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 assailed, 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 asserts "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 mass 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 classes 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 reasserts 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 class 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 classify.
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 Archæan gneiss," others are the western Quartzite, &c.; but traced eastwards, "the crystalline characters become more and more pronounced, 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 Archæan (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, "Archæan;" 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 associated 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 classify the Hebridean, especially by Hicks,[9] but into this, space 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.