﻿344 
  DE. 
  F. 
  H. 
  HATCH 
  OX 
  THE 
  CHAEACTEES 
  OE 
  EOCKS 
  

  

  Dark-coloured 
  grains 
  of 
  irregular 
  shape, 
  having 
  a 
  metallic 
  lustre 
  

   in 
  reflected 
  light, 
  but 
  which 
  are 
  translucent 
  with 
  a 
  deep 
  green 
  

   colour, 
  belong 
  to 
  the 
  spinel-group 
  (pleonaste 
  or 
  hercynite). 
  

  

  4. 
  Pyeoxexe-geaxtjlite 
  (Lehmann) 
  ; 
  Teap-geanttlite 
  (Naumann). 
  

  

  Specimens 
  of 
  this 
  rock 
  were 
  obtained 
  by 
  Mr. 
  Baron 
  from 
  the 
  vil- 
  

   lage 
  of 
  Ambohibao, 
  four 
  or 
  five 
  miles 
  N.W. 
  of 
  Antananarivo, 
  where 
  

   it 
  occurs 
  in 
  masses 
  in 
  the 
  gneiss. 
  It 
  is 
  a 
  dark-coloured 
  crystalline 
  

   and 
  granular 
  aggregate, 
  in 
  which 
  glassy, 
  striated 
  felspar, 
  red 
  garnet, 
  

   and 
  a 
  predominant 
  jet-black 
  material 
  can 
  be 
  distinguished 
  with 
  the 
  

   unaided 
  eye. 
  jS"o 
  parallel 
  structure 
  is 
  visible 
  in 
  the 
  hand-specimen. 
  

   In 
  both 
  macroscopical 
  and 
  microscopical 
  characters 
  it 
  is 
  very 
  similar 
  

   to 
  the 
  pyroxene-granulites 
  that 
  occur 
  in 
  such 
  abundance 
  among 
  the 
  

   metamorphic 
  rocks 
  of 
  Saxony 
  *, 
  Brittany 
  f, 
  and 
  Sutherlandshire 
  %. 
  

   It 
  is 
  also 
  related 
  to 
  the 
  hypersthene-gabbros 
  of 
  Baltimore 
  described 
  

   by 
  Gr. 
  H. 
  Williams 
  § 
  ; 
  these, 
  however, 
  contain 
  no 
  garnet. 
  True 
  

   pyroxene-granulites 
  with 
  garnet 
  occur, 
  however, 
  according 
  to 
  Wil- 
  

   liams, 
  at 
  Claymont, 
  Del., 
  U.S. 
  

  

  Under 
  the 
  microscope 
  the 
  following 
  minerals 
  were 
  detected 
  : 
  — 
  

   plagioclase, 
  pyroxene, 
  hornblende, 
  garnet, 
  and 
  iron-ore. 
  These 
  

   minerals 
  form 
  a 
  holo-crystalline, 
  granulitic 
  aggregate, 
  in 
  which 
  

   granules 
  of 
  one 
  and 
  the 
  same 
  mineral 
  show 
  a 
  tendency 
  to 
  group 
  

   together 
  ; 
  in 
  all 
  cases 
  the 
  structure 
  is 
  allotriomorphic, 
  the 
  boun- 
  

   daries 
  of 
  each 
  mineral 
  being 
  formed 
  by 
  those 
  of 
  its 
  neighbours. 
  The 
  

   felspar 
  is 
  a 
  plagioclase 
  of 
  ideal 
  freshness 
  and 
  perfect 
  clearness. 
  It 
  

   presents 
  well-developed 
  twin-striation 
  (on 
  both 
  the 
  albite- 
  and 
  the 
  

   pericline-types) 
  and 
  high 
  extinction-angles, 
  and 
  probably 
  belongs 
  to 
  

   the 
  labradorite 
  series. 
  

  

  The 
  predominating 
  mineral 
  is 
  pyroxene. 
  It 
  occurs 
  usually 
  in 
  

   rounded 
  grains, 
  which 
  occasionally 
  enclose 
  lath-shaped 
  portions 
  of 
  

   the 
  felspar. 
  It 
  rarely 
  presents 
  crystalline 
  contours. 
  On 
  rotation 
  

   of 
  the 
  section 
  over 
  the 
  polarizer 
  the 
  greater 
  proportion 
  of 
  the 
  grains 
  

   are 
  found 
  to 
  be 
  strongly 
  pleochroic 
  (a 
  and 
  /3 
  = 
  red 
  tints; 
  y=pale 
  

   green) 
  ; 
  but 
  a 
  considerable 
  number 
  remain 
  of 
  an 
  unvarying 
  grass- 
  

   green 
  colour. 
  The 
  latter 
  are 
  certainly 
  a 
  monoclinic 
  pyroxene 
  — 
  

   omphacite 
  or 
  diallage 
  ; 
  while 
  there 
  is 
  very 
  little 
  doubt 
  that 
  the 
  

   pleochroic 
  mineral 
  is 
  hypersthene. 
  It 
  exactly 
  resembles, 
  for 
  instance, 
  

   the 
  rhombic 
  pyroxene 
  of 
  the 
  Baltimore 
  gabbros, 
  which 
  has 
  been 
  

   isolated 
  and 
  submitted 
  to 
  analysis 
  by 
  Williams 
  ||. 
  

  

  The 
  hornblende 
  is 
  a 
  dark-coloured, 
  greenish-brown 
  variety 
  occur- 
  

   ring 
  in 
  isolated, 
  irregularly 
  shaped 
  grains. 
  The 
  pleochroism 
  is 
  as 
  

   follows: 
  — 
  a=bright 
  straw-yellow; 
  /3=dark 
  greenish 
  brown 
  ; 
  y= 
  

   deep 
  brownish 
  green. 
  

  

  * 
  E. 
  Datke, 
  " 
  Die 
  Diallaggranulite 
  der 
  sacksiscken 
  Granulitformation," 
  

   Zeitschr. 
  d. 
  deutschen 
  geol. 
  Ges., 
  1877, 
  p. 
  274; 
  J. 
  Lekniann, 
  "Die 
  Entstekung 
  

   der 
  altkrystallinischen 
  Sckiefergesteine,' 
  Bonn, 
  1884, 
  p. 
  228. 
  

  

  t 
  Ch. 
  Barrois, 
  " 
  Les 
  Pyroxenites 
  des 
  lies 
  du 
  Morbihan," 
  Ann. 
  Soc. 
  Geol. 
  du 
  

   Nord, 
  xv. 
  1887, 
  p. 
  69. 
  

  

  \ 
  " 
  Eecent 
  Work 
  of 
  the 
  Geol. 
  Survey 
  in 
  the 
  N.W. 
  Higklands," 
  Q. 
  J. 
  G. 
  S. 
  

   1888, 
  p. 
  388. 
  

  

  § 
  Bull. 
  U.S. 
  Geol. 
  Survey, 
  No. 
  28 
  (1886). 
  || 
  Loc. 
  cit 
  

  

  