THE PRE- CAMBRIAN ROCKS, BRITISH ISLES. g 



lines just referred to in the Cambrian strata, and in the overlying 

 schists, and that the general tectonic structures and lithological 

 characters of the eastern schists differed in many respects from 

 those of the Lewisian gneiss. 



The problems in tectonic geology presented 'by the compli- 

 cated structures of the northwest of Scotland have been 

 ably worked out by the officers of the Geological Survey, to 

 whose report in the Quarterly Journal of the Geological Society 

 for 1888, I would refer for full details. It has been shown 

 that, besides stupendous dislocations and horizontal displace- 

 ments, the rocks have been cut into innumerable slices which 

 have been driven over each other from the eastward, while at 

 the same time there has been such a general shearing of the 

 whole region that for many hundreds of square miles the 

 original rock -structures have been entirely effaced, and have 

 been replaced by new divisional planes, which, when they 

 approach the underlying Cambrian strata, are roughly parallel 

 with the bedding planes of these strata. 



In this region, therefore, we have striking proofs of a stupen- 

 dous post -Cambrian regional metamorphism. But there is still 

 much uncertainty regarding the geological age of the rocks 

 which have been affected by it. There can be no doubt that 

 large masses of the old gneiss, torn up from below, have been 

 thrust bodily westward for many miles, and are now seen with 

 their dykes and pegmatites resting on the Durness limestones and 

 quartzites. It is equally certain that in other districts huge 

 slices of the Torridon sandstones have been similarly treated. 

 But where all trace of original structure has disappeared, we 

 have, as yet, no means of definitely determining from what for- 

 mation the present eastern schists have been produced. The 

 ordinary gneissose and quartzose flagstones do not appear to me 

 to be such rocks as could ever be manufactured by any chemi- 

 cal or mechanical process out of the average type of Lewisian 

 gneiss. I have long held the belief that they were originally 

 sediments, but whether they represent altered Torridon Sand- 

 stone, or some clastic formations which may have followed the 



