﻿COLLECTED 
  IN 
  MADAGASCAR 
  BY 
  REV. 
  R. 
  BARON. 
  355 
  

  

  phyritic 
  crystals. 
  The 
  presence 
  of 
  hornblende 
  in 
  the 
  original 
  magma 
  

   is 
  marked 
  by 
  pseudomorphs 
  of 
  iron-ore. 
  

  

  Andesitic 
  lavas 
  occur 
  at 
  Andranonatoha 
  and 
  on 
  a 
  hill 
  in 
  the 
  

   volcanic 
  region 
  to 
  the 
  west 
  of 
  Lake 
  Itasy. 
  They 
  are 
  of 
  a 
  darker 
  

   colour 
  than 
  the 
  trachytes, 
  namely, 
  greyish 
  brown. 
  They 
  contain 
  

   striated 
  felspar, 
  which 
  generally 
  gives 
  lath-shaped 
  sections, 
  but 
  also 
  

   occurs 
  in 
  large 
  irregular 
  masses, 
  imbedded 
  in 
  a 
  ground-mass 
  com- 
  

   posed 
  of 
  prisms 
  of 
  yellowish-green 
  augite, 
  granules 
  of 
  magnetite, 
  

   and 
  microlites 
  of 
  felspar. 
  A 
  glassy 
  base 
  does 
  not 
  appear 
  to 
  be 
  

   present, 
  unless 
  it 
  be 
  as 
  extremely 
  thin 
  films 
  between 
  the 
  felspars 
  of 
  

   the 
  ground-mass. 
  Large 
  porphyritic 
  crystals 
  of 
  hornblende 
  floated 
  

   in 
  the 
  molten 
  magma 
  when 
  it 
  was 
  first 
  erupted 
  : 
  but 
  they 
  have 
  

   been 
  converted 
  into 
  granules 
  of 
  iron-ore 
  ; 
  and 
  only 
  small 
  fragments 
  

   of 
  the 
  original 
  hornblende-substance 
  remain 
  to 
  mark 
  its 
  former 
  

   presence. 
  

  

  To 
  summarize 
  briefly, 
  we 
  have 
  found 
  the 
  older 
  crystalline 
  series 
  

   of 
  Madagascar, 
  as 
  represented 
  by 
  Mr. 
  Baron's 
  collection, 
  to 
  consist 
  

   of 
  foliated 
  rocks 
  (which 
  are 
  described 
  as 
  granitite-gneiss 
  and 
  tonalite- 
  

   gneiss), 
  and 
  rocks 
  in 
  which 
  no 
  trace 
  of 
  foliation 
  can 
  be 
  detected, 
  

   comprising 
  granite, 
  olivine-norite,pyroxene-granulite, 
  and 
  pyroxenite. 
  

   The 
  majority 
  of 
  the 
  granites 
  are 
  of 
  the 
  granitite 
  type, 
  but 
  true 
  granite 
  

   (i. 
  e. 
  granite 
  with 
  tivo 
  micas) 
  also 
  occurs. 
  The 
  basic 
  members 
  of 
  the 
  

   unfoliated 
  series 
  are 
  interesting 
  on 
  account 
  of 
  their 
  striking 
  mineral 
  

   combinations. 
  The 
  clear 
  aspect 
  and 
  bright 
  colouring 
  of 
  their 
  con- 
  

   stituent 
  grains 
  often 
  make 
  them 
  objects 
  of 
  surpassing 
  beauty 
  when 
  

   viewed 
  under 
  the 
  microscope. 
  Of 
  deeper 
  interest 
  is 
  the 
  fact 
  that 
  

   these 
  basic 
  types, 
  so 
  well 
  known 
  in 
  other 
  areas 
  of 
  crystalline 
  schists, 
  

   — 
  in 
  Saxony, 
  Brittany, 
  Scandinavia, 
  Scotland, 
  and 
  on 
  the 
  Hudson 
  

   Biver 
  — 
  constitute 
  in 
  Madagascar, 
  as 
  at 
  Kilama-njaro 
  on 
  the 
  adjacent 
  

   mainland, 
  so 
  large 
  a 
  part 
  of 
  the 
  ancient 
  floor 
  on 
  which 
  the 
  sedimentary 
  

   rocks 
  were 
  laid 
  down 
  and 
  through 
  which 
  the 
  volcanic 
  lavas 
  were 
  

   erupted. 
  

  

  The 
  volcanic 
  rocks 
  consist 
  mainly 
  of 
  basaltic 
  types, 
  only 
  a 
  few 
  

   specimens 
  of 
  trachyte 
  and 
  andesite 
  being 
  represented 
  in 
  Mr. 
  Baron's 
  

   collection. 
  The 
  basalts 
  vary, 
  as 
  regards 
  composition, 
  Avith 
  respect 
  

   to 
  the 
  presence 
  or 
  absence 
  of 
  quartz, 
  olivine, 
  porphyritic 
  and 
  micro- 
  

   litic 
  hornblende, 
  and 
  biotite. 
  One 
  curious 
  type 
  contains 
  idio- 
  

   morphic 
  crystals 
  of 
  hornblende 
  as 
  a 
  constituent 
  of 
  the 
  ground- 
  

   mass. 
  A 
  felspar-free 
  variety 
  or 
  mag 
  ma-basalt 
  is 
  also 
  represented. 
  

   This 
  rock 
  contains 
  only 
  a 
  small 
  quantity 
  of 
  olivine, 
  and 
  is 
  therefore 
  

   intermediate 
  in 
  composition 
  between 
  the 
  limburgite 
  of 
  Kosenbusch 
  

   and 
  the 
  augitite 
  of 
  Dolter. 
  

  

  