IN THE NORTH-WEST HIGHLANDS OP SCOTLAND. 429 



these movements in the Cambrian strata near Dundonald that it is 

 difficult to trace the line between the schistose sandstones and the 

 eastern schists. 



VI. Metamorphism resulting from the Post-Lower- 

 SiLURiAN Movements. 



Having described the evidence in proof of enormous terrestrial 

 movements along the chain of mountains between Eriboll and 

 Ullapool, we now proceed to consider the relation of these dis- 

 placements to regional metamorphism *. In reviewing the effects of 

 these movements, we will describe the evidence furnished by (1) 

 the Archaean, (2) the Cambrian, (3) the Silurian, and (4) the igneous 

 rocks intrusive in the Cambrian and Silurian strata. From these 

 various lines of evidence it will be seen that with each successive 

 maximum thrust there is a progressive alteration in the displaced 

 materials as we pass eastwards to the horizon of the micaceous 

 flagstones overlying the Moine Thrust-plane. 



1. Metamorjphism of Archcean Rocks. 



The great slice of Archsean rocks brought forward by the Glencoul 

 Thrust does not present any striking evidence of deformation except 

 close to the lines of disruption. To the north of Glencoul the 

 original banding of the gneiss is as distinct as that in the Archaean 

 area to the west of the Post-Lower- Silurian displacements, the general 

 strike being W.N.W., and the dip of the foliation being S.W., at high 

 angles. It is a significant fact that although the north-west 

 dolerite-dykes are numerous in the undisturbed Archaean area 

 between Ben Stack and Glencoul, none is met with in the thrust- 

 gneiss, above this thrust-plane, till we pass southwards to Glencoul. 

 Indeed the absence of these basic dykes and the presence of broad 

 Aeins of red pegmatite point to the conclusion that the displaced 

 gneiss between Strathcarran and Glencoul resembles that in the 

 Archaean area to the north of Ben Stack. By protracting the angle 

 which the basic dykes in the undisturbed gneiss on Ben Stack 

 make with the direction of movement in the displaced masses, it is 

 clear that the disrupted rocks must have travelled for several miles 

 from the east. 



Overlying the thrust-plane, a thin band of slaty schist or highly 

 sheared gneiss is frequently met with, the strike of the foliation 

 being more or less parallel with that of the thrust-plane. Ad- 

 vancing eastwards towards the limit of the displaced mass, these 

 new divisional planes are more strikingly developed, owing to the 

 number of more or less powerful thrusts. In the latter case, the 

 new planes of schistosity may be either parallel with those of the 



* It ought to be borne in mind that, though the movements affecting the 

 Cambrian sandstones and the fossiliferous quartzites and limestones are now 

 regarded as of Post-Lower-Silurian date, it may ultimately be possible to fix 

 tbeir age with greater precision, when the fossils from the Durness Limestone 

 have been correlated with those of other countries. 



