﻿COLLECTED 
  IN 
  MADAGASCAR 
  BY 
  REV. 
  R. 
  BARON. 
  345 
  

  

  The 
  most 
  striking- 
  constituent 
  of 
  the 
  rock 
  is 
  the 
  garnet. 
  It 
  occurs 
  

   in 
  irregular 
  grains, 
  which 
  are 
  of 
  much 
  greater 
  size 
  than 
  the 
  remaining 
  

   minerals. 
  In 
  thin 
  section 
  they 
  are 
  pale 
  pink 
  ; 
  hut 
  in 
  the 
  hand- 
  

   specimen 
  they 
  have 
  a 
  bright 
  reddish-brown 
  colour. 
  Inclusions 
  of 
  

   pyroxene 
  and 
  iron-ore 
  are 
  not 
  unfrequent 
  : 
  felspar 
  also 
  occurs, 
  but 
  

   rather 
  more 
  rarely. 
  

  

  5. 
  Diallage-hypeesthene-Rock 
  (Pyroxenite, 
  Dana). 
  

  

  Rocks 
  of 
  this 
  composition 
  occur 
  in 
  the 
  valley 
  at 
  the 
  eastern 
  foot 
  

   of 
  the 
  mountain 
  of 
  Ambiniviny, 
  200 
  miles 
  JN". 
  of 
  the 
  capital, 
  and 
  at 
  a 
  

   place 
  20 
  miles 
  S.E. 
  of 
  the 
  village 
  of 
  Andranosamonta, 
  N". 
  Mada- 
  

   gascar. 
  They 
  are 
  finely 
  granular 
  and 
  noncrystalline 
  rocks 
  and 
  

   composed 
  of 
  green 
  striated 
  monoclinic 
  pyroxene 
  (diallage) 
  and 
  

   a 
  strongly 
  pleochroic 
  rhombic 
  pyroxene 
  (hypersthene). 
  By 
  the 
  

   addition 
  of 
  olivine 
  these 
  rocks 
  would, 
  and 
  doubtless 
  do, 
  pass 
  into 
  

   peridotites 
  and 
  thence 
  into 
  picrites, 
  norites, 
  and 
  gabbros. 
  The 
  

   name 
  pyroxenite, 
  first 
  used 
  by 
  J. 
  R. 
  Dana 
  *, 
  who 
  applied 
  it 
  to 
  rocks 
  

   very 
  similar 
  to 
  the 
  one 
  described, 
  is 
  a 
  very 
  suitable 
  designation 
  

   for 
  this 
  type 
  of 
  rock. 
  Barrois 
  f, 
  however, 
  has 
  used 
  this 
  name 
  to 
  

   designate 
  rocks 
  which 
  he 
  admits 
  to 
  be 
  typical 
  pyroxene-granulites, 
  

   thus 
  creating 
  a 
  needless 
  redundancy 
  in 
  terminology. 
  DolterJ, 
  

   who 
  also 
  adopted 
  the 
  term 
  pyroxenite 
  for 
  a 
  variety 
  of 
  magma- 
  

   basalt 
  (viz. 
  those 
  without 
  olivine), 
  has 
  since, 
  at 
  Rosenbusch's 
  sugges- 
  

   tion, 
  abandoned 
  it 
  in 
  favour 
  of 
  " 
  augitite." 
  

  

  Pyroxenites 
  (Dana), 
  of 
  the 
  Madagascar 
  type, 
  occur 
  as 
  members 
  

   of 
  the 
  Cortlandt 
  series 
  on 
  the 
  Hudson 
  River, 
  near 
  Peekskill, 
  N. 
  Y. 
  §. 
  

   Williams 
  also 
  mentions 
  their 
  occurrence 
  among 
  the 
  Baltimore 
  

   rocks 
  || 
  ; 
  and 
  of 
  these 
  he 
  intends, 
  I 
  believe, 
  soon 
  to 
  publish 
  an 
  account. 
  

   Mr. 
  Teall 
  informs 
  me 
  that 
  these 
  interesting 
  rocks 
  are 
  also 
  found 
  

   in 
  the 
  Gneissose 
  series 
  of 
  the 
  Highlands 
  of 
  Scotland. 
  

  

  II. 
  THE 
  YOLCANIC 
  ROCKS. 
  

  

  The 
  volcanic 
  rocks 
  here 
  to 
  be 
  described 
  have 
  been 
  erupted 
  mainly 
  

   from 
  vents 
  occurring 
  in 
  the 
  older 
  crystalline 
  rocks. 
  These 
  vents 
  are 
  

   very 
  abundant 
  in 
  the 
  near 
  neighbourhood 
  of 
  the 
  high 
  mountain 
  of 
  

   Ankaratra, 
  which 
  is 
  itself 
  of 
  volcanic 
  origin. 
  For 
  instance, 
  in 
  the 
  

   region 
  immediately 
  to 
  the 
  west 
  of 
  Lake 
  Itasy, 
  at 
  the 
  western 
  foot 
  

   of 
  the 
  mountain, 
  there 
  are 
  numerous 
  extinct 
  volcanoes, 
  with 
  craters, 
  

   lava-streams, 
  and 
  all 
  the 
  usual 
  accompaniments 
  of 
  intense 
  volcanic 
  

  

  * 
  " 
  Geol. 
  Relations 
  of 
  the 
  Limestone 
  Belts 
  of 
  Westchester 
  Co., 
  New 
  York," 
  

   Amer. 
  Journ. 
  Sci. 
  (3) 
  xx. 
  (1880) 
  p. 
  197. 
  

  

  t 
  "Les 
  Pyroxenites 
  des 
  lies 
  du 
  Morbihan," 
  Ann. 
  Soc. 
  Geol. 
  du 
  Nord, 
  t. 
  xv. 
  

   (1887) 
  p. 
  69. 
  

  

  \ 
  ' 
  Die 
  Vulcaneder 
  Capverden,' 
  Graz, 
  1882, 
  p. 
  137, 
  and 
  Neues 
  Jahrb. 
  1883, 
  i. 
  

   p. 
  404. 
  

  

  § 
  " 
  On 
  the 
  Norites 
  of 
  the 
  ' 
  Cortlandt 
  Series 
  ' 
  on 
  the 
  Hudson 
  River, 
  near 
  

   Peekskill, 
  N. 
  Y.," 
  Amer. 
  Journ. 
  Sci. 
  (3) 
  xxxiii. 
  (1887) 
  p. 
  194. 
  

  

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

   Geol. 
  Survey, 
  No. 
  28 
  (1886), 
  p. 
  55. 
  

  

  