﻿226 
  J. 
  W. 
  JT7DD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OP 
  SCOTLAND. 
  

  

  the 
  following 
  : 
  — 
  "Were 
  these 
  lavas 
  poured 
  out 
  upon 
  a 
  terrestrial 
  sur- 
  

   face 
  '? 
  or 
  were 
  they 
  the 
  product 
  of 
  a 
  series 
  of 
  submarine 
  eruptions 
  ? 
  

   On 
  this 
  question 
  very 
  diverse 
  opinions 
  have 
  been 
  maintained 
  by 
  

   different 
  authors, 
  the 
  majority, 
  however, 
  being 
  in 
  favour 
  of 
  the 
  sub- 
  

   marine 
  origin 
  of 
  the 
  rocks. 
  Tbis 
  opinion 
  has, 
  in 
  a 
  great 
  degree, 
  

   arisen 
  from 
  the 
  long 
  prevalent 
  idea 
  that 
  the 
  lavas 
  in 
  question 
  were 
  

   contemporaneous 
  with 
  the 
  Secondary 
  strata 
  with 
  which 
  they 
  are 
  

   often 
  so 
  intimately 
  associated. 
  We 
  have 
  already 
  pointed 
  out 
  how 
  

   this 
  opinion 
  has 
  been 
  gradually 
  dissipated— 
  first 
  by 
  the 
  discovery 
  

   of 
  the 
  Ardtun 
  fossils 
  by 
  the 
  Duke 
  of 
  Argyll, 
  and 
  secondly 
  through 
  

   the 
  recognition 
  by 
  Professor 
  Geikie 
  of 
  the 
  intrusive 
  character, 
  and 
  

   therefore 
  subsequent 
  age, 
  of 
  the 
  sheets 
  of 
  igneous 
  rock 
  which 
  lie 
  in 
  

   the 
  midst 
  of 
  the 
  Mesozoic 
  strata. 
  "With 
  the 
  supposed 
  evidences 
  of 
  

   the 
  Secondary 
  age 
  of 
  these 
  lavas 
  the 
  grounds 
  on 
  which 
  their 
  sub- 
  

   marine 
  origin 
  was 
  maintained 
  have 
  also 
  disappeared 
  ; 
  and 
  from 
  the 
  

   overwhelming 
  and 
  irresistible 
  mass 
  of 
  evidence 
  which 
  I 
  am 
  now 
  

   able 
  to 
  adduce 
  upon 
  the 
  subject, 
  it 
  will, 
  I 
  think, 
  be 
  accepted 
  as 
  

   conclusively 
  demonstrated 
  that 
  the 
  lavas 
  were 
  unquestionably 
  of 
  

   subaerial 
  or 
  terrestrial 
  origin. 
  

  

  The 
  first 
  point 
  to 
  which 
  it 
  is 
  necessary 
  to 
  allude, 
  is 
  the 
  total 
  

   absence 
  of 
  marine 
  sediments 
  and 
  fossils, 
  of 
  contemporary 
  age, 
  

   interstratified 
  with 
  the 
  great 
  lava 
  sheets 
  wbich 
  we 
  are 
  consider- 
  

   ing. 
  It 
  is 
  true 
  that 
  this 
  evidence 
  is 
  of 
  a 
  negative 
  character 
  ; 
  but 
  

   when 
  we 
  reflect 
  on 
  the 
  duration 
  of 
  the 
  periods 
  required 
  for 
  the 
  

   gradual 
  accumulation 
  of 
  these 
  enormous 
  masses 
  of 
  lava, 
  and 
  on 
  the 
  

   ample 
  proofs 
  which 
  exist 
  of 
  the 
  occurrence 
  of 
  long 
  intervals 
  of 
  time 
  

   between 
  the 
  outflow 
  of 
  sheets 
  now 
  directly 
  superimposed 
  the 
  one 
  

   upon 
  the 
  other, 
  we 
  cannot 
  but 
  be 
  impressed 
  by 
  the 
  consideration 
  

   that, 
  if 
  such 
  an 
  accumulation 
  of 
  lavas 
  took 
  place 
  upon 
  the 
  sea- 
  

   bottom, 
  beds 
  of 
  stratified 
  materials 
  containing 
  marine 
  organisms 
  

   must, 
  at 
  some 
  points 
  at 
  least, 
  have 
  been 
  deposited 
  in 
  the 
  intervals 
  

   between 
  the 
  successive 
  outflows 
  of 
  igneous 
  rock. 
  The 
  actual 
  com- 
  

   parison 
  of 
  the 
  volcanic 
  rocks 
  of 
  the 
  Hebrides 
  and 
  Ulster 
  with 
  others 
  

   of 
  undoubted 
  submarine 
  character, 
  like 
  those 
  of 
  Central 
  Scotland 
  

   (which 
  we 
  shall 
  have 
  to 
  refer 
  to 
  more 
  particularly 
  in 
  the 
  sequel, 
  

   and 
  in 
  which 
  the 
  interbedding 
  of 
  masses 
  of 
  aqueous 
  and 
  igneous 
  

   origin 
  respectively 
  is 
  such 
  a 
  constant 
  and 
  characteristic 
  feature), 
  

   lends 
  additional 
  weight 
  to 
  the 
  presumption 
  against 
  the 
  submarine 
  

   origin 
  of 
  the 
  former. 
  

  

  But 
  this 
  presumption, 
  derived 
  from 
  indirect 
  and 
  negative 
  evidence, 
  

   must 
  be 
  regarded 
  as 
  a 
  conclusion 
  satisfactorily 
  established 
  by 
  all 
  

   who 
  will 
  examine 
  and 
  weigh 
  the 
  strong, 
  numerous 
  and 
  cumulative 
  

   proofs 
  of 
  a 
  direct 
  kind 
  which 
  can 
  be 
  adduced 
  in 
  its 
  support. 
  These 
  

   it 
  will 
  be 
  necessary 
  briefly 
  to 
  detail. 
  

  

  The 
  highly 
  vesicular 
  and 
  scoriaceous 
  character 
  of 
  many 
  of 
  the 
  

   Tertiary 
  lavas 
  seems 
  to 
  be 
  inconsistent 
  with 
  the 
  supposition 
  that 
  

   they 
  have 
  been 
  poured 
  out 
  under 
  a 
  considerable 
  depth 
  of 
  water, 
  

   the 
  pressure 
  of 
  which 
  would 
  probably 
  prevent 
  that 
  extensive 
  libera- 
  

   tion 
  of 
  volatile 
  materials 
  which 
  has 
  so 
  evidently 
  taken 
  place. 
  Some 
  

   of 
  the 
  lava 
  beds 
  must, 
  in 
  consequence 
  of 
  this 
  operation, 
  have 
  origin- 
  

  

  