﻿498 
  Transactions. 
  — 
  Geology. 
  

  

  with 
  beds 
  of 
  agglomerates, 
  ashes, 
  tufas, 
  and 
  laterites 
  can 
  be 
  traced 
  to 
  tbe 
  

   very 
  summit. 
  Still 
  clearer 
  sections 
  are 
  open 
  to 
  our 
  inspection 
  if 
  we 
  follow 
  

   the 
  barranco 
  or 
  entrance 
  into 
  the 
  harbour, 
  forming 
  sometimes 
  vertical 
  

   cliffs 
  of 
  considerable 
  altitude, 
  and 
  where 
  the 
  whole 
  series 
  of 
  beds 
  can 
  easily 
  

   be 
  followed. 
  However, 
  the 
  most 
  interesting 
  and 
  complete 
  insight 
  wa3 
  

   obtained 
  in 
  the 
  railway 
  tunnel 
  passing 
  through 
  the 
  caldera 
  wall, 
  and 
  of 
  

   which, 
  as 
  the 
  work 
  gradually 
  advanced, 
  I 
  prepared 
  a 
  careful 
  section. 
  The 
  

   succession 
  and 
  dip 
  of 
  the 
  lava 
  streams 
  and 
  the 
  intervening 
  beds 
  can 
  also 
  be 
  

   made 
  out 
  by 
  following 
  the 
  slopes 
  of 
  the 
  ridges 
  between 
  the 
  deep 
  valleys 
  

   washed 
  out 
  on 
  the 
  outer 
  side 
  of 
  the 
  crater 
  wall, 
  where 
  it 
  will 
  be 
  found 
  that 
  

   the 
  lava 
  streams 
  forming 
  the 
  lip 
  of 
  the 
  crater 
  have 
  generally 
  a 
  slighter 
  

   inclination 
  than 
  those 
  lower 
  down, 
  the 
  dip 
  of 
  the 
  upper 
  ones 
  being 
  only 
  nine 
  

   degrees 
  on 
  the 
  average. 
  In 
  the 
  tunnel 
  the 
  dip 
  is 
  greater, 
  an 
  inclination 
  of 
  

   twenty 
  degrees 
  not 
  being 
  uncommon. 
  It 
  is 
  evident 
  that 
  the 
  building 
  up 
  of 
  

   such 
  a 
  huge 
  system 
  during 
  numerous 
  eruptions, 
  often 
  of 
  great 
  magnitude, 
  

   could 
  not 
  be 
  accomplished 
  without 
  a 
  great 
  destruction 
  of 
  portions 
  of 
  the 
  

   beds 
  previously 
  formed 
  taking 
  place, 
  the 
  point 
  of 
  eruption 
  in 
  the 
  crater 
  

   shifting 
  continuously 
  about 
  the 
  centre. 
  If, 
  at 
  the 
  same 
  time, 
  we 
  examine 
  

   the 
  lava 
  streams 
  and 
  the 
  interstratified 
  agglomerate 
  and 
  ash 
  beds 
  along 
  the 
  

   water's 
  edge, 
  we 
  have 
  to 
  come 
  to 
  the 
  conclusion 
  that 
  all 
  the 
  eruptions 
  by 
  

   which 
  the 
  caldera 
  wall 
  was 
  formed 
  from 
  summit 
  to 
  bottom, 
  occurred 
  under 
  

   the 
  same 
  physical 
  conditions. 
  

  

  Examining 
  into 
  the 
  formation 
  of 
  the 
  Lyttelton 
  caldera, 
  and 
  beginning 
  

   our 
  observations 
  in 
  the 
  harbour, 
  we 
  find 
  that 
  many 
  lava 
  streams 
  have 
  been 
  

   preserved 
  which 
  have 
  cooled 
  in 
  their 
  ascent 
  ; 
  others 
  lie 
  horizontal 
  for 
  a 
  

   short 
  distance, 
  and 
  are 
  then 
  seen 
  to 
  descend, 
  conforming 
  to 
  the 
  gradient 
  of 
  

   the 
  underlying 
  lava 
  streams 
  or 
  agglomerate 
  beds. 
  In 
  many 
  instances 
  we 
  

   have 
  also 
  clear 
  evidence 
  that 
  considerable 
  destruction 
  of 
  the 
  beds 
  previously 
  

   formed 
  had 
  taken 
  place 
  before 
  new 
  streams 
  flowed 
  over 
  the 
  lip 
  of 
  the 
  crater, 
  

   or 
  before 
  beds 
  of 
  ashes, 
  scoriae 
  and 
  lapilli, 
  were 
  deposited 
  anew. 
  The 
  

   tunnel 
  section 
  in 
  this 
  respect 
  is 
  also 
  very 
  instructive. 
  Thus, 
  in 
  course 
  of 
  

   time, 
  the 
  great 
  crater 
  wall 
  was 
  formed, 
  rising 
  to 
  an 
  altitude 
  of 
  nearly 
  2,000 
  

   feet, 
  and 
  having 
  a 
  diameter 
  of 
  more 
  than 
  five 
  miles 
  at 
  its 
  crest. 
  It 
  is 
  clear 
  

   that 
  close 
  to 
  the 
  vent, 
  from 
  which 
  scoria 
  and 
  ashes 
  were 
  thrown 
  out 
  in 
  

   large 
  quantities, 
  the 
  greatest 
  thickness 
  of 
  the 
  agglomerate 
  beds 
  ought 
  to 
  be 
  

   formed, 
  and 
  this, 
  in 
  fact, 
  is 
  the 
  case, 
  as 
  the 
  largest 
  beds, 
  having 
  sometimes 
  

   a 
  thickness 
  of 
  several 
  hundred 
  feet, 
  are 
  situated 
  within 
  the 
  inner 
  side 
  of 
  the 
  

   caldera 
  wall. 
  The 
  lava-^streams 
  here 
  between 
  these 
  agglomerates 
  are 
  irre- 
  

   gular 
  in 
  their 
  direction, 
  and 
  mostly 
  of 
  small 
  dimensions. 
  The 
  more 
  we 
  

   advance 
  towards 
  the 
  outer 
  slopes 
  of 
  the 
  caldera 
  wall, 
  the 
  less 
  frequent 
  

   )jecome 
  these 
  agglomei'atic 
  or 
  tufaceous 
  layers, 
  whilst 
  the 
  lava-streams^ 
  

  

  