﻿GEOLOGY 
  OF 
  MADAGASCAR 
  309 
  

  

  The 
  gneiss 
  *, 
  in 
  many 
  places, 
  is 
  of 
  so 
  highly 
  metamorphosed 
  a 
  

   character 
  that, 
  at 
  first 
  sight, 
  one 
  would 
  conclude 
  it 
  to 
  be 
  granite 
  ; 
  

   but 
  an 
  examination 
  of 
  other 
  portions 
  of 
  the 
  mass 
  soon 
  reveals 
  its 
  

   nature. 
  In 
  and 
  about 
  the 
  capital, 
  for 
  instance, 
  the 
  gneiss, 
  which 
  

   is 
  hornblendic, 
  is 
  generally 
  so 
  highly 
  metamorphosed 
  that, 
  without 
  

   due 
  care, 
  its 
  real 
  character 
  may 
  be 
  overlooked, 
  as, 
  indeed, 
  is 
  shown 
  

   by 
  the 
  fact 
  that 
  it 
  is 
  almost 
  always 
  spoken 
  of 
  as 
  granite. 
  It 
  may 
  be 
  

   called 
  granitic 
  gneiss. 
  Still 
  the 
  rock, 
  in 
  certain 
  parts 
  of 
  the 
  island, 
  

   even 
  where 
  comparatively 
  large 
  sections 
  are 
  exposed 
  to 
  view, 
  pre- 
  

   sents 
  such 
  a 
  massive 
  character, 
  not 
  having 
  even 
  the 
  slightest 
  trace 
  

   of 
  foliation, 
  that, 
  could 
  one 
  feel 
  sure 
  that 
  its 
  texture 
  were 
  the 
  same 
  

   throughout 
  the 
  mass, 
  one 
  would 
  unhesitatingly 
  speak 
  of 
  it 
  as 
  gra- 
  

   nite. 
  Frequently 
  the 
  rock 
  appears 
  as 
  though 
  it 
  were 
  very 
  slightly 
  

   streaked, 
  and 
  may 
  be 
  called 
  granitic 
  gneiss. 
  

  

  The 
  gneiss, 
  being 
  so 
  abundant 
  and 
  covering 
  such 
  a 
  wide 
  area, 
  is, 
  

   as 
  might 
  be 
  expected, 
  various 
  in 
  texture 
  and 
  mineral 
  composition. 
  

   For 
  many 
  miles 
  round 
  the 
  capital 
  it 
  is 
  chiefly 
  of 
  a 
  greyish 
  or 
  bluish 
  

   colour, 
  while 
  in 
  the 
  mountains 
  of 
  Antaramanana 
  and 
  Vavavato 
  and 
  

   other 
  places 
  it 
  is 
  reddish 
  or 
  pinkish, 
  owing 
  to 
  the 
  flesh-coloured 
  

   orthoclase 
  contained 
  in 
  it. 
  A 
  great 
  deal 
  of 
  it, 
  moreover, 
  is 
  horn- 
  

   blendic, 
  while 
  in 
  some 
  districts, 
  notably 
  about 
  Lake 
  Itasy, 
  it 
  is 
  

   garnetiferous, 
  the 
  garnets 
  belonging 
  to 
  the 
  species 
  almandine. 
  

   They 
  are 
  of 
  the 
  kind 
  known 
  as 
  Cape 
  garnets, 
  but 
  are 
  of 
  no 
  com- 
  

   mercial 
  value. 
  As 
  for 
  the 
  mica 
  contained 
  in 
  the 
  gneiss, 
  it 
  seems 
  to 
  

   be 
  chiefly 
  biotite. 
  Muscovite, 
  however, 
  exists 
  abundantly, 
  and 
  may 
  

   be 
  sometimes 
  found 
  in 
  plates 
  several 
  inches 
  in 
  length. 
  

  

  The 
  most 
  abundant 
  of 
  the 
  accessory 
  minerals 
  existing 
  in 
  the 
  

   gneiss 
  is 
  probably 
  magnetite. 
  This 
  is 
  found 
  in 
  such 
  quantities 
  in 
  

   certain 
  localities 
  as 
  to 
  render 
  observations 
  taken 
  with 
  the 
  prismatic 
  

   compass 
  totally 
  unreliable. 
  In 
  the 
  part 
  of 
  the 
  country 
  east 
  of 
  

   Imerina 
  known 
  as 
  Amoronkay, 
  which 
  is 
  on 
  the 
  western 
  flanks 
  of 
  

   the 
  great 
  hill-range 
  spoken 
  of 
  above, 
  this 
  magnetic 
  iron 
  is 
  specially 
  

   abundant. 
  It 
  is 
  here 
  that 
  the 
  natives, 
  after 
  separating 
  it 
  from 
  the 
  

   gangue 
  by 
  washing, 
  work 
  it 
  in 
  their 
  rude 
  way, 
  manufacturing 
  chiefly 
  

   spades, 
  which 
  are 
  taken 
  for 
  sale 
  to 
  various 
  parts 
  of 
  the 
  island. 
  It 
  

   is 
  also 
  worked 
  in 
  the 
  same 
  way 
  in 
  Eastern 
  Betsileo, 
  the 
  beds 
  

   there 
  being 
  probably 
  only 
  a 
  continuation 
  of 
  those 
  at 
  Amoronkay. 
  

   Abundance 
  of 
  magnetite 
  is 
  also 
  found 
  a 
  little 
  to 
  the 
  west 
  of 
  Ambo- 
  

   hibeloma 
  (near 
  the 
  village 
  of 
  Anjamanga), 
  about 
  40 
  miles 
  to 
  the 
  

   west 
  of 
  the 
  capital; 
  at 
  Ambohitrandraina 
  hill 
  and 
  Ambohimanoa 
  

   mountain, 
  ten 
  or 
  a 
  dozen 
  miles 
  to 
  the 
  north-west 
  of 
  the 
  capital 
  ; 
  

   in 
  Antsihanaka, 
  to 
  the 
  north 
  of 
  Lake 
  Alaotra; 
  and 
  in 
  many 
  

   other 
  places. 
  Indeed 
  there 
  are 
  many 
  localities 
  where 
  it 
  is 
  so 
  abun- 
  

   dant 
  that, 
  were 
  there 
  coal 
  to 
  be 
  found 
  anywhere 
  in 
  its 
  neighbour- 
  

   hood, 
  it 
  might 
  be 
  expected 
  to 
  form 
  at 
  some 
  future 
  day 
  a 
  great 
  

   source 
  of 
  wealth. 
  It 
  exists, 
  in 
  fact, 
  in 
  grains 
  in 
  the 
  rock, 
  in 
  greater 
  

   or 
  less 
  proportion, 
  throughout 
  the 
  whole 
  of 
  the 
  area 
  occupied 
  by 
  the 
  

   gneiss, 
  and 
  by 
  its 
  oxidation 
  imparts 
  the 
  red 
  colour 
  to 
  the 
  soil. 
  In 
  

  

  * 
  A 
  petrographical 
  description 
  of 
  several 
  varieties 
  is 
  given 
  in 
  the 
  accom- 
  

   panying 
  paper 
  by 
  Dr. 
  Hatch 
  (pp. 
  340, 
  341). 
  

  

  