512 



NA TURE 



[September 19, 1901 



in the course of his investigations Nicol's views underwent a 

 process of evolution, and that even in the form in which he 

 ultimately presented them he did not grasp the whole truth. 

 We now know that he was in error when he regarded portions 

 of the Archoean gneiss, occurring in the displaced masses, as 

 igneous rocks intruded during the earth-movements, and that 

 he failed to realise the evidence bearing on dynamic meta- 

 morphism resulting from these movements. But I do not 

 doubt that the verdict of the impartial historian will be that 

 Nicol displayed the qualities of a great stratigraphist in grap- 

 pling with the tectonics of one of the most complicated mountain 

 chains in Europe. 



The period now under review embraces the reopening of that 

 controversy in 187S by Dr. Hicks, and its close in 1^884 after 

 the publication of the " Report on the Geology of the North- 

 west of Sutherland," by the Geological Survey (Nature, vol. 

 xx.kI. p. 29, November 1S84). The Survey work has confirmed 

 Prof. Bonney's identification of the Lewisian gneiss and Torri- 

 don Sandstone in Glen Logan, Kinlochewe {Quart. Join n. Geol. 

 Soc, vol. xxxvi. p. 93), brought into that position by a reversed 

 fault ; and Dr. Callaway's conclusions regarding overthrust 

 faulting at Loch Broom, in Assynt and in Glencoul (ibid., vol. 

 xxxix. p. 416). Special reference must be made to the remark- 

 able series of papers by Prof. Lapworth on "The Secret of the 

 Highlands," in which he demonstrated the accuracy of Nicol's 

 main conclusions, and pointed out that the stratigraphical 

 phenomena are but the counterpart of those in the Alps, as de- 

 scribed by Heim (Geol. Mag., December 2, vol. x. pp. 120, 193, 

 337). His researches, moreover, led him to a departure from 

 Prof. NicoVs views regarding the age, composition, and mode of 

 formation of the Eastern Schists, for in the paper which he com- 

 municated to the Geologists' Association in 1884 he announced 

 that their present foliated and mineralogical characters had 

 been developed by the crust-movements which operated in that 

 region since the time of the Durness quartzites and limestones 

 (Proc. Geol. Assoc, vol. viii. p. 438; Geol. Mag., December 3, 

 vol. ii. 1885, p. 97). Allusion must be made also to his great 

 paper " On the Discovery of the Olenelliis Fauna in the Lower 

 Cambrian Rocks of Britain," in which he not only chronicled 

 the finding of this fauna at the top of the basal quartzite in 

 Shropshire, but suggested the correlation of the Durness quartz- 

 ites and limestones with the Cambrian rocks elsewhere (Geol. 

 Mag., December 3, vol. v. pp. 484-4S7). That suggestion was 

 strikingly confirmed within three years afterwards by the dis- 

 covery of the Olenellus fauna in Ross-shire. 



The detailed mapping of the belt of Cambrian strata has 

 proved the striking uniformity of the rock sequence. There is 

 little variation in the lithological characters or thicknesses of 

 the various zones. Basal quartzites, pipe-rock, Fucoid-beds, 

 Serpulite (Salterella) grit, limestone, and dolomite form the 

 invariable sequence, for a distance of a hundred miles, to the 

 west of the line of earth-movements. This feature is also 

 characteristic of the fossiliferous zones, for the sub-zones of the 

 pipe-rock, the Olenellus fauna in the Fucoid-beds, and the 

 Salterella limestone have been traced from Eriboll to Skye. 

 Owing to the interruption of the sequence by reversed faults or 

 thrusts, the higher fossiliferous limestone zones are never met 

 with between Eriboll and Kishorn, but they occur in Skye, 

 where they were first detected by Sir A. Geikie (Quart. Journ. 

 Geol. Soc. vol. xliv. p. 62). 



Regarding the palreontological divisions of the system, my 

 colleague, Mr. Peach, concludes "that the presence of three 

 species of Olenellus in the Fucoid-beds and Serpulite-grit of 

 the North-west Highlands, nearly allied to the American form 

 Olenellus Thomsoni — the type species of the genus — together 

 with Hyolithes, Salterella, and other organisms found with it, 

 prove that these beds represent the Georgian terrane of 

 America, which, as shown by Walcott, underlies the Paradox- 

 ides zone." Hence he infers that there can be no doubt of the 

 Lower Cambrian age of the beds yielding the Olenellus fauna 

 in the North-west Highlands. Mr. Peach further confirms 

 Salter's opinion as to the American facies of the fossils obtained 

 from the higher fossiliferous zones of the Durness dolomite and 

 limestone. He states that " the latter fauna is so similar 

 to, if not identical with, that occurring 1 in Newfoundland, 

 Mingan Islands, and Point Levis, beneath strata yielding the 

 Phyllograftus fauna of Arenig age, that the beds must be 

 regarded as belonging to the higher divisions of the Cambrian 

 formation." 



The intrusive igneous rocks of the Assynt region, of later date 



than Cambrian time, and yet older than the post-Cambrian 

 movements, have been specially studied by Mr. Teall, who has 

 obtained results of special importance from a petrological point 

 of view. This petrographical province embraces the plutonic 

 complex of Cnoc na Sroine and Loch Borolan, and the 

 numerous sills and dykes that traverse the Cambrian and Torri- 

 donian sediments, and even the underlying platform of Lewisian 

 gneiss. He infers that the plutonic rocks have been formed by 

 the consolidation of alkaline magmas rich in soda. At the one 

 end of the series is the quartz-syenite of Cnoc na Sroine, and at 

 the other the basic augite-syenite, nepheline-syenite, and boro- 

 lanite. The basic varieties occur on the margin, and the acid 

 varieties in the centre. The sills and dykes comprise two well- 

 marked types, camptonites or vogesites, and felsites with alkali 

 felspar and Kgirine, which he believes to represent the dyke 

 form of the magmas that gave rise to the plutonic mass (Geol. 

 Mag., December 4, vol. vii. p. 385, 1900). 



The striking feature in the geology of the North-west High- 

 lands is the evidence relating to those terrestrial movements that 

 affected that region in post-Cambrian times, which are without 

 a parallel in Britain. The geological structures produced by 

 these displacements are extremely complicated, but the vast 

 amount of evidence obtained in the course of the survey of that • 

 belt clearly proves that, though the sections vary indefinitely 

 along the line of complication, they have certain features in 

 common which throw much light on the tectonics of that 

 mountain chain. Some of these features may thus be briefly 

 summarised. 



( 1 ) By means of lateral compression or earth-creep the strata 

 are thrown into a series of inverted folds which culminate in 

 reversed faults or thrusts. 



(2) Without incipient folding, the strata are repeated by a 

 series of minor thrusts or reversed faults which lie at an oblique 

 angle to the major thrust-planes and dip in the direction from 

 which the pressure came, that is, from the east. 



(3; By means of major thrusts of varying magnitude the follow- 

 ing structures are produced : (a) the piled-up Cambrian strata 

 are driven westwards along planes formed by the underlying un- 

 disturbed materials ; (b) masses of Lewisian gneiss, Torridon 

 Sandstone, and Cambrian rocks are made to override the under- 

 lying piled-up strata ; (c) the Eastern Schists are driven west- 

 wards and, in some cases, overlap all major and minor thrusts 

 till they rest directly on the undisturbed Cambrian strata. 



When to these features are added the effects of normal fault- 

 ing and prolonged denudation, it is possible to form some con- 

 ception of the evolution of those extraordinary structures which 

 are met with in that region. Some of the features just described 

 occur in other mountain chains affected by terrestrial movement, 

 as in the Alps and in Provence ; but there is one which appears 

 to be peculiar to the North-west Highlands. It is the remark- 

 able overlap of the Moine Thrust-plane — the most easterly of 

 the great lines of displacement. Along the southern confines of 

 the wild and complicated region of Assynt, that plane can be 

 traced westwards for a distance of six miles to the Knockan 

 clifif, where the micaceous flagstones rest on the Cambrian lime- 

 stone. In Durness we find an outlier of the Eastern Schists re- 

 posing on Cambrian limestone, there preserved by normal faults, 

 at a distance of about ten miles from the mass of similar schists 

 east of Loch Eriboll, with which it was originally continuous. 



Though many of these structures appear incredible at first, it 

 is worthy of note that some have been reproduced experiment- 

 ally by Mr. Cadell { Trans. Roy. Soc. Edinburgh, vol. x.xxv. 

 p. 337). He took layers of sand, loam, clay, and plaster of 

 Paris, and after the materials had set into hard brittle laminae, 

 in imitation of sedimentary strata, he applied horizontal pressure 

 under varying conditions. The results, some of which may here 

 be given, were remarkable. 



(1) The compressed mass tends to find relief along a series of 

 gently inclined thrust-planes, which dip towards the side from 

 which pressure is exerted. 



(2) After a certain amount of heaping up along a series of 

 minor thrust-planes, the heaped-up mass tends to rise and ride 

 forward bodily along major thrust-planes. 



(3) The front portion of a mass being pushed along a thrust- 

 plane tends to bend over and curve under the back portion. 



(4) A thrust-plane below may pass into an anticline above ; 

 and a major thrust-plane above may and probably always does 

 originate in a fold below. 



Now these important experiments confirm the conclusion 

 reached by the (_;eological Survey from a study of the pheno- 



NO. 1664, VOL. 64] 



