TRANSACTIONS OF SECTION C. 62l 



Phyllograptus fauna of Arenig age, that the beds must be regarded as belonging 

 to the higher divisions of the Cambrian formation.' 



The intrusive igneous rocljs of the Assynt region, of later date than Cambrian 

 time, and yet older than the post-Cambriau 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 Torridoniau sediments, and even the underlying platform of 

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

 solidiition 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, aud borolanite. The basic varitties occur on ihe 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 regirine, which 

 lie believes to represent the dyke form of the magmas that gave rise to the plutonic 

 mass.' 



The striking feature in the geology of the North-west Highlands 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 following structures 

 are produced : («) the piled up Cambrian strata are driven westwards along 

 planes formed by the underlying undisturbed 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 westwards 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 eftects of normal faulting and prolonged 

 denudation, it is possible to form some conception of the evolution of those extra- 

 ordinary structures which are met with in that region. Some of the features just 

 described occur in other mountain chains aflected 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 remarkable 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 tlie Knockan clifi', where the micaceous flagstones rest 

 on the Cambrian limestone. In Durness we find an outlier of the Eastern Schists 

 reposing on Cambrian limestone, there preserved by normal faults, at a distance of 

 about ten iniles from the mass of similar schists east of Loch EriboU, 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 experimentally by Wr. Cadall.'^ He took layers 

 of sand, loam, clay, and plaster of Paris, and after the materials had set into hard 

 brittle laminas, in imitation of sedimentary strata, he applied horizontal pressure 

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

 remarkable. 



' Gcol. Mag., December 4, vol. vii. p. 385 (1900). 

 ' Trans. Eoyal Soc. Edinhirgh, vol. xxxv. p. 337. 



