﻿COLLECTED 
  IN 
  HADAGASCAE 
  BY 
  EEV. 
  E. 
  BAEON. 
  341 
  

  

  and, 
  adopting 
  the 
  method 
  of 
  nomenclature 
  used 
  by 
  Mr. 
  Teall 
  *, 
  I 
  

   propose 
  to 
  call 
  it 
  Granitite-gneiss. 
  Accessory 
  constituents 
  of 
  this 
  

   rock 
  are 
  iron-ore 
  and 
  zircon, 
  the 
  latter 
  occurring 
  in 
  minute 
  prisms 
  

   imbedded 
  in 
  the 
  felspar. 
  

  

  A 
  very 
  fine-grained 
  light-coloured 
  gneiss, 
  belonging 
  to 
  this 
  series, 
  

   occurs 
  near 
  the 
  mountains 
  of 
  Vombohitra, 
  eighty 
  miles 
  north 
  of 
  

   the 
  capital. 
  In 
  addition 
  to 
  orthoclase 
  and 
  plagioclase, 
  this 
  rock 
  

   contains 
  microcline, 
  distinguishable 
  by 
  its 
  characteristic 
  reticu- 
  

   lated 
  structure. 
  

  

  Other 
  specimens 
  present 
  a 
  well-marked 
  banded 
  structure. 
  A 
  gneiss 
  

   of 
  this 
  character 
  occurs 
  at 
  a 
  place 
  to 
  the 
  north-east 
  of 
  Lake 
  ltasy; 
  

   it 
  is 
  composed 
  of 
  alternating 
  light- 
  and 
  dark-coloured 
  layers, 
  the 
  

   former 
  consisting 
  mainly 
  of 
  quartz 
  and 
  felspar 
  (microcline, 
  ortho- 
  

   clase, 
  and 
  a 
  little 
  oligoclase), 
  the 
  latter 
  being 
  rich 
  in 
  biotite. 
  This 
  

   rock 
  contains 
  apatite 
  as 
  an 
  accessory 
  constituent. 
  Another 
  banded 
  

   rock 
  occurs 
  at 
  the 
  village 
  of 
  Ambohidratrimo, 
  ten 
  miles 
  N.W. 
  of 
  the 
  

   capital. 
  Small 
  red 
  garnets 
  occur 
  in 
  this 
  rock. 
  

  

  b. 
  Basic 
  Series 
  (Tonalite- 
  Gneiss). 
  

  

  A 
  dark-coloured 
  gneiss, 
  from 
  Antobj^, 
  near 
  the 
  mountain 
  Vavavato, 
  

   seventy 
  miles 
  S.W. 
  of 
  the 
  capital, 
  may 
  be 
  taken 
  as 
  a 
  type 
  of 
  this 
  

   series. 
  It 
  is 
  a 
  well 
  foliated 
  rock, 
  consisting 
  of 
  lenticular 
  layers 
  of 
  

   black 
  ferro-magnesian 
  minerals 
  alternating 
  with 
  thinner 
  bands 
  of 
  

   quartz 
  and 
  a 
  striated 
  felspar. 
  

  

  Hornblende 
  occurs 
  in 
  large 
  plates 
  and 
  in 
  small 
  irregular 
  flakes. 
  

   Its 
  pleochroism 
  is 
  as 
  follows 
  : 
  — 
  

  

  af= 
  straw-yellow; 
  

   ft 
  == 
  rich 
  grass-green 
  ; 
  

   y 
  = 
  bluish 
  green. 
  

  

  y>/3>a. 
  

   Maximum 
  extinction-angle 
  = 
  15°. 
  

  

  Associated 
  with 
  the 
  hornblende 
  is 
  a 
  uniaxial 
  brown 
  mica, 
  occur- 
  

   ring 
  in 
  thin 
  plates. 
  This 
  mica 
  is 
  strongly 
  pleochroic 
  ; 
  rays 
  vibrating 
  

   parallel 
  to 
  a 
  being 
  pale 
  yellow, 
  those 
  parallel 
  to 
  (3 
  and 
  y, 
  blackish 
  

   brown. 
  

  

  Magnetite 
  and 
  zircon 
  occur 
  as 
  accessories. 
  

  

  The 
  effects 
  of 
  mechanical 
  metamorphism 
  may 
  be 
  traced 
  in 
  this 
  

   rock 
  in 
  a 
  mosaic-like 
  aggregation 
  of 
  granules 
  of 
  secondary 
  quartz 
  

   and 
  felspar, 
  surrounding 
  the 
  primary 
  felspar-crystals, 
  and 
  con- 
  

   trasting 
  with 
  them 
  by 
  their 
  greater 
  freshness 
  and 
  pellucidity. 
  

  

  2. 
  Geanite. 
  

  

  The 
  specimens 
  of 
  granite 
  collected 
  by 
  Mr. 
  Baron 
  belong 
  chiefly 
  to 
  

   the 
  granitite 
  type, 
  that 
  is 
  to 
  say, 
  they 
  contain 
  only 
  dark 
  mica. 
  

   Only 
  one 
  specimen 
  (from 
  the 
  mountain 
  Yombohitra, 
  eighty 
  miles 
  N. 
  

  

  * 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xliv. 
  1888, 
  p. 
  314. 
  

   t 
  Axes 
  of 
  elasticity. 
  

  

  