﻿354 
  DE. 
  F. 
  H. 
  HATCH 
  OX 
  THE 
  CHAEACTEES 
  OE 
  EOCKS 
  

  

  Bohemia 
  (Boricky), 
  Cape 
  Yerd 
  (Dolter) 
  and, 
  quite 
  recently, 
  from 
  

   Kilima-njaro 
  (Hyland). 
  

  

  For 
  those 
  rich 
  in 
  olivine 
  Rosenbusch 
  has 
  used 
  the 
  term 
  lhnburgite 
  ; 
  

   while 
  the 
  olivine-frec 
  type 
  has 
  been 
  designated 
  augitite 
  by 
  Dolter. 
  

   The 
  rock 
  from 
  Ambohipolo 
  lies 
  between 
  the 
  two, 
  since 
  olivine 
  is 
  

   present 
  but 
  in 
  very 
  small 
  quantity. 
  A 
  type 
  more 
  nearly 
  allied 
  to 
  

   the 
  limburgites 
  occurs 
  near 
  the 
  mountain 
  of 
  Yavavato. 
  This 
  rock 
  

   consists 
  of 
  a 
  ground-mass 
  composed 
  of 
  glass, 
  augite, 
  and 
  magnetite, 
  

   in 
  which 
  are 
  imbedded 
  numerous 
  porphyritic 
  crystals 
  of 
  perfectly 
  

   fresh 
  olivine. 
  

  

  Fig. 
  13. 
  — 
  Trichites 
  in 
  brown 
  Glass 
  of 
  Magma-xisalt. 
  ( 
  x 
  300.) 
  

  

  

  2. 
  Trachytes 
  and 
  Andesites. 
  

  

  Trachytic 
  rocks 
  occur 
  among 
  the 
  volcanic 
  rocks 
  of 
  Madagascar; 
  

   but 
  they 
  are 
  not 
  so 
  well 
  represented 
  as 
  the 
  basalts 
  in 
  Mr. 
  Baron's 
  

   collection. 
  Deserving 
  of 
  mention, 
  however, 
  are 
  specimens 
  of 
  sani- 
  

   dine-trachyte 
  from 
  the 
  top 
  of 
  Ambohitrakoholahy, 
  a 
  peak 
  of 
  

   Ankaratra. 
  

  

  This 
  trachyte 
  is 
  a 
  greyish-white 
  rock, 
  rather 
  compact, 
  but 
  still 
  

   rough 
  to 
  the 
  touch. 
  It 
  contains 
  porphyritic 
  crystals 
  of 
  glassy 
  sanidine 
  

   imbedded 
  in 
  a 
  ground-mass 
  composed 
  almost 
  entirely 
  of 
  minute 
  

   microlites 
  of 
  felspar, 
  the 
  arrangement 
  of 
  which 
  in 
  wavy 
  lines 
  imparts 
  

   to 
  it 
  a 
  marked 
  fluxion-structure. 
  Other 
  constituents 
  are 
  very 
  

   sparingly 
  present 
  : 
  a 
  few 
  small 
  crystals 
  of 
  hornblende 
  and 
  biotite, 
  the 
  

   original 
  substance 
  of 
  which 
  is 
  almost 
  completery 
  replaced 
  by 
  opaque 
  

   iron-ore 
  ; 
  red 
  specks 
  of 
  haematite 
  and 
  disseminated 
  particles 
  of 
  iron- 
  

   ore 
  complete 
  the 
  list. 
  

  

  A 
  columnar 
  trachyte 
  occurs 
  at 
  the 
  south-west 
  foot 
  of 
  Ankaratra. 
  

   This 
  rock 
  differs 
  from 
  the 
  preceding 
  in 
  the 
  nature 
  of 
  the 
  ground-mass, 
  

   which, 
  instead 
  of 
  being 
  microlitic, 
  is 
  composed 
  of 
  minute, 
  depolariz- 
  

   ing 
  particles 
  (felspar) 
  in 
  cryptocrystalline 
  aggregation. 
  In 
  this 
  

   ground-mass 
  are 
  scattered 
  isolated 
  granules 
  of 
  magnetite 
  and 
  small 
  

   prisms 
  of 
  green 
  hornblende. 
  Sanidine 
  occurs 
  in 
  numerous 
  por- 
  

  

  