﻿J. 
  W. 
  JCDD 
  ON 
  THE 
  SECONDARY 
  BUCKS 
  OF 
  SCOTLAND. 
  259 
  

  

  11. 
  Dimensions 
  of 
  the 
  great 
  Tertiary 
  Volcanoes. 
  — 
  It 
  might 
  at 
  first 
  

   sight 
  appear 
  impossible 
  to 
  arrive 
  at 
  any 
  souud 
  conclusions 
  upon 
  this 
  

   question, 
  concerning, 
  as 
  it 
  does, 
  the 
  physical 
  features 
  of 
  a 
  period 
  so 
  

   remote 
  from 
  the 
  present. 
  Nevertheless, 
  as 
  I 
  hope 
  to 
  be 
  able 
  to 
  

   show, 
  so 
  far 
  is 
  this 
  from 
  being 
  the 
  case, 
  that 
  we 
  have 
  several 
  different 
  

   kinds 
  of 
  data, 
  foremost 
  among 
  which 
  must 
  be 
  ranked 
  those 
  derived 
  

   from 
  the 
  subsidence 
  which 
  we 
  have 
  shown 
  to 
  have 
  taken 
  place 
  in 
  

   the 
  case 
  of 
  the 
  Mull 
  volcano, 
  enabling 
  us 
  to 
  arrive 
  at 
  least 
  at 
  an 
  

   approximate 
  estimate. 
  

  

  We 
  are 
  able 
  in 
  the 
  case 
  of 
  each 
  of 
  the 
  great 
  Tertiary 
  volcanoes 
  to 
  

   to 
  arrive 
  at 
  some 
  idea 
  concerning 
  the 
  areas 
  covered 
  by 
  their 
  bases. 
  

   The 
  base 
  of 
  the 
  volcano 
  of 
  Mull 
  must 
  have 
  had 
  a 
  circumference 
  of 
  

   at 
  least 
  forty 
  miles 
  ; 
  Etna, 
  which 
  has 
  a 
  greatly 
  truncated 
  form, 
  

   nevertheless 
  rises 
  to 
  the 
  height 
  of 
  10,900 
  feet 
  from 
  a 
  base 
  only 
  thirty 
  

   miles 
  in 
  circumference*; 
  a 
  similar 
  relation 
  between 
  the 
  base 
  and 
  

   altitude 
  of 
  the 
  great 
  volcanoes 
  of 
  Sicily 
  and 
  Mull 
  would 
  lead 
  us 
  to 
  

   infer 
  that 
  the 
  elevation 
  of 
  the 
  latter 
  was 
  at 
  least 
  14,500 
  feet. 
  

  

  By 
  a 
  careful 
  study 
  of 
  the 
  present 
  inclinations 
  of 
  the 
  lava 
  streams 
  

   which 
  flowed 
  from 
  the 
  Mull 
  volcano, 
  we 
  may 
  obtain 
  another 
  approx- 
  

   imation 
  to 
  the 
  minimum 
  possible 
  elevation 
  of 
  its 
  summit. 
  From 
  an 
  

   examination 
  of 
  all 
  the 
  data, 
  especially 
  those 
  furnished 
  by 
  the 
  sec- 
  

   tions 
  along 
  the 
  shores 
  of 
  Loch 
  Scridain 
  and 
  Loch-na-Kael, 
  and 
  care- 
  

   fully 
  availing 
  myself 
  of 
  every 
  cheek 
  in 
  making 
  the 
  calculations, 
  I 
  find 
  

   that, 
  if 
  the 
  rocks 
  were 
  restored 
  to 
  the 
  positions 
  which 
  they 
  occupied 
  

   before 
  the 
  great 
  central 
  subsidence 
  took 
  place, 
  the 
  present 
  summit 
  

   of 
  Beinn 
  More, 
  Avhich 
  is 
  3172 
  feet 
  above 
  the 
  sea, 
  would 
  be 
  raised 
  to 
  

   to 
  an 
  elevation 
  of 
  at 
  least 
  6000 
  feet— 
  and, 
  further, 
  that 
  a 
  central 
  

   cone 
  reconstructed 
  on 
  the 
  basis 
  thus 
  obtained 
  would 
  have 
  an 
  eleva- 
  

   tion 
  of 
  more 
  than 
  10,000 
  feet. 
  

  

  But 
  this 
  must 
  be 
  regarded 
  as 
  only 
  the 
  lowest 
  possible 
  estimate 
  ; 
  for 
  

   in 
  making 
  it 
  I 
  have 
  totally 
  neglected 
  four 
  different 
  considerations, 
  

   all 
  tending 
  to 
  increase 
  our 
  estimate, 
  but 
  by 
  what 
  exact 
  amounts 
  it 
  is 
  

   impossible 
  to 
  state. 
  These 
  are 
  : 
  — 
  

  

  (1) 
  The 
  gradual 
  increase 
  of 
  the 
  inclination 
  of 
  the 
  lava-beds 
  on 
  

   the 
  higher 
  slopes 
  of 
  the 
  mountain. 
  

  

  (2) 
  The 
  effect 
  of 
  the 
  great 
  faults 
  before 
  alluded 
  to 
  as 
  contributing 
  

   to 
  the 
  central 
  subsidence, 
  the 
  influence 
  of 
  which, 
  though 
  not 
  exactly 
  

   determinable, 
  was 
  certainly 
  very 
  considerable. 
  

  

  (3) 
  The 
  originally 
  greater 
  elevation 
  of 
  the 
  surfaces 
  on 
  which 
  the 
  

   volcanic 
  rocks 
  lie. 
  This 
  is 
  proved 
  by 
  the 
  fact 
  that 
  many 
  rocks 
  which 
  

   were 
  certainly 
  of 
  terrestrial 
  origin 
  are 
  now 
  below 
  the 
  sea-level. 
  

  

  (4) 
  The 
  great 
  masses 
  of 
  agglomerates 
  and 
  lavas 
  which 
  originally 
  

   formed 
  the 
  summit 
  and 
  flanks 
  of 
  the 
  mountain, 
  but 
  have 
  now 
  been 
  

   removed 
  by 
  denudation. 
  

  

  In 
  speaking, 
  however, 
  of 
  the 
  height 
  of 
  a 
  volcano 
  it 
  must 
  be 
  re- 
  

   membered 
  that 
  we 
  are 
  dealing 
  with 
  a 
  quantity 
  subject 
  to 
  constant 
  

   and 
  wide 
  variations. 
  During 
  a 
  single 
  paroxysm, 
  hundreds, 
  nay, 
  

   even 
  thousands, 
  of 
  feet 
  of 
  materials 
  may 
  be 
  blown 
  from 
  the 
  suni- 
  

  

  * 
  In 
  this 
  estimate 
  I 
  of 
  course 
  only 
  include 
  the 
  mass 
  of 
  rocks 
  constituting 
  the 
  

   mountain 
  proper, 
  and 
  not 
  the 
  lava- 
  covered 
  plains 
  around 
  it. 
  

  

  