IN THE NORTH-WEST HIGHLANDS OF SCOTLAND. 399 



the old foliation has been completely destroyed and we find 

 belts of newer foliation trending W. or N.W., or, it may be, a 

 combination of the old and the new schistosities, accompanied by 

 violent contortion. Or, again, the original gneiss has undergone 

 partial or complete reconstruction along the old foliation-planes, 

 whether they happen to be inclined at high or low angles. Hence 

 it is only within limited areas that we can study the characters of 

 the original gneisses. We believe, and the evidence in the field 

 warrants the belief, that the highly basic, pyroxenic, and horn- 

 blendic gneisses at one time extended over the whole area, and that 

 the lithological varieties now met with, so different from the older 

 set, have been produced by the deformation of the original gneisses *. 



8. Evidence proving the Pre-Oambrian Age of these Movements. 



There is an overwhelming amount of evidence to prove that these 

 various mechanical movements, which have so powerfully affected 

 the basic dykes and the Archaean gneiss, took place prior to the 

 deposition of the Cambrian (Torridon) sandstones. The various 

 disruption-planes and the belts of secondary foliation can be traced 

 from the sea-coast, across the Archaean area, till they are buried 

 underneath the pile of Cambrian sandstones and Silurian quartz- 

 ites. Neither the red sandstones nor the quartzites along the 

 western escarpment show the slightest trace of having been affected 

 by these movements. The disruption-lines, like the basic dykes, 

 disappear at the base of the great cliff of Palaeozoic sedimentary 

 deposits. We are therefore forced to conclude that these move- 

 ments have no connexion with the gigantic Post-Lower-Silurian 

 displacements, and that the rocks had assumed their present cha- 

 racters in Pre-Cambrian time. 



The Archaean area is still further complicated by a double system 

 of normal faults, one set trending N.W. and S.E., and the other set 

 N.E. and S.W. Most of these are probably later than the Post- 

 Lower-Silurian movements. 



Among the Archaean rocks certain dykes are met with which are 

 all probably later than the deposition of the Cambrian and Silurian 

 strata of Sutherland. These include some of mica-diabase occurring 

 in the Archaean gneiss and Cambrian strata, and the porphyritic 

 quartz -felsites of the same age as the Canisp " porphyry " to be 

 referred to presently, and probably certain dykes of olivine-diabase. 



9. Summary of the foregoing Researches in the Archcean RocJcs. 



The series of phenomena revealed by these researches in the 

 Archaean rocks in the north-west of Sutherland may be tabulated 

 as follows : — 



(1) The eruption of a great series of igneous rocks of a more or 



* From the evidence adduced in the foregoing pages regarding the effects of 

 the later Pre-Cambrian movements, it is obvious that they are merely the differ- 

 ential results of an enormous thrust of these Archsean rocks, generally from 

 the E.N.E. towards the W.S.W. 



Q.J.G.S. No. 175. 2e 



