﻿W. 
  G. 
  Foye 
  — 
  Geology 
  of 
  the 
  Lau 
  Islands. 
  347 
  

  

  Andivathi 
  ash 
  beds 
  rest 
  on 
  an 
  irregular 
  surface 
  of 
  limestone 
  

   and 
  dip 
  S.E. 
  toward 
  the 
  eroded 
  and. 
  nearly 
  vertical 
  walls 
  of 
  

   limestone, 
  forming 
  the 
  coast 
  of 
  Vanua 
  Mbalavu 
  less 
  than 
  a 
  

   hundred 
  yards 
  away. 
  The 
  irregularity 
  of 
  the 
  surface 
  beneath 
  

   the 
  ash 
  makes 
  it 
  apparent 
  that 
  the 
  limestone 
  was 
  elevated 
  and 
  

   eroded 
  before 
  the 
  volcanic 
  rocks 
  of 
  the 
  second 
  period 
  were 
  

   extruded. 
  

  

  The 
  central 
  ridge 
  south 
  of 
  Koro 
  Mbasanga 
  is 
  more 
  deeply 
  

   lateritized 
  than 
  the 
  rocks 
  of 
  the 
  highest 
  peak, 
  and 
  bits 
  of 
  silici- 
  

   fied 
  coral 
  often 
  occur 
  in 
  the 
  laterite. 
  The 
  writer 
  inferred 
  that 
  

   the 
  corals 
  were 
  residuals 
  of 
  a 
  limestone 
  cover 
  which 
  once 
  man- 
  

   tled 
  the 
  volcanic 
  rocks. 
  The 
  andesites, 
  lateritized 
  to 
  a 
  depth 
  

   of 
  20 
  feet 
  or 
  more, 
  strengthen 
  this 
  inference 
  since 
  it 
  is 
  apparent 
  

   that 
  they 
  are 
  of 
  a 
  very 
  different 
  age 
  than 
  the 
  rocks 
  of 
  Koro 
  

   Mbasanga. 
  A 
  petrographic 
  study 
  shows 
  that 
  the 
  latter 
  rocks 
  

   are 
  olivine 
  basalts 
  whereas 
  the 
  former 
  are 
  andesites. 
  

  

  The 
  limestone 
  area 
  at 
  the 
  northern 
  end 
  of 
  the 
  island 
  covers 
  

   10 
  to 
  12 
  square 
  miles. 
  Its 
  surface 
  is 
  very 
  irregular 
  and 
  in 
  

   places 
  almost 
  impassable 
  because 
  of 
  the 
  tangled 
  mass 
  of 
  solu- 
  

   tion 
  remnants. 
  It 
  is 
  covered 
  with 
  a 
  red 
  residual 
  soil, 
  having 
  

   an 
  average 
  thickness 
  of 
  one 
  or 
  two 
  feet. 
  Skeats 
  found 
  that 
  17 
  

   limestones 
  from 
  this 
  district 
  showed 
  less 
  than 
  l/10th 
  of 
  one 
  

   per 
  cent 
  of 
  insoluble 
  matter. 
  It 
  follows 
  that 
  the 
  limestone 
  lost 
  

   by 
  solution 
  must 
  be 
  reckoned 
  in 
  hundreds 
  of 
  feet. 
  

  

  The 
  general 
  elevation 
  of 
  the 
  limestone 
  plateau 
  is 
  300 
  to 
  350 
  

   feet. 
  Nearly 
  perpendicular 
  walls 
  bound 
  this 
  area 
  along 
  the 
  

   coast 
  and 
  in 
  these 
  w 
  T 
  alls 
  may 
  be 
  traced 
  a 
  slight 
  folding 
  of 
  very 
  

   low 
  curvature. 
  There 
  is 
  no 
  evidence 
  here 
  that 
  the 
  limestones 
  

   have 
  been 
  deposited 
  on 
  the 
  eroded 
  stumps 
  of 
  Tertiary 
  strata, 
  

   as 
  Agassiz's 
  theory 
  would 
  postulate. 
  Coral 
  heads 
  occur 
  in 
  place 
  

   from 
  top 
  to 
  bottom 
  of 
  these 
  cliffs 
  and 
  are 
  usually 
  surrounded 
  

   by 
  a 
  paste 
  of 
  shell 
  and 
  coral 
  rubble 
  similar 
  to 
  that 
  filling 
  in 
  

   around 
  the 
  heads 
  of 
  a 
  modern 
  reef. 
  Since 
  these 
  cliffs 
  are 
  

   higher 
  than 
  the 
  vertical 
  limit 
  within 
  which 
  corals 
  grow 
  beneath 
  

   the 
  sea, 
  and 
  since, 
  moreover, 
  they 
  must 
  have 
  retreated 
  laterally, 
  

   as 
  well 
  as 
  vertically, 
  by 
  solution 
  and 
  wave-cutting, 
  the 
  writer 
  

   sees 
  no 
  escape 
  from 
  the 
  conclusion 
  that 
  they 
  were 
  formed 
  by 
  

   vertical 
  subsidence. 
  

  

  Distinctive 
  features 
  of 
  the 
  limestone 
  of 
  the 
  island 
  are 
  the 
  

   narrow 
  fingering 
  bays, 
  inclosed 
  by 
  nearly 
  vertical 
  walls 
  of 
  lime- 
  

   stone, 
  extending 
  far 
  inland. 
  Many 
  of 
  the 
  bay-heads 
  rest 
  against 
  

   the 
  volcanic 
  rocks 
  and 
  the 
  writer 
  believes 
  that 
  underground 
  

   solution 
  cavities, 
  formed 
  along 
  the 
  contact 
  between 
  the 
  lime- 
  

   stone 
  and 
  volcanic 
  rocks, 
  have 
  in 
  most 
  cases 
  been 
  the 
  initial 
  

   stage 
  in 
  the 
  production 
  of 
  such 
  bays. 
  Most 
  of 
  the 
  rainfall 
  

   percolates 
  easily 
  into 
  the 
  porous 
  limestone 
  and 
  flows 
  away 
  to 
  

   the 
  sea 
  on 
  the 
  surface 
  of 
  the 
  volcanic 
  rocks. 
  Lar^e 
  solution 
  

  

  