﻿1XX 
  PROCEEDINGS 
  OF 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  [May 
  1 
  897, 
  

  

  that 
  the 
  majority 
  of 
  the 
  pre-Cambrian 
  rocks 
  at 
  St. 
  David's 
  were 
  

   of 
  volcanic 
  origin 
  ; 
  and 
  petrological 
  notes 
  in 
  confirmation 
  of 
  this 
  

   view 
  were 
  added 
  by 
  Mr. 
  T. 
  Davies, 
  Prof. 
  Bonney, 
  and 
  Prof. 
  Judd. 
  

   In 
  working 
  out 
  these 
  conclusions 
  I 
  was 
  also 
  greatly 
  assisted 
  by 
  

   Prof. 
  Hughes, 
  Mr. 
  Hudleston, 
  and 
  the 
  late 
  Mr. 
  Tawney. 
  Up 
  to 
  

   this 
  time 
  the 
  oldest 
  volcanic 
  rocks 
  anywhere 
  found 
  in 
  Britain 
  were 
  

   supposed 
  to 
  be 
  of 
  Lower 
  Silurian 
  age 
  ; 
  and 
  Sir 
  A. 
  Ramsay, 
  in 
  his 
  

   Presidential 
  Address 
  to 
  the 
  British 
  Association 
  in 
  1880, 
  so 
  far 
  from 
  

   even 
  then 
  accepting 
  our 
  views, 
  stated 
  that 
  'the 
  oldest 
  volcanic 
  

   products 
  ' 
  of 
  which 
  he 
  had 
  any 
  personal 
  knowledge 
  were 
  of 
  

   1 
  Lower 
  Silurian 
  age.' 
  (Swansea 
  Meeting, 
  p. 
  7.) 
  

  

  Since 
  that 
  time, 
  in 
  several 
  areas 
  in 
  Britain, 
  it 
  has 
  been 
  shown 
  

   that 
  the 
  pre-Cambrian 
  rocks 
  are 
  mainly 
  made 
  up 
  of 
  igneous 
  

   materials, 
  and 
  even 
  the 
  so-called 
  Laurentian 
  rocks 
  of 
  the 
  North- 
  

   west 
  Highlands 
  of 
  Scotland 
  are 
  now 
  known 
  to 
  contain 
  but 
  few 
  beds 
  

   which 
  can 
  in 
  any 
  way 
  be 
  claimed 
  as 
  having 
  had 
  a 
  sedimentary 
  

   origin, 
  as 
  explained 
  in 
  the 
  following 
  quotation 
  from 
  Sir 
  A. 
  Geikie's 
  

   Address 
  delivered 
  before 
  this 
  Society 
  in 
  the 
  year 
  1891 
  : 
  — 
  

  

  c 
  With 
  the 
  possible 
  exception 
  of 
  a 
  strip 
  of 
  ground 
  in 
  the 
  Gairloch 
  

   district 
  — 
  which 
  includes 
  graphite-schist, 
  garnetiferous 
  mica-schist, 
  

   limestone, 
  and 
  a 
  few 
  other 
  remarkable 
  rocks 
  — 
  no 
  portion 
  of 
  the 
  

   fundamental 
  gneiss 
  has 
  anywhere 
  yielded 
  a 
  trace 
  of 
  materials 
  that 
  

   can 
  be 
  supposed 
  to 
  be 
  of 
  sedimentary 
  origin. 
  Everywhere 
  the 
  rock 
  

   is 
  thoroughly 
  crystalline, 
  and 
  presents 
  no 
  structure 
  that 
  in 
  any 
  way 
  

   suggests 
  an 
  alteration 
  of 
  clastic 
  constituents. 
  Here 
  and 
  there 
  

   it 
  can 
  be 
  traced 
  into 
  bands 
  and 
  bosses 
  which, 
  being 
  either 
  non- 
  

   foliated 
  or 
  foliated 
  only 
  in 
  a 
  slight 
  degree, 
  present 
  the 
  ordinary 
  

   characters 
  of 
  true 
  eruptive 
  masses. 
  In 
  Sutherland 
  and 
  Ross-shire 
  

   these 
  amorphous 
  patches 
  occur 
  abundantly. 
  Their 
  external 
  margins 
  

   are 
  not 
  well 
  defined, 
  and 
  they 
  pass 
  insensibly 
  into 
  the 
  ordinary 
  

   gneiss, 
  the 
  dark 
  basic 
  massive 
  rocks 
  shading 
  off 
  into 
  the 
  coarse- 
  

   basic 
  gneisses, 
  and 
  the 
  pegmatites 
  of 
  quartz 
  and 
  felspar 
  which 
  

   traverse 
  them 
  merging 
  into 
  bands 
  of 
  grey 
  quartzose 
  gneiss. 
  

  

  ' 
  So 
  far, 
  therefore, 
  as 
  present 
  knowledge 
  goes, 
  the 
  Lewisian 
  gneiss 
  

   of 
  the 
  North-west 
  Highlands 
  of 
  Scotland 
  was 
  originally 
  a 
  mass 
  of 
  

   various 
  eruptive 
  rocks. 
  It 
  has 
  subsequently 
  undergone 
  a 
  succession 
  

   of 
  deformations 
  from 
  enormous 
  stresses 
  within 
  the 
  terrestrial 
  crust, 
  

   which 
  have 
  been 
  investigated 
  with 
  great 
  care 
  by 
  my 
  colleagues 
  

   of 
  the 
  Geological 
  Survey.' 
  l 
  

  

  1 
  Quart. 
  Joui*n. 
  Geol. 
  Soc. 
  vol. 
  xlvii. 
  Proc. 
  68. 
  

  

  