﻿206 
  prof. 
  j. 
  w. 
  ju/dd 
  on 
  the 
  tertiary 
  volcanoes 
  

  

  younger 
  of 
  the 
  gabbro-bosses 
  may 
  be 
  of 
  later 
  date 
  than 
  the 
  oldest 
  of 
  

   the 
  basalt-flows 
  — 
  those, 
  namely, 
  which 
  have 
  escaped 
  denudation. 
  

   AVe 
  are 
  equally 
  agreed 
  as 
  to 
  the 
  enormous 
  periods 
  occupied 
  by 
  the 
  

   eruption 
  of 
  the 
  basaltic 
  lavas, 
  and 
  as 
  to 
  the 
  vast 
  amount 
  of 
  denu- 
  

   dation 
  which 
  they 
  have 
  since 
  suffered. 
  

  

  Y. 
  The 
  Alternative 
  Theory 
  of 
  " 
  Fissure-Eruptions." 
  

  

  "We 
  have 
  now, 
  I 
  think, 
  dealt 
  with 
  the 
  arguments 
  which 
  Dr. 
  Geikie 
  

   brings 
  forward 
  as 
  tending 
  to 
  throw 
  doubt 
  on 
  the 
  theory 
  of 
  the 
  

   origin 
  of 
  the 
  volcanic 
  rocks 
  of 
  the 
  AVestern 
  Isles 
  of 
  Scotland, 
  which 
  

   was 
  proposed 
  by 
  me 
  in 
  1874. 
  So 
  far 
  as 
  I 
  can 
  understand 
  them, 
  

   the 
  observations 
  made 
  by 
  the 
  Director-General 
  and 
  the 
  officers 
  of 
  

   the 
  Geological 
  Survey 
  — 
  especially 
  as 
  to 
  the 
  order 
  of 
  appearance 
  of 
  

   the 
  several 
  varieties 
  of 
  igneous 
  rock 
  in 
  the 
  area 
  in 
  question, 
  and 
  in 
  

   the 
  closely 
  related 
  districts 
  of 
  Antrim 
  and 
  Carlingford 
  — 
  are 
  quite 
  

   capable 
  of 
  being 
  completely 
  reconciled 
  with 
  the 
  views 
  I 
  have 
  pro- 
  

   pounded. 
  But 
  Dr. 
  Geikie 
  has, 
  in 
  1880, 
  proposed 
  a 
  rival 
  theory 
  * 
  ; 
  

   and 
  to 
  this 
  theory, 
  in 
  support 
  of 
  which 
  a 
  number 
  of 
  arguments 
  are 
  

   now 
  adduced, 
  we 
  must 
  proceed 
  to 
  direct 
  attention. 
  

  

  A. 
  Can 
  great 
  Basaltic 
  Plateaux 
  he 
  formed 
  by 
  ordinary 
  

   Volcanic 
  Action 
  ? 
  

  

  It 
  maybe 
  well 
  at 
  the 
  outset 
  to 
  consider 
  an 
  a 
  priori 
  objection 
  

   which 
  has 
  been 
  frequently 
  raised, 
  — 
  in 
  which 
  doubt 
  is 
  thrown 
  

   upon 
  the 
  possibility 
  of 
  thick 
  plateaux 
  of 
  basaltic 
  lava, 
  covering 
  many 
  

   thousands 
  of 
  square 
  miles, 
  having 
  been 
  poured 
  out 
  from 
  ordinary 
  

   volcanic 
  vents. 
  

  

  Now, 
  in 
  seeking 
  to 
  explain 
  the 
  volcanic 
  phenomena 
  of 
  the 
  Hebrides 
  

   and 
  the 
  north 
  of 
  Ireland, 
  no 
  illustrations 
  drawn 
  from 
  modern 
  

   examples 
  of 
  eruptive 
  activity 
  are 
  likely 
  to 
  be 
  of 
  greater 
  value 
  and 
  

   appropriateness 
  than 
  those 
  derived 
  from 
  the 
  area 
  which 
  comprises 
  the 
  

   Faroe 
  Isles 
  and 
  Iceland, 
  and 
  which 
  stretches 
  away 
  northward 
  into 
  

   Greenland. 
  As 
  I 
  have 
  already 
  shown, 
  those 
  districts 
  form 
  parts 
  of 
  

   a 
  great 
  petrographical 
  province, 
  and 
  the 
  various 
  types 
  of 
  lava 
  

   poured 
  out 
  at 
  successive 
  periods 
  are 
  wonderfully 
  similar 
  throughout 
  

   the 
  whole 
  of 
  the 
  vast 
  area. 
  For 
  some 
  reason, 
  not 
  yet 
  explained, 
  

   however, 
  the 
  eruptive 
  activity, 
  which 
  at 
  the 
  central 
  part 
  of 
  the 
  area 
  

   (Iceland) 
  is 
  still 
  in 
  full 
  vigour, 
  appears 
  to 
  have 
  died 
  out, 
  probably 
  in 
  

   the 
  Pliocene 
  period, 
  both 
  in 
  the 
  northern 
  part 
  of 
  the 
  province 
  

   (Greenland) 
  and 
  at 
  its 
  southern 
  extremity 
  (the 
  British 
  Isles). 
  

  

  I 
  know 
  of 
  no 
  valid 
  grounds 
  whatever 
  for 
  doubting 
  that 
  the 
  great 
  

   plateaux 
  of 
  basaltic 
  lava 
  which 
  cover 
  so 
  large 
  an 
  area 
  in 
  Iceland 
  

   have 
  been 
  poured 
  out 
  by 
  a 
  long 
  succession 
  of 
  outflows 
  from 
  ordinary 
  

   volcanic 
  vents. 
  Within 
  the 
  comparatively 
  short 
  period 
  covered 
  by 
  

   Icelandic 
  history, 
  we 
  have 
  the 
  example 
  of 
  the 
  eruption 
  of 
  1783 
  at 
  

   Yarmardalr 
  (usually 
  spoken 
  of 
  as 
  the 
  Skaptar 
  Jokull 
  eruption). 
  

   From 
  the 
  carefully 
  prepared 
  accounts 
  of 
  this 
  eruption, 
  drawn 
  up 
  by 
  

  

  * 
  ' 
  Nature,' 
  vol. 
  xxiii. 
  p. 
  3 
  (November 
  4, 
  1880). 
  

  

  