﻿COLLECTED 
  IN 
  MADAGASCAR 
  BY 
  EEV. 
  R. 
  BARON. 
  343 
  

  

  extinction, 
  is 
  probably 
  an 
  effect 
  of 
  dynamic 
  metamorphism. 
  The 
  

   extinction-angles 
  indicate 
  a 
  basic 
  felspar 
  (labradorite 
  or 
  anorthite). 
  

   Inclusions 
  of 
  gas 
  and 
  liquid 
  are 
  abundant 
  ; 
  they 
  are 
  arranged 
  along 
  

   planes 
  which 
  intersect 
  at 
  varying 
  angles 
  and 
  bear 
  no 
  apparent 
  

   morphological 
  relation 
  to 
  the 
  enclosing 
  crystal. 
  Besides 
  these 
  inclu- 
  

   sions, 
  which 
  are, 
  comparatively 
  speaking, 
  large, 
  there 
  is 
  also 
  present 
  

   in 
  the 
  felspar 
  a 
  fine 
  dusty 
  material 
  which, 
  under 
  the 
  highest 
  powers, 
  

   is 
  incapable 
  of 
  distinct 
  resolution 
  and, 
  under 
  low 
  powers, 
  produces 
  a 
  

   slight 
  turbidity. 
  These 
  minute 
  bodies 
  have 
  every 
  appearance 
  of 
  

   being 
  original, 
  and 
  the 
  enclosing 
  felspar 
  is 
  perfectly 
  fresh 
  and 
  un- 
  

   altered, 
  Similar 
  dust-like 
  inclusions 
  in 
  felspar 
  have 
  been 
  described 
  

   by 
  numerous 
  authors, 
  recently 
  by 
  G. 
  H. 
  Williams* 
  in 
  the 
  hyper- 
  

   sthene-gabbro 
  of 
  Baltimore. 
  

  

  The 
  pyroxene 
  occurs 
  in 
  irregular 
  masses, 
  wedged 
  in 
  between 
  the 
  

   felspar-grains. 
  Since 
  it 
  presents 
  no 
  crystalline 
  contours, 
  exact 
  

   optical 
  determination 
  is 
  impossible, 
  but 
  its 
  properties 
  are 
  those 
  cha- 
  

   racteristic 
  of 
  hypersthene. 
  It 
  possesses, 
  for 
  instance, 
  the 
  strongly 
  

   marked 
  pleochroism 
  peculiar 
  to 
  this 
  mineral 
  (reddish 
  brown 
  to 
  pale 
  

   sea-green), 
  and 
  exhibits 
  also, 
  in 
  places, 
  an 
  accumulation 
  of 
  thin 
  

   brown 
  plates, 
  the 
  orientation 
  of 
  which 
  along 
  definite 
  crystallogra- 
  

   phic 
  planes 
  determines 
  the 
  metallic 
  sheen 
  which 
  distinguishes 
  

   the 
  hypersthene 
  of 
  plutonic 
  rocks 
  from 
  that 
  occurring 
  in 
  volcanic 
  

   lavas. 
  In 
  certain 
  spots 
  in 
  the 
  crystals 
  the 
  development 
  of 
  plate-like 
  

   inclusions 
  has 
  been 
  so 
  great 
  as 
  to 
  produce 
  a 
  fibrous 
  appearance. 
  In 
  

   these 
  places 
  the 
  pleochroism 
  is 
  also 
  considerably 
  intensified. 
  

  

  Brown 
  hornblende 
  is 
  occasionally 
  associated 
  with 
  the 
  pyroxene. 
  

   This 
  mineral 
  forms 
  irregularly 
  contoured 
  masses, 
  which 
  present 
  a 
  

   well-developed 
  prismatic 
  cleavage 
  and 
  contain 
  inclusions 
  of 
  iron-ore. 
  

   Its 
  pleochroism 
  is 
  strong, 
  rays 
  vibrating 
  parallel 
  to 
  a 
  being 
  pale 
  

   yellow, 
  those 
  parallel 
  to 
  /3 
  a 
  rich 
  reddish 
  brown. 
  

  

  The 
  olivine 
  is 
  quite 
  fresh, 
  with 
  the 
  exception 
  that 
  along 
  the 
  

   cleavage- 
  cracks 
  it 
  is 
  stained 
  with 
  hydrated 
  oxide 
  of 
  iron. 
  Some 
  

   of 
  the 
  prisms 
  are 
  packed 
  with 
  granular 
  and 
  rod-like 
  inclusions, 
  

   showing 
  a 
  parallel 
  arrangement. 
  Occasionally 
  the 
  olivine-grains 
  

   present 
  a 
  double 
  zoning, 
  the 
  innermost 
  layer 
  being 
  hypersthene, 
  

   succeeded 
  by 
  a 
  fringe 
  of 
  a 
  pale 
  green, 
  fibrous 
  hornblende 
  (actinolite). 
  

   Similar 
  cases 
  of 
  zoning 
  have 
  been 
  described 
  by 
  A. 
  E. 
  Tornebohm 
  f, 
  

   Frank 
  D. 
  Adams 
  J, 
  G. 
  H. 
  Williams 
  §, 
  and 
  J. 
  J. 
  H. 
  Teall 
  ||. 
  

   G.H.Williams 
  calls 
  the 
  zones 
  " 
  reactionary 
  rims," 
  and 
  regards 
  them 
  

   as 
  representing 
  a 
  reaction 
  between 
  the 
  olivine 
  and 
  the 
  felspar, 
  while 
  

   the 
  latter 
  was 
  undergoing 
  crystallization, 
  the 
  amphibole 
  and 
  pyroxene 
  

   being 
  intermediate 
  products. 
  

  

  * 
  The 
  " 
  Gabbros 
  and 
  associated 
  Hornblende-rocks 
  of 
  Baltimore," 
  Bull. 
  U.S. 
  

   Geol. 
  Survey, 
  No. 
  28, 
  1886, 
  p. 
  21. 
  

  

  t 
  " 
  Ueber 
  die 
  wichtigeren 
  Diabas- 
  u. 
  Gabbro-Gesteine 
  Schwedens," 
  Neues 
  

   Jahrb. 
  1877, 
  p. 
  383. 
  

  

  \ 
  " 
  The 
  Anorthosite 
  Kocks 
  of 
  Canada," 
  Brit. 
  Assoc. 
  Eep. 
  March 
  1886, 
  p. 
  666, 
  

   also 
  ' 
  American 
  Naturalist,' 
  Nov. 
  1885, 
  p. 
  1087. 
  

  

  § 
  « 
  Peridotites 
  near 
  Peakskill, 
  N.Y.," 
  Amer. 
  Journ. 
  of 
  Sci. 
  (3) 
  vol. 
  xxxi. 
  (1886) 
  

   p. 
  35. 
  

  

  || 
  British 
  Petrography, 
  p. 
  176. 
  

  

  