﻿Haast. 
  — 
  On 
  the 
  Geological 
  Structure 
  of 
  Banks 
  Peninsula. 
  605 
  

  

  those 
  during 
  wiiicli 
  the 
  caldera 
  walls 
  were 
  built 
  up. 
  It 
  is 
  evident 
  that 
  a 
  

   great 
  j)ortion 
  of 
  the 
  lava-streams 
  and 
  agglomeratic 
  beds 
  which 
  once 
  formed 
  

   the 
  crater 
  of 
  the 
  volcanic 
  system 
  of 
  Lyttelton 
  Harbour, 
  must 
  have 
  been 
  

   blown 
  away, 
  or 
  at 
  least 
  removed 
  during 
  one 
  of 
  those 
  violent 
  outbursts 
  of 
  

   subterranean 
  forces 
  necessary 
  to 
  clear 
  the 
  choked 
  vent 
  of 
  the 
  volcano, 
  

   similar 
  to 
  those 
  by 
  which 
  in 
  recent 
  times 
  the 
  upper 
  portions 
  of 
  active 
  

   volcanoes 
  have 
  repeatedly 
  been 
  destroyed 
  under 
  the 
  eyes 
  of 
  the 
  trembling 
  

   population 
  in 
  the 
  neighbourliood. 
  

  

  For 
  an 
  explanation 
  we 
  might 
  go 
  back 
  to 
  Durocher's 
  views, 
  that 
  all 
  

   igneous 
  rocks, 
  even 
  the 
  most 
  modern 
  lavas, 
  are 
  derived 
  from 
  two 
  distinct 
  

   magmas 
  which 
  co-exist 
  below 
  the 
  solid 
  crust 
  of 
  the 
  globe, 
  each 
  of 
  them 
  

   occupying 
  a 
  well-defined 
  position. 
  According 
  to 
  this 
  distinguished 
  French 
  

   chemist, 
  the 
  uppermost 
  portion 
  is 
  occupied 
  by 
  the 
  acidic 
  magma, 
  which, 
  be- 
  

   sides 
  being 
  of 
  lighter 
  specific 
  gravity, 
  possesses 
  a 
  larger 
  amount 
  of 
  silica 
  

   and 
  less 
  iron 
  osyde 
  than 
  the 
  other 
  or 
  basic 
  magma. 
  From 
  the 
  upper 
  layer 
  

   the 
  granites, 
  porphyries, 
  and 
  trachytes, 
  according 
  to 
  his 
  views, 
  are 
  derived, 
  

   ^he 
  zone 
  of 
  contact 
  producing 
  rocks 
  of 
  an 
  intermediate 
  character, 
  such 
  as 
  

   trachydolerites. 
  If 
  this 
  theory 
  is 
  correct, 
  we 
  have 
  to 
  admit 
  that 
  not 
  only 
  

   the 
  dyke 
  rocks 
  were 
  injected 
  in 
  rents 
  formed 
  during 
  earthquakes, 
  or 
  imme- 
  

   diately 
  before 
  volcanic 
  eruptions 
  had 
  taken 
  place 
  from 
  the 
  opened 
  chimney 
  of 
  

   the 
  volcano, 
  but 
  that 
  in 
  each 
  case 
  the 
  molten 
  matter 
  was 
  furnished 
  both 
  from 
  

   the 
  upper 
  and 
  lower 
  stratum 
  of 
  incandescent 
  matter 
  below 
  the 
  hard 
  crust 
  of 
  

   the 
  globe. 
  There 
  is, 
  however, 
  one 
  great 
  difficulty 
  which 
  crops 
  up 
  here, 
  and 
  

   which 
  I 
  wish 
  to 
  point 
  out, 
  and 
  that 
  is 
  the 
  presence 
  of 
  dykes 
  of 
  basic 
  rocks 
  

   and 
  of 
  others 
  of 
  an 
  intermediate 
  character. 
  If 
  all 
  the 
  radiating 
  fissures 
  

   without 
  exception 
  had 
  been 
  filled 
  up 
  by 
  acidic 
  rocks, 
  this 
  would 
  go 
  far 
  to 
  

   prove 
  the 
  existence 
  of 
  such 
  an 
  upper 
  acidic 
  incandescent 
  magma 
  ; 
  in 
  which 
  

   case 
  we 
  should 
  be 
  forced 
  to 
  the 
  conclusion 
  that 
  the 
  chimney 
  of 
  the 
  volcano 
  

   reached 
  lower 
  down 
  to 
  the 
  lower 
  or 
  basic 
  layer. 
  But 
  it 
  is 
  difficult 
  to 
  under- 
  

   stand 
  how 
  all 
  the 
  radiating 
  fissures 
  over 
  an 
  area 
  of 
  12 
  miles 
  in 
  diameter 
  

   could 
  pass 
  through 
  the 
  solid 
  crust 
  of 
  the 
  earth 
  and 
  through 
  the 
  fluid 
  acidic 
  

   magma, 
  and 
  how 
  the 
  lower 
  basic 
  rocks 
  could 
  be 
  injected 
  into 
  them 
  from 
  

   below 
  without 
  disturbing 
  the 
  acidic 
  magma, 
  which 
  certainly 
  should 
  have 
  

   been 
  forced 
  up 
  before. 
  This 
  difficulty 
  might, 
  however, 
  be 
  met 
  by 
  the 
  sug- 
  

   gestion 
  that 
  the 
  radiating 
  fissures 
  in 
  this 
  instance 
  did 
  not 
  reach 
  so 
  far 
  down 
  

   as 
  the 
  fluid 
  acidic 
  magma, 
  and 
  that 
  the 
  material 
  for 
  the 
  formation 
  of 
  the 
  

   dykes 
  had 
  been 
  furnished 
  from 
  the 
  crater 
  itself, 
  but 
  it 
  is 
  scarcely 
  conceivable 
  

   that 
  for 
  a 
  distance 
  of 
  six 
  miles 
  and 
  for 
  an 
  altitude 
  of 
  several 
  thousand 
  feet 
  the 
  

   molten 
  matter 
  would 
  have 
  been 
  forced 
  in 
  all 
  directions 
  from 
  the 
  central 
  axis 
  

   of 
  eruption 
  along 
  these 
  fissures, 
  often 
  only 
  a 
  few 
  feet 
  wide. 
  Mr, 
  E. 
  Mallet,* 
  

  

  * 
  Transactions 
  of 
  the 
  Eoyal 
  Society. 
  Phil. 
  Trans. 
  1873. 
  

  

  