﻿310 
  EEV. 
  E. 
  EAEOX 
  OK" 
  THE 
  

  

  some 
  places 
  nodules 
  of 
  this 
  magnetite 
  are 
  found 
  almost 
  as 
  large 
  as 
  

   one's 
  fist. 
  In 
  certain 
  localities, 
  notably 
  in 
  "West 
  Yalalafotsy, 
  the 
  

   country 
  is 
  covered 
  with 
  innumerable 
  pebbles 
  composed 
  of 
  quartz 
  

   and 
  magnetite, 
  so 
  much 
  so, 
  indeed, 
  as 
  to 
  render 
  walking 
  difficult. 
  

   Sometimes 
  one 
  may 
  meet 
  with 
  a 
  kind 
  of 
  ferruginous 
  conglomerate, 
  

   formed 
  by 
  the 
  percolation 
  of 
  water 
  charged 
  with 
  iron 
  through 
  sand 
  

   and 
  pebbles. 
  This 
  conglomerate 
  may 
  often 
  be 
  seen 
  by 
  stream-sides 
  ; 
  

   but 
  in 
  some 
  places 
  away 
  from 
  streams 
  it 
  is 
  found 
  in 
  considerable 
  

   quantity. 
  In 
  the 
  valle} 
  T 
  between 
  the 
  villages 
  of 
  Isoavinimerina 
  

   and 
  Ambohimandray 
  in 
  Imerina 
  there 
  is 
  a 
  bed 
  of 
  pisolitic 
  iron-ore, 
  

   which 
  the 
  natives 
  know 
  as 
  tai-mamba 
  or 
  taolan-tany 
  (" 
  crocodile- 
  

   dung," 
  or 
  " 
  bones 
  of 
  the 
  earth 
  '*). 
  

  

  Iron 
  pyrites 
  also 
  exists 
  as 
  an 
  accessory 
  mineral 
  in 
  the 
  gneiss. 
  

   This 
  may 
  frequently 
  be 
  seen 
  in 
  small 
  glittering 
  specks 
  in 
  freshly 
  

   fractured 
  surfaces 
  of 
  the 
  rock. 
  Large 
  cubical 
  crystals 
  are 
  found 
  iu 
  

   certain 
  localities. 
  Molybdenite 
  is 
  also 
  an 
  accessory 
  mineral 
  occa- 
  

   sionally 
  to 
  be 
  met 
  with 
  in 
  the 
  gneiss. 
  

  

  In 
  descriptions 
  of 
  the 
  central 
  provinces 
  of 
  Madagascar 
  we 
  not 
  

   unfrequently 
  see 
  it 
  stated 
  that 
  there 
  exist 
  extensive 
  deposits 
  of 
  clay. 
  

   Dr. 
  Mullens, 
  for 
  instance, 
  in 
  his 
  ' 
  Twelve 
  Months 
  in 
  Madagascar,'* 
  

   several 
  times 
  mentions 
  such 
  deposits. 
  In 
  'The 
  Great 
  African 
  

   Island/ 
  too, 
  it 
  is 
  said 
  : 
  — 
  " 
  A 
  very 
  large 
  extent 
  of 
  this 
  portion 
  of 
  

   Madagascar 
  is 
  covered 
  with 
  bright 
  red 
  clay 
  through 
  which 
  the 
  

   granite 
  and 
  basaltic 
  rocks 
  protrude." 
  The 
  same 
  statement 
  is 
  re- 
  

   peated 
  in 
  Mr. 
  Shaw's 
  book, 
  ; 
  Madagascar 
  and 
  Prance.' 
  This 
  clay, 
  

   however, 
  is 
  merely 
  the 
  decayed 
  rock, 
  chiefly 
  gneiss, 
  reddened 
  with 
  

   the 
  oxidized 
  magnetite 
  above 
  alluded 
  to 
  ; 
  and 
  from 
  the 
  fact 
  that 
  

   foliation 
  of 
  the 
  gneiss 
  is 
  not 
  always 
  entirely 
  obliterated 
  in 
  the 
  de- 
  

   cayed 
  portions 
  of 
  the 
  rock, 
  Dr. 
  Mullens 
  has 
  been 
  tempted 
  to 
  call 
  it 
  

   " 
  sedimentary 
  clay." 
  This 
  decay 
  or 
  weathering 
  of 
  the 
  rocks 
  has, 
  in 
  

   some 
  places, 
  reached 
  an 
  enormous 
  depth. 
  In 
  one 
  locality 
  north 
  of 
  

   Andringitra 
  I 
  found 
  that 
  the 
  gneiss 
  had 
  decomposed 
  into 
  clay 
  to 
  a 
  

   depth 
  of 
  180 
  feet. 
  It 
  is 
  owing 
  to 
  this 
  decomposed 
  condition 
  of 
  the 
  

   rock 
  that 
  the 
  heavy 
  rains 
  in 
  the 
  wet 
  season 
  scoop 
  out 
  those 
  deep 
  and 
  

   unsightly 
  ravines 
  in 
  the 
  hill-sides 
  which 
  are 
  so 
  common 
  in 
  the 
  inte- 
  

   rior 
  of 
  the 
  island, 
  and 
  which 
  are 
  occasionally 
  used 
  as 
  cattle-pens 
  by 
  

   fencing 
  in 
  the 
  lower 
  end. 
  This 
  weathering, 
  moreover, 
  explains 
  the 
  

   occurrence 
  of 
  those 
  large 
  "boulders" 
  which 
  may 
  frequently 
  be 
  

   seen, 
  even 
  on 
  hill 
  tops, 
  and 
  which 
  have 
  been 
  more 
  than 
  once 
  con- 
  

   sidered 
  as 
  erratic 
  blocks 
  due 
  to 
  glacial 
  action, 
  but 
  which, 
  of 
  

   course, 
  are 
  merely 
  masses 
  of 
  hard 
  rock 
  rounded 
  by 
  weathering 
  that 
  

   have 
  hitherto 
  resisted 
  decomposition. 
  It 
  may 
  perhaps 
  be 
  worthy 
  of 
  

   notice 
  that 
  the 
  gneiss, 
  a 
  little 
  to 
  the 
  north 
  of 
  Marotandrano, 
  in 
  the 
  

   district 
  of 
  Androna, 
  weathers, 
  in 
  some 
  places, 
  into 
  spheroidal 
  forms 
  

   of 
  a 
  foot 
  or 
  more 
  in 
  diameter. 
  

  

  The 
  other 
  members 
  of 
  the 
  crystalline 
  schists 
  are 
  of 
  much 
  less 
  

   frequent 
  occurrence 
  than 
  the 
  gneiss, 
  and 
  as 
  yet 
  comparatively 
  little 
  

   is 
  known 
  either 
  as 
  to 
  their 
  locality 
  or 
  their 
  exact 
  mineral 
  character. 
  

   Such 
  data, 
  however, 
  as 
  I 
  have 
  been 
  able 
  to 
  gather, 
  imperfect 
  though 
  

   they 
  be, 
  are 
  here 
  given. 
  Clay-slate 
  is 
  found 
  at 
  least 
  in 
  one 
  locality 
  

  

  