﻿268 
  J. 
  W. 
  JTJDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND. 
  

  

  wards 
  more 
  clearly 
  illustrated 
  by 
  Prof. 
  Geikie, 
  that 
  besides 
  the 
  great 
  

   central 
  eruptive 
  masses 
  of 
  granite 
  there 
  are 
  other 
  portions 
  of 
  the 
  

   same 
  rock, 
  usually 
  of 
  a 
  finer-grained 
  character 
  and 
  passing 
  into 
  

   many 
  varieties 
  of 
  felsite, 
  the 
  relations 
  of 
  which 
  to 
  the 
  Primary 
  and 
  

   Secondary 
  strata 
  among 
  which 
  they 
  lie 
  is 
  of 
  a 
  totally 
  different 
  kind. 
  

   Instead 
  of 
  violently 
  breaking 
  through 
  and 
  disturbing 
  them, 
  these 
  

   intrusive 
  masses 
  appear 
  to 
  form 
  hills 
  simply 
  overlying 
  the 
  stratified 
  

   rocks, 
  but 
  occasionally 
  sending 
  off 
  veins 
  into 
  them 
  and 
  entangling 
  

   portions 
  of 
  them 
  in 
  their 
  mass 
  ; 
  and 
  in 
  place 
  of 
  effecting 
  a 
  meta- 
  

   morphism 
  in 
  the 
  sedimentary 
  rocks 
  extending 
  for 
  a 
  great 
  distance 
  

   around, 
  they 
  produce 
  only 
  a 
  comparatively 
  small 
  and 
  local 
  action 
  

   upon 
  them. 
  These 
  features, 
  it 
  must 
  be 
  confessed, 
  seem 
  at 
  first 
  sight 
  

   sufficiently 
  anomalous 
  ; 
  and 
  some 
  authors 
  have 
  even 
  gone 
  so 
  far 
  as 
  

   to 
  describe 
  these 
  granites 
  as 
  having 
  been 
  poured 
  out 
  upon 
  the 
  exist- 
  

   ing 
  surface 
  " 
  like 
  ordinary 
  trap 
  rocks." 
  

  

  A 
  carefulstudy 
  and 
  comparison 
  of 
  all 
  the 
  phenomena 
  presented 
  

   by 
  masses 
  of 
  this 
  kind, 
  numerous 
  examples 
  of 
  which 
  occur 
  in 
  con- 
  

   nexion 
  with 
  the 
  different 
  volcanoes 
  we 
  have 
  been 
  describing, 
  affords 
  

   us 
  a 
  very 
  simple 
  explanation 
  of 
  the 
  supposed 
  anomalies. 
  These 
  

   masses 
  of 
  fine-grained 
  granite 
  passing 
  into 
  felsite 
  are 
  really 
  portions 
  

   proceeding 
  from 
  the 
  great 
  central 
  masses 
  and 
  intruded 
  between 
  stra- 
  

   tified 
  rocks. 
  In 
  Skye, 
  and 
  also 
  in 
  some 
  other 
  cases, 
  the 
  overlying 
  

   strata 
  have 
  been 
  removed 
  by 
  denudation 
  ; 
  and 
  the 
  intrusive 
  rocks, 
  

   which 
  have 
  by 
  their 
  hardness 
  resisted 
  denuding 
  influences, 
  now 
  ap- 
  

   pear 
  uncovered, 
  and 
  as 
  if 
  actually 
  deposited 
  upon 
  the 
  strata 
  below. 
  

   That 
  the 
  explanation 
  now 
  given 
  of 
  this 
  phenomenon 
  is 
  the 
  true 
  one, 
  

   I 
  have 
  been 
  able 
  to 
  verify 
  by 
  the 
  comparison 
  of 
  a 
  very 
  great 
  number 
  

   of 
  cases, 
  in 
  which 
  every 
  stage 
  of 
  the 
  series 
  of 
  operations 
  here 
  de- 
  

   scribed 
  as 
  terminating 
  in 
  the 
  production 
  of 
  the 
  anomalous 
  features 
  

   in 
  question, 
  can 
  be 
  clearly 
  traced. 
  Admirable 
  examples 
  showing 
  

   that 
  these 
  masses 
  of 
  granite 
  and 
  felsite 
  were 
  actually 
  intruded 
  

   among 
  the 
  stratified 
  rocks, 
  are 
  seen 
  in 
  Paasay, 
  Ardnamurchan, 
  

   and 
  Mull 
  ; 
  I 
  may 
  especially 
  cite 
  the 
  cases 
  of 
  Stron 
  Beg 
  and 
  Csaig- 
  

   nure, 
  situated 
  in 
  the 
  two 
  last-mentioned 
  of 
  these 
  districts 
  respec- 
  

   tively, 
  as 
  illustrating 
  the 
  features 
  and 
  relations 
  of 
  such 
  masses 
  in 
  a 
  

   peculiarly 
  interesting 
  manner. 
  A 
  mass 
  of 
  this 
  kind, 
  probably 
  origin- 
  

   ally 
  connected 
  with 
  the 
  volcano 
  of 
  Pum, 
  occurs 
  at 
  the 
  northern 
  part 
  

   of 
  the 
  island 
  of 
  Eigg 
  ; 
  Professor 
  Geikie 
  notices 
  the 
  resemblance 
  of 
  

   the 
  rock 
  of 
  which 
  it 
  is 
  composed 
  to 
  the 
  " 
  quartziferous 
  porphyries 
  " 
  

   of 
  Skye 
  and 
  Paasay, 
  and 
  he 
  justly 
  states 
  that 
  the 
  mass 
  " 
  appears 
  

   to 
  have 
  risen 
  approximately 
  along 
  the 
  bedding 
  of 
  the 
  Oolitic 
  strata, 
  

   and 
  thus 
  to 
  form 
  of 
  itself 
  a 
  large 
  rude 
  bed 
  "*. 
  

  

  In 
  the 
  very 
  different 
  modes 
  of 
  their 
  subterranean 
  disposition, 
  the 
  

   igneous 
  rocks 
  of 
  acid 
  and 
  basic 
  composition 
  respectively 
  present 
  a 
  

   remarkable 
  parallel 
  to 
  the 
  behaviour 
  of 
  the 
  two 
  classes 
  of 
  lavas 
  to 
  

   which 
  they 
  give 
  rise. 
  While 
  the 
  acid 
  igneous 
  rocks, 
  when 
  intruded 
  

   between 
  strata, 
  tend 
  to 
  form 
  thick 
  lenticular 
  masses, 
  which 
  are 
  

   generally 
  confined 
  to 
  within 
  moderate 
  distances 
  from 
  the 
  great 
  centres 
  

   of 
  eruption, 
  the 
  basaltic 
  rocks, 
  on 
  the 
  other 
  hand, 
  under 
  like 
  con- 
  

   * 
  Quart. 
  Journ. 
  Geol. 
  Soo. 
  vol. 
  xxvii. 
  (1871) 
  p. 
  294. 
  

  

  