﻿REV. 
  R. 
  BARON 
  ON 
  THE 
  GEOLOGY 
  OE 
  MADAGASCAR. 
  305 
  

  

  18. 
  Notes 
  on 
  the 
  Geology 
  of 
  Madagascar. 
  By 
  the 
  Rev. 
  E. 
  Baron, 
  

   F.L.S., 
  F.G.S. 
  With 
  an 
  Appendix 
  on 
  the 
  Fossils, 
  by 
  R. 
  B. 
  

   Newton, 
  Esq., 
  F.G.S. 
  (Read 
  March 
  6, 
  1889.) 
  

  

  [Communicated 
  by 
  the 
  Director-General 
  of 
  the 
  Geological 
  Survey.] 
  

  

  [Plates 
  XIII. 
  & 
  XIV.] 
  

  

  General 
  Description. 
  

  

  Madagascar 
  is 
  as 
  yet 
  almost 
  a 
  terra 
  incognita 
  to 
  the 
  geologist, 
  

   nothing, 
  so 
  far 
  as 
  I 
  am 
  aware, 
  but 
  notices 
  of 
  the 
  most 
  fragmentary 
  

   kind 
  ever 
  having 
  appeared 
  in 
  regard 
  to 
  its 
  geological 
  features 
  * 
  ; 
  

   and, 
  indeed, 
  until 
  the 
  country 
  is 
  surveyed 
  by 
  competent 
  men, 
  we 
  

   must 
  be 
  content 
  with 
  descriptions 
  of 
  the 
  most 
  general 
  character. 
  

   In 
  the 
  absence 
  of 
  something 
  more 
  complete, 
  I 
  present 
  the 
  following; 
  

   notes, 
  drawn 
  up 
  from 
  personal 
  observation, 
  to 
  which 
  1 
  have 
  added 
  

   here 
  and 
  there 
  a 
  few 
  remarks 
  taken 
  from 
  other 
  sources, 
  as 
  a 
  slight 
  

   contribution 
  to 
  our 
  knowledge 
  of 
  the 
  geology 
  of 
  this 
  great 
  island. 
  

  

  The 
  central 
  portion 
  of 
  Madagascar 
  is 
  generally 
  regarded 
  as 
  consist- 
  

   ing 
  chiefly 
  of 
  granite. 
  Mr. 
  Wallace, 
  for 
  instance, 
  in 
  his 
  ' 
  Island 
  Life 
  ' 
  

   (p. 
  384), 
  says 
  of 
  it 
  : 
  "A 
  lofty 
  granitic 
  plateau, 
  from 
  80 
  to 
  160 
  miles 
  

   wide, 
  and 
  from 
  3000 
  to 
  5000 
  feet 
  high, 
  occupies 
  its 
  central 
  portion, 
  

   on 
  which 
  rise 
  peaks 
  and 
  domes 
  of 
  basalt 
  and 
  granite 
  to 
  a 
  height 
  of 
  

   nearly 
  9000 
  feet 
  ; 
  " 
  and 
  in 
  the 
  same 
  book 
  there 
  is 
  a 
  physical 
  sketch- 
  

   map 
  in 
  which 
  the 
  whole 
  of 
  the 
  interior 
  of 
  the 
  island, 
  from 
  about 
  14° 
  to 
  

   23° 
  S. 
  lat., 
  is 
  represented 
  as 
  an 
  " 
  elevated 
  granitic 
  region." 
  Now 
  if 
  

   we 
  use 
  the 
  terms 
  " 
  granite 
  " 
  and 
  " 
  granitic 
  " 
  in 
  a 
  wide 
  and 
  popular 
  

   sense, 
  and 
  include 
  in 
  them 
  the 
  various 
  members 
  of 
  the 
  crystalline 
  

   series 
  of 
  rocks, 
  the 
  description 
  may 
  be 
  regarded 
  as 
  correct 
  ; 
  for 
  by 
  

   far 
  the 
  greater 
  part 
  of 
  the 
  eastern 
  half 
  of 
  Madagascar 
  consists 
  of 
  

   gneiss 
  and 
  other 
  crystalline 
  rocks, 
  though 
  gneiss 
  very 
  largely 
  pre- 
  

   dominates. 
  Granite 
  occurs 
  frequently 
  in 
  the 
  form 
  of 
  bosses 
  and, 
  in 
  

   some 
  places, 
  apparently 
  intercalated 
  with 
  the 
  crystalline 
  schists 
  ; 
  

   diorite 
  is 
  also 
  frequently 
  met 
  with, 
  but 
  gneiss 
  is 
  certainly 
  the 
  pre- 
  

   vailing 
  rock. 
  The 
  area 
  occupied 
  by 
  these 
  ciystalline 
  rocks, 
  though 
  

   not 
  precisely 
  known, 
  reaches 
  on 
  the 
  east 
  as 
  far 
  as 
  the 
  coast 
  : 
  on 
  the 
  

   west 
  it 
  extends, 
  in 
  some 
  places 
  at 
  least, 
  as 
  far 
  as 
  46° 
  E. 
  longitude 
  f 
  ; 
  

   and 
  it 
  runs 
  in 
  a 
  northerly 
  and 
  southerly 
  direction 
  from 
  about 
  13° 
  50' 
  

   to 
  24° 
  S. 
  latitude. 
  In 
  other 
  words, 
  the 
  region 
  occupied 
  by 
  these 
  crys- 
  

   talline 
  (chiefly 
  metamorphic) 
  rocks 
  has 
  a 
  length 
  of 
  about 
  730 
  miles, 
  

   and 
  an 
  average 
  breadth 
  of 
  probably 
  not 
  less 
  than 
  150, 
  being 
  a 
  total 
  

   area 
  of 
  over 
  100,000 
  square 
  miles. 
  Indeed 
  it 
  is 
  not 
  at 
  all 
  improbable 
  

   that 
  the 
  area 
  may 
  cover 
  as 
  much 
  as 
  130,000 
  square 
  miles. 
  Fully 
  

   a 
  half 
  of 
  the 
  island, 
  therefore, 
  and 
  that 
  the 
  eastern 
  half, 
  consists 
  of 
  

  

  * 
  The 
  MS. 
  of 
  the 
  present 
  paper 
  was 
  prepared 
  long 
  before 
  Cortese's 
  contribu- 
  

   tion 
  in 
  the 
  Bollettino 
  del 
  R. 
  Com. 
  Geol. 
  1888, 
  Nos. 
  3 
  & 
  4, 
  was 
  published, 
  

   t 
  Its 
  most 
  westerly 
  limit 
  is 
  probably 
  about 
  45° 
  50'. 
  

  

  z2 
  

  

  