IN TKE NORTH-WEST HIGHLANDS OF SCOTLAND. 395 



An interesting^ example occurs on the hills to the east of Stoer, 

 where a dolerite-dyke trending 'N.W. is traversed by a thrust-plane 

 nearly at rip^ht angles to its course. Wrenched out of its path for 

 some distance in accordance with the line of shearing, the dyke has 

 been completely recrystallized and converted into hornblende-schist, 

 which is not continuous. Along the path of the newer shearing the 

 dyke is represented by lenticular strips of schist, the dip of the 

 foliation being the same as that of the adjacent gneiss, which at 

 this point has also undergone reconstruction owing to the same 

 movement. 



In all the cases above described belonging to these three systems 

 of disruption-lines, the divisional planes developed in the hornblende- 

 schist are either vertical or highly inclined. But we have yet to 

 call attention to the presence of nearly horizontal foliation in these 

 basic dykes. 



In the neighbourhood of Stoer various dolerite-dykes, trending 

 N.W. and occurring in the heart of gently inclined gneiss, are 

 traversed by divisional planes crossing them at low angles and 

 curving downwards towards the edges. The adjacent gneiss, 

 though nearly flat or lying at low angles, has also been reconstructed. 

 In such cases the dykes lose many of their characteristic features ; 

 they no longer present their wall-like form, but weather with much 

 the same contour as the gneiss. 



These horizontal movements, resulting in gently inclined thrust- 

 planes and the development of horizontal schistosity, are not un- 

 common throughout the Archaean area. But there can be no doubt 

 that thev have an intimate relation with the vertical disruption- 

 lines, for it sometimes happens that the one merges into the other. 

 A remarkable example of this phenomenon is met with about a mile 

 to the N'.F.W. of Kylestrome. The general direction of the thrust- 

 plane is W. 47° S., the hade being sometimes nearly flat, sometimes 

 forming an angle of 30° with the horizon : while on the north side 

 of the dolerite-dyke which it traverses the thrust-plane is inclined at 

 a high angle. This instance is of special importance, because it 

 illustrates those sharp curves so common in the far later disruption- 

 lines which traverse the Silurian limestones and quartzites ; nay, 

 further, it shows how the same phenomena were repeated after 

 long cycles of time in the production of the eastern schists. 



7. Effects of these Movements on the Gneiss. 



We now come to consider the effects of these various systems 

 of movement on the original Archaean gneiss. 



a. On approaching one of these vertical lines of movement the 

 original gneiss suddenly loses its low angle, becomes highly inclined, 

 and dips either towards or away from the disruption-line. Still 

 more frequently the gneiss is thrown into a series of sharp folds 

 forming a belt of contorted strata close to the lines of shearing. 

 No matter what may have been the strike of the original gneisses, 

 the axes of the folds are always more or less parallel with the 



