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  KEY. 
  K. 
  BAEON 
  ON 
  THE 
  

  

  great 
  and 
  monotonous 
  stretches 
  of 
  gneiss, 
  interspersed 
  here 
  and 
  

   there 
  with 
  other 
  metamorphic 
  rocks, 
  and 
  occasionally 
  granitic 
  

   bosses, 
  basaltic 
  masses, 
  and 
  volcanic 
  cones. 
  

  

  The 
  strata 
  have 
  been 
  plicated 
  by 
  lateral 
  pressure 
  into 
  numerous 
  

   anticlinal 
  and 
  synclinal 
  folds 
  or 
  mountain-waves, 
  the 
  dominant 
  axes 
  

   of 
  which, 
  as 
  also 
  the 
  strike 
  of 
  the 
  rocks 
  of 
  which 
  they 
  are 
  composed, 
  

   run 
  about 
  15° 
  to 
  the 
  east 
  of 
  north, 
  or 
  in 
  a 
  direction 
  parallel 
  with 
  

   the 
  eastern 
  sea-board, 
  and 
  also 
  corresponding 
  with 
  the 
  longitudinal 
  

   axis 
  of 
  the 
  island 
  *. 
  Hence 
  the 
  roads 
  from 
  Central 
  Madagascar 
  to 
  

   the 
  east 
  run 
  over 
  an 
  endless 
  series 
  of 
  more 
  or 
  less 
  parallel 
  hill-ranges, 
  

   while 
  those 
  to 
  the 
  north 
  and 
  south 
  pass 
  along 
  valleys 
  or 
  compara- 
  

   tively 
  level 
  country. 
  These 
  mountain-waves 
  attain 
  their 
  highest 
  

   elevation 
  at 
  a 
  distance 
  of 
  about 
  60 
  or 
  70 
  miles 
  from 
  the 
  east 
  coast, 
  

   where 
  they 
  run 
  in 
  a 
  linear 
  direction 
  nearly 
  north 
  and 
  south 
  for 
  

   probably 
  800 
  miles 
  or 
  more. 
  Here 
  is 
  situated 
  the 
  great 
  watershed 
  

   of 
  the 
  island, 
  and 
  from 
  here 
  also 
  the 
  folds 
  of 
  rock 
  gradually 
  dwindle 
  

   into 
  undulations 
  of 
  smaller 
  dimensions, 
  until 
  finally 
  they 
  disappear 
  

   below 
  the 
  Indian 
  Ocean 
  on 
  the 
  eastern 
  side, 
  and 
  on 
  the 
  western 
  

   pass 
  under 
  Secondary 
  and 
  Tertiary 
  formations, 
  of 
  which 
  something 
  

   will 
  be 
  said 
  by 
  and 
  by. 
  

  

  The 
  great 
  dorsal 
  ridge 
  which 
  forms 
  the 
  watershed 
  of 
  the 
  island 
  

   consists 
  of 
  gneiss, 
  with 
  a 
  dip 
  towards 
  the 
  west 
  at 
  an 
  average 
  angle 
  

   of 
  probably 
  30° 
  ; 
  for 
  a 
  great 
  part 
  of 
  its 
  course 
  the 
  ridge 
  has 
  on 
  its 
  

   eastern 
  side 
  a 
  steep 
  declivity 
  of 
  about 
  1500 
  feet. 
  About 
  20 
  or 
  30 
  

   miles 
  east 
  of 
  that 
  portion 
  of 
  the 
  ridge 
  which 
  bounds 
  Imerina 
  there 
  

   runs 
  for 
  some 
  distance 
  another 
  ridge 
  more 
  or 
  less 
  parallel 
  with 
  it, 
  but 
  

   with 
  the 
  dip 
  of 
  the 
  gneiss 
  apparently 
  to 
  the 
  east, 
  and 
  between 
  the 
  

   two 
  ridges 
  there 
  is 
  a 
  more 
  or 
  less 
  level 
  plain 
  or 
  valley, 
  which 
  

   extends, 
  with 
  few 
  interruptions, 
  for 
  about 
  130 
  miles, 
  the 
  southern 
  

   half 
  being 
  known 
  as 
  Ankay, 
  and 
  the 
  northern 
  as 
  Antsihanaka. 
  

   This 
  longitudinal 
  valley 
  has 
  served 
  as 
  a 
  receptacle 
  for 
  the 
  accumu- 
  

   lation 
  of 
  water 
  and 
  detritus, 
  brought 
  down 
  chiefly 
  from 
  the 
  high 
  

   ground 
  to 
  the 
  west, 
  in 
  fact 
  it 
  has 
  formed 
  the 
  site 
  of 
  a 
  lake. 
  In 
  

   an 
  article 
  in 
  the 
  ' 
  Antananarivo 
  Annual 
  ' 
  for 
  1885 
  I 
  made 
  the 
  

   following 
  remarks 
  with 
  regard 
  to 
  this 
  great 
  valley 
  : 
  — 
  " 
  The 
  largest 
  

   plain 
  in 
  Central 
  Madagascar 
  is 
  that 
  of 
  Ankay. 
  . 
  . 
  . 
  This 
  plain, 
  now 
  

   cut 
  and 
  scored 
  by 
  the 
  river 
  Mangoro 
  and 
  its 
  tributaries, 
  forms 
  the 
  

   bed 
  of 
  an 
  ancient 
  lake, 
  which 
  once 
  extended 
  for 
  a 
  distance 
  of 
  pro- 
  

   bably 
  30 
  or 
  40 
  miles, 
  running 
  in 
  a 
  direction 
  north 
  and 
  south 
  between 
  

   two 
  lines 
  of 
  hills. 
  Alaotra 
  in 
  Antsihanaka 
  (about 
  20 
  miles 
  long 
  by 
  

   4 
  or 
  5 
  wide) 
  is 
  perhaps 
  the 
  remnant 
  of 
  this 
  ancient 
  lake 
  or, 
  at 
  any 
  

   rate, 
  one 
  in 
  serial 
  connexion 
  with 
  it." 
  Towards 
  the 
  end 
  of 
  the 
  year 
  

   1886 
  1 
  passed 
  through 
  the 
  plain 
  of 
  Antsihanaka, 
  and 
  found 
  that, 
  not 
  

   only 
  were 
  my 
  conjectures 
  correct 
  with 
  regard 
  to 
  the 
  former 
  exten- 
  

   sion 
  of 
  the 
  lake, 
  but 
  that 
  the 
  reality 
  far 
  exceeded 
  even 
  my 
  conjectures. 
  

   In 
  fact 
  I 
  discovered 
  undoubted 
  evidence 
  that 
  this 
  ancient 
  sheet 
  of 
  

   water 
  originally 
  reached 
  at 
  least 
  as 
  far 
  north 
  as 
  latitude 
  15° 
  30' 
  in 
  

  

  * 
  It 
  is 
  of 
  interest 
  to 
  note 
  that 
  the 
  dominant 
  strike 
  of 
  the 
  crystalline 
  schists 
  in 
  

   Eastern 
  Africa 
  is 
  more 
  or 
  less 
  parallel 
  with 
  that 
  of 
  the 
  same 
  series 
  of 
  rocks 
  in 
  

   Madagascar. 
  

  

  