436 EECEXT WOEK OF TflE GEOLOGICAL STJEVEr 



5. Succession of Strata above tlie Moine T7irust-j)lane, 



"We must now describe briefly the strata overlying the Moine Thrust- 

 plane, stretching eastwards by the Kyle of Tongue to Strathnaver. 

 For nearly six miles there is a belt of strata of remarkably uniform 

 character, consisting of flaggy quartzose mica-schists or fine-grained 

 gneiss, tyiDically developed on the ILoine between Loch Eriboll and 

 the Kyle of Tongue. These strata (the "younger gneiss" and 

 " quartzose flagstones " of Murchison) have been traced continuously 

 from the north coast of Sutherland to Loch Broom, showing little 

 variation in lithological character. In the heart of the mass there 

 is a prominent zone of hornblendic and micaceous schist, studded 

 with garnets, traceable from the north coast to the Kyle of Tongue 

 and thence round the north and west slopes of Ben Hope. There can 

 be little doubt that this zone has once been an extensive sheet of 

 igneous rock, because at various localities patches of the original 

 igneous mass are still met with, consisting of diorite or diabase. 

 Overlying this belt of garnetiferous schist, bands which can still be 

 recognized as sheared Archaean gneiss can be followed for some 

 distance. 



Like the crushed slates, schists, and sheared gneiss (mylonites) un- 

 derlying the ]y!oine Thrust-plane, these flaggy crystalline schists and 

 gneisses are inclined at gentle angles to the E.S.E. That they form 

 an enormous pile of material is evident from the fact that they rise 

 from the sea-level to the crest of Ben Hope (3040 feet). But a 

 careful examination of the path along which they have travelled 

 shows that the divisional planes or foliation-surfaces lie at an 

 oblique angle to the thrust-plane (see fig. 20, where the lloine schists 

 are represented as lying at an oblique angle to the plane of the ]Moine 

 Thrust). It is obvious, therefore, that the thickness of the Moine 

 schists cannot be estimated after the manner of ordinary sedimen- 

 tary strata. This conclusion is confirmed by a study of the struc- 

 tures presented by these schists. The main divisional planes 

 truncate minor planes, like the major and minor thrusts in the dis- 

 placed Silurian strata. In other words, as the schists were being 

 driven forward the materials were piled on each other to an enor- 

 mous thickness. Further, on closer examination, it is observable 

 that the different mineral constituents lie at an angle to the main 

 foliation-planes. There is also evidence to show that the terrestrial 

 movements were intermittent, because the first divisional planes in 

 the Moine schists are frequently truncated b}^ subsequent thrusts. 

 During pauses in the disturbances dykes and thin sheets of various 

 igneous (granitoid) rocks were injected across the foliation-planes, 

 and these intrusions have been in turn sheared by later movements. 



These crystalline schists and flaggy gneisses display parallel lines 

 on the foliation-surfaces, indicating the same direction of movement 

 as those in the sheared Silurian strata and crushed slates and schists 

 (mylonites), while the constituent minerals are orientated along these 

 lines. It is obvious, therefore, that the present strike, dip, and 

 lithological characters of these crystalline schists and flaggy gneisses 



