﻿204 
  PROF. 
  J. 
  W. 
  JUDD 
  ON 
  THE 
  TERTIARY 
  VOLCANOES 
  

  

  Secondly. 
  It 
  is 
  asserted 
  that 
  there 
  is 
  an 
  absence 
  of 
  masses 
  of 
  

   pyroclastic 
  materials 
  (tuffs 
  and 
  dust) 
  such 
  as 
  we 
  should 
  expect 
  to 
  

   find 
  around 
  great 
  volcanic 
  centres. 
  

  

  In 
  reply 
  to 
  this 
  I 
  may 
  remark 
  that 
  some 
  volcanoes, 
  like 
  those 
  of 
  

   Hawaii, 
  exhibit 
  very 
  little 
  evidence 
  indeed 
  of 
  explosive 
  activity, 
  

   and 
  that 
  their 
  cones 
  are 
  almost 
  wholly 
  built 
  up 
  by 
  successively 
  out- 
  

   welling 
  lava- 
  currents. 
  But, 
  as 
  a 
  matter 
  of 
  fact, 
  there 
  are 
  very 
  

   abundant 
  evidences 
  of 
  explosive 
  action 
  at 
  these 
  great 
  centres. 
  My 
  

   critic 
  has 
  completely 
  adopted 
  the 
  account 
  which 
  I 
  gave 
  in 
  1874 
  of 
  

   the 
  agglomerates 
  of 
  Cnoc 
  nam 
  Fitheach 
  in 
  Skye, 
  and 
  of 
  the 
  volcanic 
  

   breccias 
  in 
  the 
  tract 
  lying 
  south 
  of 
  Allival 
  and 
  Rarkeval 
  in 
  Rum. 
  

   (See 
  T. 
  R. 
  S. 
  E. 
  1888, 
  pp. 
  107 
  & 
  134, 
  and 
  compare 
  Q. 
  J. 
  G. 
  S. 
  1874, 
  

   pp. 
  253-255.) 
  

  

  When 
  it 
  is 
  admitted 
  that 
  at 
  one 
  of 
  the 
  great 
  centres 
  of 
  eruption, 
  

   Skye, 
  there 
  are 
  masses 
  of 
  volcanic 
  agglomerate 
  developed 
  on 
  such 
  a 
  

   scale 
  that 
  it 
  is 
  possible 
  to 
  interpret 
  them 
  as 
  indicating 
  the 
  exis- 
  

   tence 
  of 
  a 
  volcanic 
  " 
  neck 
  " 
  two 
  miles 
  in 
  diameter 
  ; 
  when, 
  at 
  a 
  second 
  

   centre, 
  Ardnamurchan, 
  one 
  mass 
  of 
  agglomerate 
  (Faskadale), 
  more 
  

   than 
  a 
  quarter 
  of 
  a 
  mile 
  in 
  diameter, 
  is 
  described, 
  and 
  another 
  

   (Maclean's 
  ]Sose) 
  more 
  than 
  half 
  a 
  mile 
  in 
  diameter 
  (T. 
  R. 
  S. 
  E. 
  

   1888, 
  p. 
  106) 
  ; 
  when, 
  at 
  a 
  third 
  centre, 
  Rum, 
  it 
  is 
  granted 
  that 
  

   volcanic 
  breccias 
  attain 
  a 
  thickness 
  of 
  200 
  feet, 
  extending 
  as 
  a 
  con- 
  

   tinuous 
  mass 
  for 
  three 
  miles, 
  and 
  in 
  a 
  more 
  interrupted 
  manner 
  

   much 
  further 
  {loc. 
  tit. 
  p. 
  134) 
  ; 
  and 
  when, 
  finally, 
  details 
  of 
  similar 
  

   occurrences 
  are 
  given 
  in 
  the 
  case 
  of 
  a 
  fourth 
  centre, 
  Mull 
  (loc. 
  cit. 
  

   p. 
  83), 
  it 
  is 
  hard 
  to 
  understand 
  how 
  the 
  deposits 
  of 
  volcanic 
  ejecta- 
  

   menta 
  can 
  be 
  spoken 
  of 
  as 
  being 
  insignificant 
  ! 
  

  

  When 
  the 
  geological 
  surveyors 
  come 
  to 
  study 
  the 
  country 
  in 
  

   detail, 
  they 
  will 
  find 
  how 
  numerous 
  and 
  extensive 
  the 
  beds 
  of 
  turf 
  

   and 
  agglomerate 
  really 
  are 
  around 
  the 
  great 
  centres. 
  Of 
  course, 
  

   such 
  deposits 
  are 
  not 
  so 
  conspicuous 
  as 
  they 
  would 
  be 
  if 
  not 
  covered 
  

   by 
  taluses 
  of 
  fallen 
  blocks 
  from 
  overlying 
  lavas; 
  and 
  they 
  have, 
  

   moreover, 
  often 
  suffered 
  greatly 
  from 
  denudation, 
  — 
  -first, 
  from 
  their 
  

   greater 
  softness, 
  and 
  secondly, 
  from 
  their 
  having 
  in 
  many 
  cases 
  

   occupied 
  the 
  highest 
  elevations. 
  Enough 
  of 
  them 
  still 
  remain, 
  how- 
  

   ever, 
  to 
  indicate 
  their 
  character 
  and 
  illustrate 
  their 
  former 
  extent. 
  

  

  Thirdly. 
  It 
  is 
  said 
  that 
  there 
  is 
  no 
  proof 
  of 
  a 
  communication 
  having 
  

   existed 
  between 
  the 
  great 
  intrusive 
  sheets 
  and 
  dykes 
  of 
  gabbro 
  and 
  

   dolerite, 
  and 
  the 
  currents 
  of 
  basalt 
  that 
  were 
  poured 
  out 
  at 
  the 
  

   surface. 
  

  

  Here, 
  too, 
  I 
  must 
  point 
  out 
  that 
  I 
  have 
  never 
  asserted 
  that 
  any 
  

   such 
  connexion 
  can 
  be 
  directly 
  traced. 
  Following 
  Darwin 
  and 
  

   Jukes, 
  I 
  argued 
  that 
  if 
  the 
  source 
  of 
  an 
  obsidian 
  lava-current 
  could 
  

   be 
  traced 
  down 
  sufficiently 
  far 
  into 
  the 
  bowels 
  of 
  the 
  earth, 
  every 
  

   gradation 
  from 
  glass 
  to 
  granite 
  might 
  he 
  observed. 
  Rut 
  in 
  the 
  case 
  

   of 
  the 
  Western 
  Isles 
  of 
  Scotland, 
  I 
  stated 
  that 
  the 
  proofs 
  of 
  the 
  

   connexion 
  between 
  the 
  Plutonic 
  rocks 
  and 
  the 
  lavas, 
  necessarily 
  

   inferential, 
  were 
  as 
  follows 
  : 
  — 
  

  

  The 
  intrusive 
  Plutonic 
  masses, 
  " 
  in 
  their 
  ultimate 
  chemical 
  com- 
  

  

  