﻿GEOLOGY 
  OF 
  MADAGASCAR 
  

  

  313 
  

  

  the 
  direction 
  of 
  least 
  resistance 
  ; 
  hence 
  their 
  longer 
  axes 
  are 
  more 
  

   or 
  less 
  parallel 
  with 
  each 
  other. 
  This 
  peculiarity 
  may 
  distinctly 
  be 
  

   seen 
  about 
  Ambatovory. 
  Grauite 
  is 
  also 
  found 
  a 
  little 
  to 
  the 
  south 
  

   of 
  Mevatanana 
  ; 
  at 
  and 
  about 
  the 
  village 
  of 
  Yotovorona 
  (or 
  Soan- 
  

   drariny), 
  between 
  the 
  capital 
  and 
  Fianarautsoa 
  ; 
  in 
  the 
  bed 
  of 
  the 
  

   river 
  Mananta, 
  on 
  the 
  road 
  from 
  the 
  capital 
  to 
  Antongodrahoja 
  ; 
  

   and 
  dykes 
  of 
  it 
  appear 
  near 
  Tsirangaina, 
  a 
  few 
  miles 
  to 
  the 
  south- 
  

   west 
  of 
  the 
  capital, 
  and 
  near 
  Ambohipiara*, 
  about 
  a 
  dozen 
  miles 
  north- 
  

   west 
  of 
  the 
  capital. 
  At 
  Yombohitra, 
  Pamoizankova, 
  and 
  the 
  locality 
  

   to 
  the 
  east 
  of 
  the 
  capital, 
  the 
  granite 
  is 
  evidently 
  eruptive 
  ; 
  but 
  in 
  

   the 
  other 
  places 
  mentioned 
  (except, 
  of 
  course, 
  where 
  it 
  occurs 
  

   in 
  dykes) 
  I 
  should 
  be 
  inclined 
  to 
  regard 
  it 
  as 
  metamorphic, 
  for 
  it 
  

   seems 
  to 
  be 
  confusedly 
  intermingled 
  with 
  the 
  gneiss, 
  and 
  shades 
  

   off 
  into 
  it 
  so 
  imperceptibly 
  that 
  it 
  is 
  often 
  quite 
  impossible 
  to 
  say 
  

   where 
  the 
  granite 
  begins 
  and 
  the 
  gneiss 
  ends. 
  

  

  About 
  fifteen 
  miles 
  to 
  the 
  north-north-west 
  of 
  Mandritsara 
  (im- 
  

   mediately 
  to 
  the 
  south 
  of 
  the 
  village 
  of 
  Ambohitromby) 
  there 
  are 
  

   one 
  or 
  two 
  dome-shaped 
  bosses 
  of 
  what 
  is 
  probably 
  eruptive 
  granite 
  t; 
  

   and 
  the 
  gneiss 
  near 
  the 
  junction 
  has 
  been 
  altered 
  by 
  contact 
  with 
  

   the 
  heated 
  mass, 
  the 
  alteration 
  having 
  been 
  accompanied 
  by 
  the 
  

   development 
  of 
  various 
  minerals. 
  

  

  Older 
  Volcanic 
  Series. 
  

  

  Let 
  us 
  now 
  proceed 
  to 
  notice 
  some 
  of 
  the 
  volcanic 
  phenomena 
  of 
  

   the 
  region. 
  The 
  scene 
  of 
  the 
  greatest 
  display 
  of 
  former 
  volcanic 
  

   activity 
  in 
  Central 
  Madagascar 
  has 
  undoubtedly 
  been 
  Ankaratra. 
  

   This 
  mountain, 
  situated 
  some 
  twenty 
  to 
  thirty 
  miles 
  to 
  the 
  south- 
  

   west 
  of 
  Antananarivo, 
  is 
  the 
  highest 
  in 
  the 
  island, 
  attaining 
  an 
  

   altitude 
  of 
  nearly 
  9000 
  feet 
  above 
  the 
  sea. 
  It 
  is 
  a 
  broad 
  and 
  ele- 
  

   vated 
  mass 
  of 
  land, 
  with 
  no 
  very 
  sharp 
  peaks 
  or 
  ridges, 
  and 
  having, 
  

   for 
  the 
  most 
  part, 
  a 
  gentle 
  slope 
  of 
  4°-8° 
  on 
  all 
  sides, 
  so 
  that 
  it 
  is 
  

   not 
  easy 
  to 
  define 
  its 
  exact 
  limits. 
  Roughly 
  speaking, 
  however, 
  it 
  

   may 
  be 
  said 
  to 
  cover 
  an 
  area 
  of 
  perhaps 
  fifty 
  square 
  miles. 
  JNow 
  

   there 
  can 
  be 
  no 
  doubt 
  that 
  Ankaratra 
  is 
  the 
  wreck 
  of 
  a 
  huge, 
  but 
  

   ancient, 
  subaerial 
  volcano. 
  There 
  are 
  at 
  present, 
  so 
  far 
  at 
  least 
  as 
  

   my 
  observations 
  go, 
  no 
  traces 
  of 
  cones 
  or 
  craters, 
  but 
  there 
  are 
  

   volcanic 
  ejectamenta 
  scattered 
  about 
  which 
  bear 
  witness 
  to 
  their 
  

   former 
  existence. 
  Prom 
  this 
  volcano 
  vast 
  floods 
  of 
  liquid 
  lava 
  

   have 
  issued 
  and 
  overflowed 
  the 
  surrounding 
  country 
  to 
  the 
  extent, 
  

   probably, 
  of 
  from 
  1500 
  to 
  2000 
  square 
  miles. 
  In 
  fact, 
  almost 
  the 
  

   whole 
  of 
  Vakin' 
  Ankaratra 
  province 
  has 
  been 
  covered 
  by 
  a 
  sheet 
  of 
  

   lava. 
  This 
  lava 
  has 
  been 
  poured 
  out 
  at 
  various 
  times, 
  several 
  beds 
  

   being 
  superimposed 
  on 
  one 
  another. 
  Some 
  of 
  the 
  lava- 
  streams 
  are 
  

   probably 
  no 
  less 
  than 
  twenty 
  or 
  twenty-five 
  miles 
  in 
  length, 
  and, 
  

   before 
  they 
  thin 
  out, 
  from 
  300 
  to 
  500 
  feet 
  in 
  thickness. 
  They 
  are 
  

   mostly 
  of 
  a 
  basaltic 
  character, 
  chiefly 
  compact, 
  though 
  occasionally 
  

  

  * 
  Described 
  by 
  Dr. 
  Hatch, 
  p. 
  342. 
  

  

  t 
  Also 
  described 
  by 
  Dr. 
  Hatch, 
  p. 
  342. 
  

  

  