﻿Haast. 
  — 
  On 
  the 
  Geological 
  Structure 
  of 
  Banks 
  Peninsula. 
  511 
  

  

  been 
  able 
  to 
  cool 
  to 
  the 
  stony 
  compact 
  form. 
  I 
  may, 
  however, 
  observe 
  

   tliat 
  the 
  boundary 
  hne 
  between 
  both 
  kinds 
  of 
  roch 
  is, 
  in 
  many 
  instances, 
  

   very 
  distinct 
  and 
  clearly 
  defined. 
  These 
  scoriacoous 
  beds 
  occur 
  throughout 
  

   the 
  tunnel; 
  they 
  are 
  sometimes 
  of 
  considerable 
  dimensions, 
  some 
  of 
  them 
  

   being 
  over 
  100 
  feet 
  thick. 
  In 
  speaking 
  of 
  the 
  formation 
  of 
  the 
  Lyttelton 
  

   caldera, 
  I 
  have 
  already 
  pointed 
  out 
  that 
  it 
  has 
  been 
  built 
  up 
  by 
  volcanic 
  

   rocks 
  belonging 
  to 
  two 
  distinct 
  divisions, 
  of 
  which 
  the 
  basic 
  rocks 
  have 
  

   furnished 
  all 
  the 
  material 
  for 
  the 
  lava-streams, 
  agglomeratic 
  and 
  tufaceous 
  

   beds, 
  whilst 
  the 
  principal 
  portion 
  of 
  the 
  dykes 
  owe 
  their 
  origin 
  to 
  the 
  acidic 
  

   division. 
  As 
  might 
  be 
  anticipated, 
  the 
  dykes 
  are 
  most 
  numerous 
  near 
  the 
  

   focus 
  of 
  eruption; 
  thus 
  we 
  find 
  the 
  greater 
  portion 
  of 
  them 
  near 
  the 
  Lyt- 
  

   telton 
  side, 
  several 
  of 
  them 
  not 
  reaching 
  to 
  the 
  roof 
  of 
  the 
  tunnel. 
  Of 
  these 
  

   dykes, 
  No. 
  29 
  is 
  the 
  most 
  important. 
  It 
  consists 
  of 
  a 
  soft 
  flaky 
  and 
  

   lustrous 
  trachyte, 
  and 
  possesses, 
  like 
  most 
  of 
  the 
  other 
  acidic 
  dykes, 
  the 
  

   characteristic 
  feature 
  that 
  it 
  is 
  accompanied 
  on 
  both 
  sides 
  by 
  a 
  selvage 
  of 
  

   tachylite, 
  sometimes 
  two 
  or 
  three 
  inches 
  thick. 
  This 
  change 
  in 
  the 
  charac- 
  

   ter 
  of 
  the 
  bed 
  rock 
  is 
  especially 
  visible 
  when 
  the 
  dykes 
  pass 
  through 
  

   agglomeratic 
  or 
  tufaceous 
  beds. 
  It 
  shows 
  clearly 
  that 
  the 
  volcanic 
  matter 
  

   ascending 
  by 
  these 
  fissures 
  was 
  in 
  such 
  an 
  intense 
  state 
  of 
  fusion 
  that 
  it 
  

   was 
  able 
  to 
  alter 
  the 
  rocks 
  on 
  both 
  sides 
  so 
  thoroughly 
  for 
  such 
  a 
  distance. 
  

   In 
  some 
  instances 
  the 
  dyke 
  rocks 
  themselves 
  have 
  a 
  selvage 
  of 
  tachylite, 
  the 
  

   bed 
  rock 
  being 
  unaltered. 
  It 
  is 
  worthy 
  of 
  notice 
  that 
  the 
  basaltic 
  dykes 
  

   have 
  not 
  produced 
  the 
  same 
  effect, 
  the 
  rocks 
  on 
  both 
  sides 
  being 
  generally 
  

   unaltered. 
  Large 
  beds 
  of 
  loess, 
  similar 
  to 
  those 
  deposited 
  on 
  the 
  inner 
  

   side 
  of 
  the 
  caldera 
  wall, 
  have 
  also 
  been 
  passed 
  through 
  on 
  the 
  Heathcote 
  

   side. 
  Of 
  minerals 
  of 
  secondary 
  origin 
  found 
  in 
  the 
  tunnel, 
  the 
  most 
  diffuse 
  

   is 
  sphferosiderite, 
  which 
  usually 
  coats 
  the 
  pores 
  and 
  cavities 
  of 
  scoriaceous 
  

   lavas. 
  Of 
  others, 
  calcareous 
  spar 
  and 
  aragonite 
  are 
  the 
  most 
  conspicuous. 
  

   The 
  latter 
  is 
  younger 
  than 
  the 
  former, 
  having 
  often 
  been 
  deposited 
  on 
  the 
  

   surface 
  of 
  the 
  calcareous 
  spar 
  coating 
  the 
  small 
  geodes. 
  In 
  a 
  few 
  localities, 
  

   hyalite 
  fills 
  small 
  clefts, 
  or 
  is 
  found 
  in 
  a 
  stalactitic 
  form. 
  

  

  I 
  shall 
  close 
  this 
  address 
  by 
  offering 
  a 
  few 
  observations 
  on 
  some 
  other 
  

   physical 
  features 
  of 
  the 
  beds 
  through 
  which 
  the 
  tunnel 
  has 
  been 
  excavated, 
  

   and 
  as 
  I 
  noted 
  them 
  on 
  the 
  large 
  section 
  during 
  the 
  survey. 
  Forty 
  chains 
  

   from 
  the 
  Heathcote 
  end, 
  a 
  scoriaceous 
  lava-stream, 
  fifteen 
  feet 
  thick, 
  and 
  

   accompanied 
  on 
  both 
  sides 
  by 
  beds 
  of 
  laterite 
  and 
  agglomerate, 
  was 
  passed, 
  

   which 
  was 
  so 
  loose 
  and 
  full 
  of 
  water 
  that 
  the 
  ground 
  had 
  at 
  once 
  to 
  be 
  

   heavily 
  timbered. 
  All 
  the 
  cavities 
  in 
  the 
  lava 
  are 
  lined 
  with 
  sphcerosiderite, 
  

   on 
  which 
  crystals 
  of 
  calcareous 
  spar 
  have 
  been 
  deposited. 
  At 
  40f 
  chains 
  

   on 
  the 
  same 
  side, 
  in 
  a 
  bed 
  of 
  laterite, 
  four 
  feet 
  above 
  the 
  floor 
  of 
  the 
  tunnel, 
  

   a 
  small 
  spring 
  was 
  struck, 
  drying 
  up 
  a 
  few 
  months 
  after 
  ; 
  35^ 
  chains 
  froiii 
  

  

  