﻿222 
  J. 
  W. 
  JtTDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND. 
  

  

  When, 
  therefore, 
  we 
  reflect 
  upon 
  the 
  remarkable 
  combination 
  of 
  

   circumstances 
  to 
  which 
  we 
  are 
  indebted 
  for 
  the 
  preservation 
  of 
  these 
  

   interesting 
  records 
  of 
  the 
  whole 
  series 
  of 
  Secondary 
  formations 
  (from 
  

   the 
  Trias 
  to 
  the 
  Upper 
  Chalk 
  inclusive) 
  we 
  cannot 
  fail 
  to 
  be 
  im- 
  

   pressed 
  by 
  the 
  accidental 
  (and 
  often, 
  indeed, 
  exceptional) 
  nature 
  of 
  

   the 
  conditions 
  upon 
  which 
  the 
  escape 
  from 
  destruction 
  of 
  fossili- 
  

   ferous 
  deposits 
  has 
  in 
  so 
  many 
  cases 
  depended. 
  It 
  would 
  be 
  difficult 
  

   to 
  adduce 
  a 
  more 
  striking 
  illustration 
  of 
  the 
  necessarily 
  great 
  imper- 
  

   fection 
  of 
  the 
  geological 
  record 
  than 
  that 
  which 
  is 
  suggested 
  by 
  these 
  

   strangely 
  preserved 
  fragments, 
  of 
  what 
  were 
  evidently 
  once 
  widely 
  

   spread 
  formations 
  representing 
  geological 
  periods 
  of 
  vast 
  duration. 
  

  

  It 
  is 
  impossible 
  rightly 
  to 
  understand 
  the 
  features 
  presented 
  by 
  the 
  

   Secondary 
  rocks 
  in 
  the 
  Western 
  Highlands 
  without 
  carefully 
  studying, 
  

   in 
  the 
  first 
  place, 
  their 
  relations 
  to 
  the 
  great 
  masses 
  of 
  igneous 
  rocks 
  

   among 
  which 
  they 
  lie. 
  These 
  relations 
  are 
  of 
  the 
  most 
  intimate 
  

   and 
  often 
  complicated 
  character. 
  Not 
  only 
  have 
  the 
  fragments 
  of 
  

   Mesozoic 
  strata 
  which 
  had 
  escaped 
  denudation 
  at 
  the 
  commence- 
  

   ment 
  of 
  the 
  Tertiary 
  period, 
  been 
  buried 
  under 
  vast 
  accumulations 
  

   of 
  lava 
  sheets 
  to 
  the 
  depth 
  of 
  hundreds 
  and 
  even 
  thousands 
  of 
  

   feet, 
  but 
  they 
  are 
  often, 
  as 
  I 
  shall 
  show 
  hereafter, 
  penetrated 
  by 
  

   igneous 
  masses 
  connected 
  with 
  three 
  distinct 
  periods 
  of 
  volcanic 
  

   activity, 
  from 
  the 
  influence 
  of 
  which 
  they 
  exhibit 
  every 
  conceivable 
  

   stage 
  of 
  metamorphism 
  ; 
  and, 
  further, 
  their 
  fragments 
  are 
  found, 
  

   often 
  in 
  great 
  abundance, 
  imbedded 
  in 
  the 
  vast 
  masses 
  of 
  scorise 
  

   and 
  ashes 
  which 
  have 
  been 
  ejected 
  from 
  the 
  volcanic 
  vents. 
  In 
  

   order, 
  therefore, 
  to 
  reconstruct 
  the 
  history 
  of 
  the 
  Mesozoic 
  period, 
  

   it 
  is 
  necessary 
  to 
  carefully 
  restore 
  and 
  reunite 
  all 
  these 
  scattered 
  

   fragments 
  of 
  evidence 
  ; 
  for 
  the 
  same 
  volcanic 
  agency 
  that 
  has 
  so 
  

   wonderfully 
  preserved 
  the 
  records, 
  has 
  at 
  the 
  same 
  time 
  unfortu- 
  

   nately, 
  in 
  too 
  many 
  instances, 
  sadly 
  mutilated 
  and 
  defaced 
  them. 
  

  

  I 
  shall 
  show, 
  moreover, 
  that 
  although 
  during 
  nearly 
  the 
  whole 
  

   of 
  the 
  Secondary 
  periods 
  the 
  volcanic 
  forces 
  were 
  dormant 
  in 
  the 
  

   district, 
  yet 
  that 
  era 
  was 
  preceded, 
  as 
  well 
  as 
  followed, 
  by 
  an 
  epoch 
  

   of 
  the 
  most 
  intense 
  and 
  prolonged 
  igneous 
  activity. 
  The 
  influences 
  

   of 
  this 
  earlier 
  period 
  of 
  volcanic 
  action 
  in 
  determining 
  the 
  characters 
  

   of 
  the 
  Secondary 
  deposits, 
  although 
  less 
  marked 
  than 
  those 
  of 
  the 
  

   succeeding 
  eruptions 
  of 
  the 
  Tertiary 
  period, 
  are 
  nevertheless 
  clearly 
  

   traceable. 
  Further, 
  many 
  of 
  the 
  peculiarities 
  and 
  anomalies 
  pre- 
  

   sented 
  by 
  the 
  Secondary 
  rocks 
  in 
  this 
  district 
  appear 
  to 
  find 
  an 
  

   adequate 
  explanation 
  in 
  the 
  circumstance 
  that 
  they 
  were 
  deposited 
  

   in 
  the 
  interval 
  between 
  these 
  two 
  periods 
  of 
  violent 
  igneous 
  activity, 
  

   and 
  that 
  the 
  areas 
  which 
  they 
  occupy 
  may 
  therefore 
  naturally 
  be 
  sup- 
  

   posed 
  to 
  have 
  been 
  subject 
  to 
  frequent 
  and 
  excessive 
  disturbance. 
  

  

  But 
  while 
  many 
  points 
  of 
  great 
  importance 
  with 
  respect 
  to 
  the 
  

   Secondary 
  rocks 
  are 
  thus 
  dependent 
  for 
  their 
  elucidation 
  on 
  a 
  care- 
  

   ful 
  study 
  of 
  the 
  volcanic 
  products 
  with 
  which 
  they 
  are 
  so 
  intimately 
  

   associated, 
  much 
  new 
  light 
  is 
  at 
  the 
  same 
  time 
  thrown 
  upon 
  the 
  

   nature, 
  age, 
  and 
  history 
  of 
  the 
  latter 
  by 
  an 
  examination 
  of 
  the 
  

   relations 
  which 
  subsist 
  between 
  them 
  and 
  the 
  interesting 
  fragments 
  

   of 
  fossiliferous, 
  and 
  thereby 
  dated, 
  rocks 
  of 
  the 
  Mesozoic 
  periods. 
  

  

  