﻿342 
  DE. 
  E. 
  H. 
  HATCH 
  ON 
  THE 
  CHAEACTEES 
  OF 
  EOCXS 
  

  

  of 
  the 
  capital, 
  a 
  boss 
  protruding 
  through 
  gneiss) 
  was 
  found 
  to 
  be 
  

   granite 
  with 
  two 
  micas. 
  This 
  rock 
  is 
  a 
  moderately 
  coarse-grained 
  

   aggregate 
  of 
  grey 
  translucent 
  quartz, 
  pink 
  orthoclase, 
  microcline 
  

   and 
  oligoclase, 
  together 
  with 
  a 
  small 
  quantity 
  of 
  muscovite 
  and 
  

   biotite, 
  the 
  latter 
  altering 
  to 
  chlorite. 
  Magnetic 
  iron-ore 
  occurs 
  as 
  

   an 
  accessory 
  constituent. 
  

  

  Granitites 
  occur 
  at 
  the 
  following 
  localities 
  : 
  — 
  a 
  place 
  from 
  10 
  to 
  15 
  

   miles 
  N.N.W. 
  of 
  the 
  town 
  of 
  Mandritsara 
  (220 
  miles 
  K 
  of 
  the 
  

   capital); 
  near 
  the 
  mountain 
  Votovorona, 
  in 
  the 
  district 
  Vakin' 
  

   Ankaratra, 
  immediately 
  S.W. 
  of 
  the 
  capital 
  ; 
  and, 
  as 
  a 
  dyke, 
  near 
  

   the 
  village 
  of 
  Ambohipiara, 
  12 
  miles 
  1ST.W. 
  of 
  the 
  capital. 
  

  

  These 
  are 
  all 
  medium-grained 
  rocks, 
  composed 
  of 
  pale 
  grey 
  quartz 
  

   and 
  reddish 
  felspar, 
  with 
  interspersed 
  lustrous 
  plates 
  of 
  black 
  

   mica. 
  

  

  With 
  the 
  aid 
  of 
  the 
  microscope 
  the 
  following 
  felspars 
  were 
  

   distinguished 
  : 
  — 
  orthoclase, 
  containing 
  included 
  patches 
  and 
  lamellae 
  

   of 
  triclinic 
  felspar 
  (microperthite), 
  microcline 
  and 
  oligoclase. 
  The 
  

   mica 
  is 
  a 
  dark, 
  small-angled 
  variety, 
  pleochroic 
  in 
  the 
  following 
  

   tints: 
  a=pale 
  yellow,/? 
  and 
  y=greenish 
  brown 
  to 
  vandyke-brown. 
  

   It 
  is 
  rich 
  in 
  inclusions, 
  in 
  the 
  neighbourhood 
  of 
  which 
  the 
  pleo- 
  

   chroism 
  is 
  much 
  intensified 
  (" 
  pleochroic 
  borders 
  " 
  or 
  " 
  halos 
  "). 
  

   Among 
  such 
  inclusions 
  were 
  observed 
  granules 
  of 
  quartz, 
  needles 
  

   of 
  apatite, 
  sphene 
  and 
  minute 
  crystals 
  of 
  zircon. 
  Interesting 
  as 
  

   showing 
  that 
  mica 
  is 
  sometimes 
  of 
  later 
  formation 
  than 
  both 
  quartz 
  

   and 
  felspar 
  is 
  the 
  fact 
  that 
  the 
  former 
  is 
  sometimes 
  found 
  in 
  allotrio- 
  

   niorphic 
  plates 
  (in 
  the 
  rock 
  from 
  the 
  first-mentioned 
  locality) 
  filling 
  

   in 
  the 
  interspaces 
  between 
  these 
  minerals, 
  and 
  retaining 
  uniform 
  

   optic 
  orientation 
  over 
  considerable 
  areas. 
  

  

  The 
  specimen 
  from 
  Ambohipiara 
  contains 
  a 
  green 
  hornblende 
  in 
  

   addition 
  to 
  the 
  mica 
  (hornblende-granitite). 
  

  

  Sphene 
  is 
  the 
  most 
  abundant 
  accessory 
  constituent 
  in 
  these 
  rocks. 
  

   It 
  occurs 
  in 
  rather 
  large, 
  slightly 
  rounded 
  grains 
  of 
  a 
  pale 
  brown 
  

   colour, 
  and 
  contains 
  included 
  granules 
  of 
  felspar, 
  proving 
  that 
  sphene 
  

   (one 
  of 
  the 
  earliest 
  minerals 
  produced 
  in 
  consolidation) 
  was 
  still 
  

   separating 
  from 
  the 
  magma 
  when 
  the 
  felspar 
  began 
  to 
  form. 
  

  

  3. 
  Olivine-noeite 
  (Rosenbusch) 
  ; 
  Hypeeite 
  (Tornebohm). 
  

  

  This 
  rock 
  occurs, 
  according 
  to 
  Mr. 
  Baron, 
  in 
  large 
  hills, 
  protru- 
  

   ding 
  through 
  the 
  gneissose 
  rocks 
  on 
  the 
  north-east 
  border 
  of 
  the 
  

   plain 
  of 
  Antsihanaka, 
  immediately 
  north 
  of 
  the 
  town 
  of 
  Ampara- 
  

   faravola, 
  about 
  110 
  miles 
  N.E. 
  of 
  the 
  capital. 
  

  

  It 
  is 
  a 
  dark-coloured 
  rock 
  of 
  granitoid 
  texture, 
  remarkable, 
  in 
  

   the 
  hand-specimen, 
  for 
  the 
  vitreous 
  lustre 
  of 
  its 
  perfectly 
  trans- 
  

   parent 
  felspar. 
  It 
  is 
  composed 
  of 
  the 
  following 
  minerals 
  : 
  — 
  plagio- 
  

   clase, 
  hypersthene, 
  olivine, 
  brown 
  hornblende, 
  and 
  green 
  spinel. 
  

  

  The 
  plagioclase 
  occurs 
  in 
  allotriomorphic 
  grains 
  of 
  varying 
  size. 
  

   It 
  is 
  twinned 
  poly 
  synthetically, 
  on 
  both 
  albite- 
  and 
  pericline-types. 
  

   The 
  curvature 
  of 
  the 
  twin-lamellse, 
  which 
  is 
  accompanied 
  by 
  an 
  

   increase 
  in 
  the 
  amount 
  of 
  striation 
  and 
  a 
  marked 
  tl 
  undulose 
  " 
  

  

  