Chapter IX.
The Geology of Loch Maree and Neighbourhood.
By William Jolly.
The geology of Loch Maree is unusually varied, interesting, and representative. It exhibits, in a limited area, the whole debated series of the succession of rocks in the North-West Highlands. This has been a fertile subject of controversy, surpassed only by the world-famous Glen Roy. It has engaged the attention and the pens of some of the most eminent British geologists, including Macculloch, Hugh Miller, Sedgwick, Sir Roderick Murchison, Professor Nicol of Aberdeen, Archibald Geikie, and a host of others not less able. After considerable discussion, chiefly between Murchison and Nicol, the authoritative name of Murchison, along with that of Archibald Geikie, who wrote a joint memoir on the subject, seemed for a time finally to settle the question in Murchison's favour; and his views were not only generally received, but were embodied in the geological maps of the district most in use. But, lately, the whole question has been reopened with greater keenness than ever, and the conclusions of the geological king have been vigorously and uncompromisingly assailed all along the line. The war is, at the present date, in full swing, but with a near prospect of final peace. The geological problem of the Highlands is by no means settled, though much additional light has been thrown on the debatable ground by the researches of the numerous and capable combatants, including recently Peach and Horne, of the Geological Survey; and their investigations will no doubt hasten the final determination of the vexed question. But a firm basis of interpretation has at length been gained, by which the geological structure of the broad tracts of the Highlands, hitherto uniformly coloured as Silurian, will be investigated under new and important lights, and a remapping of the Highland area erelong achieved, with such permanent results as have hitherto been impossible.
The conditions of the problem are extremely well exhibited round Loch Maree. Here we are presented, as Dr Archibald Geikie truly observes, with "a series of sections of singular clearness." He confesses that he knows of "no locality where the geologist may better acquaint himself with the order of superposition of the ancient crystalline rocks of the Highlands, or with the dislocations and metamorphism which they have undergone." These will now be briefly explained. The whole subject may, without much difficulty, be understood by the ordinary reader, if he will use a geological map of Scotland, such as Nicol's or Geikie's, which he will also find useful as a guide.
A.—The Series of Rocks in the North-West Highlands.
The rocks round Loch Maree are shortly the following:—
I. The Hebridean Gneiss.—The Long Island from the Butt of the Lews to Barra Head consists almost entirely of a species of gneiss, very much metamorphosed. It occurs in the Inner Hebrides in Tiree and Coll, in Sleat, in Raasay, and Rona, off Portree, but very little in Skye, one of our youngest isles. It is found in patches on the Mainland on the western shores of Ross and Sutherland, and stretches from Torridon to Cape Wrath, whose contorted cliffs it forms. It has been variously designated Hebridean, from being chiefly found in the Hebrides; Lewisean, from forming the most of the Lewis, a less acceptable name; Archæan, from being the earliest system; Pre-Cambrian, as being earlier than the Cambrian sandstone immediately above it; and Fundamental, from its constituting the lowest rock strata in the British Isles. Murchison identified it with the lowest geological series, the Laurentian, which is so named from being extensively developed on the St Lawrence in Canada. It is best, however, to designate the rocks by a geographical and non-theoretical term, like Hebridean.