Dochartie and on to Achnasheen.
It would be out of place here to enter into the various opinions offered to explain the remarkable facts connected with these "Eastern rocks,"
their nature, and their relations to the western. The papers on Loch Maree are already very numerous, and opinions are still conflicting; and the Survey has not yet published its memoir on the Loch Maree district.
Dr Hicks, for example, held that these "Eastern rocks" generally are metamorphosed forms of the Hebridean reappearing, but that the Hebridean occurs at the junction of Glen "Logan" and Glen Dochartie, and that along the floor of the latter, the Hebridean, but not the limestone, is overlaid by certain "blue flags and sandstones, and argillaceous, quartziferous, and micaceous flaggy beds" in succession, up to the head of Glen Dochartie. These along with the Limestone he cla.s.ses as Silurian, placing the underlying Quartzite with the Cambrian. At the head of Glen Dochartie, the Silurians disappear, he held, by a possible fault, and the Hebridean or "Pre-Cambrian" as he prefers to call it, again rea.s.serts itself up to the summit of Ben Fyn and eastwards. He writes me, however (1886), that in the light of recent investigations, he is prepared to cla.s.s the Glen Dochartie rocks with the Hebridean, like those at the head of the glen; though he would not yet affirm their exact place in the broad Pre-Cambrian series, which he has lately attempted to cla.s.sify.
In his recent utterance, Geikie maintains that these "Eastern rocks"
have undergone such intense alteration that their original characters have been in great measure effaced. Some of them are "unquestionably part of the Archaean gneiss," others are the western Quartzite, &c.; but traced eastwards, "the crystalline characters become more and more p.r.o.nounced, until we cannot tell, at least from examination in the field, what the rocks may have originally been. They are now fine flaggy micaceous gneisses and mica-schists, which certainly could not have been developed out of any such Archaean (that is Hebridean) gneiss as is now visible to the west. Whether they consist in part of higher members of the Silurian series in a metamorphic condition remains to be seen."
We have now described the whole succession of rocks in our district, from Gairloch and Poolewe to the head of Glen Dochartie, and given some idea of the difficult problems they present and the theories offered for their solution. The succession up to the Limestone is accepted. The Hebridean is now variously designated "Pre-Cambrian;" and by Callaway, Geikie, and others, "Archaean;" the determination of Murchison as "Laurentian" being generally avoided. The Torridon Red is accepted as "Cambrian" by most, and recently by Geikie and his colleagues; though there are differences of opinion as to the precise period in that series to which they belong. The Quartzite and its a.s.sociated beds are placed by Hicks and others with the "Cambrian;" and by others, including Geikie, with the Lower Silurian or Ordovician: but their position above the Torridon and below the Limestone is undoubted. The Limestone is conceded to lie above the Quartzite, but its nature and age are not yet settled, some holding it to be dolomitic and unlike the Durness limestone; Heddle for a time heading these, though now agreeing with the Survey; others, like Hicks, holding the limestones to be the same or, like the Survey geologists, so related as to form one system, which they call "Durness-Eriboll limestone." The "Logan rock" is variously interpreted,--some reckoning it to be igneous and intrusive; others, to be metamorphosed Hebridean; and others, to be granitic and syenitic. The "Eastern gneisses and schists" are still undetermined as to character, relations, or age, opinions being very various and conflicting; though there is a general agreement as to their belonging to some portion of the Hebridean series. Attempts have been made to cla.s.sify the Hebridean, especially by Hicks,[9] but into this, s.p.a.ce prevents our entering here.
My own opinion on this much controverted succession, during nigh twenty years" careful study of the whole field from Skye to Eriboll and more or less minute examination of the disputed sections, has been increasingly in favour of Nicol"s general position. The proofs of Murchison"s contention of the superposition and newer age of the "Eastern gneisses"
I always regarded imperfect, as often expressed both privately and publicly. Nicol"s general contentions as to the unlikelihood of highly metamorphic schistose and gneissic rocks, like the Eastern, being transformed, while older rocks remained so little affected as the Cambrian and others beneath, gained growing weight. Every fresh examination of the ground increased the probability of their apparent superposition being merely overfoldings of the western rocks. The displacements, the investigations of more recent observers have shewn to be much greater than all earlier students, including myself, ever imagined.
Great honour has lately been done Professor Nicol for his enlightened perception of the true solution of this difficult problem at so early a date, "against a phalanx of eminent geological authorities." Professor Judd, at the meeting of the British a.s.sociation at Aberdeen last year (1885), in reviewing this geological problem in a masterly address, justly observes, and in so doing felicitously expresses general opinion:--"Calmly reviewing, in the light of our present knowledge, the grand work accomplished single-handed by Nicol, I have no hesitation in a.s.serting that, twenty-six years ago, he had mastered the great Highland problem in all its essential details." "The Murchisonian theory of Highland succession," he finally concludes, "is now, by general consent, abandoned."
C.--OTHER NOTEWORTHY GEOLOGICAL PHENOMENA.
There are other noteworthy phenomena connected with the geology of Loch Maree deserving attention, which will be now shortly described:--