﻿OF 
  THE 
  WESTERN 
  ISLES 
  OE 
  SCOTLAND. 
  207 
  

  

  a 
  Danish 
  Commission 
  appointed 
  for 
  the 
  purpose, 
  and 
  which 
  have 
  been 
  

   confirmed 
  by 
  the 
  observations 
  of 
  many 
  travellers 
  who 
  have 
  since 
  

   visited 
  the 
  district, 
  we 
  learn 
  that 
  two 
  streams 
  of 
  very 
  liquid 
  basaltic 
  

   lava 
  having 
  lengths 
  of 
  from 
  40 
  to 
  50 
  miles, 
  with 
  breadths 
  varying 
  

   from 
  7 
  to 
  15 
  miles, 
  and 
  an 
  average 
  depth 
  of 
  100 
  feet, 
  were 
  pjured 
  

   out 
  during 
  this 
  eruption. 
  Professor 
  Bischoff 
  has 
  made 
  a 
  very 
  remark- 
  

   able 
  estimate, 
  which 
  shows 
  that 
  the 
  quantity 
  of 
  material 
  brought 
  

   from 
  within 
  the 
  earth's 
  crust 
  and 
  spread 
  over 
  the 
  surface 
  in 
  this 
  

   single 
  outburst 
  exceeds 
  the 
  bulk 
  of 
  Mont 
  Blanc 
  *. 
  

  

  It 
  is 
  scarcely 
  necessary 
  to 
  point 
  out 
  that 
  a 
  succession 
  of 
  flows 
  

   of 
  such 
  magnitude 
  as 
  that 
  of 
  Skaptar 
  Jokull, 
  continued 
  through 
  the 
  

   long 
  periods 
  of 
  time 
  covered 
  by 
  the 
  Tertiary 
  epoch, 
  would 
  be 
  amply 
  

   sufficient 
  to 
  produce 
  the 
  great 
  basaltic 
  plateaux 
  of 
  Iceland, 
  without 
  

   calling 
  in 
  the 
  aid 
  of 
  any 
  sudden 
  and 
  overwhelming 
  extravasation 
  of 
  

   lava, 
  to 
  which 
  we 
  can 
  find 
  nothing 
  approaching 
  a 
  parallel 
  among 
  the 
  

   volcanic 
  phenomena 
  of 
  the 
  present 
  time. 
  

  

  It 
  is 
  only 
  fair 
  to 
  Dr. 
  Geikie 
  to 
  point 
  out 
  that 
  his 
  own 
  observations, 
  

   and 
  those 
  of 
  other 
  members 
  of 
  the 
  Geological 
  Survey, 
  are 
  clearly 
  

   shown 
  to 
  support 
  the 
  conclusion 
  that 
  the 
  plateaux 
  of 
  basalt 
  in 
  Antrim 
  

   and 
  the 
  Western 
  Isles 
  are 
  built 
  up 
  of 
  a 
  great 
  number 
  of 
  separate 
  lava- 
  

   streams, 
  some 
  of 
  a 
  volume 
  comparable 
  to 
  the 
  lava-sheets 
  of 
  " 
  Skaptar 
  

   Jokull," 
  others 
  of 
  much 
  smaller 
  bulk, 
  with 
  not 
  a 
  few 
  of 
  quite 
  insig- 
  

   nificant 
  dimensions. 
  

  

  After 
  pointing 
  out 
  that 
  the 
  thickness 
  of 
  the 
  individual 
  lava-sheets 
  

   in 
  Antrim 
  and 
  the 
  Scottish 
  Isles 
  varies 
  from 
  60 
  or 
  70 
  feet 
  to 
  only 
  

   6 
  or 
  8 
  feet, 
  Dr. 
  Geikie 
  goes 
  on 
  to 
  say 
  : 
  — 
  

  

  " 
  Each 
  bed 
  appears, 
  on 
  a 
  cursory 
  inspection, 
  to 
  retain 
  its 
  average 
  

   thickness, 
  and 
  to 
  be 
  continuous 
  for 
  a 
  long 
  distance. 
  But 
  I 
  believe 
  

   that 
  this 
  persistence 
  is 
  in 
  a 
  great 
  measure 
  deceptive. 
  It 
  is 
  not 
  often 
  

   that 
  we 
  can 
  follow 
  the 
  same 
  bed 
  with 
  absolutely 
  unbroken 
  con- 
  

   tinuity 
  for 
  more 
  than 
  a 
  mile 
  or 
  two. 
  Even 
  in 
  the 
  most 
  favourable 
  

   conditions, 
  such 
  as 
  are 
  afforded 
  by 
  a 
  bare 
  sea-cliff 
  on 
  which 
  every 
  

   bed 
  can 
  be 
  seen, 
  there 
  occur 
  small 
  faults, 
  gullies 
  where 
  the 
  rocks 
  are 
  

   for 
  the 
  time 
  concealed, 
  slopes 
  of 
  debris, 
  and 
  other 
  failures 
  of 
  con- 
  

   tinuity 
  ; 
  while 
  the 
  rocks 
  are 
  generally 
  so 
  like 
  each 
  other 
  that 
  on 
  the 
  

   further 
  side 
  of 
  any 
  such 
  interruption 
  it 
  is 
  not 
  always 
  possible 
  to 
  

   make 
  sure 
  that 
  we 
  are 
  still 
  tracing 
  the 
  same 
  bed 
  of 
  basalt 
  which 
  we 
  

   may 
  have 
  been 
  previously 
  following. 
  On 
  the 
  other 
  hand, 
  a 
  careful 
  

   examination 
  of 
  one 
  of 
  these 
  great 
  natural 
  sections 
  will 
  usually 
  supply 
  

   us 
  with 
  proofs 
  that, 
  while 
  the 
  bedded 
  character 
  may 
  continue 
  well 
  

   marked, 
  the 
  individual 
  beds 
  die 
  out, 
  and 
  are 
  replaced 
  by 
  others 
  of 
  

   similar 
  character. 
  On 
  the 
  south 
  coast 
  of 
  Mull, 
  for 
  instance, 
  cases 
  

   may 
  be 
  observed 
  where 
  the 
  basalt 
  of 
  one 
  sheet 
  abruptly 
  wedges 
  out 
  

  

  * 
  If 
  it 
  should 
  be 
  asserted, 
  as 
  has 
  sometimes 
  been 
  done, 
  that 
  the 
  Icelandic 
  

   eruptions 
  are 
  themselves 
  not 
  ordinary 
  volcanic 
  outbursts, 
  but 
  " 
  fissure-eruptions,' 
  ' 
  

   it 
  is 
  only 
  necessary 
  to 
  refer 
  to 
  the 
  accounts 
  given 
  of 
  the 
  " 
  Skaptar 
  Jokull 
  " 
  out- 
  

   break 
  by 
  the 
  Danish 
  Commission 
  and 
  subsequent 
  authors. 
  The 
  explosive 
  

   action 
  was 
  so 
  violent 
  as 
  to 
  produce 
  phenomena 
  quite 
  comparable 
  to 
  those 
  

   occur. 
  ing 
  at 
  Krakatoa 
  in 
  1883, 
  and 
  lines 
  of 
  cinder-cones 
  were 
  thrown 
  up, 
  just 
  

   as 
  in 
  the 
  eruptions 
  which 
  take 
  place 
  on 
  the 
  flanks 
  of 
  Etna, 
  

  

  Q.J.G.S. 
  No. 
  178. 
  e 
  

  

  