﻿604 
  Transactions. 
  — 
  Geology. 
  

  

  of 
  wliich 
  most 
  of 
  the 
  dykes 
  on 
  the 
  eastern 
  side 
  of 
  the 
  Lyttelton 
  Harboui? 
  

   consist, 
  are 
  formed 
  generally 
  of 
  a 
  peculiarly 
  lustrous 
  and 
  flaky 
  rock, 
  some- 
  

   times 
  vesicular, 
  with 
  small 
  crystals 
  of 
  sanidine. 
  This 
  rock 
  has 
  a 
  light 
  

   greyish 
  colour, 
  and 
  its 
  small 
  cavities 
  are 
  lined 
  by 
  sphserosiderite. 
  On 
  both 
  

   sides 
  of 
  the 
  dyke 
  the 
  rock 
  is 
  generally 
  tabular 
  — 
  parallel 
  to 
  the 
  direction 
  of 
  

   the 
  flow, 
  and 
  is 
  massive 
  in 
  the 
  centre 
  with 
  polyhedric 
  joints, 
  of 
  which 
  the 
  

   principal 
  ones 
  appear 
  at 
  right 
  angles 
  to 
  the 
  flow. 
  There 
  are 
  also 
  a 
  few 
  

   trachytic 
  dykes, 
  principally 
  small 
  ones, 
  where 
  the 
  sides, 
  for 
  half 
  an 
  inch 
  to 
  

   one 
  inch, 
  consist 
  of 
  a 
  rather 
  brittle 
  obsidian, 
  doubtless 
  the 
  effect 
  of 
  rapid 
  

   cooling. 
  Some 
  very 
  thin 
  thread-like 
  dykes, 
  about 
  one 
  to 
  two 
  inches 
  thick, 
  

   consist 
  entu'ely 
  of 
  that 
  peculiar 
  form 
  of 
  acidic 
  volcanic 
  rock. 
  

  

  In 
  studying 
  the 
  position 
  of 
  the 
  dykes 
  it 
  becomes 
  manifest 
  that 
  they 
  have 
  

   been 
  formed 
  at 
  different 
  times 
  ; 
  however, 
  the 
  altitude 
  of 
  their 
  uppermost 
  

   portion 
  does 
  not 
  indicate 
  their 
  age. 
  I 
  have 
  no 
  doubt 
  that 
  many 
  of 
  them, 
  

   which 
  scarcely 
  reach 
  above 
  high 
  water-mark, 
  are 
  not 
  older 
  than 
  others 
  of 
  

   the 
  same 
  petrological 
  nature, 
  which 
  reach 
  to 
  the 
  very 
  summit 
  of 
  the 
  caldera 
  

   wall. 
  In 
  the 
  present 
  state 
  of 
  our 
  knowledge 
  it 
  is 
  impossible 
  to 
  solve 
  this 
  

   interesting 
  question 
  in 
  all 
  its 
  bearings, 
  and 
  I 
  can 
  therefore 
  only 
  suggest 
  

   that 
  dykes 
  containing 
  rocks 
  of 
  exactly 
  the 
  same 
  lithological 
  character 
  

   have 
  most 
  probably 
  been 
  formed 
  during 
  the 
  same 
  eruption. 
  It 
  is 
  also 
  

   evident 
  that 
  a 
  number 
  of 
  dykes 
  were 
  formed 
  long 
  before 
  the 
  whole 
  of 
  the 
  

   caldera 
  wall 
  was 
  built 
  up, 
  and 
  that 
  they 
  were 
  partly 
  destroyed 
  during 
  one 
  

   of 
  the 
  next 
  eruptions. 
  One 
  clear 
  instance 
  of 
  the 
  occurrence 
  of 
  such 
  older 
  

   dykes 
  is 
  to 
  be 
  found 
  near 
  Cliff's 
  Cove 
  in 
  Lyttelton 
  Harbour, 
  where 
  several 
  

   trachy-doleritic 
  dykes 
  were 
  injected 
  when 
  the 
  rest 
  of 
  the 
  caldera 
  wall 
  was 
  at 
  

   least 
  1,000 
  feet 
  lower 
  than 
  at 
  present. 
  They 
  pass 
  through 
  a 
  basaltic 
  lava- 
  

   etream, 
  which 
  latter 
  was 
  afterwards 
  partly 
  destroyed 
  along 
  with 
  them, 
  the 
  

   whole 
  possessing 
  now 
  nearly 
  a 
  straight 
  surface, 
  upon 
  which 
  a 
  large 
  bed 
  of 
  

   agglomerate 
  has 
  been 
  deposited. 
  However, 
  what 
  is 
  of 
  the 
  greatest 
  interest 
  

   in 
  the 
  history 
  of 
  the 
  volcanic 
  systems 
  under 
  consideration 
  is 
  the 
  predomi- 
  

   nating 
  acidic 
  character 
  of 
  the 
  dykes 
  when 
  compared 
  with 
  the 
  basic 
  lava- 
  

   streams. 
  In 
  Vesuvius 
  and 
  Etna 
  all 
  the 
  dykes 
  are 
  formed 
  by 
  the 
  same 
  kind 
  

   of 
  rock 
  as 
  the 
  lava-streams 
  are 
  composed 
  of, 
  but 
  they 
  are 
  generally 
  more 
  

   compact, 
  having, 
  as 
  Lyell 
  suggests, 
  cooled 
  and 
  consolidated 
  under 
  greater 
  

   pressure. 
  It 
  is 
  evident 
  that 
  they 
  owe 
  their 
  existence 
  to 
  the 
  same 
  subter- 
  

   ranean 
  efforts 
  by 
  which 
  the 
  lava-streams 
  were 
  ejected 
  from 
  the 
  mouth 
  of 
  

   the 
  crater, 
  the 
  fissures 
  in 
  which 
  they 
  were 
  formed 
  being 
  evidently 
  filled 
  up 
  

   from 
  the 
  same 
  focus, 
  and 
  about 
  the 
  same 
  time 
  as 
  the 
  eruption 
  of 
  the 
  lava- 
  

   streams 
  took 
  place. 
  But 
  such 
  a 
  simple 
  process 
  cannot 
  be 
  admitted 
  for 
  the 
  

   greater 
  portion 
  of 
  the 
  dykes 
  of 
  Banks 
  Peninsula, 
  which 
  must 
  owe 
  theu* 
  

   existence 
  to 
  paroxysmal 
  perturbations 
  in 
  the 
  earth's 
  crust, 
  distinct 
  from 
  

  

  