Gairloch in North-West Ross-Shire / Its Records, Traditions, Inhabitants, and Natural History, with a Guide to Gairloch and Loch Maree, and a Map and Illustrations

by John H. Dixon

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 associated with the limestone strike, and assumes 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 classification 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 passed 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," associated 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 associated 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, classified 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 Assynt and elsewhere; but as these do not occur in our district, they need not be further described.