﻿Haast. 
  — 
  0)1 
  the 
  Geological 
  Structure 
  of 
  Banks 
  Peninsula. 
  503 
  

  

  eruptions, 
  molten 
  matter 
  in 
  a 
  high 
  state 
  of 
  fusion 
  generally 
  rushed 
  up 
  in 
  

   the 
  fissures 
  which 
  had 
  been 
  formed 
  at 
  the 
  time, 
  radiating 
  from 
  the 
  focus 
  

   hke 
  the 
  spokes 
  of 
  a 
  wheel. 
  An 
  examination 
  of 
  these 
  dyke 
  rocks 
  will 
  show 
  

   at 
  a 
  glance 
  that 
  most 
  of 
  them 
  are 
  quite 
  different 
  in 
  composition 
  and 
  

   character 
  from 
  those 
  of 
  which 
  the 
  lava-streams 
  have 
  been 
  formed. 
  The 
  

   latter, 
  as 
  already 
  explained, 
  with 
  one 
  notable 
  exception,* 
  all 
  consist 
  of 
  true 
  

   basic 
  rocks 
  — 
  basalts 
  often 
  assuming 
  a 
  doleritic 
  texture, 
  the 
  dyke 
  rocks 
  

   being 
  generally 
  acidic, 
  having 
  either 
  the 
  composition 
  of 
  a 
  trachyte 
  or 
  

   domite. 
  "We 
  are 
  able 
  to 
  judge 
  of 
  the 
  more 
  or 
  less 
  high 
  state 
  of 
  fusion 
  in 
  

   which 
  the 
  molten 
  matter 
  ascended 
  the 
  open 
  fissures 
  from 
  the 
  eifect 
  pro- 
  

   duced 
  on 
  the 
  walls 
  on 
  both 
  sides. 
  The 
  trachytic 
  matter 
  forming 
  the 
  dykes, 
  

   which 
  are 
  principally 
  developed 
  on 
  the 
  eastern 
  side 
  of 
  the 
  caldera 
  wall, 
  has 
  

   evidently 
  been 
  in 
  such 
  a 
  condition 
  that 
  it 
  could 
  exercise 
  a 
  most 
  powerful 
  

   effect 
  on 
  both 
  walls 
  of 
  the 
  fissure, 
  the 
  rocks 
  often, 
  for 
  several 
  inches, 
  being 
  

   changed 
  to 
  tachylite, 
  a 
  peculiar 
  basic 
  volcanic 
  glass, 
  quite 
  distinct 
  from 
  

   obsidian. 
  This 
  change 
  in 
  the 
  character 
  of 
  the 
  rock 
  is 
  most 
  observable 
  

   when 
  the 
  dykes 
  pass 
  along 
  tufaceous 
  or 
  agglomeratic 
  beds. 
  Here 
  the 
  

   reddish 
  or 
  light 
  purple 
  rocks 
  have 
  been 
  altered 
  to 
  a 
  black 
  vitreous 
  mass, 
  

   containing 
  small 
  crystals 
  of 
  felspar. 
  The 
  domitic 
  dykes, 
  mostly 
  confined 
  

   to 
  the 
  western 
  half 
  of 
  the 
  caldera 
  wall, 
  seem 
  not 
  to 
  have 
  exercised 
  such 
  a 
  

   great 
  influence 
  as 
  the 
  former, 
  as 
  in 
  most 
  instances 
  the 
  walls 
  on 
  both 
  sides 
  

   of 
  the 
  dykes 
  are 
  only 
  slightly 
  hardened. 
  However, 
  there 
  is 
  no 
  constant 
  

   rule 
  ; 
  large 
  dykes, 
  as 
  for 
  instance 
  the 
  huge 
  domitic 
  dyke 
  at 
  Governor's 
  

   Bay, 
  running 
  for 
  a 
  considerable 
  distance 
  parallel 
  to 
  the 
  coast, 
  and 
  forming 
  

   such 
  a 
  conspicuous 
  object 
  along 
  the 
  picturesque 
  beach 
  road 
  lately 
  con- 
  

   structed, 
  have 
  scarcely 
  made 
  any 
  alteration 
  on 
  either 
  side, 
  whilst 
  smaller 
  

   dykes 
  of 
  the 
  same 
  rock, 
  only 
  a 
  few 
  feet 
  in 
  thickness, 
  are 
  sometimes 
  

   accompanied 
  by 
  a 
  well-defined 
  selvage 
  of 
  tachylite. 
  The 
  same 
  may 
  be 
  said 
  

   of 
  the 
  basaltic 
  dykes, 
  of 
  which, 
  however, 
  by 
  far 
  the 
  greatest 
  part 
  has 
  caused 
  

   no 
  visible 
  alteration 
  along 
  the 
  walls 
  on 
  either 
  side. 
  The 
  trachytic 
  varieties, 
  

  

  * 
  This 
  exception 
  consists 
  of 
  a 
  trachytic 
  lava-stream 
  of 
  considerable 
  size, 
  and 
  having 
  

   an 
  average 
  thickness 
  of 
  eighty 
  feet, 
  ■which 
  is 
  interstratified 
  between 
  two 
  others 
  of 
  a 
  basic 
  

   character. 
  This 
  peculiar 
  stream 
  occurs 
  between 
  Lyttelton 
  and 
  the 
  pass 
  to 
  Sumner. 
  It 
  

   is 
  the 
  only 
  trachyte 
  lava 
  known 
  to 
  me 
  as 
  having 
  flowed 
  from 
  any 
  of 
  the 
  different 
  centres 
  

   of 
  eruption 
  of 
  Banks 
  Peninsula, 
  all 
  the 
  other 
  acidic 
  rocks, 
  as 
  I 
  shall 
  show 
  in 
  the 
  sequel, 
  

   having 
  been 
  ejected 
  into 
  fissures 
  of 
  more 
  recent 
  date. 
  This 
  lava-stream 
  consists 
  of 
  a 
  

   white 
  vesicular 
  trachyte 
  rich 
  in 
  quartz, 
  resembling 
  closely 
  some 
  of 
  the 
  domites 
  of 
  the 
  

   Auvergne, 
  from 
  which, 
  however, 
  it 
  is 
  distinguished 
  by 
  its 
  larger 
  amount 
  of 
  silica, 
  

   although 
  it 
  approaches 
  it 
  again 
  in 
  its 
  considerable 
  percentage 
  of 
  potash. 
  A 
  vertical 
  

   dyke, 
  about 
  eight 
  feet 
  thick, 
  of 
  a 
  peculiar 
  flaky 
  silky 
  trachyte, 
  passes 
  through 
  this 
  lava- 
  

   stream, 
  narrowing, 
  however, 
  in 
  its 
  upper 
  portion. 
  Although 
  this 
  acidic 
  lava 
  is 
  rather 
  

   soft 
  and 
  friable 
  in 
  small 
  pieces, 
  it 
  has 
  nevertheless 
  resisted 
  the 
  disintegrating 
  agencies 
  at 
  

   work 
  far 
  better 
  than 
  the 
  hard 
  basaltio 
  lavas 
  and 
  agglomerates 
  in 
  its 
  neighbourhood. 
  

  

  