﻿OF 
  THE 
  WESTERN 
  ISLES 
  OF 
  SCOTLAND. 
  211 
  

  

  area. 
  Where 
  only 
  a 
  few 
  scattered 
  islets 
  now 
  remain, 
  we 
  both 
  

   believe 
  that 
  extended 
  and 
  thick 
  plateaux 
  of 
  basalt 
  must 
  have 
  once 
  

   spread 
  over 
  the 
  whole 
  intervening 
  area. 
  I 
  have 
  maintained 
  that, 
  

   away 
  from 
  the 
  great 
  central 
  vents 
  of 
  the 
  district, 
  these 
  basaltic 
  

   lavas 
  formed 
  masses 
  which 
  were, 
  in 
  places, 
  2000 
  feet 
  in 
  thickness, 
  

   but 
  that, 
  around 
  the 
  vents, 
  they 
  attained 
  to 
  a 
  far 
  greater 
  depth. 
  

   My 
  critic 
  thinks 
  that 
  even 
  where 
  there 
  were 
  no 
  great 
  vents, 
  the 
  

   plateaux 
  were 
  as 
  much 
  as 
  3000 
  feet 
  in 
  thickness 
  ! 
  (T. 
  R. 
  S. 
  E. 
  

   1888, 
  pp. 
  91, 
  99 
  ; 
  compare 
  Q. 
  J. 
  G. 
  S. 
  1874, 
  pp. 
  244, 
  255, 
  &c). 
  

  

  Now 
  even 
  if 
  we 
  accept 
  a 
  large 
  proportion 
  of 
  the 
  basic 
  dykes 
  as 
  

   being 
  of 
  Tertiary 
  age, 
  we 
  must 
  take 
  into 
  consideration 
  several 
  

   very 
  important 
  facts. 
  

  

  First. 
  — 
  Many 
  of 
  these 
  dykes, 
  for 
  large 
  portions 
  of 
  their 
  course, 
  do 
  

   not, 
  even 
  at 
  the 
  present 
  day, 
  reach 
  the 
  surface 
  at 
  all. 
  This 
  fact 
  was 
  

   long 
  ago 
  pointed 
  out 
  by 
  Winch, 
  and 
  has 
  been 
  confirmed 
  by 
  the 
  obser- 
  

   vations 
  of 
  the 
  Geological 
  Surveyors 
  and 
  of 
  Mr. 
  Teall. 
  The 
  recent 
  

   admirable 
  studies 
  carried 
  on 
  by 
  Mr. 
  G. 
  Barrow 
  in 
  the 
  North 
  of 
  

   Yorkshire 
  have 
  demonstrated 
  the 
  frequency 
  of 
  this 
  upward 
  dying- 
  

   out 
  of 
  the 
  Cleveland-dyke 
  over 
  considerable 
  distances 
  *. 
  But 
  the 
  

   present 
  surface 
  of 
  the 
  country 
  must 
  be 
  many 
  hundreds 
  or 
  thousands 
  

   of 
  feet 
  below 
  that 
  which 
  existed 
  in 
  early 
  Tertiary 
  times 
  ; 
  and 
  if 
  we 
  

   could 
  restore 
  the 
  vast 
  masses 
  of 
  rock 
  removed 
  by 
  denudation, 
  the 
  

   abrupt 
  upward 
  termination 
  of 
  the 
  dykes 
  would 
  doubtless 
  be 
  found 
  

   to 
  be 
  of 
  much 
  more 
  frequent 
  occurrence 
  ; 
  and 
  it 
  is 
  of 
  course 
  only 
  

   where 
  the 
  dykes 
  actually 
  reached 
  the 
  old 
  surface 
  early 
  in 
  the 
  Tertiary 
  

   period 
  that 
  they 
  could 
  have 
  served 
  as 
  lava-conduits. 
  

  

  Secondly. 
  — 
  While 
  a 
  few 
  of 
  these 
  dykes 
  are 
  50 
  feet 
  or 
  even 
  more 
  in 
  

   breadth, 
  the 
  great 
  majority 
  of 
  them 
  are 
  of 
  far 
  smaller 
  dimensions, 
  

   many 
  of 
  them, 
  indeed, 
  being 
  quite 
  insignificant 
  ; 
  and 
  it 
  has 
  always 
  

   appeared 
  to 
  me 
  difficult 
  to 
  believe 
  that 
  such 
  enormous 
  floods 
  of 
  lava 
  

   could 
  have 
  been 
  erupted 
  through 
  channels 
  so 
  narrow. 
  

  

  Thirdly. 
  — 
  If 
  it 
  be 
  argued 
  that 
  even 
  comparatively 
  narrow 
  fissures 
  

   might 
  have 
  given 
  rise 
  to 
  enormous 
  outflows 
  of 
  lava, 
  provided 
  they 
  

   were 
  of 
  the 
  nature 
  of 
  habitual 
  vents, 
  then 
  a 
  fresh 
  difficulty 
  has 
  to 
  

   be 
  faced. 
  In 
  such 
  cases 
  we 
  might 
  surely 
  expect 
  that 
  the 
  long-con- 
  

   tined 
  flow 
  of 
  molten 
  materials 
  through 
  the 
  same 
  channel 
  would 
  

   have 
  given 
  rise 
  to 
  the 
  most 
  striking 
  and 
  profound 
  contact-metamor- 
  

   phism 
  on 
  the 
  sides 
  of 
  the 
  fissure. 
  But 
  this 
  is 
  what 
  we 
  never 
  find 
  to 
  

   be 
  the 
  case; 
  as 
  the 
  author 
  of 
  the 
  recent 
  memoir 
  justly 
  remarks, 
  

   " 
  evidence 
  of 
  any 
  serious 
  amount 
  of 
  alteration 
  is 
  singularly 
  scarce, 
  

   a 
  slight 
  induration 
  of 
  the 
  rocks 
  on 
  either 
  side 
  of 
  a 
  dyke 
  is 
  generally 
  

   all 
  that 
  can 
  be 
  detected 
  " 
  (loc. 
  cit. 
  pp. 
  62, 
  63). 
  

  

  It 
  was 
  on 
  these 
  and 
  similar 
  grounds 
  that 
  I 
  long 
  ago 
  rejected 
  the 
  

   suggestion 
  so 
  very 
  frequently 
  advanced 
  by 
  some 
  of 
  the 
  older 
  geolo- 
  

   gists, 
  and 
  which 
  has 
  been 
  revived 
  at 
  various 
  times 
  by 
  my 
  critic, 
  that 
  

   the 
  plateau-basalts 
  or 
  " 
  trap-rocks 
  " 
  are 
  masses 
  of 
  igneous 
  material 
  

   which 
  have 
  come 
  up 
  through 
  the 
  dyke-fissures 
  and 
  overflowed 
  the 
  

   surface. 
  In 
  the 
  case 
  of 
  the 
  Western 
  Isles 
  of 
  Scotland, 
  at 
  all 
  events, 
  

  

  * 
  The 
  Geology 
  of 
  North 
  Cleveland 
  (1888), 
  pp. 
  60-64. 
  

  

  