﻿270 
  J. 
  W. 
  JUDD 
  ON 
  IHE 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND. 
  

  

  great 
  volcanoes 
  their 
  almost 
  infinite 
  numbers, 
  the 
  complexities 
  of 
  

   their 
  intricate 
  interfacings, 
  and 
  the 
  manner 
  in 
  which 
  those 
  of 
  later 
  

   date 
  cut 
  across 
  and 
  often 
  laterally 
  displace 
  portions 
  of 
  the 
  earlier 
  

   ones, 
  give 
  rise 
  to 
  a 
  series 
  of 
  phenomena 
  of 
  the 
  most 
  instructive 
  and 
  

   interesting 
  character. 
  As 
  a 
  general 
  rule, 
  the 
  larger 
  the 
  dyke 
  the 
  

   more 
  coarsely 
  crystalline 
  is 
  the 
  mass 
  of 
  rock 
  which 
  composes 
  it 
  ; 
  

   and 
  in 
  many 
  cases 
  the 
  middle 
  of 
  the 
  dyke 
  is 
  composed 
  of 
  a 
  coarser- 
  

   grained 
  rock 
  than 
  the 
  sides. 
  

  

  Frequent 
  allusion 
  has 
  already 
  been 
  made 
  to 
  the 
  metamorphism 
  

   produced 
  in 
  the 
  Primary 
  and 
  Secondary 
  strata, 
  and 
  also 
  in 
  pre- 
  

   existing 
  rocks, 
  by 
  the 
  passage 
  through 
  them 
  of 
  masses 
  of 
  igneous 
  

   rock. 
  Careful 
  examination 
  will 
  serve 
  to 
  detect 
  such 
  alteration 
  at 
  the 
  

   surfaces 
  of 
  contact 
  of 
  almost 
  every 
  igneous 
  mass 
  ; 
  but 
  in 
  the 
  degree 
  of 
  

   this 
  action 
  there 
  is 
  the 
  greatest 
  diversity 
  in 
  different 
  cases. 
  Some- 
  

   times 
  it 
  consists 
  of 
  a 
  scarcely 
  perceptible 
  induration 
  extending 
  for 
  a 
  

   distance 
  of 
  a 
  few 
  lines 
  only 
  from 
  the 
  surfaces 
  of 
  the 
  igneous 
  mass, 
  

   and 
  unattended 
  by 
  any 
  change 
  in 
  chemical 
  characters 
  ; 
  but 
  in 
  other 
  

   cases 
  the 
  soft 
  sediments 
  of 
  the 
  Lias, 
  Oolite 
  and 
  Cretaceous 
  forma- 
  

   tions 
  have 
  had 
  every 
  trace 
  of 
  their 
  abundant 
  fossil 
  contents 
  wholly 
  

   obliterated, 
  and 
  their 
  masses 
  converted 
  into 
  rocks 
  undistinguishable 
  

   in 
  appearance 
  and 
  characters 
  from 
  the 
  most 
  highly 
  metamorphic 
  

   Primary 
  rocks, 
  with 
  which, 
  indeed, 
  they 
  have 
  in 
  some 
  cases 
  been 
  

   confounded. 
  

  

  A 
  careful 
  study 
  of 
  these 
  phenomena 
  of 
  local 
  metamorphism 
  enables 
  

   us 
  to 
  enunciate 
  the 
  general 
  law 
  which 
  governs 
  its 
  action 
  to 
  be 
  as 
  

   follows 
  : 
  — 
  The 
  degree 
  of 
  metamorphism 
  and 
  the 
  distance 
  to 
  which 
  

   its 
  action 
  extends 
  from 
  the 
  intrusive 
  mass 
  is 
  usually 
  proportioned 
  

   to 
  the 
  mass 
  of 
  the 
  latter. 
  Thus, 
  the 
  maximum 
  of 
  change 
  is 
  seen 
  

   in 
  the 
  vicinity 
  of 
  the 
  great 
  eruptive 
  mountain 
  masses, 
  and 
  the 
  

   minimum 
  near 
  the 
  smaller 
  intrusive 
  veins 
  and 
  dykes 
  ; 
  while 
  around 
  

   the 
  various 
  intrusive 
  sheets 
  and 
  bosses 
  every 
  intermediate 
  degree 
  of 
  

   alteration 
  is 
  exhibited. 
  Even 
  among 
  dykes 
  we 
  can 
  often 
  observe 
  

   that 
  the 
  amount 
  of 
  induration 
  in 
  the 
  strata 
  which 
  they 
  traverse, 
  and 
  

   the 
  distance 
  to 
  which 
  this 
  extends 
  from 
  their 
  surfaces, 
  are 
  directly 
  

   proportioned 
  to 
  their 
  width. 
  There 
  are, 
  it 
  is 
  true, 
  occasional 
  appa- 
  

   rent 
  exceptions 
  to 
  this 
  rule 
  ; 
  but 
  these 
  are 
  in 
  most 
  cases 
  capable 
  of 
  

   easy 
  explanation. 
  Thus 
  masses 
  of 
  rock 
  traversed 
  by 
  a 
  complete 
  plexus 
  

   of 
  veins 
  and 
  dykes, 
  such 
  as 
  are 
  seen 
  at 
  some 
  points 
  in 
  Ardnamurchan, 
  

   undergo 
  a 
  very 
  great 
  amount 
  of 
  change, 
  their 
  fossils 
  being 
  almost 
  

   wholly 
  obliterated 
  and 
  very 
  decided 
  chemical 
  Ghanges 
  induced 
  in 
  

   their 
  mass, 
  these 
  changes 
  being 
  evidently 
  the 
  result 
  of 
  the 
  cumulative 
  

   action 
  of 
  the 
  numerous 
  small 
  intrusions. 
  The 
  establishment 
  of 
  this 
  

   law 
  of 
  metamorphic 
  action 
  serves 
  to 
  confirm 
  what, 
  indeed, 
  is 
  suffi- 
  

   ciently 
  obvious, 
  that 
  the 
  metamorphism 
  in 
  surrounding 
  rocks 
  results 
  

   from 
  the 
  passage 
  of 
  heat 
  from 
  the 
  intrusive 
  masses 
  of 
  molten 
  matter 
  

   as 
  these 
  gradually 
  pass 
  into 
  the 
  solid 
  condition. 
  

  

  I 
  can 
  scarcely 
  conceive 
  of 
  any 
  series 
  of 
  phenomena 
  more 
  striking 
  in 
  

   character, 
  and 
  certainly 
  of 
  none 
  more 
  interesting 
  and 
  suggestive 
  to 
  

   a 
  geologist, 
  than 
  those 
  presented 
  by 
  the 
  wonderful 
  complexities 
  pro- 
  

   duced 
  by 
  the 
  mutual 
  interferences 
  of 
  different 
  eruptive 
  rock-masses, 
  

  

  