﻿OF 
  THE 
  WESTERN 
  ISLES 
  OF 
  SCOTLAND. 
  209 
  

  

  mainly 
  to 
  post-volcanic 
  movements, 
  or 
  at 
  least 
  to 
  movements 
  which, 
  

   if 
  not 
  later 
  than 
  all 
  the 
  phases 
  of 
  volcanic 
  action, 
  must 
  have 
  suc- 
  

   ceeded 
  the 
  outpouring 
  of 
  the 
  plateau-basalts 
  " 
  {Joe. 
  cit. 
  p. 
  89). 
  

   I 
  have 
  again 
  and 
  again 
  insisted 
  upon 
  the 
  same 
  fact 
  in 
  the 
  case 
  of 
  

   the 
  basaltic 
  plateaux 
  of 
  the 
  Western 
  Isles 
  of 
  Scotland, 
  and 
  have 
  

   especially 
  dwelt 
  upon 
  the 
  dip 
  of 
  the 
  lava-sheets 
  towards 
  the 
  central 
  

   vents 
  (Q. 
  J. 
  G. 
  S. 
  1874, 
  pp. 
  256-258.), 
  and 
  the 
  correctness 
  of 
  these 
  

   observations 
  the 
  author 
  of 
  the 
  recent 
  memoir 
  appears 
  not 
  only 
  to 
  

   fully 
  admit, 
  but 
  also 
  to 
  adopt, 
  for 
  he 
  says 
  : 
  — 
  " 
  There 
  appears 
  also 
  

   to 
  have 
  been 
  a 
  general 
  tendency 
  to 
  sagging 
  subsequent 
  to 
  the 
  gabbro 
  

   protrusions, 
  and 
  the 
  inward 
  dip 
  thereby 
  produced 
  has 
  probably 
  been 
  

   instrumental 
  in 
  effacing 
  at 
  least 
  the 
  more 
  gentle 
  outward 
  inclina- 
  

   tions 
  caused 
  by 
  the 
  uprise 
  of 
  the 
  eruptive 
  rock." 
  (T. 
  11. 
  S. 
  E. 
  1888, 
  

   p. 
  142.) 
  

  

  B. 
  The 
  Age 
  of 
  the 
  Basic 
  Dykes 
  of 
  North 
  Britain. 
  

  

  The 
  author 
  of 
  the 
  recent 
  memoir, 
  in 
  seeking 
  to 
  give 
  an 
  explanation 
  

   of 
  the 
  volcanic 
  phenomena 
  of 
  the 
  Western 
  Isles 
  of 
  Scotland, 
  begins 
  

   by 
  laying 
  much 
  stress 
  on 
  the 
  great 
  importance 
  of 
  the 
  numerous 
  dykes 
  

   which 
  traverse 
  the 
  central 
  and 
  southern 
  districts 
  of 
  Scotland 
  and 
  

   the 
  North 
  of 
  England. 
  Eor 
  the 
  sweeping 
  generalization 
  which 
  he 
  

   announces, 
  however, 
  that 
  those 
  dykes 
  or, 
  at 
  all 
  events, 
  the 
  great 
  

   majority 
  of 
  them, 
  were 
  formed 
  during 
  the 
  Tertiary 
  period, 
  I 
  confess 
  

   I 
  can 
  find 
  no 
  evidence. 
  That 
  the 
  great 
  mass 
  of 
  the 
  basaltic 
  dykes 
  

   in 
  the 
  Western 
  Isles 
  of 
  Scotland, 
  and 
  in 
  Antrim, 
  many 
  of 
  which 
  can 
  

   be 
  shown 
  to 
  radiate 
  from 
  the 
  great 
  centres 
  of 
  eruption, 
  are 
  of 
  this 
  

   age, 
  I 
  fully 
  admit 
  ; 
  but 
  when 
  the 
  numerous 
  dykes 
  of 
  the 
  Central 
  

   Yalley 
  of 
  Scotland, 
  of 
  Eifeshire, 
  of 
  Eorfarshire, 
  the 
  Southern 
  High- 
  

   lands, 
  the 
  Borderland, 
  and 
  the 
  northern 
  counties 
  of 
  England 
  are 
  

   all 
  claimed 
  as 
  fissures 
  from 
  which 
  ejections 
  took 
  place 
  in 
  Tertiary 
  

   times, 
  it 
  would 
  have 
  been 
  satisfactory 
  to 
  have 
  had 
  the 
  grounds 
  of 
  

   such 
  a 
  very 
  startling 
  conclusion 
  a 
  little 
  more 
  clearly 
  stated. 
  Such 
  

   field- 
  evidence 
  as 
  is 
  available 
  only 
  suffices 
  to 
  prove 
  that 
  while 
  some 
  

   are 
  later 
  than 
  Silurian 
  or 
  Ordovieian 
  strata, 
  others 
  are 
  younger 
  than 
  

   Old-Red-Sandstone 
  or 
  Carboniferous 
  times. 
  

  

  Two 
  sets 
  of 
  facts 
  on 
  the 
  other 
  hand 
  make 
  me 
  think 
  this 
  sweeping 
  

   generalization 
  a 
  very 
  doubtful 
  one 
  indeed. 
  Mr. 
  Airport's 
  valuable 
  

   researches 
  concerning 
  the 
  Carboniferous 
  Dolerites 
  showed 
  how 
  

   remarkably 
  fresh 
  are 
  some 
  of 
  the 
  basic 
  rocks 
  of 
  this 
  age 
  ; 
  and 
  his 
  

   studies 
  also 
  demonstrated 
  the 
  fact 
  that 
  the 
  materials 
  filling 
  many 
  of 
  

   the 
  dykes 
  are 
  perfectly 
  similar 
  to 
  some 
  of 
  the 
  lavas 
  which 
  were 
  

   undoubtedly 
  erupted 
  during 
  the 
  Carboniferous 
  period 
  in 
  the 
  South 
  of 
  

   Scotland. 
  The 
  author 
  of 
  the 
  memoir 
  we 
  are 
  now 
  considering, 
  it 
  is 
  

   true, 
  at 
  one 
  time 
  maintained 
  that 
  at 
  Arthur's 
  Seat 
  (where 
  some 
  of 
  tie 
  

   doleritic 
  lavas 
  of 
  freshest 
  appearance 
  are 
  found) 
  there 
  was 
  evidence 
  of 
  

   eruption 
  having 
  occurred 
  in 
  the 
  Tertiary 
  period 
  ; 
  but 
  this 
  conclusion 
  

   he 
  has 
  long 
  since 
  abandoned. 
  I 
  see 
  no 
  ground 
  myself 
  for 
  doubting 
  that 
  

   many, 
  and 
  perhaps 
  the 
  great 
  majority, 
  of 
  the 
  basaltic 
  dykes 
  of 
  the 
  

   Southern 
  Highlands, 
  of 
  the 
  Central 
  Yalley 
  of 
  Scotland, 
  of 
  the 
  Border- 
  

  

  e2 
  

  

  