Presidential Address to Geological Section. 459 



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 summarized : — 



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 : (a) 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 underlying 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 effects of normal faulting and 

 prolonged denudation, it is possible to form some conception 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 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 the Knockan cliff, 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 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 experimentally by Mr. Cadell.^ 

 He took layers of sand, loam, clay, and plaster of Paris, and after the 

 materials had set into hard brittle lamiua3, 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 Geological Survey from a study of the phenomena in the field, viz., that 

 under the influence of horizontal compression or earth-creep the rocks in 

 that region behaved like brittle rigid bodies which snapped across, were 

 piled up, and driven westwards in successive slices. But, further, these 

 displacements were accompanied by difl'erential movement of the materials 

 which resulted in the development of new structures. These phenomena 

 culminate along the belt of rocks in immediate association with the Moine 



1 Traus. Roy. Soc. Edinburgh, vol. xxxv, p. 337. 



