﻿290 
  J. 
  W. 
  JTTDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND. 
  

  

  traversed 
  by 
  granite 
  veins, 
  are 
  found 
  actually 
  imbedded 
  in 
  the 
  gra- 
  

   nitic 
  masses. 
  And, 
  further, 
  it 
  appears 
  that 
  considerable 
  tracts 
  of 
  

   stratified 
  rocks, 
  preserving 
  all 
  their 
  usual 
  characters, 
  were 
  actually 
  

   enveloped 
  by 
  the 
  fluid 
  igneous 
  protrusions 
  ; 
  and 
  thus 
  we 
  have 
  often 
  

   the 
  most 
  complex 
  entanglements 
  of 
  the 
  granitic 
  and 
  stratified 
  rocks. 
  

   These 
  facts 
  are 
  very 
  clearly 
  exemplified 
  in 
  the 
  fine 
  coast-sections 
  of 
  

   the 
  Ross 
  of 
  Mull, 
  and 
  more 
  obscurely, 
  owing 
  to 
  the 
  want 
  of 
  sections, 
  

   at 
  many 
  points 
  in 
  the 
  Grampian 
  Mountains. 
  

  

  But 
  besides 
  these 
  great 
  granitic 
  masses 
  and 
  the 
  veins 
  in 
  immediate 
  

   connexion 
  with 
  them, 
  the 
  whole 
  district 
  for 
  many 
  miles 
  around 
  bears 
  

   witness 
  to 
  the 
  igneous 
  activity 
  to 
  which 
  their 
  origin 
  is 
  referred. 
  In 
  

   all 
  directions 
  smaller 
  masses 
  of 
  granite, 
  syenite-granite, 
  or 
  felsite 
  

   are 
  found, 
  either 
  forced 
  between, 
  or 
  cutting 
  across 
  the 
  strata 
  ; 
  while 
  

   dykes 
  of 
  similar 
  materials, 
  passing 
  into 
  felstones, 
  are 
  found 
  traversing 
  

   the 
  surrounding 
  rocks, 
  in 
  all 
  directions 
  and 
  often 
  to 
  great 
  distances 
  

   from 
  the 
  great 
  central 
  igneous 
  masses. 
  Such 
  veins 
  and 
  dykes 
  usually 
  

   occur 
  in 
  prodigious 
  numbers 
  near 
  the 
  great 
  granite 
  protrusions, 
  as 
  

   is 
  so 
  well 
  seen 
  in 
  the 
  passes 
  of 
  Glencoe 
  and 
  Brander 
  ; 
  but 
  their 
  fre- 
  

   quency 
  diminishes 
  in 
  proportion 
  as 
  we 
  recede 
  from 
  the 
  central 
  

   intrusive 
  masses. 
  

  

  The 
  smaller 
  intrusive 
  masses 
  and 
  the 
  dykes 
  effect 
  a 
  certain 
  amount 
  

   of 
  local 
  metamorphism 
  in 
  the 
  rocks 
  which 
  they 
  traverse 
  ; 
  but 
  this 
  is 
  

   far 
  more 
  limited 
  in 
  extent 
  and 
  less 
  striking 
  in 
  character 
  than 
  that 
  

   produced 
  by 
  the 
  great 
  central 
  granitic 
  masses. 
  Where, 
  however, 
  

   the 
  stratified 
  rocks 
  are 
  traversed 
  by 
  a 
  complete 
  plexus 
  of 
  dykes, 
  as 
  

   in 
  Glencoe, 
  the 
  amount 
  of 
  alteration 
  produced 
  by 
  them 
  in 
  rocks 
  

   through 
  which 
  they 
  pass 
  is 
  often 
  very 
  considerable. 
  

  

  The 
  period 
  at 
  which 
  these 
  intrusions 
  of 
  igneous 
  rocks 
  (which 
  are 
  

   almost 
  uniformly 
  of 
  acid 
  or 
  felspathic 
  composition) 
  took 
  place 
  we 
  can 
  

   approximately 
  determine. 
  No 
  one 
  can 
  study 
  the 
  distribution 
  of 
  the 
  

   various 
  rocks 
  which 
  make 
  up 
  the 
  Lower 
  Silurian 
  strata 
  of 
  the 
  High- 
  

   lands, 
  now 
  that 
  the 
  true 
  succession 
  of 
  these 
  beds 
  has 
  been 
  so 
  clearly 
  

   demonstrated 
  by 
  the 
  labours 
  of 
  Sir 
  Roderick 
  Murchison 
  and 
  Profes- 
  

   sor 
  Geikie, 
  without 
  perceiving 
  that 
  the 
  igneous 
  protrusions 
  were 
  

   posterior, 
  not 
  only 
  to 
  the 
  deposition 
  of 
  the 
  Lower 
  Silurian 
  rocks, 
  but 
  

   also 
  to 
  the 
  disturbance 
  and 
  metamorphism 
  by 
  which 
  they 
  have 
  

   acquired 
  their 
  present 
  characters. 
  This 
  necessary 
  corollary 
  to 
  the 
  

   main 
  conclusions 
  arrived 
  at 
  concerning 
  the 
  succession 
  of 
  the 
  Lower 
  

   Silurian 
  strata 
  was 
  clearly 
  recognized 
  by 
  the 
  authors 
  whom 
  I 
  have 
  

   cited*. 
  

  

  On 
  the 
  other 
  hand, 
  that 
  these 
  granitic 
  protrusions 
  took 
  place 
  prior 
  

   to 
  the 
  deposition 
  of 
  both 
  the 
  Secondary 
  and 
  Tertiary 
  strata, 
  is 
  suf- 
  

   ficiently 
  shown 
  by 
  the 
  fact 
  that 
  no 
  traces 
  of 
  the 
  extensive 
  network 
  

   of 
  veins 
  and 
  dykes 
  proceeding 
  from 
  them 
  are 
  in 
  any 
  instance 
  found 
  

   traversing 
  these 
  younger 
  rocks. 
  

  

  Thus 
  it 
  appears 
  certain 
  that 
  the 
  igneous 
  rocks 
  of 
  the 
  Grampian 
  

  

  Mountains 
  must 
  be 
  referred 
  to 
  some 
  part 
  of 
  the 
  Newer 
  Palaeozoic 
  

  

  periods. 
  Pebbles 
  of 
  these 
  granitic 
  rocks 
  are 
  entirely 
  absent 
  from 
  

  

  the 
  conglomerates 
  of 
  the 
  Old 
  Red 
  Sandstone, 
  which, 
  however, 
  are 
  

  

  * 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xvii. 
  p. 
  228. 
  

  

  