﻿340 
  DE. 
  F. 
  H. 
  HATCH 
  ON" 
  THE 
  CHAEACTEES 
  OP 
  EOCKS 
  

  

  19. 
  Notes 
  on 
  the 
  Peteogeaphical 
  Chaeactees 
  of 
  some 
  Eoces 
  collected 
  

   in 
  Madagascae 
  by 
  the 
  Bev. 
  E. 
  Baeon. 
  By 
  Frederick 
  H. 
  Hatch, 
  

   Ph.D., 
  F.G.S. 
  (Read 
  March 
  6, 
  1889.) 
  

  

  The 
  rock-specimens 
  here 
  to 
  be 
  described 
  were 
  collected 
  by 
  the 
  Rev. 
  

   E. 
  Baron 
  during 
  his 
  extensive 
  travels 
  through 
  Madagascar. 
  They 
  

   do 
  not 
  by 
  any 
  means 
  represent 
  the 
  whole 
  of 
  his 
  collection 
  ; 
  hut 
  care 
  

   has 
  been 
  taken 
  to 
  make 
  the 
  collection 
  include 
  all 
  the 
  more 
  impor- 
  

   tant 
  types 
  of 
  crystalline 
  and 
  volcanic 
  rocks. 
  

  

  The 
  paper 
  is 
  divided 
  into 
  two 
  parts, 
  the 
  first 
  dealing 
  with 
  the 
  

   older 
  crystalline 
  rocks, 
  and 
  the 
  second 
  with 
  the 
  comparatively 
  recent 
  

   volcanic 
  eruptions. 
  

  

  I. 
  THE 
  OLDEE 
  CEYSTALLINE 
  EOCKS. 
  

  

  The 
  mountainous 
  portion 
  of 
  Madagascar, 
  extending 
  from 
  north 
  to 
  

   south 
  through 
  the 
  eastern 
  half 
  of 
  the 
  island, 
  is 
  a 
  ridge 
  of 
  old 
  crys- 
  

   talline 
  rocks, 
  on 
  the 
  western 
  flanks 
  of 
  which 
  sedimentary 
  formations 
  

   have 
  been 
  deposited, 
  and 
  through 
  which 
  volcanic 
  rocks 
  have 
  been 
  

   erupted. 
  Mr. 
  Baron's 
  collection 
  comprises 
  both 
  foliated 
  rocks 
  

   (gneisses) 
  and 
  rocks 
  in 
  which 
  there 
  is 
  no 
  parallel 
  structure 
  visible 
  

   in 
  the 
  hand-specimen 
  (granite, 
  gabbro 
  or 
  norite, 
  pyroxene-granulite, 
  

   and 
  pyroxenite). 
  

  

  In 
  the 
  following 
  pages 
  the 
  petrographical 
  characters 
  of 
  these 
  rocks 
  

   will 
  be 
  described 
  briefly, 
  the 
  order 
  being 
  as 
  here 
  given. 
  

  

  1. 
  Giraiss. 
  

  

  Considered 
  petrographically, 
  the 
  specimens 
  of 
  gneiss, 
  collected 
  by 
  

   Mr. 
  Baron, 
  may 
  he 
  divided 
  into 
  an 
  acid 
  and 
  a 
  basic 
  series, 
  the 
  former 
  

   being 
  characterized 
  by 
  the 
  presence 
  of 
  abundant 
  quartz 
  with 
  ortho- 
  

   clase 
  as 
  the 
  dominant 
  felspar, 
  the 
  latter 
  by 
  the 
  subordination 
  of 
  the 
  

   quartz 
  and 
  the 
  predominance 
  of 
  plagicclase 
  felspar. 
  

  

  a. 
  Acid 
  Series 
  {Granitite- 
  Gneiss). 
  

  

  To 
  this 
  division 
  belongs 
  a 
  type 
  of 
  gneiss 
  very 
  common 
  in 
  Mada- 
  

   gascar 
  and 
  occurring, 
  for 
  instance, 
  in 
  the 
  immediate 
  neighbourhood 
  

   of 
  the 
  capital. 
  It 
  is 
  a 
  medium-grained 
  rock 
  with 
  a 
  granitoid 
  

   texture, 
  foliation 
  not 
  being 
  very 
  marked 
  in 
  the 
  hand-specimen. 
  

   The 
  orthoclase 
  presents 
  a 
  well-developed 
  microperthite-structure, 
  

   inclusions 
  of 
  triciinic 
  felspar 
  appearing 
  in 
  short 
  oblong 
  patches 
  or 
  

   in 
  long 
  narrow 
  lamella?, 
  according 
  to 
  the 
  direction 
  of 
  the 
  section. 
  

   Besides 
  orthoclase, 
  the 
  rock 
  usually 
  contains 
  a 
  small 
  quantity 
  of 
  

   triciinic 
  felspar 
  (oligoclase). 
  White 
  mica 
  is 
  completely 
  absent. 
  

   Black 
  mica 
  and 
  deep 
  olive-green 
  hornblende 
  occur 
  in 
  moderate 
  

   quantity. 
  The 
  rock 
  therefore 
  has 
  the 
  composition 
  of 
  granitite, 
  

  

  