﻿COLLECTED 
  IN 
  MADAGASCAR 
  BY 
  REV. 
  R. 
  BARON". 
  349 
  

  

  The 
  doleritic 
  type 
  is 
  also 
  represented. 
  Such 
  a 
  rock 
  occurs, 
  for 
  

   instance, 
  at 
  Mojanga 
  on 
  the 
  north-west 
  coast 
  of 
  Madagascar. 
  In 
  

   this 
  rock 
  there 
  is 
  a 
  marked 
  ophitic 
  structure, 
  large 
  allotriomorphic 
  

   grains 
  of 
  augite 
  being 
  penetrated 
  by 
  prisms 
  and 
  microlites 
  of 
  felspar 
  ; 
  

   the 
  rock 
  is 
  nearly 
  noncrystalline, 
  glassy 
  matter 
  being 
  present 
  only 
  

   in 
  thin 
  films 
  between 
  such 
  felspar 
  needles 
  as 
  are 
  not 
  enclosed 
  in 
  

   the 
  augite. 
  

  

  A 
  very 
  curious 
  rock, 
  which, 
  though 
  diverging 
  widely 
  from 
  the 
  

   normal 
  structure 
  of 
  the 
  olivine 
  basalts, 
  may 
  here 
  be 
  conveniently 
  

   described, 
  occurs 
  three 
  or 
  four 
  miles 
  east 
  of 
  the 
  village 
  of 
  Eeno- 
  

   arivo, 
  in 
  the 
  district 
  of 
  Yalalafotsy, 
  60 
  miles 
  west 
  of 
  the 
  capital. 
  

  

  It 
  is 
  a 
  compact 
  rock 
  composed 
  of 
  augite, 
  felspar, 
  olivine, 
  and 
  

   magnetite. 
  

  

  The 
  most 
  abundant 
  constituent 
  is 
  augite. 
  This 
  mineral 
  occurs 
  

   in 
  small, 
  sharply 
  contoured, 
  prismatic 
  crystals 
  of 
  a 
  pale 
  brown 
  colour. 
  

   Under 
  the 
  microscope 
  the 
  crystals 
  give 
  lath-shaped 
  sections 
  with 
  

   pyramidal 
  terminations. 
  Twinning 
  parallel 
  to 
  (100) 
  is 
  frequent. 
  

   In 
  several 
  cases 
  it 
  is 
  polysynthetic, 
  as 
  in 
  the 
  triclinic 
  felspars. 
  The 
  

   extinction-angle 
  has 
  a 
  maximum 
  value 
  of 
  about 
  42°. 
  

  

  These 
  little 
  crystals 
  of 
  augite 
  are 
  imbedded 
  in 
  a 
  matrix 
  of 
  felspar. 
  

   The 
  latter 
  presents 
  no 
  trace 
  of 
  twin-striation. 
  In 
  places 
  it 
  is 
  slightly 
  

   kaolinized. 
  The 
  relation 
  between 
  the 
  felspar 
  and 
  augite 
  may 
  be 
  said 
  

   to 
  be 
  " 
  ophitic 
  ;" 
  but 
  it 
  differs 
  from 
  that 
  structure, 
  as 
  represented 
  in 
  

   the 
  dolerites, 
  in 
  that 
  in 
  the 
  case 
  under 
  consideration 
  the 
  felspar 
  

   is 
  moulded 
  upon 
  idiomorphic 
  crystals 
  of 
  augite, 
  instead 
  of 
  augite 
  

   upon 
  felspar, 
  as 
  in 
  the 
  dolerites. 
  Similar 
  relations 
  between 
  felspar 
  

   and 
  augite 
  have 
  been 
  described 
  by 
  Rosenbusch 
  * 
  in 
  reference 
  to 
  

   vogesites, 
  and 
  by 
  Teall 
  f 
  in 
  certain 
  Scotch 
  traps. 
  

  

  The 
  order 
  of 
  consolidation 
  in 
  the 
  Madagascar 
  rock 
  appears 
  to 
  

   have 
  been 
  as 
  follows 
  : 
  — 
  

  

  f 
  Magnetite. 
  

   1st 
  phase 
  -j 
  Olivine. 
  2nd 
  phase 
  . 
  . 
  Felspar. 
  

  

  [ 
  Augite. 
  

  

  b. 
  Olivine- 
  Basalt 
  with 
  Hornblende 
  and 
  Biotite 
  in 
  PorpTiyritic 
  

   Crystals, 
  (ffoniblende-olivine-basalt.) 
  

  

  A 
  rock 
  of 
  this 
  composition 
  occurs 
  in 
  the 
  volcanic 
  region 
  west 
  of 
  

   Lake 
  Itasy. 
  Its 
  predominant 
  constituent 
  is 
  augite, 
  occurring 
  in 
  

   large 
  black 
  crystals, 
  imbedded 
  in 
  a 
  slightly 
  vesicular 
  ground-mass, 
  

   the 
  latter 
  being 
  made 
  up 
  of 
  microlites 
  of 
  felspar, 
  granules 
  of 
  olivine, 
  

   augite 
  and 
  iron-ore, 
  together 
  with 
  a 
  small 
  quantity 
  of 
  interstitial 
  

   glass. 
  

  

  Under 
  the 
  microscope 
  the 
  augite 
  appears 
  sometimes 
  in 
  irregular 
  

   masses, 
  sometimes 
  in 
  characteristic, 
  octagonal 
  sections, 
  bounded 
  by 
  the 
  

   faces 
  (110,100,010). 
  Included 
  material 
  is 
  abundant, 
  namely: 
  — 
  glass, 
  

   magnetite, 
  olivine, 
  and, 
  more 
  rarely, 
  a 
  microlite 
  of 
  brown 
  hornblende. 
  

  

  * 
  Die 
  massigen 
  Gesteine, 
  1887, 
  p. 
  316. 
  

  

  t 
  British 
  Petrography, 
  1888, 
  pp. 
  188, 
  194, 
  214. 
  

  

  